-TERM
Having, under the pressure of observational evidence and an aesthetical
desire
to keep the inflationary scenario of the early universe as simple as
possible, admitted the existence of a constant positive
-term, it is
natural to take a step beyond Einstein's original hypothesis and
consider the additional possibility
that the -term is
not an exact constant, but rather, describes a new dynamical
degree of freedom (perhaps even a new form of matter). Really, neither
observational data, nor inflationary considerations tell us that a
"cosmological constant" is constant (though, as discussed above, it
should change sufficiently slowly with time, in particular, slower than
the Ricci tensor). In fact the effective
-term which
appears in the inflationary scenario of
the early universe is never an exact constant and rarely even an
approximate constant of motion.
(A recent analysis of observational data in the light of a time
dependent
may be found in
[203].)
To quantitatively describe this new degree of freedom (or a new form of
matter), some phenomenological models of a dynamical
-term have to be
introduced. The word "phenomenological" means that no attempt to derive
these models
from an underlying quantum field theory is being made, in contrast to
examples discussed in previous sections.
Historically, many phenomenological
-models were proposed
since 1986 (not counting the "C-field" of Hoyle and Narlikar (1962)
which was perhaps the earliest, though unsuccessful,
attempt to introduce a dynamical
-term in cosmology).
Depending upon their level of "fundamentality", these phenomenological
methods may be classified into 3 main groups:
1) Kinematic models.
Here
is simply assumed to be a function of either the cosmic time t
or the scale factor a(t) of the FRW cosmological model.
2) Hydrodynamic models.
Here a -term
is described by a barotropic fluid with some equation of state
p(
) (dissipative terms may also be present).
3) Field-theoretic models.
The -term is
assumed to be a new physical classical field (which we shall
call a lambda-field) with some phenomenological Lagrangian.
Of course, models from the last group are in a sense also the
most fundamental. In
particular, they may be used in a non-FRW setting.
Additionally, their quantization is straightforward. However, if we restrict
ourselves to a FRW model with small perturbations, the three different
way of describing a
-term could lead
to converging results. Note also, that
it was recently proposed to call a dynamical
-term "quintessence"
[24]
(irrespective of the specifities of modelling, so this
notion is wider than the notion of a lambda-field), though we don't
consider it to be obligatory.
In the case of field-theoretic models, the most simple and natural
is the model of a scalar field
with some self-interaction
potential
V(), minimal coupling
to gravity and no (or very weak) coupling to
other known physical fields. The latter requirement follows not only from
simplicity, but also from observational evidence (see
[27] for
a recent analysis of upper bounds on coupling of
to the electromagnetic
field). The assumption of minimal coupling to gravity may be relaxed,
but only slightly (see
[33]
for constraints on the
coefficient 
in
case of the
R2/2
coupling). Since
the minimally coupled scalar field model has proven to be
extremely successfuly in the case of
the inflationary scenario, one might be tempted to use it for the
description
of a -term. As a
result, an overwelming part of recent theoretical
activity has focussed on the scalar field model
(more appropriately, on this class of
models differing between themselves by the form of the scalar field
potential V()).