Sunday, August 17, 2008

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:

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

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