An infinitely good read

Via CP at the Knight Science Journalism Tracker (a highly recommended site for general science news) comes the suggestion for this excellent article by Science News editor Tom Siegfried: Success in coping with infinity could strengthen case for multiple universes.

Despite the strangeness and fanciful-seemingness of some of the ideas suggested in the title and the article itself, it's a pretty good summary of some current thinking about life, the universe, and everything. It's even understandable on one level – as long as you don't insist on knowing the mathematical details of things like string theory and cosmic inflation.

In outline, some reputable physicists, including some of the originators of inflationary cosmology, are arguing that they may have mathematical "proof" that there must exist multiple universes. The argument is based on the idea that without an infinite number of existing universes, similar in some respects to ours, yet possibly different in radical ways, the probability is nil that all the characteristics of our universe could be so precisely tuned as to allow the existence of sentient life.

While some may question whether sentient life does in fact exist in our universe, just take that as an assumption for the present. The idea just described is sometimes known as the Anthropic Principle. There are several forms of this principle, and all are rather controversial, in some degree or another, in the minds of people (such as physicists and philosophers) who think about such things.

Tom Siegfried is reporting (among other things) that physicists like Alan Guth and Andre Linde think they may have found a way to prove mathematically that an infinity of multiple universes must exist in order to explain the highly unlikely existence of sentient life in the one universe we know about.

Using the word "unlikely" here means that what's involved in a rigorous argument has to use the mathematical theory of probability. And to employ that theory, it is necessary to define what's called a "measure", which is a way of assigning a specific number to the relative size of a subset of a larger set. What has to be done, to support a mathematical argument for some validity in an application of the Anthropic Principle for deducing the necessity of multiple universes, is to find a suitable measure that makes it exceedingly unlikely that the universe we are aware of, with its particular forms of life as we know them, could exist if there were only one universe (or a finite number of them).

There are certain unobvious problems that have to be dealt with, for example the problem of "Boltzmann brains". That refers to something exactly like a human brain that could arise purely by chance in a universe that's infinitely large and infinitely old.

There are physicists who object violently to the idea of the Anthropic Principle, in any form, as an explanation for why the universe we can perceive seems to have the properties which allow the existence of sentient life. An alternative that many of these physicists prefer is the existence of mathematical principles that uniquely determine the properties of our universe – rather than have it all be a matter of chance, which leads to an infinite number of universes, each with very different characteristics and physical laws. Einstein, too, was a believer in the existence of deterministic principles, long before inflation was even thought of, but also before it was recognized just how finely-tuned a universe has to be to support life.

Unfortunately, a huge problem remains with this deterministic view, that our universe, if it's one of at most a finite number of other possible universes, can have the life-friendly properties we (think we) observe. Namely, why should mathematical principles dictate exactly this kind – and only this kind – of universe? Yet if such principles could allow more than a finite number of other universes, we'd be back in the multiple universe scenario, whether or not it's the scenario string theory seems to call for.

Further reading

Boltzmann brains and the scale-factor cutoff measure of the multiverse – August 2008 arXiv paper by Alan H. Guth, Andrei Linde, Alexander Vilenkin, and others

Life, the Universe, and Everything: A Conference Looks to Ultimate Origins – Sky and Telescope report on the conference

The cosmic "dark ages"

In brief, "dark ages" refers to the period after recombination occurred, about 380,000 years after the big bang, creating the cosmic microwave background (CMB), up to and partly including the time that the first stars had formed, perhaps as early as four hundred million years later, and caused the reionization of much of the neutral hydrogen in the universe.

That's a mouthful, but it's important to understand in order to have a useful discussion of the conditions that existed when the first stars and first galaxies in the universe formed. Many important open questions in astrophysics right now have to do with the nature of these events. Since I expect to discuss some of these questions, it's necessary to say some things about the "dark ages". That's what this note is about.

Let's start with the CMB. I'm going to give just a sketch. You might want to consult other references if you need more detail.

The period of time in which the CMB emerged is also known as the period of recombination. This was not an instantaneous process, but it did proceed relatively quickly, and is often thought of as a single event.

Basically, before recombination matter (mostly hydrogen and helium) and energy (light, i. e. photons) existed in thermal equilibrium. That is, the following reaction could occur with equal probability in either direction:

H + γ ⇄ p + e^-

Here, H stands for a hydrogen atom (with one electron), γ is a photon, p is a proton, and e^- is an electron. (We'll ignore helium, for simplicity.) What this formula says is that a photon of sufficient energy could dislodge an electron from a hydrogen atom to form a proton and a free electron, and with equal probability protons and free electrons could combine to form a hydrogen atom and a photon.

As the universe cooled after the big bang, the average temperature of the matter-energy plasma steadily dropped. Before the period of recombination, hydrogen atoms could exist, but not for very long, because most photons had enough energy to completely dislodge an electron, so the reaction went from left to right as often as from right to left.

Over a period of time lasting a few tens of thousands of years the situation changed so that most photons no longer had enough energy to dislodge an electron. (A low energy photon could still interact or "scatter" with a hydrogen atom by raising an electron to a higher energy level, but we can gloss over that detail.) Note that the average or "typical" photon energy may be a lot lower than what's needed to dislodge an electron, as long as there are still enough higher-energy photons around. This is a statistical situation governed by the Maxwell-Boltzmann equation, but all that really matters is that eventually photons and neutral hydrogen atoms "decouple" statistically.

So at some point you have recombination, when electrons combine with protons to make neutral hydrogen, and the process (mostly) doesn't reverse. Occasionally, a photon and a hydrogen atom may still interact, but as the universe expands, it's increasingly less likely for a photon and an atom to come close enough to interact, until the probability of interaction is essentially zero. This second stage is sometimes referred to as "decoupling" of matter and photons.

Thus the period of "dark ages" began right after the relatively brief process of recombination and decoupling. For simplicity, we date this point to a single time when the process was about half complete, roughly 380,000 years after the big bang.

This period is termed "dark", even though there were plenty of photons around, because there were as yet no stars or other compact sources of illumination. (There's another reasons for calling it "dark", which we'll get to in a moment.)

CMB photons have a nearly perfect black-body distribution. There is a clear peak of maximum energy in this distribution. What we observe is that this peak occurs at about 2 mm, in the microwave part of the spectrum. But the time of decoupling, 380,000 years after the big bang, corresponds to a redshift of z≈1100, so at the time of decoupling the peak photon energy was around a wavelength of just 2.2 μm (2200 nm) in the infrared part of the electromagnetic spectrum. (If you need to refresh your memory about how redshift works, check here.) So "dark" is not exactly the right term to use, but compared to abundant light from stars, it's not unreasonable.

After the time of recombination/decoupling, most hydrogen and helium atoms were neutral and un-ionized. This went on for several hundred million years. One of the most interesting open questions is about determining more exactly how long this lasted. We can reasonably guess what probably brought the dark ages to an end: formation of the first stars in the universe. But we don't have a good idea of just when this started, or how long the process took.

In this period, matter was beginning slowly to come together in higher-density clumps under the force of gravity. Dark matter, which outweighed ordinary matter then (as now), by a ratio of about 5.5:1 speeded up this process.

It was precisely this formation of regions of higher matter density that enabled the first stars to form. But because of this same higher density of matter around newborn stars, the abundant high-energy photons produced by these stars were again likely to interact with the nearby un-ionized matter, which scattered them and reduced their energy – dimming the light of these first stars.

One of the large uncertainties concerns the characteristics of this first generation of stars. We have no direct evidence about them. What we think we know about them is based on theoretical models rather than direct observation. (We discussed formation of the first stars back here.) However, it's widely believed that these stars were unlike stars formed later, right up to the present time. The first stars were probably quite large (maybe as much as 200 solar masses), very hot, and very bright. Because they burned their fuel so rapidly, their lifetimes would be very short, perhaps less than a million years.

Some of the light from extremely hot, massive stars such as those of the first generation is well into the ultraviolet part of the spectrum, around 90 nm. Such photons have an energy above 13.6 eV (91.2 nm wavelength), enough to completely ionize hydrogen. If these stars formed, say, 400 million years after the big bang, at a redshift of z≈11, the wavelength of their light would be shifted to the area beyond 1100 nm, which is in the infrared. That's beyond the range of human eyes, or most astronomical instruments. Consequently, we would have a very hard time detecting light from the first stars, even if they weren't so far away (over 13 billion light-years) and obscured by clouds of atomic hydrogen. That time period would still seem "dark" to us – there would be very little we could "see".

