# The Naked Scientists Forum

### Author Topic: Does the universe expanding have a rate and an edge?  (Read 2877 times)

#### Jack McArdle

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##### Does the universe expanding have a rate and an edge?
« on: 08/05/2011 21:30:02 »
Jack McArdle  asked the Naked Scientists:

Hi!

Although I do not study physics (A level Chem/Bio student), I often read about it for the sake of expanding my scientific knowlege. I was recently looking at theories about the structure of the universe, and came across the idea that it could be spherical, hyperbolic or flat depending on the value of omega for -1<x<1. However, if the universe could potentialy be spherical would gravity not eventualy stop its expansion?

Thanks!

Jack

What do you think?

==

Jack, next time we would appreciate if you asked the same exact question only one time, instead of three times, using different captions. Could be a system error though?

Cheers :)
Yoron.

« Last Edit: 09/05/2011 05:35:43 by yor_on »

#### Phractality

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##### Re: Does the universe expanding have a rate and an edge?
« Reply #1 on: 08/05/2011 22:15:07 »
All those models tacitly assume that all the mass in the universe is evenly distributed within a finite volume of space. That being the case, you can calculate a critical density. If the density is greater, gravity should eventually stop and reverse the expansion. However, we now believe the expansion is accelerating, so the old formula is known to be wrong. Either the critical density is much greater than previously believed, or all the mass is not contained in a finite volume of space and there is no critical density.

If the universe is isotropic and homogeneous at the largest scales, then gravity pulls equally in all directions. But if the universe is finite, then due to the large-scale warp of space-time, all directions intersect at the singularity of the big bang, so gravity does slow the expansion.

If the universe is infinite, isotropic and homogeneous at the largest scales, then gravitational potential is the same everywhere at the largest scales. With no large-scale gradient of gravitational potential, there can be no large-scale warp of space-time. Gravity still pulls equally in all directions, but no two directions can ever intersect, so gravity does not slow the expansion.

#### granpa

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##### Re: Does the universe expanding have a rate and an edge?
« Reply #2 on: 09/05/2011 01:48:47 »

#### yor_on

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##### Re: Does the universe expanding have a rate and an edge?
« Reply #3 on: 09/05/2011 04:03:35 »
What you seem to be wondering about is the Friedmann-Lemaître-Robertson-Walker (FLRW) model. That model assumes that the universes shape will depend on the density, where 'Omega is the average density of the universe divided by the critical energy density, i.e. that required for the universe to be flat (zero curvature).

I saw someone write that

"The gravitational energy of the universe is precisely equal to its kinetic energy (the energy an object possesses by virtue of its motion) for a particular value of the mass density in the universe. This value, which separates eternal expansion from eventual contraction, is called the critical density"

discussing it but that definition seems quite shaky to me as any motion in this universe has to be related to some other 'frame of reference' to make sense. And we don't have a 'gold standard' anywhere, as far as I know. so to define a objects kinetic energy is a relation to another frame of reference. there is no way to define a uniformly moving objects speed without involving subjective parameters, that is, defining a 'system'.

But I'm not entirely sure if this definition is what they went out from? It seem to build on the universes isotropic apparition, that everything looks much the same, no matter from where, and on what you look on in space. "it describes a homogeneous, isotropic expanding or contracting universe that may be simply connected or multiply connected."

The assumption that 'the kinetic energy is exactly equal to the gravitational energy' seem to make a certain sense though, if we look at it as a symmetric solution, and ignore the idea of 'gravity' as an 'energy'. In Einsteins universe gravity is not an energy as far as I know, it's a geometry. But the universe seems to have a symmetry, like an equilibrium that keeps it in balance, so from that point of view I agree. Maybe we have some guys that have looked closer on this?

#### MikeS

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##### Does the universe expanding have a rate and an edge?
« Reply #4 on: 15/05/2011 09:03:55 »

"The gravitational energy of the universe is precisely equal to its kinetic energy (the energy an object possesses by virtue of its motion) for a particular value of the mass density in the universe.

You could say
"The gravitational energy of the universe is precisely equal to its kinetic energy plus its potential energy for a particular value of the mass density in the universe. This would be true but as all mass in the universe is subject to a gravitational field, it must be moving in which case the term potential energy is not required.

#### Phractality

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##### Does the universe expanding have a rate and an edge?
« Reply #5 on: 15/05/2011 20:34:12 »
Kinetic energy is determined by velocity relative to an arbitrary reference frame.

Potential energy is relative to an arbitrary ground state.

It makes no sense to equate the two in reference to the universe.

#### MikeS

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• Posts: 1044
##### Does the universe expanding have a rate and an edge?
« Reply #6 on: 16/05/2011 07:36:26 »
"The gravitational energy of the universe is precisely equal to its kinetic energy"

Kinetic energy is determined by velocity relative to an arbitrary reference frame.

Potential energy is relative to an arbitrary ground state.

It makes no sense to equate the two in reference to the universe.

It might not make sense but it still exists.  Just because we cant define an arbitrary reference frame and hence can't measure it does not mean it does not exist.  It quite obviously does.  If, for example, the universe were to collapse to a singularity then all matter must have kinetic energy to return it to that point.

#### The Naked Scientists Forum

##### Does the universe expanding have a rate and an edge?
« Reply #6 on: 16/05/2011 07:36:26 »