All Questions
Tagged with cosmology special-relativity
25
questions with no upvoted or accepted answers
3
votes
0
answers
39
views
Is it valid to add energy densities of *interacting* perfect fluids?
In several papers on interacting perfect fluids in cosmology, the authors assume that we still can add the energy densities and pressures of the individual fluids, as if there wasn't any interaction:
\...
- 7,592
3
votes
0
answers
109
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Can Lorentz symmetry be broken?
I am used to symmetry breaking in condensed matter, where we have a preferred reference frame (superconductivity, ferromagnets, etc). But what about Lorentz symmetry? What would such symmetry breaking ...
- 2,625
2
votes
0
answers
50
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Does shape dynamics introduce a temporal order on events separated by a space-like interval such that their simultaneity is not relative?
I've read some popular books and articles that reference shape dynamics (e.g., The Singular Universe and the Reality of Time, https://www.pbs.org/wgbh/nova/article/shape-dynamics/). I have also ...
- 181
2
votes
0
answers
64
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Proving that the CMBR remains thermal after a Lorentz boost
Suppose $S$ is a frame in which the CMBR is thermal and isotropic. I'm taking this to mean it follows the Planck Blackbody distribution $$n(\omega)=\frac{1}{\pi^2 c^3}\frac{\omega^2}{e^{\beta\hbar\...
- 248
2
votes
0
answers
48
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Does expansion of space over time assume a particular space/time dichotomy?
Regarding the expansion of the Universe, Wikipedia states:
The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe ...
- 222
2
votes
0
answers
81
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Cosmological or Doppler redshift in distance calculations
If I want to determine the distance to a far away galxy with a high redshift,
then using relativistic doppler shift will not give the true distance as it
does not take into account the expansion of ...
- 164
2
votes
1
answer
46
views
How to include the effect of cosmological expansion in the close to speed of light calculations of a relativistic rocket?
I calculated time ago as undergraduated and post it at my blog the calculations for traveling through the Universe in a (special) relativist way. At what level and how should we include the effect of ...
- 6,611
1
vote
0
answers
44
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Expansion of the universe and superluminal speeds - an analogy
To illustrate, let's imagine the whole universe having a coordinate grid. Essentially a great big mesh grid of interlocking meter sticks.
Now... Einstein's theory of relativity tells us that nothing ...
- 41
1
vote
0
answers
43
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Could inhomogeneties and/or topological defects break the fundamental symmetries of the universe?
I have heard that some types of inhomogeneties and topological defects (like cosmic strings) in cosmology have been proposed to be able to break fundamental symmetries of nature such as the Poincaré, ...
- 2,462
1
vote
0
answers
60
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Why is the age of the universe $=D/v$, despite that $v$ is not constant with time?
I am watching a series of lectures by the Noble prize laureate Brian Schmidt and Paul Francis and in this episode (at 4:20) they make the simple assumption that a galaxy receding from us due to the ...
- 157
1
vote
0
answers
61
views
Co-moving distance and universal time
I am reading about Cosmology, in particular, the concept of co-moving distance. I am trying to derive the equation for calculating the co-moving distance with the following argument.
If the source ...
- 301
1
vote
0
answers
70
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As space-time expands between two stars, what happens to the gravitational potential energy seemingly lost as U~1/r => ZERO.?
As the space-time between two stars grows (the accelerating expansion of the universe) the gravitational potential energy between two stars is reduced as 1/r -> ZERO (r is the distance between stars).
...
1
vote
0
answers
80
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What does the temperature of the early universe tell me?
I am re-reading Weinberg's book "The First Three Minutes". In the Introduction he makes this statement: "At about one-hundreth of a second, the earliest time about which we can speak with any ...
- 1,577
1
vote
0
answers
122
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Time-Independent Gravitational Equation?
Is it possible to calculate gravitational induced position change without requiring the use of time (and therefore, acceleration) anywhere in the equation?
If such an equation were to be discovered, ...
- 121
1
vote
1
answer
129
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What is the average recessional velocity of an object in the universe?
I’m trying to get a better grasp on cosmological horizons and have a question regarding recessional velocity. In particular:
What is the average recessional velocity of a cosmological object (...
- 487