The discovery of the dark matter particle would
resolve a long-standing mystery in astronomy,
provide insights into dark matter’s role in galaxy
formation and structure, and be the first signature of new physics beyond the Higgs.
When Einstein introduced his theory of general
relativity, he added a cosmological constant term.
This term generated a repulsive force that countered the pull of gravity and kept the universe
static and stable. In the 1920s, Hubble’s discoveries showed that the universe was expanding, and physicists dropped the cosmological
Motivated by observational evidence favoring
a low-density universe and theoretical prejudice
that favored a flat universe, enthusiasm for a cosmological constant revived in the 1970s and 1980s
in the astronomy community (54–56). Physicists
recognized that the value of the cosmological
constant was a profound problem in fundamental physics (57).
A universe dominated by a cosmological constant is a strange place to live. We think of gravity
as an attractive force. If you throw a ball upwards, gravity slows its climb out of the Earth’s
gravitational well. Similarly, gravity (in the absence of a cosmological constant) slows the expansion rate of the universe. Imagine your surprise
if you threw a ball upwards and it started to
accelerate! This is the effect that a cosmological
constant has on the universe’s rate of expansion.
Supernova observations provided critical evidence for the universe’s acceleration. Supernovae
are bright stellar explosions of nearly uniform
peak luminosities (58). Thus, they serve as beacons that can be used to determine the light-travel
distance to their host galaxies. By determining
distance as a function of galaxy redshift, the
supernova observations measure the expansion
rate of the universe as a function of time. In the
late 1990s, supernova observers reported the surprising result that the expansion rate of the universe is accelerating (59, 60).
Over the past 15 years, the observational evidence for cosmic acceleration has continued to
grow. Measurements of the baryon acoustic scale,
both in the microwave background (3–8) and in
the galaxy distribution (9) as a function of redshift, traced the scale of the universe back to a
redshift of 1100. Measurements of the growth
rate of structure as a function of redshift also reinforced the case for cosmic acceleration.
Why is the universe accelerating? The most
studied possibility is that the cosmological con-
stant (or equivalently, the vacuum energy of emp-
ty space) is driving cosmic acceleration. Another
possibility is that there is an evolving scalar field
that fills space (like the Higgs field or the inflaton
field that drove the rapid early expansion of the
universe) (61). Both of these possibilities are
lumped together in “dark energy.” Because all of
the evidence for dark energy uses the equations
of general relativity to interpret our observations
of the universe’s expansion and evolution, an
alternative conclusion is that a new theory of
gravity is needed to explain the observations (38).
Possibilities include modified gravity theories with
extra dimensions (62).
Future observations can determine the source
of cosmic acceleration and determine the nature
of dark energy. Our observations can measure
two different effects: the relationship between
distance and redshift and the growth rate of struc-
ture (63). If general relativity is valid on cosmo-
logical scales, then these two measurements
should be consistent. These measurements will
also determine the basic properties of the dark
Astrophysicists are currently operating several
ambitious experiments that aim to use measurements of galaxy clustering and supernova observations to measure distance and gravitational
lensing observations to measure the growth rate
of structure (64, 65, 66). These are complemented
by microwave background observations (67, 68, 69)
that will provide independent measurements of
gravitational lensing and more precise measurements of cosmic structure. In the next decade,
even more powerful observations will map the
large-scale structure of the universe over the past
10 billion years and trace the distribution of matter over much of the observable sky background
(70, 71, 72). These observations will provide deeper
insights into the source of cosmic acceleration.
Although general relativity is now a hundred-year-old theory, it remains a powerful, and controversial, idea in cosmology. It is one of the basic
assumptions behind our current cosmological
model: a model that is both very successful in
matching observations, but implies the existence
of both dark matter and dark energy. These signify that our understanding of physics is incomplete. We will likely need a new idea as profound
as general relativity to explain these mysteries
and require more powerful observations and experiments to light the path toward our new
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D.N.S. is supported by grants from the National Science
Foundation and NASA.