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

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

Special issue of General Relativity and Gravitation on dark energy

Speaking of astrophysical theories that still have many skeptics, how about dark energy? Whatever your opinion of the theory, here's a real treasure trove of information – a whole issue of the journal General Relativity and Gravitation, and all the articles are available for free. But for how long I don't know, so better go get it now:

General Relativity and Gravitation: Special issue on dark energy

From the introduction:

General Relativity and Gravitation has put together a special issue on “dark energy” in cosmology, because it is a major challenge to gravitational physics and actually to all of theoretical physics. We look at the observational side (the astrophysical data for dark energy and alternative explanations of that data), phenomenological models for dark energy and possible tests of these models, and the quantum gravity side (why do we expect a very large cosmological constant? what are the possible explanations of why it is small?). The idea is not to pursue one particular approach to this important problem, but rather to produce a survey of significant approaches that have been taken as regards each of its aspects.

Tags: dark energy, general relativity

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

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

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

The energy of empty space that isn't zero

Physicist and well-known science writer Lawrence Krauss a few months ago organized a symposium for a selected elite of physicists and cosmologists to discuss their work. Here he writes about some of the ideas that the notables talked about. It's not exactly easy to follow if you don't have some background, and it would take a book to fill in the gaps. (There are several now that do this, some by Krauss himself.) But otherwise he gives a nice snapshot of some of the things we're ignorant of at the frontiers of physics and cosmology. "Dark energy" is just one of those things. (See here, and references therein.) There's also string theory, "the landscape", quantum gravity, "cosmic inflation", and more.

I do take exception to one comment Krauss makes. He observes that while all the evidence points to a small, non-zero amount of dark energy in the universe, it is smaller by the enormous factor of 10¹²⁰ than what it should be if it were really "vacuum energy". He goes on to note that symmetry principles might explain how the energy represented could be cancelled out exactly so that it is effectively zero. However, he says, "what you couldn't understand was how to cancel a number to a hundred and twenty decimal places and leave something finite left over. You can't take two numbers that are very large and expect them to almost exactly cancel leaving something that's 120 orders of magnitude smaller left over."

I don't follow that, because all it would take is some highly improbable symmetry breaking process, such as that which (apparently) accounts for the very slight excess of matter over antimatter that must have existed very soon after the big bang. (This asymmetry is about 3 parts in 10⁹.) It would have to be a process even more improbable. But then, we know things can happen that have arbitrarily small probability. For instance, the event that all gas molecules of a large room simultaneously occupy an arbitraily small volume in some corner of the room for a non-zero instant of time. Simply adjust the volume and time period in which this occurs, and you can manufacture an event that has a probability of 1 in 10¹²⁰. QED.

--------------------------

Tags: dark energy, cosmology, Lawrence Krauss

It's all relative

In the previous message we mentioned an interesting article on relativity: Relativity at the centenary, which pointed out that gravitational physics has become an experimental science. There's a lot going on with studies of black holes, the search for gravitational waves, detailed tests of special and general relativity. And on the theoretical side, there's a lot of activity in the (as yet still unsuccessful) quest for a theory of quantum gravity.

Of course, the theory of relativity is intimidating to a lot of people, but it doesn't need to be. Special relativity actually involves little more that basic physics (ideas like mass, velocity, force, energy) and high school algebra with rudimentary calculus. The mathematics needed for general relativity is somewhat more sophisticated. But the basic concept described by the math is pretty simple: concentrations of matter cause space to curve, and the motion of two (or more) massive objects that interact gravitationally can be understood as "straight lines" in the curved space.

Once you've read some overviews, like the Wikipedia articles just referred to, you will have the basic ideas needed to learn more about relativity. Fortunately, there's an excellent online reference with a large number of review articles that explain in more detail many of the most interesting topics in the contemporary theory of relativity. It's called Living Reviews in Relativity -- and it's all free.

