[Eelpie]

a naive question probably,

obviously the expansion of the universe now in proportional terms is very very slow (would be interested to know how slow, how much further would an object exactly a billion light years away be after a year), but presumably even objects much closer together are moving away from each other and we could in principle observe the expansion of space? if the theoretical expansion between two points is below a planck unit how would it occur exactly?

This post has been edited by Eelpie: 11 February 2012 - 02:26 PM

[Eelpie]

sorry may have used terms like "local" wrongly...I know they have specific meaning in physics but I am using them as a layman would...also is relativity the right area for this post...I always assumed that general relativity was intricately related to theories about the expansion of the universe...

[IM Egdall]

Eelpie, on 11 February 2012 - 08:38 AM, said:

a naive question probably,

obviously the expansion of the universe now in proportional terms is very very slow (would be interested to know how slow, how much further would an object exactly a billion light years away be after a year), but presumably even objects much closer together are moving away from each other and we could in principle observe the expansion of space? if the theoretical expansion between two points is below a planck unit how would it occur exactly?

Within galaxy clusters, gravity offsets the expansion. So the expansion is evident between galaxy clusters but not inside them. Thus the stars and planets etc. inside our Milky Way galaxy are not expanding.

See link for how fast the universe is expanding:

http://helios.gsfc.n...ex.html#fastexp

This post has been edited by IM Egdall: 11 February 2012 - 04:08 PM

[Widdekind]

IM Egdall, on 11 February 2012 - 04:04 PM, said:

Within galaxy clusters, gravity offsets the expansion. So the expansion is evident between galaxy clusters but not inside them. Thus the stars and planets etc. inside our Milky Way galaxy are not expanding.

See link for how fast the universe is expanding:

http://helios.gsfc.n...ex.html#fastexp

May I please "kibitz" for clarification ? I understand, that "mass tells space-time how to curve; and curved space-time tells mass how to move" (Wheeler. Journey into Gravity & Spacetime). Er go, different mass distributions, generate different space-time curvatures. So, I understand, that in "gravitationally bound" structures, e.g. galaxies, space-time is non-expanding; whereas, in deep space between the galaxies, space-time is expanding. From this understanding, I would say, not that "gravity inside galaxies 'offsets' expansion"; but rather that bound objects have fundamentally different space-time curvatures, where-with-in the fabric of space-time is non-expanding.

Your use of the word "offsets" suggests some sort of "internal struggle", between a vaguely "Schwarzschild-like" space-time, attributable to local galaxy masses; and a Hubble-expanding "Friedmann" space-time, attributable to the global cosmological mass distribution. Is there such a "struggle" of competing factors, within the fabric of space-time, within galaxies ? The vaguely "Schwarzschild-like" space-time, inside a galaxy, as numerically calculated, in isolation, would presumably be vaguely "static" (on galactic scales). But, in real life, galaxies are embedded into the expanding space-time fabric of our universe. So, is your use of the word "offsets" an accurate description ? E.g. does the Hubble expansion "tug at the edges" of galaxies ??

[IM Egdall]

Widdekind, on 12 February 2012 - 09:45 AM, said:

May I please "kibitz" for clarification ? I understand, that "mass tells space-time how to curve; and curved space-time tells mass how to move" (Wheeler. Journey into Gravity & Spacetime). Er go, different mass distributions, generate different space-time curvatures. So, I understand, that in "gravitationally bound" structures, e.g. galaxies, space-time is non-expanding; whereas, in deep space between the galaxies, space-time is expanding. From this understanding, I would say, not that "gravity inside galaxies 'offsets' expansion"; but rather that bound objects have fundamentally different space-time curvatures, where-with-in the fabric of space-time is non-expanding.

Your use of the word "offsets" suggests some sort of "internal struggle", between a vaguely "Schwarzschild-like" space-time, attributable to local galaxy masses; and a Hubble-expanding "Friedmann" space-time, attributable to the global cosmological mass distribution. Is there such a "struggle" of competing factors, within the fabric of space-time, within galaxies ? The vaguely "Schwarzschild-like" space-time, inside a galaxy, as numerically calculated, in isolation, would presumably be vaguely "static" (on galactic scales). But, in real life, galaxies are embedded into the expanding space-time fabric of our universe. So, is your use of the word "offsets" an accurate description ? E.g. does the Hubble expansion "tug at the edges" of galaxies ??

My understanding is the expansion does "tug" throughout galaxies. It is pushing space apart within our galaxy right now. But gravity (spacetime curvature) is pulling it together within our galaxy. Gravity within our galaxy is much more powerful, so it dominates -- thus stopping the expansion of space within our galaxy.


See linkfor calculations on expansion within galaxies: http://answers.yahoo...05063620AAapPXk


So

[DrRocket]

Widdekind, on 12 February 2012 - 09:45 AM, said:

So, I understand, that in "gravitationally bound" structures, e.g. galaxies, space-time is non-expanding; whereas, in deep space between the galaxies, space-time is expanding.

Spacetime is not expanding.

Spacetime is static. It includes all of space and all of time. It does not change. It can't.

It is space that is expanding. Even that requires explanation.

If one assumes that spacetime, the universe, is homogeneous and isotropic, then spacetime can be decomposed as a one-parameter foliation by spacelike hypersurfaces. That time-like parameter serves as a surrogate for "time" and the hypersurfaces as a surrogate for "space". They inherit a true Riemannian metric from the full Lorentzian metric of spacetime. "Expansion of space" refers to the metric expansion of those space-like hypersurfaces as a function of the timelike parameter, as measured by the inherited metric.

[Eelpie]

IM Egdall, on 11 February 2012 - 04:04 PM, said:

Within galaxy clusters, gravity offsets the expansion. So the expansion is evident between galaxy clusters but not inside them. Thus the stars and planets etc. inside our Milky Way galaxy are not expanding.

See link for how fast the universe is expanding:

http://helios.gsfc.n...ex.html#fastexp

Thanks very much.

I always thought that where two objects were in "equilbrium" wrt to each other gravity was typically predominantly offset by angular momentum? Or is it a combination

It seems odd that it should exactly offset inside superclusters....

If we took two points in an "empty" bit of space ie between superclusters and these were 1m apart, then according to your formula and ignoring gravity (I am assuming points not particles with mass) after a billion billionth of a second space should have expanded but by less than a planck length. How is that possible? Or do we have to think in terms of quantum mechanics when thinking about small units of time? Is a billion billionth small?

[Eelpie]

ok another way of putting the same question a different way....

if we had two objects where the expansion of the universe offset the extent to which they were gravitationally attracted to each other how would they behave? would tiny bits of space be created but then be pushed away by gravity so that the distance was unchanged?

many thanks!