Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here: http://www.routledge-ny.com/books.cfm?isbn=0415923549
The theory of general relativity has to be modified if it is to be applied to situations where the matter density is extremely high, in order to take account of the effects of to quantum physics (see also quantum gravity). In cosmology, this basically means that form when quantum effects manifest themselves on the scale of the horizon.
When do we expect quantum corrections to become significant? Of course, in the absence of a complete theory (or even any theory at all) of quantum gravity, it is not possible to give a precise answer to this question. On the other hand, we can make fairly convincing general arguments that yield estimates of the timescales and energy scales where we expect quantum gravitational effects to be large, and for which therefore we should distrust calculations based only on the classical (non-quantum) theory of general relativity. It turns out that the limit of validity of general relativity in the Friedmann models is fixed by the Planck time, which is of the order of 10-43 seconds after the Big Bang.
The Planck time tp, is the time for which quantum fluctuations governed by the Heisenberg uncertainty principle exist on the scale of the Planck length, lp = ctp, where c is the speed of light. From these two scales we can construct other Planck quantities such as the Planck mass, mp, the Planck energy, Ep, and so on. Starting from the Heisenberg uncertainty principle in the form
and ignoring any factors of 2 from
now on, we can see that on
dimensional grounds alone we can identify the energy term with some
mass mp through the relation E =
mpc2. Assuming that
t can be
represented as the Planck time tp, we have
We can express the Planck mass as a Planck density
p times a
Planck
volume (or rather the cube of a Planek length). We want to bring
gravity into these considerations, in the shape of the Newtonian
gravitational constant G. We can do this by noting that the free-fall
collapse time for a self-gravitating body of density
p is given
by t2
= 1 / Gp. Replacing m by
p(ctp)3 and then
p by
1 / Gtp2 in the above
expression leads to
which finally leads us to an expression for the Planck time in terms
of fundamental constants only:
which is around 10-43 seconds. The Planck length is simply this
multiplied by the speed of light, c, and is consequently around
10-33
cm. This, for example, is about the size of the cosmological horizon
at the Planck time (assuming that the concept of a horizon is
meaningful at such an early time). The Planck density is phenomenally
high: about 1096 grams per cubic centimetre. Interestingly, however,
the Planck mass itself is not an outrageous number: mp
= (hc / G) = 10-5
g. We can carry on with this approach to calculate the Planck energy
(about 1019 GeV) and the Planck temperature (the
Planck energy divided
by the Boltzmann constant, which gives about 1032 K).
In order to understand the physical significance of the Planck time
and all the quantities derived from it, it is useful to think of it in
the following manner, which ultimately coincides with the derivation
given above. We can define the Compton time for a particle of
mass m
to be tC = h / mc2; this represents
the time for which it is permissible
to violate the conservation of energy by an amount equal to the mass
of the particle, as deduced from the uncertainty principle. For
example, a pair of virtual particles of mass m can exist for a time of
about tC. We can also defined the Compton
radius of a body of mass m
to be equal to the Compton time times the velocity of light:
lC = ctC
= h / mc. Obviously both these quantities decrease as m
increases. These scales indicate when phenomena which are associated
with quantum physics are important for an object of a given mass.
Now, the Schwarzschild radius of a body of mass m is given
by lS =
2Gm / c2. This represents, to within an order of
magnitude, the radius
that a body of mass in must have for its rest-mass energy
mc2 to equal
to its internal gravitational potential energy U
Gm2 / lS. General
relativity leads us to the conclusion that no particle (not even a
photon) can escape from a region of radius lS around a
body of mass m;
in other words, speaking purely in terms of classical mechanics, the
escape velocity from a body of mass m and radius
lS is equal to the
velocity of light. We can similarly define a Schwarzschild time to be
the quantity tS = lS / c =
2Gm/c3; this is simply the time taken by
light to travel a proper distance lS. A body of
mass m and radius lS
has a free-fall collapse of the order of tS. Note that
both tS and lS
increase as m increases.
We can easily verify that, for a mass equal to the Planck mass, the
Compton and Schwarzschild times are equal to each other and to the
Planck time.
Likewise, the relevant length scales are all equal. For a mass greater
than the Planck mass, that is to say for a macroscopic body,
tC < tS
and lC < lS, and quantum corrections
are expected to be negligible in
the description of the gravitational interactions between different
parts of the body. Here we can describe the self-gravity of the body
using general relativity or even, to a good approximation, Newtonian
theory. On the other hand, for bodies of the order of the Planck
mass, that is to say for microscopic entities such as elementary
particles, tC > tS and
lC > lS, and quantum corrections will be
important in a description of their self-gravity. In the latter case
we must use a theory of quantum gravity in place of general relativity
or Newtonian gravity.
At the cosmological level, the Planck time represents the moment
before which the characteristic timescale of the expansion is such
that the cosmological horizon, given roughly by lP,
contains only one
particle (with mass equal to the Planck mass) for which
lC
lS. On
the same grounds as above, we therefore have to take into account
quantum effects on the scale of the cosmological horizon.
It is interesting to note the relationship between the Planck
quantities and the properties of black holes. According to theory, a
black hole of mass M, because of quantum effects, emits Hawking
radiation like a black body. The typical energy of photons
emitted by
the black hole is kT, where the temperature T is given by
The time needed for such a black hole to evaporate completely (i.e. to
lose all its rest-mass energy Mc2 via Hawking
radiation) is given by
By taking these two equations and inserting M =
mp, we arrive at the
interesting conclusion that a Planck-mass black hole evaporates on a
timescale of the order of the Planck time.
These considerations show that quantum gravitational effects are
expected to be important not only at a cosmological level at the
Planck time, but also continuously on a microscopic scale for
processes operating over distances of about lp and
times of about
tp. In particular, the components of the metric
describing space-time
geometry will suffer fluctuations of the order of
lp / l on a length
scale l and of the order of tp / t on a timescale
t. At the Planck time,
the fluctuations are 100% on the spatial scale lp of
the horizon and
on the timescale tp of the expansion. We might imagine
the Universe at
very early times as behaving like a collection of Planck-mass black
holes, continually evaporating and recollapsing in a Planck time. This
picture is very different from the idealised, perfect-fluid universe
described by the Friedmann models, and it would not be surprising if
deductions from these equations, such as the existence of a
singularity, were found to be invalid in a full quantum description.
FURTHER READING:
Kolb, E.W. and Turner, M.S., The Early Universe (Addison-Wesley,
Redwood City, CA, 1990).