However, the first stars radiated so much energy, especially at ultraviolet wavelengths, that over time they effectively reionized all of the hydrogen in their vicinity. And this is (probably) what brought about the epoch of reionization in the universe, effectively ending the "dark ages".

The first stars were probably not part of galaxies, though we don't know for sure. If that's the case, they would be even harder to observe. The first objects we will be able to detect from this period almost certainly will be galaxies or quasars (which are galaxies with a very active central black hole). So the question of when galaxies began to form is separate, but equally puzzling. There is evidence that the first galaxies in fact did form before the end of the dark ages – because we can actually observe a few that show evidence of un-ionized hydrogen.

What we can say for sure is that the first stars must have consisted only of the primordial elements hydrogen and helium, since heavier elements (except for a small trace of lithium) formed and dispersed only when the first stars exploded as supernovae. But that did happen rather quickly, since the first stars were very luminous, and consequently had very brief lives.

As noted, we aren't very sure about when this first generation of stars appeared, because we can't yet observe them directly. But we do have some evidence concerning the epoch of reionization, and hence we have some idea of when it ended.

One kind of evidence involves studying the spectra of some of the most distant objects we are currently able to observe – quasars. We have detected a number of quasars at redshifts between 6 and 7. This range represents a time period from about 780 to 950 million years after the big bang.

There are absorption lines in the spectra of these quasars, and they tell us not only about the redshift, but give other information as well. Among the most important lines are those due to hydrogen that's not fully ionized, such as lines of the Lyman series. For the most part these lines are due to hydrogen in the vicinity of the source, in which case the lines are quite sharp and distinct.

But suppose there is a substantial amount of incompletely ionized hydrogen between us and the source. If this gas is at a distance sufficiently less (in terms of redshift), the absorption lines will be fuzzy instead of sharp. This effect is called a Gunn-Peterson trough.

In 2001 a quasar was identified at z=6.28, which showed a Gunn-Peterson trough, while other quasars with z≤6 did not. This suggests that reionization was mostly complete by 950 million years after the big bang, but not by 900 million years. [1][2]

Quasars, by themselves, are a possible contributor to reionization, in addition to the earliest stars. Quasars certainly produce enough high-energy photons. However, the question is whether there were enough quasars in existence during the epoch of reionization to account for the effect. Since only the very brightest quasars can currently be observed at that distance (z≥6), it's not possible to reliably estimate how many quasars altogether were around then. Rough estimates suggest there weren't enough.

There is another source of evidence for reionization, one very different from the Gunn-Peterson trough. This involves a very detailed study of anisotropies (irregularities at small angular scales) in the CMB. The CMB has many anisotropies due to conditions existing from the earliest moments after the big bang. However, if reionization occurred, certain kinds of additional characteristic anisotropies will also be present. These result from polarization of CMB light due to Thomson scattering of photons by free electrons (if such exist in sufficient numbers). Since free electrons are a by-product of reionization, they provide a very good marker, if they can be detected.

Unfortunately, the earliest data analysis (in 2003) from the Wilkinson Microwave Anisotropy Probe suggested that reionization occurred in the range 11<z<30, which corresponds to a mere 100 million to 420 million years after the big bang. This is not compatible with the quasar evidence. It's also rather implausible at the high z end, if reionization was caused by the first stars.

Fortunately, however, later data analysis (released in 2008) restated the range for reionization to 7≤z≤11. [3] z=7 still doesn't quite mesh with the quasar data, but it's pretty close. z=11 corresponds to 420 million years after the big bang, which is quite plausible for appearance of the first stars. Note that if this range is correct, then the reionization process took a lot of time, maybe 400 to 500 million years. First generation stars almost certainly weren't around that long. Such stars have very short lifetimes, and new stars of this kind can't form, because the gas from which they could form would contain considerable amounts of elements heavier than helium, precluding the formation of more stars like those of the first generation.

A third possible source of evidence comes from surveys looking for very faint, high-redshift galaxies (not quasars). Some objects have been found up to z=7.5 – about 710 million years after the big bang. [4] What isn't clear is whether these objects were either abundant enough or had hot enough stars to contribute significantly to reionization. But when the James Webb Space Telescope goes to work, sometime after mid-2013, we should be able to find many more early galaxies. The planned upgrade to the Hubble Space Telescope this year would also help – if it occurs.

Further reading:

[1] Evidence for Reionization at z ~ 6: Detection of a Gunn-Peterson Trough in a z = 6.28 Quasar – 2001 research article on first evidence for reionization (open access)

[2] First Light: Astronomers Use Distant Quasar To Probe Cosmic "Dark Age," Universe Origins (8/8/01) – press release describing the preceding research

[3] A New Day in Precision Cosmology (3/11/08) – news article describing analysis of WMAP data, including information on reionization

[4] Largest Sample Of Very Distant Galaxies Ever Seen Provide New Insights Into Early Universe (7/24/08) – press release

Tags: cosmology, cosmic dark ages

Evidence for dark energy accumulates

Dark energy (in its most plausible form as a "cosmological constant") has been a hypothetical possibility almost since Einstein's publication of his general theory of relativity in 1916. (Check here for our previous discussions of dark energy.)

However, it has been just over 10 years (since late 1997) that there has been strong evidence for the existence of dark energy. This evidence came from the observation of Type 1a supernovae. Such supernovae are expected on theoretical grounds to have roughly the same absolute brightness in all cases. This is because they result from the accumulation of hydrogen on the surface of white dwarf stars. This hydrogen is "stolen" by the white dwarf from a larger companion star, and as soon as a sufficient amount accumulates, a thermonuclear explosion occurs, destroying the white dwarf and producing a supernova.

Because all Type 1a supernovae should have approximately the same absolute brightness, it is possible to compare their observed brightness with what would be expected as a result of the absolute brightness and their estimated distance. The distance of a Type 1a supernova can be estimated from the redshift of its spectral lines, and assumptions about how fast the universe is expanding.

Up until 1997 it had generally been assumed that the universe was expanding, but at a slowly decreasing rate. However, what was determined in 1997 was that distant Type 1a supernovae had an observed brightness that was dimmer than would be expected on the assumption that the expansion of the universe was decelerating. Instead, the most natural assumption was that the expansion was accelerating, which would mean that the distant supernovae were farther away than expected, and hence dimmer.

There was a lot of uncertainty in the initial measurements of supernova brightness, as well as questions about the suitability of assumptions made in order to calculate the expected brightness. However, there were two other lines of evidence that supported the idea of a cosmological constant (and hence, dark energy).

One line of evidence was obtained from observations of the angular size of hot and cold spots in the cosmic microwave background (CMB) radiation. The actual size of these fluctuation can be calculated theoretically based on certain reasonable assumptions. However, the size that we observe depends on the curvature of the universe. For instance, if the curvature is positive, like a convex lens, then the angular size of the fluctuations will be magnified and appear larger than calculations predict. But it turns out that the observed size is very close to what is predicted, meaning that the universe must be nearly flat. And from other considerations, the universe can be "flat" only if there is a much higher energy density than can be accounted for in terms of all suspected types of matter, even dark matter. This extra energy density is best accounted for in terms of the dark energy.

A third line of evidence comes from the observed distribution of galaxies and galaxy clusters. The effect of dark energy to cause the expansion of the universe to accelerate also causes galaxies and clusters of galaxies to be spread farther apart than we would otherwise expect – and this additional spread is exactly what is observed.

However, the idea of dark energy, especially if it is based on a cosmological constant, is fairly radical, because we have no theoretical way to explain what dark energy is or why it should exist. Therefore, the more evidence we have that it does in fact exist the better.

So it's quite welcome that a fourth line of evidence for the existence of dark energy is now much more strongly supported by data in a new study. The new evidence is based on more precise measurements of what is called the integrated Sachs-Wolfe effect. This effect is also found in observations of the CMB, but observations of a very different kind.

The effect is predicted to be manifested as microwave photons of the CMB pass through regions of the universe with densities that are higher or lower than the overall average. Consider a region of higher density, such as a supercluster of galaxies. As the photon enters the region, its energy will increase, because it is exchanging gravitational potential energy for electromagnetic energy, like a rock gains kinetic energy falling in Earth's gravitational field. The photon's energy gain is manifested in a shorter wavelength.

