All Questions
48
questions
1
vote
1
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76
views
Why is the First Law of Thermodynamic related to Fluid Equation?
In Cosmology, there is a equation called Fluid Equation:
$$\dot{{\varepsilon}}+3\frac{\dot{a}}{a}(\varepsilon+P)=0.$$
It is derived by taking time derivative of the First Law of Thermodynamic:
$\dot{E}...
- 411
0
votes
1
answer
37
views
Does the energy of a photon in comoving space change?
Assuming a flat FLRW universe that is expanding: In comoving space, does the energy of a photon decrease or stay constant? A physical argument for this would be nice.
- 1,222
1
vote
0
answers
64
views
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
0
votes
2
answers
75
views
Does dark energy get used up in the expansion of the universe?
Now, I am a beginner in Cosmology, so I am not sure if this makes sense.
Since the universe is expanding at an accelerating rate, and thus distant objects are also accelerating away. In that sense, ...
- 109
0
votes
1
answer
134
views
Expansion of Universe and validity of law of conservation of energy [duplicate]
It has been proved by the Red Shift that universe is expanding. But if the universe is actually expanding, it needs energy to do so. I also do not know that with expansion in universe if mass ...
2
votes
0
answers
26
views
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?
2
votes
0
answers
78
views
Can gravitational waves gain energy in an expanding FRW spacetime?
I was reading this paper (Green's functions for gravitational waves in FRW spacetimes: https://arxiv.org/abs/gr-qc/9309025) I had a specific question about one statement in the paper that I would like ...
- 2,462
2
votes
2
answers
224
views
Energy from spacetime expansion?
Are there any processes or mechanisms in the universe involving spacetime expansion where energy is produced somehow out of this expansion?
I ask this in part due to this article by Sean Carroll 1 ...
- 2,462
0
votes
1
answer
395
views
The expansion of universe releases vacuum energy
The quote below has been taken from the Wikipedia article on cosmological constant, https://en.wikipedia.org/wiki/Cosmological_constant.
Einstein included the cosmological constant as a term in his ...
- 717
0
votes
1
answer
71
views
Cosmological energy conservation
The expanding universe is gaining energy through increasing dark energy, and losing energy from red shift. Are these two effects comparable in magnitude?
- 1,719
7
votes
1
answer
672
views
Energy extraction from universe expansion? [duplicate]
I recently learned that energy is not conserved according to general relativity. Inspired, I went looking for a way to harness this interesting feature of the theory and came across an idea that ...
- 73
1
vote
0
answers
19
views
Universe expansion and energy conservation principle [duplicate]
If photons lose energy due to expanding of the universe where the lost energy is stored? Does the energy conversation principle is violated?
- 119
2
votes
0
answers
41
views
What mechanisms could keep Universe at 0 energy?
In my understanding is right, there is no obvious guarantee of the conservation of energy on the cosmological scale. The expansion of space-time causes the red-shift of radiation and therefore the ...
- 180
0
votes
1
answer
181
views
Do Friedmann's equations really result in energy conservation?
I was looking into Friedmann's equations for the evolution of the universe and encountered that if one combines them they result in
$$dE + dpV = 0$$
as noted here in equ. 29.11. and the author claims ...
- 157
1
vote
0
answers
95
views
Why energy cannot be created nor destroyed if the Universe is still growing in size? [duplicate]
Why energy cannot be created nor destroyed if the Universe is still growing in size?There is creation of new space and infact this space is so energetic that causes 'Hubble flow' of matter...
- 1,815