Evidence from Type Ia Supernovae for an Accelerating Universe and Dark Energy
Alexei Filippenko, Univ. California-Berkeley
Abstract:
The measured distances of type Ia (white dwarf) supernovae as a function of
redshift (z) have shown that the expansion of the Universe is currently
accelerating, probably due to the presence of dark energy (X) having a negative
pressure, such as Einstein's cosmological constant (Lambda). Combining all of
the data with existing results from large-scale structure surveys, we find a
best fit for Omega_M and Omega_X of 0.28 and 0.72 (respectively), in excellent
agreement with the values derived independently from WMAP measurements of the
cosmic microwave background radiation. A number of possible systematic effects
(dust, supernova evolution) do not seem to eliminate the need for Omega_X >
0. Moreover, analyses of very distant supernovae (z = 1.0-1.7) reveal an early
epoch of deceleration, followed by acceleration. Several groups are now in the
process of measuring hundreds of supernovae with z = 0.2-0.8, to determine the
equation-of-state parameter of the dark energy, w_X = P/(rho c^2). Thus far,
the best-fit value is w_X = -1, and its first derivative (dw/dz) is consistent
with zero, suggesting that the dark energy may indeed be the cosmological
constant or something nearly indistinguishable from it. Any viable theory of
quantum gravity will need to be consistent with this result.