Galaxy superclusters are very large, from 100 to 500 million light-years in diameter. So in the time it takes a photon to cross a supercluster, the expansion of the universe will reduce the average matter density of the supercluster. The net effect is that the photon will lose less energy as it is leaving the supercluster than it gained when it entered. So the photon has a net energy gain in the process.

The universe also contains "supervoids", which are regions of size similar to superclusters where there are few galaxies, and the average matter density is less than the overall average. While a photon is passing through a supervoid, it will experience a net energy loss. On top of these energy gains and losses, a photon also gradually loses energy due to the expansion of the universe (as the photon wavelength gradually increases). There are still gains and losses after making allowance for this expansion effect. Moreover, the energy gains or losses are magnified if the expansion is accelerating.

The integrated Sachs-Wolfe effect is essentially these magnified energy gains and losses. The existence of this effect is a testable prediction of the existence of dark energy. Another way to think of the effect is as a measure of the extent that a supercluster or supervoid is expanding under the influence of dark energy, whereas there should be no expansion in the absence of dark energy. Importantly, this effect is independent of the brightness-distance relationship for Type 1a supernovae.

The new evidence for dark energy, then, is that very careful measurements of the energy of CMB photons in the directions of known superclusters and supervoids detect the existence of the integrated Sachs-Wolfe effect with very high probability, and hence another prediction based on the existence of dark energy is verified.

In the present study, about 3000 superclusters and 500 supervoids were initially selected from the Sloan Digital Sky Survey. This is out of around 10 million superclusters estimated to exist in the visible universe. Out of this sample, 50 superclusters and 50 supervoids having the largest density variation from the average were selected for closer examination.

The maximum distance of a chosen cluster was a redshift of about .5, corresponding to a distance of about 5 billion light-years. Because of the huge size of a supercluster, a typical supercluster would have an angular diameter, as seen from Earth, of about 1/25 of full circle, or 14 degrees. The researchers decided to consider circles of angular radius 4 degrees around the center of a cluster as containing the bulk of the cluster. Such circles are still about 16 times the diameter of the full Moon (1/2 angular degree).

Within each circle, the average temperature of CMB photons was measured, and compared to the overall average. The variations were very small – about 10^-5K, compared to average CMB photon temperature of 2.73K – about 3 parts in a million. Nevertheless, the measurements were accurate enough that the probability of this variation being measured by chance is only about 1 in 200,000.

This is not the first research effort that has produced evidence for the integrated Sachs-Wolfe effect. However, it is based on cleaner data, and has the lowest probability of falsely showing an effect based only on chance.

News articles:

• Most Direct Evidence of Dark Energy Detected (8/11/08)
  
• The most direct signal of dark energy? (8/8/08)
  
• Supervoids and clusters reveal dark energy (8/7/08)
  
• Dark Energy's Early Fingerprints (8/6/08)
  
• Dark Energy's Fingerprint Found in Distant Galaxies (8/5/08)
  
• Dark Energy Signs Seen in Giant Clusters and Voids (8/4/08)
  
• UH team sees ‘dark energy’ trail (8/4/08)
  
• Caught in the Act: Dark Energy Expanding the Universe (8/4/08)
  
• Unmasking Dark Energy (8/1/08)
  
• Scientists Find Direct Evidence of “Dark Energy” in Supervoids and Superclusters (7/31/08)
  
• Dark energy 'imaged' in best detail yet (5/23/08)
  

Further reading:

Supervoids and Superclusters – Web pages produced by the research team, with illustrations and background information

An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect – short technical paper describing the research

Dark Energy Detected with Supervoids and Superclusters – longer, more leisurely presentation of the research, by the research team

Tags: dark energy, Sachs-Wolfe effect

Mystery deepens over origin of biggest black holes

Mysteries are popular, so here's a good one, about another popular subject, black holes.

Mystery deepens over origin of biggest black holes (5/19/08)

Where did the universe's biggest black holes come from? One idea suggests the behemoths began as smaller "seed" black holes that gobbled up surrounding gas. But new computer simulations suggest these seeds were born with practically nothing around them to eat, deepening the puzzle over how the biggest black holes came to be. ...

How these supermassive black holes grew so big so fast has been a major puzzle. Some astronomers have suggested that they grew from smaller black holes of about 100 times the Sun's mass, left behind when the universe's first stars collapsed at the end of their lives.

Note that this isn't the only way that black holes could have formed, not even black holes of about this size. That's good, because what this study seems to show is that it would be difficult for a black hole formed from a collapsed early star to accrete enough matter to grow into a supermassive black hole (SBH).

But the universe's first stars were not born until a few hundred million years after the big bang. Even though they lived only a few million years before collapsing to form black holes, this does not leave much time for these seeds to grow into the monstrous black holes powering quasars.

The puzzle now appears to have deepened, with new computer simulations suggesting that these seed black holes were born with little food around them from which to gain weight.

Tom Abel of Stanford University in California, US, and his colleagues made computer models simulating the first generation of stars. These first stars are thought to have been very massive and luminous, weighing about 300 times as much as the Sun. The simulations reveal that the stars' prodigious radiation would have blown away the gas around them.
Early famine

As a result, the black holes that formed when the stars died a few million years later would have had very little to eat. In the simulations, it took about 100 million years for the gas to fall back towards the first black holes and provide them with something to eat. The time lost due to the early famine makes it even harder to imagine how these black holes could have swelled to billions of times the Sun's mass soon thereafter.

Let's just suppose this simulation result makes the formation of SBHs from such seeds very unlikely. Are there other reasonable possibilities?

Sure there are. One is that black holes don't exist at all, so neither do SBHs. Never mind that most galaxies seem to contain very massive objects, whose mass is 10⁵ solar masses or more. (Normally written as 10⁵M_⊙.) The Milky Way itself has a black hole at its center, whose mass is estimated at 3.7×10⁶M_⊙.

Such massive objects need an explanation too, which would have at least the same difficulties as for SBHs. But we've already noted that the evidence for black holes is very solid (see here), so let's rule this out.

Here's another recent hypothesis:

Biggest black holes may grow inside 'quasistars' (11/29/07)

The biggest black holes in the universe might have grown within the bellies of giant stars, a new study suggests. If these hole-bearing "quasistars" exist, then they might be bright enough to see from across the universe.

Quasistars are one attempt to explain the existence of supermassive black holes, which astronomers have detected at the hearts of most large galaxies, and whose origin is still unknown.

It is calculated that this could account for objects up to 10⁴M_⊙ in size. One problem here is that possible quasistars have never actually been observed, and observation is expected to be difficult at best, since they would have formed so soon after the big bang, and consequently now be very distant (like more than 10¹⁰ light-years).

Another problem is that SBHs can be far larger than this. The largest SBH known so far is about 1.8×10¹⁰M_⊙. (See here, here.) SBHs that formed in quasistars would still need to accrete something like 10⁶ times their initial mass to grow into the largest known SBH, let alone any larger ones that might be out there. (For comparison, the mass of the Milky Way, a medium-size galaxy, is about 10¹²M_⊙. See here, here.)

So a quasistar origin for SBHs doesn't seem too likely either, but if you want to pursue it further, the relevant paper is here.

We haven't yet even mentioned one obvious possibility: primordial black holes (PBHs). That is, black holes that formed directly very soon after the big bang itself. Exactly at what time this might have been depends very much on how PBHs might have formed, and there's plenty of uncertainty about that.

In fact, we have no evidence yet that PBHs exist at all. Since we don't know exactly how they might have formed, we don't know how big they might be, which strongly affects what we should look for. If PBHs were small enough, they should eventually "evaporate", as Stephen Hawking suggested, by the process of Hawking radiation.

There's much debate as to what happens when a sufficiently small PBH evaporates completely (see here, here, here, here, here). But such an event is generally supposed to include emission of gamma-rays, and these might be detected by the newly-launched GLAST mission. (See here.)

The most likely scenario for the formation of a PBH is as a result of gravitational collapse of overdense regions existing because of density fluctuations dating from the earliest instants after the big bang. Since fluctuations are the key, formation of PBHs is governed by probability, and the larger the PBH, the lower its probability, and hence the fewer that form altogether. While there's no obvious upper limit to the size of a PBH, the formation of one around 10⁴M_⊙ would be very improbable. Too improbable? No one knows.

