All Questions
Tagged with space-expansion causality
19
questions
2
votes
3
answers
132
views
Can a light signal from Earth reach a galaxy outside the Hubble Horizon?
Is this video on the FLRW metric (timestamp 29:00 minutes) mistaken in its claim that a light signal from Earth cannot catch up with a galaxy outside the Hubble horizon, due to the horizon receding at ...
- 6,112
0
votes
0
answers
62
views
Does inflation theory assume a finite universe?
Inflation theory has it that the early universe was causally connected, and could “mix”, hence explaining relative homogeneity of the CMB. The universe then rapidly expanded and became causally ...
- 139
5
votes
1
answer
378
views
What if the cosmological principle does not hold at larger scales?
Quite naturally, the observable Universe is the only bit of the Universe we can extract information from, as light from farther away has not reached us yet, and there are zones from which we'll never ...
- 1,616
1
vote
1
answer
73
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Is cosmological expansion paradoxical?
Suppose that there are three points in space. The distance between any two of the closest points is less than or equal to the cosmological horizon. Such is the case that information cannot be ...
- 236
1
vote
1
answer
162
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What is the farthest light from Earth can reach, given accelerating expansion?
At what current distance is the nearest point Q at which the photon sent from Earth right now never reaches it?
There is the concept of a Hubble volume, which "is a spherical region of the ...
- 21
13
votes
2
answers
3k
views
Why does an Alcubierre drive, travelling FTL, violate causality, if the universe expanding FTL doesn't?
An Alcubierre drive seems to be plausible as a means to travel faster than light, because
it doesn't move the object itself, but the space around it.
it's said that matter and information can't move ...
- 151
2
votes
1
answer
155
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Why do some diagrams of the particle horizon, observable universe etc show the past light cone as hitting ~20glyr out from us at time zero?
This question is mainly in reference to this question:
Is the observable universe equivalent to 'our' light cone?,
and the answer, which is great.
But what I can't wrap my head around is why ...
- 21
-1
votes
1
answer
98
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Minkowski Space and the Expansion of the Universe
I was watching a video about Minkowski space. It stated that no past event, outside of the light cone in space can be a cause of an action inside the light cone. Is this due to the expansion of the ...
- 2,706
1
vote
1
answer
178
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When do special relativity causally linked reference frames split into general relativity un-linked?
I'm looking for the "a-ha" moment in trying to understand how the super-luminal apparent speeds of the universe inflation theory are allowed.
Specifically, the short explanation is that &...
- 113
0
votes
1
answer
196
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Are unreachable parts of the universe 100% unreachable or do we just say so?
In Kurzgesagt's video TRUE limits of Humanity is it stated that most of the universe will be forever unreachable because the universe is expanding faster than the speed of light. This is possible ...
- 123
-3
votes
1
answer
132
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Does the speed of universal expansion lead to time going backwards? [closed]
After reading that time stops at the speed of light and at event horizons I wondered if when the speed of light is exceeded does time loop backwards ...
The thought struck me too that the term space-...
- 740
0
votes
2
answers
135
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Hubble radius and communication between two observers
According to Dodelson, Modern Cosmology (p.146)
There is a subtle distinction between the comoving horizon $\eta$ and
the comoving Hubble radius $(aH)^{-1}$. If particles are separated by
...
4
votes
4
answers
2k
views
Time dilation due to space expansion
As we observe a remote galaxy, we see it with a redshift. The most distant galaxy discovered to date is GN-z11 visible with the redshift of $z=11.09$. For simplicity, let's assume no gravitational ...
- 12.7k
1
vote
1
answer
221
views
Time dilation beyond cosmic horizon
The observable universe is limited by a cosmic horizon. Galaxies beyond the horizon move away from us faster than light, so we cannot see them. If we could see a planet close to our horizon, we would ...
- 12.7k
3
votes
0
answers
59
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Superluminal speed at cosmic horizon [duplicate]
The observable universe is limited by a cosmic horizon. Galaxies beyond the horizon move away from us faster than light, so we cannot see them. Imagine a spaceship in the vicinity of our horizon. If ...
- 12.7k