Ten years ago this winter, two teams of astronomers, one led by CfA scientists, astonished the world with their results showing that the universe would expand forever. Soon afterwards they added to the amazement by presenting evidence that the universe is not only expanding outward, it is accelerating outward. Their observations studied supernovae -- the phenomenally bright deaths of massive stars -- in galaxies so far away that their motions are due to the properties of the cosmos itself.
There are two explanations commonly advanced to explain the outward acceleration of the universe. The first asserts that, as Einstein once speculated, gravity itself caused objects to repel one another when they are far enough apart. This feature of gravity is called the cosmological constant. The second explanation is called "dark energy." It hypothesizes, based on our current understanding of elementary particle physics, that the vacuum has properties that provide energy, and this "dark energy" causes the universe to expand. Each of these two explanations has its own set of ancillary implications which can be used to probe which one (or neither or both) is correct. For example, a gravitational cause would be constant in time, whereas a vacuum energy process probably is not. These differences are embodied in what physicists call the "equation of state" of dark energy, but so far the precision with which the equation of state is known allows for either explanation.
A team of thirty-seven astronomers including seven from the CfA have been working on a large supernova study called ESSENCE (Equation of
State: Supernovae trace Cosmic Expansion). CfA astronomer Michael Wood-Vasey is the first author of the first paper by this team. In the article, which appeared in the latest issue of the Astrophysical Journal, the team reports its detailed analysis of sixty supernovae that went off in distant galaxies between 2002 and 2005. They find that they can reduce the uncertainty in the measurement of the equation of state to about 13%; their conclusions are constrained by precision with which they know the amount of dust obscuring the light of each supernova in its host galaxy. Unfortunately, this uncertainty is still large enough to permit either explanation for the cosmic acceleration. In the next few years, however, the team expects to be able to reduce their uncertainties even further. Their results will have fundamental implications for our understanding of the laws of basic physics and the nature of our universe.