Your picture of universe's timeline

[marcus]

Which is closest to how you are most apt to think of the universe's timeline:

bounded interval with definite beginning and end?

half-bounded, with a definite beginning but no end?

or with a definite end but no beginning?

infinite in both directions, no beginning and no end?

 

[jcsd]

Marcus, in cosmology bounded means that a universe has spatial boundaries. The term that's most apt is open and closed (though which as I said on another thread only strictly refer to certain models of Friedman-Lamaitre universes), though these both have a beginning in time (i.e. the big bang).

 

[zoobyshoe]

Using these more apt terms, then,
the universe, having opened, will
eventually close. But it will then
fly open again. I think this is
a very old idea, but the notion
of the universe expanding and contracting, something like a
breath, is appealing.

 

[marcus]

Originally posted by jcsd
Marcus, in cosmology bounded means that a universe has spatial boundaries. The term that's most apt is open and closed (though which as I said on another thread only strictly refer to certain models of Friedman-Lamaitre universes), though these both have a beginning in time (i.e. the big bang).

what wording would you suggest for simply talking about
segments of the line?

Its not so much a cosmology discussion as it is distinguishing four possible kinds of intervals of the real line.
infinite in both directions (-oo, +oo)
infinite to the right [a, +oo)
infinite to the left (-oo, b]
finite [a, b]

I think until around 1965 a lot of people pictured time as (-oo, +oo)

then the CMB was observed and somehow got confused with the idea of a "primeval fire-ball" and took hold of people's imagination and it became fashionable to picture time as [a, +oo)

But objectively the only reason to picture it that way is that the Friedmann equations quit working at t = 0.

And that, when you consider it, is a pretty poor reason. We know they, and the Einstein GR equation they're derived from, are just an approximation----and should at least be quantized, which people have been trying to do since Wheeler started it in 1962.

Now it looks as if when the spacetime model gets quantized it will NOT quit working at t = 0. So picturing time as [a, +oo)
may well turn out to have been a temporary fad.

 

[marcus]

Originally posted by zoobyshoe
Using these more apt terms, then,
the universe, having opened, will
eventually close. But it will then
fly open again. I think this is
a very old idea, but the notion
of the universe expanding and contracting, something like a
breath, is appealing.

it's a good image
and the idea may outlast the strictly scientific models that are currently available---no scientific model is the final word
on anything
so certain poetic visions can actually turn out to be more durable
than scientific theories, in the long run
for some reason I find this amusing, it's kind of
a joke that nature plays on scientists (in my view at least)

 

[Doctor Luz]

I think in a beginning in time, however, is necessary this beginning the big bang?

 

[marcus]

Originally posted by Doctor Luz
I think is a beginning in time, however, is necessary this beginning the big bang?

I like this question. I hope you have some ideas to offer.
Could you suggest some alternatives to the big bang?

Or could you suggest some different kinds of "big bang"?

Maybe there are different ways to imagine a beginning in time, and in that case, the words "big bang" do not have a clear meaning unless one can be a little more specific.

 

[Doctor Luz]

I think time has a begining. And I believe in Big Bang.

But what if the beginning in time (call it t=0) is not coincident with the big bang?. Maybe the origin of time is previous to the big bang. How can we know that big bang wat at t=0?

 

[Tail]

I'd say it's infinite in both directions.

 

[marcus]

Originally posted by Tail
I'd say it's infinite in both directions.

Yeah that's how I picture it too
it's just the simplest picture
given the lack of evidence to the contrary

the mere fact that one set of equations breaks down
when you extrapolate back too far with it
shouldn't be taken as proving that there is an edge to time
with nothingness beyond.

Maybe there is! but one model blowing up is hardly proof,
so try another model. how hard can it be?

and if earlier it collapsed and came thru fire and
began expanding well maybe that is just how it
refreshes itself. it tells us the story and we listen,
not the other way around,
be well tail

 

[selfAdjoint]

I didn't choose. You should always have a "None of the Abovw (Expalin)" choice.

In this case there are conflicting theories (BB versus cyclical brane cosmologies versus versions of LQG that track back through the BB to the other side...)