Other mechanisms hypothesized for formation of PBHs tend to feature rather exotic things like topological defects (cosmic strings or domain walls) or certain kinds of phase transitions.

But this is all so speculative that it's not much help for evaluating whether PBHs might make a good source of SBHs. A great reference for stuff about PBHs is this: Primordial Black Holes: Do They Exist and Are They Useful?.

The bottom line is that closer study of SBHs, which almost certainly do exist, should eventually lead us to some concrete evidence for things that are presently much more speculative.

Tag: black holes, supermassive black holes, primordial black holes

Searching for dark energy... at the South Pole

Not all experimental astrophysical studies require elaborate, incredibly expensive equipment deployed at the L2 Sun-Earth Lagrangian point, like WMAP. A lot can be done with a microwave antenna just 10 meters across... if it's located at the South Pole.

Cosmologists Probe Mystery Of Dark Energy With South Pole Telescope

What can the SPT tell us about the past and future of dark energy? John E. Carlstrom, director of KICP and the S. Chandrasekhar Distinguished Professor in Astronomy and Astrophysics at the University of Chicago, says the telescope is examining clusters of galaxies to learn what role dark energy played in their evolution. “One of the important things we need to learn about dark energy is what influence it has had on structure,” Carlstrom says. If scientists can learn how the density of clusters changed over time, he says they can determine “constraints on the equation of state of dark energy.” That is, they can get a more precise idea of whether dark energy is taking us toward a big rip, a big crunch or something in between.

The telescope is looking specifically for the Sunyaev-Zel’dovich (SZ) effect, a distortion of the CMB radiation caused by the highly energized gas of galaxy clusters. When photons originating from the CMB traverse the clusters, they interact with electrons and tend to scatter, creating slight variations in temperature -- shadows against the microwave background – that the SPT detects with a battery of 1,000 sensors chilled to near absolute zero.

The SPT will survey about a fifth of the entire southern sky and is expected to detect thousands of clusters. Analyzing follow-up data from optical telescopes, the scientists will determine the mass, distance and age of the clusters. They will then map the clusters in space and time to see how their density and structure evolved over billions of years under the competing pulls of gravity and dark energy. They hope to learn how much power dark energy exerted in the early universe, how it evolved to dominate the universe now, and by extension, how much power it may wield in the future.

But the SPT isn't adapted only for studies of dark energy. As a sensitive microwave telescope, it can also make detailed observations of the cosmic microwave background, much as WMAP does.

The SPT’s activity will not end with this survey of galaxy clusters. Another project in the works will use the telescope to scan the CMB for tiny fluctuations in its polarization. Like visible light, the microwave radiation from the Big Bang has waves moving in electromagnetic fields at different angles, some up-and-down and other side-to-side. Observations with another South Pole instrument, the degree angular scale interferometer (DASI), have confirmed that the CMB is polarized as expected from prevailing theories about the physics of the Big Bang. Researchers now want to use the more sensitive SPT to look for minute variations in the CMB polarization that mark the presence of huge gravity waves.

Stephan Meyer, associate director of KICP and Professor in Astronomy and Astrophysics at the University of Chicago, says these waves are “a reasonable fraction of the size of the universe” in length and would have been generated in the “inflationary epoch” of the Big Bang. This was the time when the universe was just 10^-50 seconds old and matter had not yet coalesced into neutrons and protons. “We don’t really understand the physics of that era,” Meyer says. A new set of sensors, able to detect polarization as well as heat, is being built by the University of Chicago and should be ready for installation on the SPT by the austral summer (the northern winter) of 2009-10.

Tags: dark energy, cosmic inflation

Readings: Cosmology and astrophysics, 4 November 2007

The text following each item is quoted material, except for editorial comments, which are in color.

FSU physicist shining a light on mysterious 'dark matter'

"Recent scientific breakthroughs have shown that most of the matter in the universe—about four-fifths—is not made up of atoms, but of something else, called 'dark matter,'" said Howard Baer, FSU's J.D. Kimel Professor of Physics. "The evidence for dark matter is now overwhelming, and the required amount of dark matter is becoming precisely known." ...

A theoretical physicist, Baer employs mathematical models and calculations, as opposed to experimental methods, in an attempt to understand the basic properties of dark matter. To that end, he travels frequently to CERN, the world's largest particle physics laboratory, located on the border between France and Switzerland. At CERN, teams of physicists from all over the world are preparing for the start-up of what will be the world's most powerful particle accelerator, the Large Hadron Collider (LHC), in 2008. With the LHC, they will conduct experiments that seek to solve some of the fundamental mysteries of science, including the identity of dark matter. In addition to searches at the LHC, the hunt for dark matter is progressing at experiments deep underground in Minnesota, under thick Antarctic ice, and even in outer space.

Another piece in the dark matter puzzle

“We took one specific theory about dark matter,” Riemer-Sørensen explains. “We look at a specific type of decaying particles, and if they represent dark matter, they will decay and transform into photons in x-rays.” The particles in question are axions, hypothetical elementary particles used in theories describing “extra” dimensions. The idea, she says, is to look for an area of the universe that has a great deal of dark matter, and then look for weak x-ray emissions. ...

So, did Riemer-Sørensen and her colleagues find the weak dark matter x-ray emissions? “We didn’t find any clear signs of x-ray emissions from axions in these regions,” she says. “And that tells us something about dark matter.” If dark matter particles do follow the reactions of decay set forth in the theory of axions as dark matter, then dark matter has an extraordinarily long lifetime. “If dark matter does decay,” Riemer-Sørensen insists, “then the lifetime of the axions is at least three million billion years, which is twenty thousand times longer than the lifetime of the universe.”

Can this experiment identify dark matter?

“Many experiments and observations all point to the existence of some form of matter that is different from the ordinary matter that makes up starts, planets and even people,” Bertone explains. Because dark matter is so prevalent in the universe, many scientists are interested in better understanding its role in fundamental physics, as well as the formation of the universe. “There are efforts to clarify the nature of dark matter.”

Bertone explains that there are three main approaches to detecting dark matter particles, which are likely to be weakly interacting massive particles (WIMPs). The first, he says, is an earthbound method using particle accelerators, like the Large Hadron Collider due to go online at CERN next year. “Scientists hope to find particles in accelerators that could be like the dark matter found in the rest of the universe.”

The next method of detection is one of indirect observation. Looking out into space, Bertone says, scientists “look for some signal due to interaction of particles amongst themselves.”

The strategy set forth in the article belongs to the third approach, which is to build a large detector and wait for a dark matter particle to interact with ordinary matter. “To show the power of this technique, we focused on an experiment called COUPP [Chicagoland Observatory for Underground Particle Physics]” Bertone says. “It is a bubble chamber, much like what has been used before in other fields.”

In the dark: science still mystified by stuff of universe

Most of the universe -- 96 percent, to be exact -- is made of dark matter and energy whose composition we simply do not fathom, a Nobel laureate told physicists gathered this week to explore the intersection of the infinitely small and the infinitely large. ...

Most physicist attending the conference here on astroparticle physics think the basic ingredient is probably some as-yet undiscovered elementary particle, a relic of the "Big Bang" that created the Universe around 13 billion years ago.

The favored candidate is the neutralino, a "supersymmetric" particle whose existence has yet to be proven. But the hunt in underway, using both direct and indirect methods, including experiments to be conducted at the Large Hadron Collider (LHC) in Switzerland.

Over the next decade, explained Katsanevas, scientists will be tackling three big questions besides dark matter: the origin of cosmic rays, the existence of gravitational waves, and the mass of neutrinos, which have provided the first solid evidence of phenomena beyond what is called the Standard Model of particle physics.

Astronomers Aim to Shine Light on Universe's 'Dark Energy'

In nearly a decade since it was discovered, a mysterious cosmic feature dubbed "dark energy" has lain like a downed redwood across the path of scientists trying to reach the holy grail of physics – a fundamental theory of matter and its basic forces. ...

"After about 10 years it's clear [dark energy] is not going away.... We have to really figure out what this is," Riess says. The past decade also has shown that "dark energy lives at the crossroads of two of our best theories of physics: quantum mechanics and general relativity."

A successful marriage of quantum theory and gravity is the last major hurdle in demonstrating that the basic four forces of nature – gravity, electromagnetism, and weak and strong forces that operate at the subatomic level – are manifestations of a single force that dominated the universe in the first few fractions of a second after the big bang. With dark energy, "nature is giving us a hint of how it does quantum gravity," Riess says.

