All Questions
254
questions
0
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2
answers
330
views
Why is the critical density of the Universe non-zero?
I'm missing something and I wondered if someone could enlighten me. As I understand it, Einstein's equations say, among other things, that matter curves spacetime. I also understand, the Universe has ...
- 2,283
3
votes
1
answer
230
views
variations of Einstein equations with conversion between gravitational and non-gravitational energy
I'm looking for existing papers studying a variation to Einstein equation that does not rely on the annoying matter conservation identity:
$$ T_{\mu \nu; \nu} = 0 $$
And instead tries to equate the ...
- 14.5k
8
votes
7
answers
5k
views
Does gravity slow the expansion of the universe?
Does gravity slow the expansion of the universe?
I read through the thread http://www.physicsforums.com/showthread.php?t=322633 and I have the same question. I know that the universe is not being ...
- 349
0
votes
1
answer
110
views
Continuum model of the n-body problem?
Consider a collection of $n$ point masses evolving under the influence of gravity. For very large $n$, one might be interested in making a continuum approximation of this system (for cosmology, say). ...
5
votes
2
answers
3k
views
Zero divergence of energy-momentum tensor and gravitational energy
Trying to teach myself general relativity and have just hit yet another confusion. I'm reading that in curved spacetime the energy-momentum tensor has zero divergence, ie
$$\nabla_{\mu}T^{\mu\nu}=0.$$...
- 3,059
1
vote
2
answers
135
views
Do Galactic Black Holes and Extra-Solar Planets change the mass balance of the universe?
With all the recent discoveries of supermassive black holes being at the center of nearly every galaxy, and the proposed "at least one planet" around (probably) every star.
Does this affect the ...
9
votes
5
answers
1k
views
Could the acceleration of universe expansion be caused by gravity itself?
Dark energy is suggested to be a repulsive force in the universe causing an accelerated expansion. If the amount of mass outside our observable universe is greater than inside (higher mass density), ...
- 91
22
votes
5
answers
28k
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Stephen Hawking says universe can create itself from nothing, but how exactly?
Stephen Hawking says in his latest book The Grand Design that,
Because there is a law such as gravity, the universe can and will create itself from nothing.
Is it not circular logic? I mean, how ...
- 329
10
votes
4
answers
2k
views
Is the "Great Attractor" an indicator of the "Multiverse"?
I have heard a bit about the Great Attractor (the gravitational anomaly that seems to be "sweeping" our universe in one direction). Someone (and forgive me, I do not recall the specifics) has ...
- 7,704
17
votes
3
answers
4k
views
Is dark matter repulsive to dark matter? Why?
I think I saw in a video that if dark matter wasn't repulsive to dark matter, it would have formed dense massive objects or even black holes which we should have detected.
So, could dark matter be ...
- 1,077
-2
votes
1
answer
781
views
History of Man Acquired knowledge of solar system [closed]
I know the basics of solar system like how earth moves around sun, and that we have so many planets, elliptical orbit of earth, and how far is sun from earth etc etc. I want to take a step back and ...
- 655
1
vote
1
answer
345
views
Why doesn't Brooklyn expand? (Or "Is the expansion of the universe kinematic?") [duplicate]
Possible Duplicate:
Why space expansion affects matter?
Imagine two tiny spacecrafts that are moving with the Hubble flow and so are moving away from each other. Let's assume that they've been ...
- 8,800
34
votes
6
answers
5k
views
Could gravity be an emergent property of nature?
Sorry if this question is naive. It is just a curiosity that I have.
Are there theoretical or experimental reasons why gravity should not be an emergent property of nature?
Assume a standard model ...
- 13.5k