Evidence that the nature of dark energy is a cosmological constant is gathering strength. By adding independent constraints on dark energy from X-ray observations of clusters of galaxies by the Chandra spacecraft, it seems unlikely that the universe will end in a Big Rip.
Image credit: X-ray NASA/CXC/SAO/A Vikhlinin et al.; optical SDSS.
Dark energy manifests itself by accelerating the expansion of the universe, an effect that was first noticed in 1998 by studying distant supernovae of type Ia (CERN Courier September 2003 p23). Additional evidence that dark energy currently constitutes more than 70% of the matter-energy content of the universe came from the Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft through the analysis of the cosmic microwave background fluctuations (CERN Courier April 2003 p11; May 2006 p12; May 2008 p8).
A first step in the characterization of dark energy is the determination of its equation of state, which describes the relation between pressure, P, and energy density, u, through the parameter w: P=wu. Unlike matter, dark energy is characterized by a negative value of w leading to a negative pressure acting as anti-gravity in the equations of general relativity. An acceleration of the expansion rate of the universe is possible if w is less than –1/3. If w is exactly –1, dark energy has the properties of the cosmological constant, with an acceleration going on forever at the same rate. For lower values of w, the acceleration would continue increasing until a dramatic Big Rip occurred, which would tear everything apart, from galaxies down to atoms and nuclei (CERN Courier May 2003 p13).
The study of clusters of galaxies provides an independent characterization of dark energy. The approach is based on measurements by the Chandra satellite of X-ray emission from hot gas in the clusters. The first results appeared four years ago (CERN Courier July/August 2004 p12). By increasing the sample to 37 distant clusters, with an average redshift of z= 0.55, and comparing them with a sample of 49 nearby ones, the team led by A Vikhlinin from the Harvard-Smithsonian Center for Astrophysics has now significantly improved the constraints on dark energy. They find an evolution of the cluster-mass function implying the existence of dark energy with an equation-of-state parameter of w= –1.14±0.21, assuming it to be constant and the universe to be flat. In combination with the latest constraints from type-Ia supernovae, baryonic acoustic oscillations and WMAP, the team obtains w= –0.991 with statistical and systematic uncertainties each of only about 0.04. This combined analysis also puts an upper limit of 0.33 eV on the masses of light neutrinos.
Only 10 years after identifying the effect of dark energy, astronomers can now combine different measurements that are consistent with each other if dark energy has the properties of the cosmological constant introduced by Einstein to counteract self-gravity in a static universe. There is still some freedom within the uncertainties for a more exotic dark energy, but the alternatives are clearly disfavoured by simplicity arguments (Occam’s razor). This means that dark energy is most likely vacuum energy, but the mystery of its low but none-zero energy density remains: why is it 120 orders of magnitude below the quantum expectation?
