236 Dark Energy
If휌휆were even slightly larger, the repulsive force would cause the Universe to expand
too fast so that there would not be enough time for the formation of galaxies or other
gravitationally bound systems. This is called thecosmological constant problem.
Second, it raises the question of why the sum
훺 0 =훺m+훺휆
is precisely 1.0 today (to within 0.3%) when we are there to observe it, after an expan-
sion of some 12 billion years when it was always greater than 1.0. The density of
matter decreases like푎−^3 ,while훺휆remains constant, so why has the cosmological
constant been fine-tuned to come to dominate the sum only now? This is referred to
as thecosmic coincidence problem.
Third, we do not have the slightest idea what the휆energy consists of, only that
it distorts the geometry of the Universe as if it were matter with strongly negative
pressure, and it acts as ananti-gravitationalforce which is unclustered at all scales.
Note that a positive휆in Equation (5.18) implies an accelerated expansion,푎>̈ 0. Since
we know so little about it, we also cannot be sure that휆is constant in time, and that
its equation of state is always푤휆=−1.
Observations. The first evidence for the need of a cosmological constant came in
1998–1999 from two independent teams monitoring high-redshift, Type Ia supernovae
[1, 2]. The expectation was to see cosmic deceleration, since gravitation is attractive.
But the teams converged on the remarkable result that, on the contrary, the cosmic
expansion was accelerating, consistent with a flat universe with훺휆≈ 0 .7. When com-
pared to local Type Ia supernovae, those observed at푧≈ 0 .5 were fainter than expected
in a matter-dominated universe, as if their light were absorbed by intervening dust.
However, many systematic checks ruled out the hypothesis of grey dust extinction that
increased towards higher redshifts. While the significance of this discovery has since
then only been confirmed with the inclusion of larger and better calibrated SN Ia data
sets the cause of the acceleration remains unknown.
At the time of the first discovery by the supernova teams the ground had been well
prepared by the CMB and large scale structure data, which already provided substan-
tial indirect evidence for a cosmological constant. As is evident from the very recent
Figure 8.7, the constraints from the supernovae and from CMB are practically orthog-
onal in the훺m,훺휆space.
The supernova results were followed within a year by the results of balloon-borne
CMB experiments that mapped the first acoustic peak and measured its angular
location, providing strong evidence for spatial flatness [3, 4]. The acoustic peak mea-
surement implied that the alternative to휆was not an open universe but a strongly
decelerating,훺푚=1 universe that disagreed with the supernova data by 0.5 mag-
nitudes, a level much harder to explain with observational or astrophysical effects.
The combination of spatial flatness and improving measurements of the Hubble
constant provided an entirely independent argument for an energetically dominant
accelerating component: a matter-dominated universe with훺tot=1 would have age
푡 0 =( 2 ∕ 3 )퐻 0 ≈ 9 .5 Gyr, too young to accommodate the 12–14 Gyr ages estimated for
globular clusters.