We need to separate issues about
the Big Bang into two.
One is that there was an event, a period of
time we know a fair amount about.
About 13.7 billion years ago.
And by direct observation by theory we know
something about the kind of matter there was,
the temperatures, the densities, how
light went through that, how it cooled down.
And a lot of cosmology is studying time from
that forward and trying to push our understanding
both by observation and my theory back as
early as we can get.
So one issue that obviously occurs to everybody
is that thing we call the Big Bang--
Was there anything before that?
Did that come out of anything?
Was there any story we can tell, 
any physical story we can tell
about the origin of the Big Bang.
The answer right now is that we don't know and
all options are on the table.
Probably 80 years ago, the Big Bang theory
was associated with the idea that,
No, there's nothing that you could say. 
Space and time itself began.
And thereforer there was nothing before it
because there was no 'before' before it.
Modern cosmological theory imagines another
possibility which is that there was something
before this thing we call the Big Bang, there
there was another state from which our observable
universe evolved and other observable universes
continue to evolve.
Different kinds of universes that maybe look
very different from ours.
So there are theories of that character.
Now, let me just say a word about the idea
that, on the first model you might say 'Doesn't
that mean the universe came from nothing?'
and that sounds very strange and part of the
strangeness has to do with people being a
little sloppy about the word 'nothing'.
Here's an old philosophical joke.
Half-joke, half-puzzle.
Someone says  "I've got proof that a ham 
sandwich is better than heaven."
And the proof has two premises, one is that
'nothing is better than heaven' and the other is
'a ham sandwich is better than nothing',
and therefore "a ham sandwich is better than heaven".
And you say, I know somethings gone wrong
with that argument, right?
What's gone wrong with it?
Well because we use the word nothing in different
ways.
And when I say 'Nothing is better than heaven', 
I'm not saying,
'There is this thing better than heaven--
we call it "nothing"'.
So when people say on the one model, the Big
Bang came from nothing, what they mean is
there isn't anything it came from, so you
don't ask the further question, well what
is this 'nothing' it came from?
What you're expressing is, there just isn't
anything, it is the complete totality,
once you trace back to a certain point, space and
time themselves don't go any further.
And if time doesn't go any further, to ask
what happened before the big bang,
it's just logically incoherent.
There can't be a before.
Now, we don't know that that's the way it
is.
But if it is that way, it's not the right
question to ask, but does that mean that nothingness
somehow gave birth to the universe?
That sounds very strange, you're going to
say, 'no nothingness didn't give birth to the
universe, there isn't anything that gave birth
to the universe.'
The universe is a kind of totality, it's a
whole, it's everything there is, and it's
limited in a certain way, it's limited in
the time extent that you can go backwards.
Now, on the other model there was something
before the Big Bang,
you can then ask, what came before it?
How did the Big Bang come out of that?
Can I trace even further back?
And at that point we're not going to find
out much by using out big telescopes
we're not going to find out much 
using the kinds of resources
that give us a lot of very interesting information
about the observable universe.
All we have to go on is theory.
And cosmology switches over from the astronomers
to the theoreticians who might say
'Yeah, I have a theory that will tell you what this
earlier state was like.'
And then you could only ask the theory, can
I keep going back?
Can I keep going back?
Does it it go back forever?
Now in the case of going backward in time,
it's interesting to notice that no matter
what you do people will be upset.
if you go back in time to a certain point and then stop, 
they're going to say well that's strange,
how could time itself begin?
But if you never stop you're going to say
well how could an infinite amount of time
have elapsed?
How could we get to now if you had to go through
an infinite amount of time to get here.
So there's perpetual discontent, this was
something that as noticed by Kant and he just
said 'Well all the answers to these questions
about the whole cosmos, the cosmological questions
about everything taken together, are going
to feel dissatisfactory.
There can't be a further explanation of everything
taken together because if you appeal to something
further, you haven't taken everything together
in the first place.
So if you think the cosmological context comes
with certain problems inherent in what it is--
if you're dealing with everything then
there are going to be certain kinds of explanations
or certain kinds of understandings which automatically
by logic, there can't be for everything taken
together, but there can be for any little part.
I can ask, of the United States, how did it
begin?
Or how did it come about.
And there are earlier things you can talk
about in a process that leads to it, but automatically
if I say I want to ask that question about
the totality of everything, well automatically
there can't be anything else I appeal to to
explain it, because then I wouldn't have had
everything in the first place.
