So how did we get from this very, very uniform state which occurred about
300,000 years after the Big Bang to all of this delicate, complex beautiful
structure of planets and stars and galaxies that we see today? The main
motor of all of that structure formation is again, gravity. Gravity works to draw
things together. So even these slight fluctuations in density, after the Big
Bang become larger and larger as the more dense places attract matter into
them. And all of the matter tends to collapse into these dense regions,
forming stars. The stars collect together into galaxies. The galaxies collect
together into galaxies clusters. And you get this tremendous variation and
complexity simply by the operation of the simple law of gravity from an
extremely uniform looking initial state. That's the modern story of structure
formation. And it's been worked out in great detail. What you do now is you
can't test this in the laboratory, so you test it on computers. You put into the
computer the laws of gravity and various initial states. And you let it evolve in
time and see what kind of structures form. Now in order to get the predictions
to match what we see, an extra ingredient has to be added, which is called Dark
Matter. There seems to be more gravitating matter in the universe than
the matter that we can actually observe. So whatever it is it doesn't emit light
it's dark. It's not hot. So this is called Cold Dark Matter. And we find that
galaxies seem to have a halo of Dark Matter around them that account for the
motions of the stars within the galaxies. This dark matter does have an effect. It
has gravitational effects. It bends light and so astronomers
now look through their telescopes for these tell-tale effects of the
gravitational light bending due to Dark Matter. The story about how we got from
the Big Bang to the present day then is on very solid ground. As far as we can go
back through our telescopes is to the Cosmic Background Radiation. We can't see
beyond that. So we have to let theory take us further back towards the Big
Bang. Now what is, at present, the most widely accepted theory is that it didn't
just happen that the Big Bang occurred and the universe expanded uniformly
after that. But that shortly after the Big Bang,
shortly after this initial state, there was a period of extreme expansion,
inflationary expansion. The size of the universe doubling and redoubling and
redoubling many times extremely quickly. And then that initial expansionary phase
ends and the slower rate of expansion that was proposed by Lemaître takes
over. And that takes us up to the present day. That's very well attested by the
evidence and by theory. But it leaves still open the question, what about the
Big Bang itself? Is there anything more to be said about it except that it was
the beginning of everything? Remember that the Big Bang, the singularity state,
was originally arrived at by analysis of Einstein's equations. But Einsteins
equations only deal with gravity. And gravity isn't the only physical force
there is. In addition to gravity there's electromagnetism, and there's the weak
and strong nuclear forces. And those forces are all dealt with by Quantum
Theory. It's hard to put Quantum Theory and General Relativity together and no
one really quite knows how to do it. So we're entering the realms of speculation
now and not settled physics. But indications are that putting Quantum
Theory together with General Relativity might
yield an entirely different picture of the Big Bang. That instead of it being
the very end of space time instead of everything coming together into a point,
that Quantum Theory will smear things out a bit. And you can imagine going even
earlier than the Big Bang. You can ask questions about where the Big Bang
itself came from? What was before it. And there are several theories that suggest
sort of state of the universe antecedent to the Big Bang. One of these theories is
called The Theory of Eternal Inflation. And it holds that there was a background
state again of this rapidly inflating, expanding kind of universe that gives
birth to these smaller bubble universes or pocket universes like we live in,
where the expansion is much slower and more sedate. It would give rise to an
infinite number of these smaller bubble universes. And we just happen to find
ourselves in one of them. So you can think of the Inflationary Theory as a
kind of scientific birth from Chaos Theory. Although the birth was from of
more than just the world we live in. There's the birth of other worlds that
we can't directly communicate with. There's another of idea which is that
antecedent to the Big Bang, there was another universe very much like ours
that expanded and then contracted or expanded and got it into a state where
it transitioned into the Big Bang. And that that, itself, the same story occurs.
So you get a Cyclic Cosmology. You get a new universe that exists and then ends
and gives birth to a new universe that exists and then ends over and over and
over. Just like in the Hindu account of the cosmos. And of course there's always
the original idea of Lemaitre that when you go back to the Big Bang you really
get to the end of everything, the end of space and time itself. And so you have a
Finite Universe Theory. So what we see is that these are the only ideas there are,
that people came up with in Mythology and in Philosophy and repeating in
Science. Because what other possibilities are there? Our structured universe might
have come from something unstructured as in the Chaos Theory. Or it might have
come from something earlier that itself was structured as in these Cyclical
Theories. Or it might not have come from anything as in the Finite Time Theories.
And there's also the possibility of the Steady State Theory that things have
always been like this. The cosmologists Fred Hoyle had held
strongly to a Steady State Theory in which the universe existed forever and
always looked something like this. We're not really going to be able to know
which of these scientific cosmologies is correct until we can work out better the
theory. Because what we have to go on is basically theory rather than observation
in this realm. And we don't know yet exactly how the Quantum Theory applies
in these very extreme conditions of high density and very high temperature.