Hubble Telescope: Solved and Unsolved Mysteries

Beyond snapping extraordinary pictures of faraway nebulas, the revolutionary Hubble Space Telescope has completely transformed our view of the universe since it was launched in 1990. By capturing the clearest, deepest images of the cosmos ever, Hubble has shed light on some long-standing mysteries perplexing scientists-while uncovering far deeper ones that have yet to be solved. ...

Dark energy has prompted new theories regarding the origin of the universe, such as one where clashing membranes of reality trigger endless cycles of cosmic death and rebirth, as well as the fate of the universe, raising the possibility that dark energy ends the universe in a Big Rip. Future progress on understanding dark energy's nature will likely require a dedicated dark energy space mission, "for sometime in the middle of the next decade, perhaps," Leckrone said.

The other mysteries mentioned in the article involve dark matter, gamma-ray bursts, direct imaging of extrasolar planets, and protoplanetary disks. These are all issues which can be studied by more or less conventional optical or infrared telescopes – much like Hubble, only more powerful. There is also a need for a more powerful ultraviolet-sensitive telescope, which is not currently even part of NASA's agenda. Although the descriptions of the "mysteries" in the article are sketchy, there are links to additional information.

Is the universe a doughnut?

Later work by Neil Cornish of Case Western University, David Spergel of Princeton University, and Glenn Starkman of the University of Maryland extended this technique to consider a wider range of possible topologies. Such a method has been applied to the WMAP results, examining the possibility that it could have a complex topology— not a toroid perhaps, but rather a dodecahedron (a bit like a soccer ball, but with all sides equivalent in size and shape). Although preliminary data (analysed in 2003) seemed to rule out this model, more recent looks at the WMAP findings have revived the idea that if you venture far enough out into space you'll return to your starting point. Hence Homer's doughnut theory may have at least a sprinkling of truth: the universe could indeed have loops.

This is a pretty good article for an overview of the shape of the universe, in spite of its facetious premise (that a cartoon world can be effectively used to explain highly technical cosmology). The genre, of popular TV shows used as points of embarcation into explanations of scientific topics, is growing. First there was The Physics of Star Trek, which is not too surprising a connection. But The Physics of the Buffyverse? And now, in effect, physics according to Homer Simpson? Having read only the first of these, I don't know that these aren't all quite good books. But what this trend says about our culture... I don't really want to go there.

However, if Robert Gilmore can write physics books based on such metaphorical worlds as those of Alice in Wonderland, The Wizard of Oz, A Christmas Carol, or Grimms' Fairy Tales, ... well, why not? Perhaps we need someone to write books of physics based on The Odyssey or The Divine Comedy? And while we're at it, let's not leave out the physics of the Niebelungenlied and the Mahabharata. I can hardly wait.

Why the Universe is All History

Some galaxies are so remote that their light hasn't had sufficient time to reach us yet, despite about 13.7 billion years of travel. There could also be more distant objects that will forever remain unknown to us.

"Because the universe is expanding and the expansion appears to be accelerating, there may be distant galaxies which if we can't see them now because their light has not had time to reach us, we will never see," Stecker said.

So we can never see the universe as it is, only as it was at various stages of its development.

New-School 'Aether' May Shed Light on Neutron Stars

Among scientists, it is widely believed that there is no such thing as an aether – a medium pervading all space that allows light waves to propagate, similar to how sound needs air or water – but a part of its spirit may live on. A group of University of Maryland (UM) physicists have proposed a modern spin on the aether of old and have used it to make new predictions about the behavior of neutron stars. ...

The UM researchers – Christopher Eling, Ted Jacobson, and Coleman Miller – describe their aether as a preferred state of rest at each point of spacetime. This preferred state would not be the result of something known, such as a gravitational field or cosmic background radiation, but may, they say, arise from the structure of empty space in quantum gravity theory. ...

The UM team use the new aether to make concrete predictions about neutron stars that differ from those generated by general relativity, Einstein’s theory of gravity. The group's calculations show that the maximum mass of neutron stars would be smaller than in general relativity and the increase in wavelength, or “redshift,” experienced by photons emitted from the stars' surfaces must be 10 percent larger.

Predicting Planets

Discovering new planets that orbit distant stars has become commonplace. But now a team of astronomers has managed to predict the orbit of an extrasolar planet — before anyone knew for certain that it existed. The last time that happened was more than 150 years ago. ...

The more-recently discovered planet is known by a rather-less-elegant name: HD 74156d. It is a gas giant, slightly more massive than Saturn, orbiting a sun-like star about 65 parsecs (212 light-years) away. Its orbit was predicted in 2004 by Rory Barnes and Sean Raymond, theoretical astronomers then at the University of Washington in Seattle. Three years later, Jacob Bean, an astronomer now at the Georg-August-Universitaet in Goettingen, Germany, announced that he had found the planet, pretty much where Barnes and Raymond said it would be.

Lonely Planets of the Cosmos

A brief letter in Nature was John Debes's inspiration. The 1999 piece, by David J. Stevenson (Caltech), proposed that planets with liquid water oceans — and even life — could exist in the cold, dark depths of interstellar space far from any star. Based on the knowledge that some fraction of planets must get gravitationally ejected from their systems during the systems' formation, the paper theorized that some of these ejected planets could, with enough internal heat, keep their atmospheres and stay warm enough to support liquid water below a thick frozen crust.

What might happen if such an outcast had a big moon? To find out, Debes (at the Carnegie Institution of Washington) ran 2,700 computer simulations based on an Earth-mass planet and a lunar-mass companion.

The Enduring Mysteries of the Sun

The sun lies at the heart of our solar system, but it still holds back many secrets from science. Unlocking these mysteries could shed light on puzzling activity seen in other stars and even safeguard lives.

It's surprising how much we don't understand about stars – from either theory or observation – considering that we live so close to one.

Tags: astrophysics, cosmology, dark matter, dark energy

Dwarf galaxies

Smallest Galaxies Ever Seen Solve a Big Problem

Mauna Kea scientists may have solved a discrepancy between the number of extremely small, faint galaxies predicted to exist near the Milky Way and the number actually observed. In an attempt to resolve the “Missing Dwarf Galaxy” problem, two astronomers used the W. M. Keck Observatory to study a population of the darkest, most lightweight galaxies known, each containing 99% dark matter. The findings suggest the “Missing Dwarf Galaxy” problem is not as severe as previously thought, and may have been solved completely.

“It seems that very small, ultra-faint galaxies are far more plentiful than we thought,” said Dr. Marla Geha, co-author of the study and a Plaskett Research Fellow at the Herzberg Institute of Astrophysics in Canada. “If you asked me last year whether galaxies this small and this dark existed, I would have said no. I’m astonished that so many tiny, dark matter-dominated galaxies have now been discovered.”

I don't really have a lot to add to this, except to remark that this is probably a more significant result than may be immediately obvious. After all, so a lot of tiny, lightweight galaxies have been found in orbit around the Milky Way – so what?

The significance is that this observation goes a long way towards solving a problem with the hypothesis that the universe contains large amounts of "cold dark matter" – perhaps 4 times as much mass in the form of "dark matter" than there is in the form of more ordinary "baryonic" matter. There is already a huge amount of evidence for the existence of dark matter, as discussed here and here.

The problem is that simulations which have been done on the evolution of galaxies under the assumption of a ratio of 4:1 dark matter to baryonic matter predict the existence of many more small "dwarf" satellite galaxies around the Milky Way than are actually observed. Intuitively, one would expect many small galaxies, since if visible galaxies consist of stars made of baryonic matter inside blobs of dark matter, there ought to be a large range of sizes, from the smallest to the largest. Instead, what has been observed until now is far too few of the smallest sizes.

The solution suggested by the results here is that galaxies that formed inside the smallest blobs of dark matter have far fewer stars than would be expected, and hence they are intrinsically dim and hard to detect, so that most very small galaxies simply haven't been noticed.

