Eleven years ago this winter two teams of astronomers, one of them led by CfA scientists, astonished the world with their announcement that the universe would expand forever. They soon added to the amazement by presenting evidence that the universe is not only expanding outward, it is accelerating outward. Their observations studied supernovae in galaxies so far away that their observed motions are primarily due to the expansion properties of the cosmos itself.
Two explanations are commonly advanced to explain the outward acceleration of the universe. The first asserts that, as Einstein once speculated, gravity itself causes objects to repel one another when they are far enough apart. This feature of gravity is described by general relativity with an arbitrary constant term, not fixed by first principles, commonly called the cosmological constant. The second explanation hypothesizes (based on our current understanding of elementary particle physics) that the vacuum has properties that provide energy to the cosmos for expansion. This energy was dubbed "dark energy," in poetic analogy to dark matter, the name given to the mysterious material that does not radiate light but reveals itself (so far at least) only via its gravitational influence on galaxies. (Dark matter is dark because it does not radiate light and is mysterious; dark energy is 'dark' only because it is mysterious.) Each of these two explanations for cosmic acceleration (which are often interchangeably referred to as dark energy) has its own set of ancillary implications that can be used to probe which one (or neither, or both) is correct.
At a news conference this week, CfA astronomers Alexey Vikhlinin, Bill Forman, Christine Jones, and Steve Murray, together with seven colleagues, announced dramatic new evidence for dark energy (their paper will appear in February). They studied the clustering of two sets of distant galaxies using X-ray observations from the Chandra X-ray Observatory and the ROSAT X-ray satellite. The two sets of clusters represent snapshots of the universe taken about four billion years apart, roughly one-third of the age of the universe. The team used sophisticated computer models to calculate how galaxies should cluster together during that time span if the accelerating cosmos were not dragging them apart. They find convincing evidence in their observations that the clustering is actually significantly less than this, but consistent with the presence of dark energy.
The new results provide the first independent, direct evidence for cosmic acceleration since the original discovery, based on distant supernovae, eleven years ago. It now looks increasingly certain that other suggested explanations for the supernova results are improbable; for example, proposals to modify Einstein's theory of relativity. The new results significantly narrow the options for key aspects of the cosmic expansion, including the putative mass of one very abundant but hard to detect particle, the neutrino. The team's results, by helping to confirm and clarify the nature of dark energy, have helped to make dark energy a little less dark.