Deep as Einstein's general theory of relativity may be, it remains silent on a profound question: Is empty space really empty? Inflation models predict that it was not so in the past, and it may not be so today either. Einstein introduced a "cosmological constant"
into his equations, to represent the possibility that even empty space has
energy and couples to gravity. The unknown magnitude of the cosmological
constant is set by parts of physics beyond Einstein's understanding---and,
at present, our own.
The new discovery that the expansion of the Universe appears to be accelerating suggests the presence of something dubbed "dark energy"
that drives space apart. It seems likely that we have roughly measured the
value of a cosmological constant or something like it.
This new discovery is already widely accepted because it explains many
observations. The first indication was that the rate of expansion of the
Universe has been increasing, revealed by Type Ia supernovae. Supporting
evidence comes from studies of global geometry, structure formation, cosmic
age, and galaxy clustering. They leave little doubt that in some sense Einstein's
"cosmological constant" is a reality. The energy of the Universe is dominated
by empty space whose gravitational effect is to pull the Universe apart.
Since we have no theory of dark energy, anything we learn is an unexpected discovery. Our current understanding of how quantum mechanics and gravity are united predicts an amount of dark energy larger than observed by a factor of 10120.
Some modern theories predict that the amount of dark energy decreases with
time, instead of staying constant as in Einstein's conception. For this
very reason, dark energy is the most exciting new development in fundamental
physics. Because dark energy seems to control the expansion of the Universe,
we cannot predict the fate of the Universe without understanding the physical
nature of dark energy. As we develop this understanding, we will be poised
to answer the profound question: will the Universe last forever?
|Composition of the Cosmos|
As we look at our Universe today, we estimate that it consists of five
percent ordinary matter (stars, planets, gas, and dust), twenty-five percent
"non-baryonic" dark matter (as-yet-undiscovered particles unlike ordinary
"baryonic" matter), and seventy percent dark energy (which can be considered to have mass, too, because energy E = mc2).
To learn how dark energy really works, we need to measure its properties
in more detail. It is spread so thin that it can only be studied in space,
where the enormous volume allows its effects to be noticed. The first step
will be to measure its density and pressure and how they change with time.
The Dark Energy Probe will deploy the best available technology to study this effect. Constellation-X, LISA, and the Inflation Probe will provide independent constraints to verify and increase the measurement precision.
The small samples provided by the Hubble Space Telescope
show that a dedicated, special-purpose instrument could provide a much better
measurement of the bulk properties of the dark matter. These determine whether
the energy is really constant, as Einstein conjectured, or whether it has
changed over cosmic time, as suggested by some string theorists. Real data on this question would help us discover where dark energy comes from, and what the future of our Universe will be.