It seems fairly likely that there was a Big Bang.
The obvious question that could be asked to challenge or define the
boundaries between physics and metaphysics is: what came before the
Physicists define the boundaries of physics by trying
to describe them theoretically and then testing that description against
observation. Our observed expanding Universe is very well described
by flat space, with critical density supplied mainly by dark matter
and a cosmological constant, that should expand forever.
If we follow this model backwards in time to when
the Universe was very hot and dense, and dominated by radiation, then
we have to understand the particle physics that happens at such high
densities of energy. The experimental understanding of particle physics
starts to poop out after the energy scale of electroweak unification,
and theoretical physicists have to reach for models of particle physics
beyond the Standard Model, to Grand Unified Theories, supersymmetry,
string theory and quantum cosmology.
This exploration is guided by three outstanding problems
with the Big Bang cosmological model:
1. The flatness problem
2. The horizon problem
3. The magnetic monopole problem
The Universe as observed today seems to enough
energy density in the form of matter and cosmological constant to provide
critical density and hence zero spatial curvature. The Einstein equation
predicts that any deviation from flatness in an expanding Universe filled
with matter or radiation only gets bigger as the Universe expands. So
any tiny deviation from flatness at a much earlier time would have grown
very large by now. If the deviation from flatness is very small now,
it must have been immeasurably small at the start of the part of Big
Bang we understand.
So why did the Big Bang start off with the deviations
from flat spatial geometry being immeasurably small? This is called
the flatness problem of Big Bang cosmology.
Whatever physics preceded the Big Bang left
the Universe in this state. So the physics description of whatever happened
before the Big Bang has to address the flatness problem.
The cosmic microwave background is the cooled remains
of the radiation density from the radiation-dominated phase of the Big
Bang. Observations of the cosmic microwave background show that it is
amazingly smooth in all directions, in other words, it is highly isotropic
thermal radiation. The temperature of this thermal radiation is 2.73°
Kelvin. The variations observed in this temperature across the night
sky are very tiny.
Radiation can only be so uniform if the photons
have been mixed around a lot, or thermalized, through particle collisions.
However, this presents a problem for the Big Bang model. Particle collisions
cannot move information faster than the speed of light. But in the expanding
Universe that we appear to live in, photons moving at the speed of light
cannot get from one side of the Universe to the other in time to account
for this observed isotropy in the thermal radiation. The horizon size
represents the distance a photon can travel as the Universe expands.
The horizon size of our Universe today is too small
for the isotropy in the cosmic microwave background to have evolved
naturally by thermalization. So that's the horizon problem.
Magnetic monopole problem
Normally, as we observe on Earth, magnets only come
with two poles, North and South. If one cuts a magnet in half, the result
will not be one magnet with only a North pole and one magnet with only
a South pole. The result will be two magnets, each of which has its
own North and South poles.
A magnetic monopole would be a magnet with
only one pole. But magnetic monopoles have never been seen? Why not?
This is different from electric charge, where we can separate an arrangement
of positive and negative electric charges so that only positive charge
is in one collection and only negative charge is in another.
Particle theories like Grand Unified Theories and
superstring theory predict magnetic monopoles should exist, and relativity
tells us that the Big Bang should have produced a lot
of them, enough to make one hundred billion times the observed energy
density of our Universe.
But so far, physicists have been unable to find even
So that's a third motivation to go beyond the Big
Bang model to look for an explanation of what could have happened when
the Universe was very hot and very small.
The Inflationary Universe>>