But why would such galaxies have so few stars? This question remains to be answered, but a plausible hypothesis is that most of the gas of these galaxies, from which stars could form, may have been literally blown away by the intense light radiated by the first very large stars that formed in the Milky Way itself:

Based on the masses measured for the new dwarf galaxies, Drs. Simon and Geha concluded the fierce ultraviolet radiation given off by the first stars, which formed just a few hundred million years after the Big Bang, may have blown all of the hydrogen gas out of the dwarf galaxies forming at that time. The loss of gas prevented the galaxies from creating new stars, leaving them very faint, or in many cases completely dark. When this effect is included in theoretical models, the numbers of expected and observed dwarf galaxies agree.

“One of the implications of our results is that up to a few hundred completely dark galaxies really should exist in the Milky Way’s cosmic neighborhood,” said Dr. Geha. “If the Cold Dark Matter model is correct they have to be out there, and the next challenge for astronomers will be finding a way to detect their presence.”

Other reports on this research: here, here

Preprint of the research paper: The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem

Preprint of a subsequent research paper describing how dark matter content of very faint dwarf galaxies might be confirmed: The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs

Tags: dark matter, dwarf galaxies

Beyond Einstein redux

You may recall a rather detailed discussion last November of NASA's Beyond Einstein program back here. In a nutshell, NASA was looking at a number of very interesting space missions related to astrophysics and cosmology. But because of the foolish emphasis being placed on manned missions to the moon and (eventually) Mars and the multi-billion-$ cost of such missions, it remained to be determined whether enough money would be left over for even a few of the science missions.

Among the proposed science missions, there were two that were well along in the planning stage – LISA (to detect and study gravitational waves), and Constellation-X (a powerful X-ray observatory to be used for studying black holes and hot gas in galaxy clusters).

In addition, there were three other projects less far along in planning: a dark energy probe, an inflation probe, and a black hole finder.

In order to prioritize and choose among these missions, the Powers That Be decided to ask the National Research Council to evaluate the various missions and report back. In April Steinn Sigurðsson at Dynamics of Cats provided an interim report on the occasion of a meeting of the committee given the assessment task.

On September 5 an answer came back from the NRC:

'Beyond Einstein' Research Should Begin with Mission to Study Dark Energy

NASA and the U.S. Department of Energy should pursue the Joint Dark Energy Mission (JDEM) as the first mission in the "Beyond Einstein" program, according to a new report from the National Research Council. Beyond Einstein is NASA's research roadmap for five proposed mission areas to study the most compelling questions at the intersection of physics and astronomy. The committee that wrote the report added that another proposed mission to detect gravitational waves using the Laser Interferometer Space Antenna (LISA) should eventually become the flagship mission of Beyond Einstein, given that it is likely to provide an entirely new way to observe the universe. However, LISA needs more testing before a launch can be planned, whereas the Joint Dark Energy Mission is ready now for a competitive selection of mission concept proposals.

So it appears that LISA and JDEM are at least still getting serious consideration for eventual mission funding. But don't forget that this is merely a recommendation to NASA and the Department of Energy (the agencies that must actually fund the projects). The projects could easily be blocked or delayed by the agencies themselves, the Executive Office of the President (especially by budget officials), or Congress.

Note that JDEM, the dark energy mission, is actually three competing proposals, among which it will still be necessary to settle on one:

So far, three specific mission plans have been studied in this area: the Supernova Acceleration Probe (SNAP), the Dark Energy Space Telescope (DESTINY), and the Advanced Dark Energy Physics Telescope (ADEPT), but the eventual JDEM could be any one of the three or be based on a different option altogether. The committee found that the underlying technology for a dark energy mission is, for the most part, in the prototype phase, and will require less development than most of the other missions. The potential gains for JDEM also outweigh its scientific risks, such as the possibility that the mission may not provide substantial insight beyond that provided by telescopes on the ground. The report recommends that NASA and DOE proceed immediately with a competition for mission proposals that will investigate the nature of dark energy with high precision.

LISA is also recommended for continued development. It's status is somewhat different in that the project is being funded jointly between NASA and the European Space Agency (ESA). And further, future plans depend on what is learned about the technology (an ambitions space-based interferometer) from a preliminary project called LISA Pathfinder, which is to be launched in 2009.

The NRC recommendation leaves the three remaining projects in limbo:

[T]he three elements of Beyond Einstein that are not being recommended for immediate implementation are still important endeavors that should receive continued support. The committee found that because the Constellation-X mission is a general-purpose x-ray observatory capable of broad contributions to astrophysics, it should be funded and assessed in a broader context than the Beyond Einstein program. The Black Hole Finder Probe and Inflation Probe missions will also make important scientific contributions; however, because of scope and technical readiness issues, they fell behind JDEM and LISA. The committee recommended that Constellation-X, Black Hole Finder Probe, and Inflation Probe receive continued support to prepare them for the next decadal survey of astronomy and astrophysics.

Additional news reports have focused mainly on the dark energy mission, for example here, here, here.

Steinn, of course, has some enlightening commentary here, here, and especially here.

The next shoe to drop is a reply from NASA, which could come at any time. It should be noted that there are possible ways and means to squeeze in some of the scientific missions which did not get recommended at this time, but that will require continued lobbying and can only be speculated on now. And everything goes up for grabs again, after January 20, 2009. One step at a time.

Tags: Beyond Einstein program, cosmology, astrophysics

Supersymmetry and big bang nucleosynthesis

The general acceptance of big bang cosmology for the past four decades rests primarily on three solid lines of evidence. First, the observation of the general expansion of the universe (using distance measurements based on "standard candles" like supernovae), is very consistent with the Friedmann equations derived from general relativity. Second, very precise measurements of inhomogeneities in the cosmic microwave background are very consistent with what is to be expected of conditions present in the universe at the time photons decoupled from matter. Third, the abundances of several light nuclei are very close to what would be expected to be produced in the process of nucleosynthesis that should have occurred around five minutes after the big bang.

As good as the agreement between theory and observation has been where nucleosynthesis is concerned, there have been various discrepancies that required some creative thinking to resolve. One of these involves the abundance of helium-3. We discussed it here.

Another example involves lithium. Although both lithium-6 and lithium-7 are calculated to have been produced in very small amounts, there was definitely some. Yet some very old stars have been observed that seem to contain no lithium at all. Where did it go? The theory here is that such stars are the result of mergers between even older stars, in which all the lithium was destroyed in the cataclysmic merger. See this.

Now there is yet another anomaly involving lithium observed in certain very old stars. The interesting thing is that one theorist is viewing this as possible evidence for very heavy supersymmetric particles that may not have yet decayed out of existence at the time of primordial nucleosynthesis.

Catalyzing Primordial Nuclear Chemistry

But a remaining puzzle is the amount of primordial lithium; both Li-6 and Li-7 are unexpectedly abundant in metal-poor stars (those with very few heavier elements). For example, a much higher than expected level of Li-6 might be pointing to a primordial origin (that is, not made later in stellar cores or in supernovas), in which case the BBN model would need to be amended. Maxim Pospelov ... of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, and University of Victoria, British Columbia suggests that the anomaly can be explained if early nucleosynthesis was aided---catalyzed---by the presence of charged heavy particles, which are common in many models of particle physics.

Tags: nucleosynthesis, big bang, supersymmetry

Hubble Finds Evidence for Dark Energy in the Young Universe

NASA's Hubble Finds Evidence for Dark Energy in the Young Universe

Scientists using NASA's Hubble Space Telescope have discovered that dark energy is not a new constituent of space, but rather has been present for most of the universe's history. Dark energy is a mysterious repulsive force that causes the universe to expand at an increasing rate.

Investigators used Hubble to find that dark energy was already boosting the expansion rate of the universe as long as nine billion years ago. This picture of dark energy is consistent with Albert Einstein's prediction of nearly a century ago that a repulsive form of gravity emanates from empty space.

So, that's the big cosmology news for today. It's very closely releated to what's discussed in the Beyond Einstein article of a couple of days ago.

Actually, in a way, it's kind of boring, since the findings are pretty much what "conventional wisdom" (of the last 6 or 7 years) has expected. No apple carts have been upset as a result of this. But further confirmataion of accepted theories is in its own way very important too.

The take-away is that NASA now has even better justification for the JDEM kind of mission to obtain better supernovae data in order to put tighter limits on the w parameter in the "equation of state" for dark energy.

I've written a lot about this stuff before in much more detail here, but perhaps I'll revisit that to highlight the most important ideas as they relate to the present news.

There are some presentation materials here from today's NASA press conference.

