All Questions
1,186
questions
-4
votes
0
answers
46
views
Simple question about finite Universe [duplicate]
If, by Big Bang, Universe was created from initial singularity, with finite "speed" of expansion of matter, shouldnt it be finite as well?
0
votes
3
answers
73
views
Are there universes where "symmetry breaking" went differently? [closed]
What have happened with other possible variants of asymmetry?
Are there other universes being run in parallel to our universe where the ball is not at C, but at B?
Stephen Wolfram told
I have found ...
- 129
2
votes
1
answer
281
views
Negative Horizon distance
Consider a flat universe, here, proper distance can be given by R-W Metric:
$$d_p (t_0) = c\int_{t_e}^{t_0}\frac{dt}{a(t)},$$ $t_e$ is the time when a photon is emitted from a distant galaxy, $t_0$ is ...
- 411
0
votes
0
answers
49
views
Dark energy and conservation of energy in General relativity [duplicate]
i know that conservation of energy in general relativity has been discussed multiple times here at PE, a popular explanation on the topic is Sean Carroll's blog "Energy is not conserved" ...
- 117
1
vote
1
answer
71
views
Age of universe vs Hubble time in Milne universe
Consider an empty universe where energy density $\varepsilon = 0$, thus the Friedmann Equation can be reduced into:
$\dot a^2= -\frac{kc^2}{R_O^2}$
$k$ is the curvature of space, $R_0$ is the radius ...
- 411
0
votes
1
answer
55
views
How to find critical density?
In Cosmology critical density is defined as the minimum density for a flat universe to keep expanding, by Friedmann Equation:
${\left({\frac {\dot {a}}{a}}\right)^{2}={\frac {8\pi G}{3}}\rho -{\frac {...
- 411
1
vote
1
answer
61
views
How to understand critical density?
In Cosmology, critical density is given by setting $\Lambda = 0$ and $k = 0$, in other words, a universe without dark energy and zero curvature. According to my understanding and Wikipedia, this ...
- 411
0
votes
1
answer
54
views
R-W Metric and null geodesic path of photon
I was reading through Introduction to Cosmology, on Chapter 3, it gives me the R-W Metric:
$ds^2 = -c^2dt^2+a(t)^2[dr^2+{S_κ}(r)^2dΩ^2]$
${S_κ}(r)$ is a function related to the curvature of space, κ ...
- 411
25
votes
9
answers
6k
views
Why are spherical shapes so common in the universe?
I have a simple question. Why are most objects in the observable universe spherical in shape? Why not conical, cubical, cuboidal for instance? I am furnishing a few points to justify this statement:
...
- 517
0
votes
0
answers
38
views
How can baryonic CP violation contribute to the explanation of the large matter-antimatter asymmetry in our universe?
I have been doing baryonic CPV experimental search in the past few years. However, I never really get a good clue on this fundamental question.
The thing is that CPV has only been found in the meson ...
- 1
2
votes
1
answer
76
views
Can glueballs and bosons survive indefinetely in space (forming structures)?
I have been recently interested in looking for possible structures (ranging from clumped structures like "stars" to diffuse clouds of gas or halos) made from standard model-particles other ...
- 2,462
1
vote
0
answers
39
views
Matter density estimates in the 1980s
Liddle (2015, p.67) writes: "From the crude estimates that a typical galaxy weighs about $10^{11}M\odot$ and that galaxies are typically about a megaparsec apart, we know that the Universe cannot ...
7
votes
3
answers
5k
views
If we consider the spacetime of the universe to be four-dimensional, does the Big Bang lie in its center?
Apologies for the (hopefully now somewhat less) clickbait-y title. Now, of course, I know that the Big Bang did not happen at any point connected to a single point in our current $3$-dimensional ...
- 1,897
3
votes
0
answers
80
views
Noether's Theorem in relativistic cosmologies [duplicate]
Is Noether's Theorem valid within the context of relativistic cosmology? If not, does this mean that the universe does not conserve energy on cosmological time and distance scales?
- 95.5k
3
votes
2
answers
153
views
What would an inter-universal medium be like?
I don't really believe in the Multiverse Theory, but hypothetically, if universes were like soap bubbles in a foam, what might that foam be like? I know this seems like an opinion-based question but ...
- 143