VANCOUVER, Canada — Chalk up another victory for the dark side.
Comparing X-ray observations of distant and nearby clusters of galaxies, astronomers say they have found new, independent evidence for the existence of dark energy, the mysterious entity that is accelerating cosmic expansion. By combining the new data with that from several other studies, the team finds that dark energy appears to have maintained the same density over time, resembling Einstein’s cosmological constant.
Some theories of dark energy suggest that the repulsive force associated with this mystery substance may grow stronger with time, causing the universe to end in a Big Rip, with every planet and person ultimately ripped apart. While the new findings indicate that dark energy has maintained a constant strength throughout cosmic history, they still allow some wiggle room and do not preclude the possibility that dark energy may vary slightly. The new X-ray study by itself allows dark energy to vary by only 50 percent from its current density, says Alexey Vikhlinin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. When combined with other studies, the new data suggest the density only varies by 10 percent.
Vikhlinin and his colleagues used NASA’s Chandra X-ray Observatory to record X-ray emissions from 86 massive clusters of galaxies, each heavier than 100 trillion suns. The team found two sets of clusters. The first, more remote and earlier group of 37 clusters dates from between 6.4 billion to 9.8 billion years after the birth of the universe. The closer group of 49 dates from later times in the cosmos, between 11.8 billion and 13 billion years after the Big Bang. Vikhlinin reported his team’s findings December 11 at the Texas Symposium on Relativistic Astrophysics in Vancouver.
Because the present-day densities of clusters are precisely known and fixed, researchers seek the fingerprints of dark energy by measuring the density of clusters back in time. At earlier times, because the universe was more compact, gravity’s pull was stronger relative to dark energy’s push. With this in mind, astronomers expect that a geometrically flat universe with dark energy would have more clusters above a certain mass in place at early times than would such a universe with no dark energy. “This is indeed what was found by Alexey’s analysis,” says Daisuke Nagai of Yale University, a member of Vikhlinin’s team.
“Clusters of galaxies are the most massive objects in the universe that can be used as tracers of the growth of structure,” Nagai says. “Dark energy, if present, tends to slow down the evolution of cluster abundance due to its repulsive force,” he says. “The rate of the evolution, in turn, depends sensitively on the nature or form of dark energy.”
Researchers first discovered evidence for dark energy 10 years ago by studying the brightness of nearby and faraway supernovas, which remain one of the preeminent features for studying dark energy.
But “the [new X-ray] findings are convincing,” and indeed provide an independent method of testing for the presence of dark energy, says theorist Gus Evrard of the University of Michigan in Ann Arbor.
With a dataset of 84 clusters, Vikhlinin and his colleagues are “working on the hairy edge of the number distribution” needed to shed light on dark energy, he adds.
Researchers have for years been using bright X-ray emissions from massive galaxy clusters as cosmological probes, notes Evrard, but it has been difficult to accurately model and interpret these diffuse, fuzzy emissions. Detailed simulations and analysis by several researchers — including Nagai, who also presented his studies at the symposium — have greatly improved the reliability of findings based on X-rays from galaxy clusters, says Evrard.
Perhaps most importantly, he adds, when researchers overlay data from other studies, including observations of the cosmic microwave background — the relic radiation from the Big Bang — and supernova observations, the X-ray findings are zeroing in on exactly the same numerical value for dark energy — finding dark energy to resemble a cosmological constant.
“The combination of all this data is shrinking the error bars,” says Evrard.
The new work “sounds very promising,” says Adam Riess of the Space Telescope Science Institute in Baltimore, a member of one of the teams that discovered dark energy a decade ago. “We’re going to need all hands on deck, all methods working pretty well if we’re going to figure out what dark energy is.”
If dark energy is a constant, it would be akin to Einstein’s cosmological constant, a term he inserted into his equations of general relativity. He later rejected the constant, reportedly calling it his “greatest blunder.”
Also at the conference, Brian Gerke of the Stanford Linear Accelerator Center in Menlo Park, Calif., reported visible-light studies of groups of galaxies observed in the Deep2 Redshift Survey, recorded at the W.M. Keck Observatory atop Hawaii’s Mauna Kea. Gerke and his colleagues collected data on several hundred groups of galaxies seen as they appeared when the universe, now 13.7 billion years old, was 4.1 billion to 7.1 billion years old. These data were compared to a sample of several thousand galaxy groups found in the universe today. Gerke’s team also finds that fewer groups are present today than would be expected if dark energy’s repulsive force did not exist. Gerke says his team found more groups — and specifically more massive ones — at earlier times in the universe than “we would expect relative to what we see today if we lived in a universe with no dark energy.”
“The physics we’re testing is exactly the same as what Alexey does, but the observational techniques are completely different. He uses X-rays, while we use optical light,” says Gerke. Both tests are “indeed completely independent from supernovas,” he adds. “Alexey gets tighter constraints than we do, though, since he still has a somewhat bigger dataset.”
