All Questions
327
questions
1
vote
2
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48
views
Viable values for the $K$ parameter in the FLRW metric
The FLWR metric is sometimes given as $$c^2 d\tau^2 = c^2 dt^2 - \frac{a(t)^2}{(1-KX^2)} dX^2. $$
I am not interested in the tangential motion so I set $d \Omega = 0$ although it is of interest in ...
- 6,102
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 ...
- 6,102
9
votes
4
answers
651
views
Understanding expansion of the Universe as things flying apart
Say that we have a Universe uniformly filled just with matter (let's not bring dark energy into this). And say that we fill it with very light particles (so that the gravitational interaction between ...
1
vote
2
answers
530
views
Please help me with this paradox [closed]
Physicists believe that some galaxies are moving away from us at faster than the speed of light. A galaxy that is moving away from us at faster than the speed of light would be moving backwards in ...
- 53
0
votes
2
answers
95
views
Could Space and Time Be Decoupled Pre- Big Bang?
The traditional view holds that both space and time emerged together from the Big Bang. However, I'm curious about the possibility that time could be eternal, with no beginning, while space began to ...
- 489
1
vote
0
answers
45
views
How to find the relation between flow-parameters and number of e-folds?
In models of single field inflation the Friedmann equation reads
$$ H(t)^2 = \frac{8\pi G}{3} \big(V(\varphi) + \frac{\dot{\varphi}^2}{2}\big) \tag{1}\label{eq1}. $$
In the slow-roll approximation one ...
- 126
1
vote
1
answer
56
views
Are comoving distances time-independent?
A comoving reference frame expands along with the universe, factoring out the effect of the Hubble expansion. Suppose a galaxy has a redshift $z = 1$ and its comoving distance DM is $11 \,\mathrm{Gly} ...
- 928
0
votes
3
answers
99
views
Does the Schwarszchild solution require the cosmological constant to stop it from expanding?
When Einstein developed his field equations for general relativity, he attempted to apply them to the entire universe. He found the universe had to expand, which at the time was believed not to be the ...
- 131
2
votes
0
answers
88
views
A problem on cosmic inflation
I analyze inflation in this following scenario: Suppose that at some very early epoch, $t_1 ≤ t ≤ t_2$ (where $t_1 ≪ t_2 ≪ t_r$ and $t_r$ is the time at the recombination epoch), the universe resides ...
- 131
-2
votes
1
answer
84
views
Why is it mysterious that space is apparently expanding at different rates everywhere? [closed]
New-ish measurements from Hubble + Webb say that the Universe is expanding at different rates everywhere: https://www.livescience.com/space/cosmology/james-webb-telescope-confirms-there-is-something-...
- 115
0
votes
0
answers
32
views
Can the integrated Sachs-Wolfe and the Rees-Sciama effects have any influence on matter?
CMB photons can be affected by the expansion of the universe through the linear integrated Sachs-Wolfe effect (ISW) 1 and the non-linear ISW effect or also called Rees-Sciama effect 1.
In particular, ...
- 2,462
0
votes
1
answer
63
views
Difference between general formula for cosmological comoving distance compared to Carroll's one
I came across two different formulas for the transverse comoving distance in cosmology from GR based on Friedmann's solution in the FLRW metric for an expanding space homogenoeus and isotropic:
a) .....
- 5
6
votes
2
answers
736
views
Is relative velocity slowed by the expansion of space?
Consider an object traveling across the universe and consider two observers billions of light years away from each other at rest with respect to their local universes. The object will pass by the ...
- 63
0
votes
0
answers
26
views
What is the change in vision and measurement between curved space and flat space, especially for measuring cosmic background radiation?
In a curved space, the light bends as it travels and acts like it is going through a lens. In a (positively) curved universe, a small object appears larger. If we know the actual size of an object, ...
8
votes
2
answers
429
views
Are the mass, diameter and age of the Universe frame dependent?
Mass of the observable Universe is known to be $1.50×10^{53}$ kg. Age is approximately known to be 13.7 billion years.The observable Universe is a sphere with diameter of roughly $8.8\times10^{26}$ m.
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