1
vote
1answer
49 views

Increasing matter density in the Friedman equations?

From what I understand, the Friedman equation is $H^2=\frac{8\pi G}{3} \rho$ and the $H$, the Hubble constant, is a measure of the rate of expansion of the universe. What I am confused about is that ...
9
votes
1answer
76 views

Is it possible to determine whether distant galaxies are gravitationally bound

In a previous question, one issue was related to the potential energy of cosmic structures. This raised in particular the question of whether these structures are gravitationally bound. If you ...
3
votes
2answers
115 views

How does the gravity well change as space expands? [duplicate]

How does the gravity well change as space expands? If we assume that the Earth's gravitational field curves flat space to create a gravity well then how does the gravity well change as space expands ...
2
votes
2answers
392 views

Wouldn't the presence of dark matter slow the expansion of the universe?

If there is a huge element of dark matter in the universe, wouldn't this extra gravity prevent the accelerated expansion of the universe?
2
votes
2answers
85 views

Dark energy vs. gravity

If dark energy is everywhere around us, then why don't we get separated? For example why don't I get separated from the pen kept in front of me? Or take a similar example in free space. Is dark ...
3
votes
1answer
68 views

Modified gravity and dark energy

Is there a model of modified gravity that either was close to or have succeeded in modelling the problem of dark energy, and suffers from no instabilities and explains the observational data. Also has ...
1
vote
0answers
21 views

Instabilities in the CDMT

Could anyone explain or refer to references on why the CDMT f(R) gravity model suffers from Instabilities any why the sign of ${\mu}^{4}$ matters.
0
votes
0answers
34 views

Is Dark Energy Pushing Us Or Pulling Us? [duplicate]

This may seem silly, but how do they know that the effects we are seeing from dark energy, (which we only assume must be there due to observed acceleration), are not from gravity pulling us out from ...
8
votes
5answers
518 views

Can entropy of Universe be constant?

If I understand entropy correctly, then for example two objects orbiting a centre of mass have lower entropy than when said objects eventually crash into each other and form a new one. So let's say ...
2
votes
1answer
257 views

What limits range of gravitational attraction concerning space expansion?

Wikipedia says: Metric expansion is a key feature of Big Bang cosmology and is modeled mathematically with the FLRW metric. This model is valid in the present era only on large scales (roughly ...
3
votes
3answers
169 views

What would be the effect on gravity if space expanded at > $c$?

If space were to expand at > $c$ (as in inflation) would that mean gravity would no longer have any effect on the curvature of space, since gravity can only propagate at $c$?
4
votes
4answers
237 views

Basic Question About General Relativity?

I'm a layman that loves Physics. I'm also horrible at math. Having said that I have many, many questions in regards to physics and General Relativity in-particular. I will try to keep my question(s) ...
0
votes
1answer
261 views

How could vacuum energy cause the expansion of the universe? [duplicate]

Is it by gravity? How could gravity produce a repulsive force?
1
vote
0answers
69 views

When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?

It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
9
votes
1answer
555 views

Gravitationally bound systems in an expanding universe

This isn't yet a complete question; rather, I'm looking for a qual-level question and answer describing a gravitationally bound system in an expanding universe. Since it's qual level, this needs a ...
2
votes
2answers
2k views

Is time speeding up due to the expansion of space?

If we just look at our local galactic cluster, if all of the galaxies that are a part of it are moving away from each other, and so the overall 'density' of the strength of gravity in the cluster is ...
1
vote
3answers
620 views

If everything in the universe doubled in size overnight, would it be noticeable?

By my understanding, if everything doubled in size, such as the Sun and the Earth, and because the space in between them (which is nothing) can't expand, would the gravities greatly change and the ...
3
votes
7answers
1k views

Does gravity slow the expansion of the universe?

Does gravity slow the expansion of the universe? I read through the thread http://www.physicsforums.com/showthread.php?t=322633 and I have the same question. I know that the universe is not being ...
5
votes
2answers
1k views

Elasticity of Space; How does the expansion of Space affect gravity?

Does space have an elastic quality? What I was thinking about was if space is expanding, is it being 'stretched', like a balloon being blown up, and if so, is this causing gravity to weaken? Imagine ...
5
votes
1answer
230 views

Is the number of wavelengths of light spanning a distance invariant with respect to spacetime distortion?

I was recently asked by a friend how the expansion of spacetime affects photons. I gave him what I feel is a satisfactory general response, but it got me wondering how, exactly to calculate this ...
9
votes
6answers
570 views

How Does Hubble's Expansion Affect Two Rope-Tied Galaxies?

Suppose we have two galaxies that are sufficiently far apart so that the distance between them increases due to Hubble's expansion. If I were to connect these two galaxies with a rope, would there be ...
10
votes
3answers
822 views

Why space expansion affects matter?

If space itself is expanding, then why would it have any effect on matter (separates distant galaxies)? Space is "nothing", and if "nothing" becomes bigger "nothing" it's still a "nothing" that ...