All Questions
113
questions
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Are there structures or systems that can have a high angular momentun that are not made by protons and/or neutrons?
Cosmic structures such as neutron stars, white dwarfs or black holes can have high amounts of angular momentum (high spin velocities). However, these are all made by protons and neutrons (black holes ...
- 2,462
11
votes
2
answers
1k
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Redshift of the CMB
Has the $z\sim 1100$ redshift of the CMB been actually measured by comparing the fingerprint (absorption spectrum) of the CMB with the theoretical radiation pattern of a $2.725\,\mathrm{K}$ blackbody, ...
- 121
0
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1
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45
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Cosmological redshift and distance calculus
I am somewhat puzzled when it comes to understanding the general relationship between the cosmological redshift of a galaxy and its distance, and none of the other questions in the forum have ...
- 161
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0
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42
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Up-tunneling of vacuum with high-energy events?
I was reading these papers by Sean Carroll (https://arxiv.org/abs/1405.0298 ; https://arxiv.org/abs/1505.02780) in which, among other things, he argues against vacuum up-tunneling occurring in the ...
- 2,462
1
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1
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190
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Why are physicists not more concerned that there are too many explanations for redshift in the universe?
There are speculative explanations for red shift such as the tired light theory, but I am not referring to those. There are three mainstream explanations
Red shift due the expansion of the universe ...
- 371
0
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57
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Why do successive redshifts multiply rather than add?
I understand the argument presented in the answer to this question on how successive redshifts combine, such that $$1+z = (1+z_1)(1+z_2) = 1 + z_1 + z_2 + z_1z_2 \\ \therefore z = z_1 + z_2 + z_1z_2.$$...
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1
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2
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129
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Is cosmological redshift associated with recession velocity when the light left or when it arrived?
Is the cosmological redshift $z$ associated with the recession velocity when the light left, when it arrived, or something in-between?
user300520
1
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0
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59
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How to derive deceleration parameter in terms of redshift?
How do I get q in terms of $z$ specifically of this form:
I want to derive the second equation from the first but I'm getting an additional $ \frac{a^2}{H^2}\ $ in the second term
$$ q= -\frac{\ddot{a}...
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45
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Clocks at different redshift: does calculating luminosity distance shift the frame of reference?
Luminosity is energy per time.
When we calculate a luminosity distance, using angular diameter distance times $(1+z)^2$, I think we are correcting the energy (from redshifted photons) by $(1+z)$ and ...
- 821
0
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0
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34
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Has the speed of light always been constant? [duplicate]
Is there any evidence to suggest that the speed of light (or its other properties) has been unchanged since the beginning?
The particular curiosity stems from the redshift observed in distant galaxies....
- 287
8
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2
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739
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At what distance can I assume peculiar velocity is negligible compared to expansion velocity, to use Hubble's law?
I am working on a high school physics assignment and am trying to figure out a method to determine if the star is a giant or main sequence from its spectral and photometric data (from SDSS). I picked ...
1
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1
answer
242
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Taylor Expansion of the scale factor in terms of redshift
I was reading about the Taylor expansion of the scale factor from visser,2004
He writes:
$$ \frac{a(t)}{a_0} = 1 +H_0 (t -t_0) - \frac{q_0}{2} {H_0}^{2}(t-t_0)^2 + \frac{j_0}{3 !} {H_0}^{3}(t-t_0)^3 ...
- 299
1
vote
4
answers
152
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How cosmologists know if the observed redshift of galaxies is due to expansion and not intrinsic to the galaxy?
Is there a way to experimentally test if the cosmological red shift observed is due to the expansion or intrinsic to the galaxy? If anyone knows how to do this or how to extract from the data please ...
- 4,170
2
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0
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When light gets red shifted by the expending universe it's energy decreases. What happens to that energy? [duplicate]
If a Photon of light is red shifted its wavelength increases and therefore its energy decreases. What happens to that energy as energy should be conserved right?
1
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1
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133
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Doesn't the increase in redshift of galaxies over larger distances mean that the expansion of the universe is slowing down? [duplicate]
This question seems too simple to not have been asked, but I cannot seem to find an explanation. So apologies if this is a duplicate.
The current explanation for the expansion of the universe as I ...
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