Tags: dark energy, cosmology

Beyond Einstein

Here's the second article in a series I'm going to do on NASA's advanced astrophysics and cosmology science program, which they've called "Beyond Einstein". The first in the series is here. It provides background on the Bush administration's lamentable intentions to delay indefinitely or even abandon most of the more advanced of NASA's pure science programs, including Beyond Einstein.

My purpose in writing about this is to stimulate interest in the program among that part of the U. S. public that pays attention to basic science, especially advanced studies of the universe at large. Because, you see, as a result of last week's elections, the character of the U. S. Congress is going to change significantly next year. There's reason to hope priorities can change. When NASA's science budgets are discussed in future years, we can advocate that Congress reinstate funds for the missions that make up the Beyond Einstein program.

The main purpose of this post is to present background information on the program. But of course, a few words need to be said first about what the Beyond Einstein program is. Fortunately, NASA's home page of the project does a really great job of providing both an overview and detailed background information. See especially the science page, the mission descriptions, and additional resources.

In a nutshell, the various missions together and separately will investigate four of the most mysterious phenomena that we know of in the universe: black holes, gravitational waves, dark energy, and cosmic inflation. These phenomena are grounded in Einstein's general theory of relativity. Yet there's a great deal we don't understand about each one – hence the name "Beyond Einstein".

This graphic from the project site sums it up (click for full-size image):



If you go to this page, you'll be able to click on individual parts of the graphic for more information. The items at the far left are space missions that have already been launched (except for GLAST, whose launch is scheduled for late 2007) or ground-based facilities (LIGO) that are currently working on different parts of the puzzle. Immediately to the right of those are two missions (LISA and Constellation-X) that are well-along in planning – but not yet approved and funded. They (as well as everything else to their right) are missions that were ditched, at least for the present, in NASA's 2007 budget.

LISA will use interferometry techniques, as does LIGO, to search for gravitational waves. But because the separation of the three observation points will be millions of kilometers, instead of a few thousand in LIGO, it will be vastly more sensitive. LISA should be able to detect gravitational waves resulting from supernovae or black hole collisions.

Constellation-X is to consist of four X-ray telescopes on a single spacecraft. It is a successor to previous space-based X-ray observatories, such as Chandra. Constellation-X will be able to study phenomena that are energetic in the X-ray part of the spectrum, such as physics in the vicinity of black holes and very hot gas found in large galaxy clusters.

The missions in the center of the chart are less far along in planning. Of the three, the dark energy probe appears to be farthest along. In fact, there are actually three possible designs in competition. In August, NASA authorized a comparative analysis of the three designs in order to identify the "best". Each of them will measure the effects of dark energy over the history of the universe by locating and studying 1000 or more Type 1a supernovae. They differ in the additional kinds of measurements they can make. However, the status of this mission (as well as the others discussed here) has recently been thrown into further uncertainty, as we'll explain in a minute.

The purpose of the inflation probe is to gather stronger evidence for the process of inflation that appears to have occurred beginning a mere 10^-35 seconds after the big bang. (As discussed here and here, back in March NASA announced that an analysis of WMAP data in fact gave preliminary evidence for inflation.) In addition, the probe will seek data that can discriminate among the many possible models which can describe inflation. There are different ways that the probe can study the problem, including a more detailed analysis of polarization in the cosmic microwave background, and a study of the evolution of large-scale structure in the universe.

The black hole finder, as the name implies, will be designed to locate and study black holes (both stellar-mass supernova remnants and supermassive black holes) in order to learn more about how they form and grow. As such, it will build upon work done by Constellation-X.

As for the two "vision missions", it's really too early for scientists and engineers to define them in any detail. Much will depend on phenomena that are better understood from the results of earlier missions, and most likely phenomena we don't even know of yet. Understandably, these missions (and certainly others like them) are decades in the future.

And this brings us to the latest news. It should be clear enough that there are plenty of overlaps and interdependencies among the various missions. The capabilities of later missions will depend critically on what we learn from earlier ones. After all, until 1997, no one seriously suspected that dark energy even existed. (And some experts still doubt its existence.)

Because of this, as well as because of the severe present constraints on NASA's science budget, The National Research Council (NRC) of the National Academies has formed a committee – at the request of NASA and the U. S. Department of Energy – to conduct an assessment of the Beyond Einstein program. The first meeting of the committee was held last week (November 6-8). The agenda is here. Further information on the committee, including its membership and staff, is here.

This is the committee's task statement:

1. Assess the five proposed Beyond Einstein missions (Constellation-X, Laser Interferometer Space Antenna, Joint Dark Energy Mission, Inflation Probe, and Black Hole Finder probe) and recommend which of these five should be developed and launched first, using a funding wedge that is expected to begin in FY 2009. The criteria for these assessments include:

a. Potential scientific impact within the context of other existing and planned space-based and ground-based missions; and

b. Realism of preliminary technology and management plans, and cost estimates.

2. Assess the Beyond Einstein missions sufficiently so that they can act as input for any future decisions by NASA or the next Astronomy and Astrophysics Decadal Survey on the ordering of the remaining missions. This second task element will assist NASA in its investment strategy for future technology development within the Beyond Einstein Program prior to the results of the Decadal Survey.

As of right now, I haven't seen any accounts of what happened at the meeting last week. If anyone out there has some actual information about the meeting, or has seen reports of it, please let me know.

What I do know is that some people are pretty worried that the real purpose of this committee is to narrow down the Beyond Einstein program to just one mission, or possibly two, because of NASA's budget problems. This might entail not merely postponing other missions, but essentially killing them altogether. The problem is that, if some level of misson activity cannot be funded on an ongoing basis, then many researchers and their institutions will have to find other things to do, and it could be very difficult to bring teams back together when, or if, funding becomes available. See two posts here and here, from Steinn Sigurðsson for examples of the kind of speculation going around.

Oh yes, there is one other thing. Along with the announcement on October 31 (before the NRC committee meeting), that a final service mission will be flown for the Hubble Space Telescope, there were strong hints that other astronomy missions are on hold. The report on this printed in Science: Hubble Gets a Green Light, With Other Missions on Hold is available only to subscribers, but says at the end:

Griffin's decision means that NASA will spend most of its astronomy budget on three major missions--the Hubble servicing flight, construction of the James Webb Space Telescope, and the Stratospheric Observatory for Infrared Astronomy (SOFIA). Technical troubles, schedule delays, and cost overruns plague the latter two. But Weiler [director of NASA's Goddard facility] says that the Webb is back on track after a rough couple of years, while SOFIA--which Griffin initially canceled only to revive in July--is slated to begin operations in 2009. Those large projects leave little room for smaller or future missions. For example, NASA halted work earlier this year on the extrasolar planet-seeking Space Interferometry Mission (SIM) in order to cover SOFIA's cost overruns. Those pressures worry some astronomers, who fear that the three missions will limit new efforts.

"Is the astronomy program with just [Webb], Hubble, and SOFIA a good astronomy program? You betcha," says Weiler. Although he acknowledges that there is a gap in smaller missions for the next few years, he notes that the cost of building the Webb will peak in 2008 and then decline over the next 5 years. "The big issue now is what to do with that wedge."

The four leading contenders appear to be the Joint Dark Energy Mission with the Energy Department, a mission called Constellation-X that features a bevy of x-ray telescopes, the Laser Interferometer Space Antenna to study black holes and the early universe, and SIM. NASA had intended to fund all in this decade and the next, but budget constraints likely will make for a competitive race.

Make of that what you will, but it certainly doesn't sound too good.

On the other hand, it certainly looks like the task of the NRC committee is to select at least one of the Beyond Einstein missions. Further, NASA is going ahead with other new astronomy projects. In addition to GLAST (launches late 2007), on October 13 there was an announcement that the Wide-field Infrared Survey Explorer will be launched in 2009 to do infrared sky maps, which would capture both nearby planetary systems undergoing formation as well as very distant galaxies – news report, further information.

So here's the bottom line I see for now: The NRC committee will take a year or so to ponder the situation. They may pick one project to go forward with initially. (Betting seems to be on the dark energy probe, because of the involvement of the Department of Energy.) Other missions in the advanced planning stage (LISA and Constellation-X) may wind up on hold, or one may be slotted as well.

The important point: there is plenty of time to make the argument before the appropriate Congressional committees that the NASA science budget should be increased enough so that the Beyond Einstein program can go forward, without having to sacrifice planning that has already been done and disrupting teams that are already in place.

