# Universe expansion -> t = - inifinite

1. Oct 30, 2007

### JPC

hey

Was thinking
since the universe is actually expanding at an increasing speed
We can ask the question : how did all the matter+energy get reunite in one small point ?

well, can we just say that if we consider , 't' : time :
t=-infinite : the beginning of everything
t = 0 : today
t = +oo : the end of everything.

That if 'd' the diameter of the universe at a time t

Lim -oo (d) = 0
lim +oo (d) = +oo

Meaning, that its all about limits

2. Oct 30, 2007

### marcus

according to the standard (LambdaCDM) model that cosmologists mostly use, the universe has NOT ALWAYS been accelerating its expansion

for a long time (billions of years) the expansion was slowing down and the Hubble rate was decreasing

so what you see now does not tell you that it has always been doing that
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a lot of work going on now is about models that have not always been expanding.

in these models there is a before-the-big-bang contraction phase and an abrupt turnaround when a critical density was reached (near the socalled Planck density)

It wouldn't have been actually a "point" as people used to imagine, but just very concentrated

If you want links to the technical papers, just say.
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the point is that just because at present it is doing such-and-such doesn't mean we can extrapolate back assuming that it has always

it's better to use a computer model with some quantum law of gravity built in, and actually run it backwards and see what happens-------or to use a equation model with solvable equations
either way you may find that there is a turnaround (gravity acting different at very high density)

Last edited: Oct 30, 2007
3. Oct 30, 2007

### JPC

but if D = F(t)
where D : diameter of the universe (aprox)
t : time

what are the variations of the function f ?
- always increasing, (with or without constant at some intervals) : (french : croissant) ?
- Increasing, then decreasing ?
- other ?

and what are the limits of f(x) at :
- infinite and + infinite ?

4. Oct 31, 2007

### JPC

No one really knows exactly ?

5. Oct 31, 2007

### Chris Hillman

Shouldn't this be in the Cosmology forum?

I think you have misunderstood careless references to "diameter of the universe", which doesn't mean what (almost certainly) you think: a more correct term is scale parameter.

Of course the time dependence of the scale parameter in the FRW lambdadusts is known (as a function of proper time as measured by the dust particles). See the last chapter of the excellent undergraduate textbook by D'Inverno for the answers you seek. You can look for old sci.physic.* posts by myself discussing in detail FRW dusts with nonzero Lambda. In some cases this parameter is most conveniently expressed parametrically but in the case of flat hyperslices it can be easily written as an explicit function of "time".

More important: if you "run time backwards" in the FRW models you come to an initial time, which we call the Big Bang. (In some special cases the same thing happens if you run time forwards, except we call that the Big Crunch.) There are some other cosmological models, such as the de Sitter lambdavacuum, in which one can "run time backwards" indefinitely. It is possible to analyse the global structure of cosmological models such as FRW models and de Sitter lambdavacuum using methods pioneered by Penrose and others. See for example Hawking and Ellis, The Large Scale Structure of Space-Time, Cambridge University Press, 1973.

Last edited: Oct 31, 2007
6. Oct 31, 2007

### JPC

Oh, yes maybe this post should be put in cosmology, but now that its posted the forum wont let me change its location.

Thanks for all the references you have given me, but im just a 12th grade student in a French semi-International Scientific section, i don't want to get lost in High level explanations until i reach the scientific level.

Im just looking for simple explanations for a general understanding. I will be looking for the details latter in my post-school studies.

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but which model as more proof, justification :
- FRW or de Sitter lambdavacuum ?

Right now the universe is expanding at an increased rate.
Is it because of black matter with a negative mass that acts as a expansion force ?

And if the Universe is accelerating, how does FRW explain that the universe will come back together in a big crush ?