 

[meteor]

I have voted for a definite beginning and a definite end
After all, I've always visualized universe like a kind of dream. And dreams have a beginning and an end
Now, it appears that people like Bojowald are suggesting that before the Big bang, the universe was contracting. But well, what caused that space started to expand another time after big Bang?
Possibly the question of the million is: WHY is the space expanding?

 

[heusdens]

Actually, the issue of a beginning or infinity of time is quite an old philisophical issue, and digs up every now and then in new form.

Read this discussion / polemic against Herr Eugen Duhring (phantasizing about a begin of time) in the Anti-Duhring by Friedrich Engels http://csf.colorado.edu/psn/marx/Archive/1877-AD/p1.htm#c5"

 

[Eh]

Originally posted by selfAdjoint
I didn't choose. You should always have a "None of the Abovw (Expalin)" choice.

In this case there are conflicting theories (BB versus cyclical brane cosmologies versus versions of LQG that track back through the BB to the other side...)

But in models like the brane collision, time is infinite in both directions.

 

[Phobos]

Interesting that less than half are choosing the standard Big Bang model (option 2).

 

[marcus]

Originally posted by Phobos
Interesting that less than half are choosing the standard Big Bang model (option 2).

If you walked into the Physics Department at Penn State you might well encounter people who understand the "Big Bang" processes (as well as anyone can) but see no reason to suppose that time began at that moment.

I guess I don't see why someone would think that imagining time
as (- oo, + oo) is a rejection of the Big Bang picture of conditions around the beginning of the ongoing expansion.

Maybe something very much like what is commonly described as BB did occur. You can believe that without believing that time began with it. The quantum equations describing BB do not have a singularity---the universe evolves right through time zero.
It is only the UNquantized general relativity equations that have
a singularity. So where is the compelling reason to suppose
there is a breakpoint or terminus?

popular account: Ashtekar "Quantum Geometry in Action..."
http://arxiv.org/math-ph/0202008

detailed discussion: Ashtekar, Lewandowski, Bojowald
"Mathematical Structure of Loop Quantum Cosmology"
http://arxiv.org/gr-qc/0304074

Its work in progress. There are 10 or so recent papers about this.
Other people must be watching this too, so the poll results are not all that surprising, or?

 

[Eh]

Interesting, Marcus. What exactly does LQG propose happens prior to the big bang, given that there now seems to be minimum size of the universe? If it cannot be in a static state, one would think a beginning is implied.

 

[marcus]

Originally posted by Eh
Interesting, Marcus. What exactly does LQG propose happens prior to the big bang, given that there now seems to be minimum size of the universe? If it cannot be in a static state, one would think a beginning is implied.

On page 16 of this paper there's a picture of a wavepacket evolving from a prior contracting phase, through the erstwhile classical singularity, and on into the expanding phase, post time zero. It's just one of a bunch of papers but it might interest you.

http://arxiv.org/gr-qc/0303073

Another perspective: a recent communcation from Martin Bojowald referred to the transition around time zero as the "bounce".

Current research shows that the disappearance of the classical time zero singularity is robust----that is, it occurs in a wide variety of LQG models constructed and described by different people.

what the conditions look like immediately before and after time zero depend on the assumptions of the model, how matter is included etc.

various papers have dealt with cases such as
isotropic case
homogeneous case
case with a matter field

so there is no single monolithic message about what prior conditions are like. I cannot speak for what "LQG proposes",
but only point out, as Ashtekar has, the robust character of
this one feature

when General Relativity (which has a singularity at time zero) is quantized, the singularity goes away

both the scale factor a(t) and the curvature become operators----selfadjoint, discrete real spectrum observables----in the quantum theory.

In whatever treatment, the curvature turns out to be a bounded operator! This is where the original 1916 GR version blew up, it had infinite curvature and stopped computing at time zero.

The best summary of this for a general audience paper is on page 8 by Ashtekar

http://arxiv.org/math-ph/0202008

matter is included in the Hamiltonian as you see on page 9
and there is a simple diagram

however a more up-to-date technical summary by Ashtekar, Lewandowski, and Bojowald came out this year and supersedes it in details

http://arxiv.org/gr-qc/0304074