The journal, which covers both theory and experiment, is now in its 9^th year. Some of the articles are valuable for understanding basic topics in cosmology. Here are some of the more generally accessible articles:

• Loop Quantum Gravity (1998)
  
• Stationary Black Holes: Uniqueness and Beyond (1998)
  
• The Cosmic Microwave Background (1998)
  
• Gravitational Wave Detection by Interferometry (2000)
  
• The Cosmological Constant (2001)
  
• Computational Cosmology: From the Early Universe to the Large Scale Structure (2001)
  
• The Thermodynamics of Black Holes (2001)
  
• Experimental Searches for Dark Matter (2002)
  
• Gravitational Waves from Gravitational Collapse (2003)
  
• Testing General Relativity with Pulsar Timing (2003)
  
• On the History of Unified Field Theories (2004)
  
• Quantum Gravity in Everyday Life: General Relativity as an Effective Field Theory (2004)
  
• Brane-World Gravity (2004)
  
• Measuring our Universe from Galaxy Redshift Surveys (2004)
  
• Quantum Gravity in 2+1 Dimensions: The Case of a Closed Universe (2005)
  
• Modern Tests of Lorentz Invariance (2005)
  
• Binary and Millisecond Pulsars (2005)
  
• Loop Quantum Cosmology (2005)
  
• The Confrontation between General Relativity and Experiment (2006)
  

Note that there have been updated versions of some articles. In each case, only the most recent update (as of this writing) and its corresponding date are listed.

Tags: relativity, special relativity, general relativity, black holes, quantum gravity, pulsars

More evidence for dark energy

Back here we provided an overview of the topic of dark energy and some of the evidence for its existence.

Recently announced research results from a project known as the Supernovae Legacy Survey (SNLS) have now provided additional evidence for dark energy, and suggested that it is more likely that the dark energy is due to Einstein's cosmological constant than to an even more exotic possibility known as quintessence.

Was Einstein's 'biggest blunder' a stellar success?

The genius of Albert Einstein, who added a “cosmological constant” to his equation for the expansion of the universe but later retracted it, may be vindicated by new research.

The enigmatic dark energy that drives the accelerating expansion of the universe behaves just like Einstein's famed cosmological constant, according to the Supernova Legacy Survey (SNLS), an international team of researchers in France and Canada that collaborated with large telescope observers at Oxford, Caltech and Berkeley. Their observations reveal that the dark energy behaves like Einstein’s cosmological constant to a precision of 10 per cent.

“The significance is huge,” said Professor Ray Carlberg of the Department of Astronomy and Astrophysics at U of T. “Our observation is at odds with a number of theoretical ideas about the nature of dark energy that predict that it should change as the universe expands, and as far as we can see, it doesn’t.”

The basic difference between dark energy in the form of a cosmological constant or of quintessence is that the former yields a constant acceleration, while the latter produces ever increasing acceleration. The latter scenario is sometimes called the "big rip" because eventually it would cause even atoms, neutrons, and protons to burst apart.

By measuring the spectra of light from distant supernovae of a special type (known as Type 1a), it is possible to determine both the distance to a supernova and its velocity relative to Earth. Whether the acceleration of the universe's expansion is constant or increasing can be deduced from this data. The evidence is that the acceleration is constant, which rules out quintessence.

Further, when the acceleration data is combined with measurements of the cosmic microwave background (called "baryon acoustic oscillation"), it is possible to determine that the dark energy makes up about 75% of the total matter and energy in the universe.