Fortunately, as a result of last week's elections, Congress will have new people in charge who should be inclined to place a higher value on basic science than those they are replacing.

Update 1 (11/13/06): According to a comment by Steinn, LISA and Con-X have been "approved", but only minimally funded.

Update 2 (11/14/06): Now Steinn says funding was cut off. In any case, they're going noplace fast at this point.

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Additional information:

Beyond Einstein: From the Big Bang to Black Holes

This is a 110 page document you can download in PDF format, and it's very much worth the effort. It's profusely illustrated (full color) and describes all of the missions and gives a good overview of the underlying science. Only problem is it was published in January 2003. But the additional science that has been learned in the last four years mostly confirms the premises of the program.

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Tags: astrophysics, cosmology, Beyond Einstein, black holes, dark energy, cosmic inflation, gravitational waves, NASA

Inflation and the cosmic microwave background

About three weeks ago I wrote a little about the cosmic microwave background (CMB), and talked about writing more. So here's a little more. The CMB is microwave radiation we can (almost) literally "see" even though it originated only about 350,000 years after the big bang. (I'll explain more in a minute about what "originated" means in this context.)

There are a number of things very wonderful and remarkable about the CMB. Not only is it one of the major pieces of evidence supporting the big bang theory in general, but it also gives us much information about things as diverse as the relative proportions of ordinary matter and dark matter in the universe, the overall curvature of the universe ("flat", "convex", or "concave"), and the ratio of the number of baryons (protons and neutrons) to photons in the universe.

Further, of particular concern here, the CMB provides a means of testing a theory – the theory of cosmic inflation – that describes a much different period of time than that of the CMB itself, a period of time that began only about 10^-35 seconds after the big bang itself. The theory of inflation comprises a range of models describing what may have happened at that time. If there is any truth at all to the theory, the CMB can help narrow the range of acceptable models.

I have a couple of older articles on this from back in March/April here and here. These deal with the announcement back then of analysis of data from the Wilkinson Microwave Anisotropy Probe (WMAP) that, among several other things, gave the first reasonable evidence for inflation.

Rather than dive right away into further explanation of that, I'm going to refer you first to this excellent recent article on the subject by Sean Carroll: Reconstructing Inflation.

What's that, you say? It sounds impressive, but you don't quite follow the details? OK, let's step back for a moment and review the basics. The picture below is a graphical representation of the main data obtained from WMAP:

This picture shows slight variations in temperature across the entire sky, at microwave frequencies, where blue represents coolest and red represents warmest. The variations are actually very small: the whole range is ± 200 microKelvins (millionths of a degree K).

Temperature differences correspond directly to differences in matter density – because a gas under higher pressure is warmer and denser than the same gas under lower pressure (the "combined gas law"). So what we see here are minute ripples of higher and lower pressure in the matter of the universe at roughly 350,000 years after the big bang. What has caused these pressure waves? Carroll's article explains

The same basic mechanism works in both cases — quantum fluctuations (due ultimately to Heisenberg’s uncertainty principle) at very small wavelengths are amplified by the process of inflation to macroscopic scales, where they are temporarily frozen-in until the expansion of the universe relaxes sufficiently to allow them to dynamically evolve.

The spots and blotches you see in this picture are shadows on the wall, as it were, of quantum fluctuations that actually occurred 10^-35 seconds after the big bang. At first, in a period that lasted perhaps only 10^-33 seconds, these fluctuations were inflated at an incredible rate. Thereafter, they continued to expand along with the rest of spacetime itself, until we see them projected on the CMB wall 350,000 years later.

To be more precise, we should note that this metaphorical CMB "wall" did not form at some single precise time. Instead, the CMB itself is a result of most of the hydrogen and helium matter in the early universe making the transition from an ionised plasma to an ordinary gas of neutral atoms, as free electrons were "captured" by the hydrogen and helium ions. Consider, for simplicity, just the hydrogen. It takes 13.6 eV (electron volts) of energy to separate an electron from a hydrogen atom. In the early universe when the typical photon had much more than this energy, atomic hydrogen could not exist for long, as most passing photons could "liberate" the electrons. But when the energy of the typical photon dropped, as the universe expanded, to the equivalent of around 13.6 eV, hydrogen atoms became stable for longer periods of time. This is known as the period of "recombination" (even though prior to this, protons and electrons had never been in a "combined" state). Once there was a lot of atomic hydrogen, photons of the most common energy levels "scattered" from the atomic hydrogen, and the universe was somewhat opaque to those photons.

But as the temperature dropped further, most photons did not have enough energy to liberate electrons from hydrogen atoms. So most photons ceased to scatter from atomic hydrogen, and the universe effectively became transparent again. Although this happened over a relatively short period of time, it was not instantaneous. By the time that most hydrogen was in an unionized state, a typical photon never again scattered off a hydrogen atom. So around any present observer, there is a "surface of last scattering". Assuming what are currently considered the most likely cosmological parameters, this corresponds to a time about 13.3 billion years ago (equivalent to a red shift of about 1100), about 350,000 years after the big bang. This surface of last scattering is what we see today as the CMB. The temperature of the universe at this time of last scattering was about 3000° Kelvin, but due to the subsequent expansion of the universe, the CMB photons now have an energy that peaks around 2.725° K, in the microwave part of the spectrum.

There is another way to represent the WMAP data for the CMB. You've probably seen it in some form. (It's in Carroll's article, if you read that.)

The vertical scale on the left is a measure of the amplitude of temperature fluctuations. The top and bottom scales are measures of angular size. 90°, for instance, is one fourth of the whole sky. The "multipole moment" (l) is an integer that corresponds to an angular measure of 180°/l. So, for instance, the peak on the above graph occurs around l=200, which is slightly less than 1°. For comparison, the angular size of the full moon viewed from earth is about .5°. What the graph is saying, roughly, is that strongest temperature fluctuations (spots in the picture above the graph), if you could see them with your naked eyes, are almost twice the angular size of a full moon. (Astrophysicists use multipole moments, since they are the relevant identifiers of "spherical harmonic" functions that are used to construct a series representation of the function which describes theoretical temperature variations, similar to the way that a Fourier series can represent a function of one real variable.)

The small dots on the graph are WMAP measurements for various values of l. They come with error bars, which are mostly too small to see, because the WMAP measurements were mostly pretty precise. The red line through the measured values is the theoretically predicted values, assuming that the temperature variations are actually the result of quantum fluctuations that occurred in the inflationary period. The locations of the two peaks to the right of the main peak are especially important, and they correspond fairly well to theoretical predictions.

Carroll's article explains how there are actually two kinds of perturbations we might potentially observe in measured quantities: "scalar" and "tensor", reflecting the fact that Einstein's equation describing gravity waves (which result from the inflation-era quantum fluctuations) is a tensor differential equation. Further, all that we can readily measure from the WMAP data are scalar perturbations:

To date, we are quite sure that we have detected the influence of scalar perturbations; they are responsible for most, if not all, of the temperature fluctuations we observe in the Cosmic Microwave Background. We’re still looking for the gravity-wave/tensor perturbations. It may someday be possible to detect them directly as gravitational waves, with an ultra-sensitive dedicated satellite; at the moment, though, that’s still pie-in-the-sky (as it were). More optimistically, the stretching caused by the gravity waves can leave a distinctive imprint on the polarization of the CMB — in particular, in the type of polarization known as the B-modes. These haven’t been detected yet, but we’re trying.

Problem is, even if the tensor modes are there, they are probably quite tiny. Whether or not they are substantial enough to produce observable B-mode polarization in the CMB is a huge question, and one that theorists are presently unable to answer with any confidence.

The WMAP experiment was capable of studying polarization of CMB microwaves only rather crudely. But a new experiment is due for launch very soon (early 2007) in the form of the European Space Agency's Planck mission. Considering that it took several years to analyze WMAP data, we may not have better information right away – but it won't be too long, if everything goes reasonably well.

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Further information about CMB:

Wilkinson Microwave Anisotropy Probe

NASA web site of WMAP, containing background information, images, and graphs.

Wayne Hu's Home Page

One of the best collections of CMB information, including an introduction, explanation of the physics, and discussion of CMB polarization.

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Tags: astrophysics, cosmology, cosmic microwave background, CMB, Microwave Anisotropy Probe, WMAP, cosmic inflation