21
votes
Accepted
When will the number of stars be a maximum?
TL; DR Somewhere between now and a few hundred billion years time. (For a co-moving volume) Now read on.
If stellar remnants are included, then the answer is very far in the future indeed, if and ...
- 146k
21
votes
Accepted
Will we start seeing galaxies disappear due to Universe expansion?
It is a common misconception that galaxies receding faster than light cannot be observed. There are two versions of this misconception:
Galaxies that are now receding faster than light cannot be seen....
- 37k
10
votes
Accepted
Is there a physical limit to how far we can go?
Your 3 points are spot on. There is a finite number of galaxies we could theoretically reach for the reasons you say. The further away the distant galaxy the greater the expansion of space between ...
- 23.9k
8
votes
Could a contracting Universe create the redshift effect observed by Hubble?
Aside from being, I suspect, totally incompatible with GR, the model fails to match observations.
The universe appears to be homogeneous and isotropic. In the model you suggest, Hubble's law would be ...
- 146k
8
votes
Accepted
Will gravitational waves too far away ever reach us?
The answer here is very similar to if you were asking about light.
In principle gravitational waves might allow us to fractions of a second after the big bang. Electromagnetic waves can see back to ...
- 146k
7
votes
Accepted
What is left of a universe where no matter and energy exist and is there a term for this?
However, due to the Law of Conservation of Energy, I suspect that energy will be all that is left, only in equilibrium meaning no ability for energy to form matter.
The law of conservation of energy ...
- 8,098
6
votes
Accepted
Will intergalactic travel be impossible in roughly 15 billion years?
Yes, over time the accelerating expansion of the universe (assuming the $\Lambda$CDM model is right) will separate gravitationally bound clusters of galaxies from each other with exponentially growing ...
- 14k
6
votes
Are there any studies that prove/disprove the Oscillating Universe Theory?
The main obstacle with the Oscillating Universe model is the pesky second law of thermodynamics — that entropy always increases within an isolated system, and never the reverse. Thus, as time ...
- 8,098
5
votes
Accepted
How can gravity lead to the Big Crunch scenario?
The amount of matter in the universe is directly related to the curvature of space itself. We can look at the Friedmann Equations to see how this works:
$$
H(t)^2 = \frac{R'(t)^2}{R(t)^2} = \frac{8\...
- 3,156
5
votes
Light beam (1 photon) in the limited universe?
A finite universe is said to have a "closed geometry", or to be "positively curved", meaning that, in principle, you may travel in a straight line and eventually return back to your starting point. In ...
- 37k
4
votes
Accepted
Has the Big Crunch been ruled out?
The Big Rip happens if the equation of state for the dark energy has $p/\rho = w<-1$, and all empirical data give us $w\approx -1$. A Big Crunch requires a pretty high value of $w$ (it must go ...
- 14k
3
votes
Could the universe expand forever even if there is no force (e.g., dark energy or eternal inflation) pushing it apart?
Without dark energy, the expansion of the universe will slow down (because of gravity). But it need not stop expanding and contract again. That's because it's possible the force of gravity is not ...
- 3,992
3
votes
Accepted
Does the critical density of the Universe also include Dark Energy?
Yes. Quoting values from wikipedia (which in turn cites Ade et.al. in Astronomy and Astrophyics 517), the contribution of matter (both Dark and visible matter) is
$$Ω_\text{mass} ≈ 0.315±0.018$$
The ...
- 115k
3
votes
Accepted
Is our universe stable or metastable?
Yes ... and no.
Experimentally, there's no indication that the laws of physics are changing (c.f. conservation of energy: if the laws of physics change with time, energy would not be conserved via ...
- 3,992
3
votes
false to stable vacuum transition
The standard picture of vacuum decay events is very much like nucleation in a phase transition: it starts somewhere, and then expands at a constant speed - lightspeed in this case. So the future ...
- 14k
3
votes
Accepted
Can gravity cause a Big Crunch, if the cosmological principle holds?
You are thinking about gravity and how it affects the universe as a whole in the wrong way.
The dynamics of the universe are governed by the Friedmann equations, which can be solved to say how the ...
- 146k
3
votes
Accepted
If the Universe is infinite, would heat death be impossible?
Heat death of the Universe doesn't mean that there is no energy in it, it means it has reached thermodynamic equilibrium and hence no useful energy in it. This could in theory occur in an infinite ...
- 1,855
2
votes
Accepted
How can the observable universe shrink in a Big Rip?
I've found the answer to this is actually very interesting and (to me, at least) surprising — it is not just the light cone that shrinks, but the cosmic event horizon as well! To explain this, let us ...
- 8,098
2
votes
Light beam (1 photon) in the limited universe?
No one can say for sure because, while we have theories which fit our observations quite well, those theories require that the universe be considerably bigger than the part we can see -- light simply ...
- 7,560
2
votes
Doesn't the energy conservation law imply, that the universe can't come to a Big Freeze?
The current model ($\Lambda$CDM) predicts that space will expand exponentially, and keep on doing it indefinitely far into the future. The end result, after the stars burn out, galaxies disperse, ...
- 14k
2
votes
Accepted
Possible scenarios for life in the death of open universe possible?
Your scheme of creating energy from the tidal forces in a pair of orbiting planets cannot work because of the law of conservation of energy (aka the 1st law of thermodynamics), which states that ...
- 13.1k
2
votes
Critical density of universe with intrinsic spatial curvature
In general relativity, there is no such thing as intrinsic curvature of spacetime that is not tied to the energy content. The Einstein field equations directly tie the curvature tensor to the stress-...
- 4,260
1
vote
Accepted
Coasting universe Hubble parameter behaviour
For scenario 2, the slope starts out positive and remains positive (expanding) but is getting less and less steep over time, asymptotically approaching a slope of zero
Indeed this description of the ...
- 4,260
1
vote
Is the universe always expanding
It will always expand (probably).
The mass and energy in the universe curves space, and it seems that there is exactly the right amount of mass and energy to keep the universe expanding forever.
...
- 115k
1
vote
What happens for a "closed" universe without any content?
You are not doing anything wrong. Positively curved empty universes are forbidden. Since as you figure it out the scale factor would be imaginary.
- 576
1
vote
What happens for a "closed" universe without any content?
I see that you are using the convention that a has units of distance and is like the radius of curvature of the universe, and you are getting that either a or t would need to be imaginary. So you can ...
- 5,300
1
vote
Could a contracting Universe create the redshift effect observed by Hubble?
So in this model, all of the observable universe is expanding. It is simply that it is part of a larger universe which is being locally stretched, as part of a an accelerating convergence to a central ...
- 10.2k
1
vote
Could a contracting Universe create the redshift effect observed by Hubble?
For a contracting universe, space would contract as a ratio, and hence even if we could only see a small distance, we would still see blue-shifted galaxies due to the fact that the galaxies further ...
- 1,013
1
vote
How long will it be until the nearby groups of galaxies are receding at speeds faster than light?
This will take about 2 trillion years.
Given everything we know now, it's not too hard to predict the eventual fate of the universe. Of course there may be some changes as our knowledge advances, but ...
- 14.9k
1
vote
How can gravity lead to the Big Crunch scenario?
In less mathy words, the Big Crunch scenario occurs if the ratio of the total density of the Universe to its expansion rate is sufficiently large$^\dagger$.
As I understand your question, you're ...
- 37k
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