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Additional resources:

Einstein's Dark Energy Accelerates the Universe -- Particle Physics and Astronomy Research Council press release

First results describing the nature of dark energy -- press release from the journal Astronomy & Astrophysics

Gemini's Nod-&-Shuffle Provides Critical Deep Spectroscopic Data for Supernova Legacy Survey -- contains a more technical description of the SNLS results

New Study of Supernovae May Absolve Einstein of Self-confessed "Biggest Blunder" -- Keck Observatory press release

The Supernova Legacy Survey: measurement of Ω_M, Ω_Λ and w from the first year data set -- original paper (PDF)

Canada-France-Hawaii Telescope Legacy Survey

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Tags: dark energy

Dark energy, quintessence

We mentioned dark energy and the cosmological constant just a few days ago in connection with "zero point energy". The distinction is that "dark energy" refers to an undefined energy of some sort that must exist in order for the universe to be geometrically "flat" in accordance with the equations of general relativity. In fact, there are now several lines of evidence that this dark energy must exist, even though its nature and origins are quite unknown.

The cosmological constant (denoted by the greek letter Λ), on the other hand, is a very specific candidate for dark energy. While it would do the job admirably, there is no independent evidence for its existence. And it has conceptual problems, namely that if (as generally supposed) the cosmological constant can be explained as zero point energy, then its apparent density is a factor of about 10¹²⁰ smaller than straightforward calculations suggest it ought to be. In other words, Λ must be nonzero but very finely tuned to a very implausibly small number. Situations like that make physicists very nervous.

However, there are many other forms that the dark energy could take, which are generally referred to as "quintessence". The idea of quintessence was proposed in 1998 by R. R. Caldwell, R. Dave, and P. J. Steinhardt. See this overview article by Caldwell and Steinhardt for more details.

This article: Dark Energy by Caldwell is an excellent recent (2004) summary of our present knowledge of dark energy, including the evidence for its existence. The evidence is:

1. Many measurements of the apparent luminosity of distant "standard candle" Type Ia supernovae show that the expansion of the universe is now accelerating, instead of decelerating as would be the case if there were no dark energy.


2. The existence of dark energy predicts a phenomenon known as the integrated Sachs-Wolfe effect, which represents a slowing of the collapse of overdense regions of the universe. This prediction has been confirmed by combining data from detailed measurements of the cosmic microwave background (CMB) and the large-scale distribution of galaxies. (The article by Caldwell has a good explanation.)


3. There is weaker circumstantial evidence from CMB and galaxy distribution data.

Given that we can now be fairly confident dark energy exists, the big question is: What is it? In particular, can we tell whether it is a result of a cosmological constant or, instead, of quintessence?

One characteristic of the cosmological constant Λ is that it is truly a constant. It is the same everywhere and for all time. Quintesence, on the other hand, gives an energy density which varies spatially (i. e., isn't homogeneous) and with time (it decreases). These differences should make it possible to distinguish the alternatives, with very sophisticated measurements of the acceleration of the universe at different time periods. The instruments and space missions that could make the measurements are under design and development.

Quintessence itself can come in many possible types, and a recent technical paper, The Limits of Quintessence, by R. R. Caldwell and Eric V. Linder distinguishes two subtypes of quintessence in some detail, and both of those from Λ. If dark energy is actually quintessence, the measurements which are being developed should be able to distinguish between the subtypes. A less technical discussion of quintessence and ways to test for it appears in this news article: Finding A Way To Test For Dark Energy.

But if you're willing to tolerate a few equations (and just a pinch of calculus), we can show the essence of the difference. Your reward for following along here is that you will be able to understand the technical articles just mentioned a little better.

The important thing is a number that's conventionally written as "w". w is simply a constant of proportionality between pressure and energy density. For any given type of matter or energy the relation is this: P = wε, where P is pressure and ε is energy (or mass) density. This equation is from the theory of gases and is known as the "equation of state".

One other equation we need is called the "fluid equation". It describes how energy density, pressure, and a third quantity called the "scale factor", denoted by "a", are related in an expanding (or contracting) universe. The scale factor can be thought of as a variable yardstick that expands or contracts in proportion as the universe does. (For much more about the scale factor and equations involving it, see this.) Here is the equation:

ε&prime + 3(&epsilon+P)a′/a = 0

The prime symbol (′) in there denotes derivative with respect to time. The derivative is zero just in case the quantity is a constant. So ε′ = 0 just in case we have the equation of state &epsilon=-P, which means w=-1.

