Invited review at "The Early Universe and Cosmological
Observations: a Critical Review", UCT, Cape Town, July 2001, to appear
in "Classical and Quantum Gravity"
astro-ph/0202076
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THE COSMOLOGICAL CONSTANT PROBLEM AND QUINTESSENCE
Varun Sahni
Inter-University Centre for Astronomy & Astrophysics,
Puné 411 007, India
E-mail: varun@iucaa.ernet.in
Abstract. I briefly review the cosmological constant problem and
the issue of dark energy (or quintessence).
Within the framework of quantum field
theory, the vacuum expectation value of the energy momentum tensor
formally diverges as k4. A cutoff at the Planck or
electroweak scale
leads to a cosmological constant which is, respectively, 10123
or 1055 times larger than the observed value,
/
8
G
10-47
GeV4.
The absence of a fundamental symmetry which could set the value
of to either
zero or a very small value
leads to the cosmological constant problem.
Most cosmological scenario's favour a large time-dependent
-term in the
past (in order to generate inflation at z >> 1010),
and a small
-term today,
to account for the current acceleration of the universe
at z 
1. Constraints arising from cosmological nucleosynthesis,
CMB and structure formation constrain
to be
sub-dominant during most of the intermediate epoch
1010 < z < 1.
This leads to the cosmic coincidence conundrum
which suggests that the acceleration of the universe is a recent phenomenon
and that we live during a special epoch when the density in
and
in matter are almost equal. Time varying models of dark energy
can, to a certain extent, ameliorate the fine tuning problem (faced by
),
but do not resolve the puzzle of cosmic coincidence.
I briefly review tracker models of dark energy, as well as more recent
brane inspired ideas and the issue of horizons in an accelerating universe.
Model independent methods which reconstruct the
cosmic equation of state from supernova observations are also assessed.
Finally, a new diagnostic of dark energy - `Statefinder', is discussed.
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