14
votes
Accepted
How is it that we are still able to talk about the speeds of other galaxies, distances and times across the universe, etc despite GR?
There are two issues mixed up in your question.
First, there is a preferred cosmic rest frame: the frame of the cosmic microwave background. This does not imply any contradiction with the principle of ...
- 33.3k
13
votes
Accepted
Will a light come back within finite years?
We don't know if the universe is finite or infinite. John Rennie's answer alludes to this in the latter parts- it is either infinite (and has always been infinite) or finite (but without an edge, so ...
- 16.6k
12
votes
Accepted
What happens when the universe runs out of fuel?
Then star formation ceases and the universe goes dark. At this stage of the universe's evolution, there'll still be plenty of hydrogen, they just don't form stars.
In theory you can create hydrogen ...
- 16.4k
8
votes
What happens when the universe runs out of fuel?
Star formation will die out long before all the hydrogen runs out. Much of it will be trapped in very low mass stars and lots more will be in the very sparse intergalactic medium (where $\sim 50$% of ...
- 111k
5
votes
What happens when the universe runs out of fuel?
Well, fate of the universe has a lot of possible scenarios, from which periodical expansion/contraction is very unlikely, because universe expands in an accelerated fashion, and it doesn't looks that ...
- 7,272
3
votes
With how many Newtons of force is the universe expanding?
To give a more concrete scenario for my question: two neutrons are in free space. At what rate will they accelerate away from each other (due to the expansion of the universe)? Does it depend on the ...
- 2,826
3
votes
Is the gravitational tidal force equivalent to expanding space?
You are neglecting the behavior of particles to the side of you, which are drawn closer. If you start with a spherical ball of coffee grounds then as it falls the ball will be stretched vertically and ...
- 64.4k
2
votes
Hubble expansion rate and reaction rates
This is a question that puzzled me once. This is simply a misleading statement. For now, a more satisfying one would be:
Once the interaction rate drop to $\Gamma\approx H$, the neutrino, for instance,...
- 21
2
votes
Comoving distance vs physical distance
The angular diameter distance is the distance to a standard ruler you would infer based on its angular size on the sky. (By "angular size", I mean the number of degrees an object takes up on ...
- 33.3k
2
votes
Will a light come back within finite years?
Well, the two answers are not talking about the same thing.
What Javier was describing is what the asker was inquiring about in their question: a "finite but unbounded" periodic universe.
...
- 1,554
1
vote
Is this right that the fate of space expansion depends on matter density inside the universe?
General relativity is the theory of how matter affects space (tells it how to bend) and space affects matter (tells it how to move). The equations governing the expansion of the Universe are derived ...
- 4,432
1
vote
Is there a consensus among cosmologists about the big freeze or any “fate of the universe” scenario?
Yes. This is called the $\Lambda$CDM scenario. It is based on current data. It predicts that the universe will expand forever, at an accelerated pace, and continue to cool while doing so: the Big ...
- 4,347
1
vote
Cosmological expansion around ropes between galaxies
You're mistaken. Mass doesn't mysteriously turn off expansion. There's just a distance regime (for a given mass) for which gravity dominates the combined effect of gravity and expansion.
Looking at ...
- 8,643
1
vote
Is redshift an accurate method for measuring distances of distant galaxies and quasars?
I have found the perfect graph which I believe can explain the concepts.
Let’s suppose we have an object located at a comoving distance $r$. This objects emits light at $t=t_1$ (or $z$) and reaches ...
- 2,826
1
vote
With how many Newtons of force is the universe expanding?
Zero for both proper acceleration and force. There is no proper acceleration associated with expansion. See: wikipedia for the difference between proper acceleration, which corresponds to forces and ...
- 8,643
1
vote
In an expanding universe, can two people communicating to each other about their cosmological horizons get around their horizon limit?
I think the right way to think about it is to rescale space into comoving coordinates $\chi$ and time into conformal time $\eta$. That way the expansion effect goes away (hidden in the coordinates). ...
- 26.8k
1
vote
What is there at a point the universe hasn't expanded past yet?
If the universe is constantly expanding that means that there is a point the univese [sic] hasn't expanded past...
Your premise is not necessarily correct.
with that what would be past that point?
...
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