Suppose the cosmological constant Λ is the dominant form of dark energy. Since Λ is a constant, the corresponding energy density ε is constant, so ε′=0 and w=-1. In other words, dark energy being entirely the result of a cosmological constant corresponds to the parameter w=-1.

But there's no a priori reason that w couldn't be just about any varying function of time. What would it be if the dark energy were solely the result of quintessence? To answer that we need one more equation, called the "acceleration equation":

a′′/a = -(4πG/3c²)(ε+3P)

The double prime denotes the second derivative, which is interpreted as acceleration. π is the constant 3.14159..., G is Newton's gravitational constant, c is the speed of light, and a, ε, and P are as before.

What this equation says is that the acceleration of the expansion of the universe is a negative number times &epsilon+3P. Since we now know observationally that the acceleration is positive, we must have &epsilon+3P<0. And since ε=wP by definition, we must have ε<-3wε, hence w<-1/3. To be consistent with observations, we must have w<-1/3 if ε is the energy density corresponding to quintessence. (This also assumes Λ=0. If dark energy consists of both quintessence and a cosmological constant, which isn't impossible, things would be much more complicated.)

The bottom line of all this is that we can distinguish between quintessence and a cosmological constant as the source of dark energy (if both are not present) just by measuring accurately enough how the universe is expanding, which will tell us what w is. If w=-1, we have a cosmological constant. If -1<w<-1/3, we have some form of quintessence. (It is also conceivable that w<-1, in which case things are really weird.)

In fact, we can put slightly tighter bounds on w, since we know roughly how much dark energy there is, and this is because we know the universe is spatially flat. Let ε_t stand for the total energy density in the universe, and let ε_m be the energy density due exclusively to matter (most of which is dark matter). Careful measurements of the motions of stars in galaxies and of galaxies gives us a value for ε_m. Knowing in addition that the universe is flat tells us what ε_t has to be, and hence that ε_m is about (1/3)ε_t. (Actually it's a little less, but that's close enough.) Since ε_d, the energy density of dark energy, accounts for all the rest, we have ε_d = (2/3)ε_t.

Since the expansion of the universe is accelerating, the acceleration equation implies ε_t+3P<0. But P=wε_d, since matter does not contribute to pressure (it has its own effective w=0), and hence P=w(2/3)ε_t. Plugging that in, we have ε_t+2wε_t<0, and so w<-1/2 (instead of w<-1/3).

Finally, we can indicate what the two subtypes of quintessence are that Caldwell and Linder identified in their paper. The types are distinguished according to whether the first time derivative of w, i. e. w′, is positive or negative. If w′<0, then w is decreasing with time and in the limit is -1, so in some sense the quintessence is "freezing" into a cosmological constant, which means that the acceleration of expansion will continue forever. On the other hand, if w′>0, then w will gradually increase from near -1, away from behaving like a cosmological constant, and this case is called "thawing". As the universe expands, both ε_d and ε_m decrease (the total amounts of quintessence and matter don't change, but the volume is increasing). The quantity ε_t+3P = ε_m + ε_d + 3wε_d = ε_m + (1+3w)ε_d. Now -2<1+3w<0 since -1<w<-1/3, so ε_t+3P approaches 0 as the densities decrease, and so acceleration gradually goes to 0 and stops.

Both subtypes of quintessence can be modeled using a variety of different types of "scalar fields", but not any fields that are part of the current standard model of particle physics.

What about the case w<-1? In that scenario, acceleration increases very rapidly, leading to what is called the "big rip", in which not only the universe itself expands, but in the distant future even stars and eventually subatomic particles are torn apart. This would correspond to yet another type of quintessence, called "phantom energy". But that's a story for another time.

Related:


How Are We to Make Progress With w?