May 23, 2001
Einstein's Biggest Blunder? The Case for an Accelerating Universe
Dr. Alex Filippenko, UC Berkeley
Type la (hydrogen-deficient) supernovae have enormous potential for the
determination of fundamental cosmological quantities. Relatively low-
redshift Type la supernovae demonstrate that the Hubble expansion is
linear , that the Hubble constant has a value of 65 +1- 2 km/s/Mpc
(statistical uncertainty), that the bulk motion of the Local Group is
consistent with the COBE result, and that the properties of dust in other
galaxies are similar to those of dust in the Milky Way. The light curves
of high-redshift (z = 0.3-1) supernovae are stretched in a manner
consistent with the expansion of space; similarly, their spectra exhibit
slower temporal evolution (by a factor of 1 +z) than those of nearby
supernovae. Our most important conclusion is that the expansion of the
Universe is accelerating, perhaps due to the presence of vacuum energy,
also referred to as Lambda or Einstein's "cosmological constant." Thus,
Einstein seems to have been right after all, but for the wrong reason (and
with the incorrect numerical value for Lambda). We derive a current
dynamical age of 14 +1- 2 billion years for the Universe, consistent with
the ages of globular star clusters. Moreover, combining our results with
existing measurements of the cosmic microwave background radiation, we
find a best fit for the normalized matter density and vacuum energy
density in the Universe of about 0.3 and 0.7, respectively. With a sum
close to unity, this agrees with the value predicted by most inflationary
models for the evolution of the Universe, and indicates that the Universe
is flat (Euclidean geometry) on large scales. A number of systematic
effects (dust, evolution) might be affecting our results and will be
discussed, but so far they don't seem to eliminate the need for nonzero
Lambda. Most recently, analysis of a supernova at redshift 1.7 provides
further support for acceleration, rather than some other astrophysical
systematic effect.
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