The study of the large-scale structure, history, and future of the universe. Cosmology is about asking and answering questions about the "big picture" - the extent, origin, and fate of everything we know.
2
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1answer
46 views
Questions about the Jeans length
I have a couple of questions about the Jeans length. Suppose the universe has a homogeneous energy density, except there's a spherical region that is overdense.
I understand that if the region is ...
2
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1answer
243 views
Does a complete theory of quantum gravity require anthropic post-selection?
Does a complete theory of quantum gravity require anthropic post-selection? Certainly the black hole complimentarity and causal patch conjectures highlights the essential role of observers, at least ...
-2
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1answer
109 views
Finite amount of matter and space
(Please bear with me.)
If the universe included multi-universes, and black holes transferred matter between them, AND yet there was a large, but FINITE amount of matter and space, How would that ...
8
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0answers
289 views
Do intergalactic magnetic fields imply an Open Universe?
According to a recent paper on the arXiv, they do. How credible is this result? The abstract says:
The detection of magnetic fields at high redshifts, and in empty
intergalactic space, support ...
6
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0answers
64 views
Does quark color contribute to “spin degeneracy” for QGP calculations?
Like the title say, does quark color matter in counting contributions in a early universe plasma (QGP), as when adding up the total plasma energy density, or is it just spin? The book I have (Pathria) ...
5
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0answers
53 views
Does the Standard Model plasma develop a spontaneous magnetisation at finite temperature?
Reference: arXiv:1204.3604v1 [hep-ph] Long-range magnetic fields in the ground state of the Standard Model plasma.
Alexey Boyarsky, Oleg Ruchayskiy, Mikhail Shaposhnikov.
The authors of this paper ...
5
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0answers
87 views
Equation of state of cosmic strings and branes
I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ...
4
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0answers
49 views
$f_{NL}$ non-Gaussianity in cosmology
In the context of cosmology, what is meant by "..arbitrary quadratic non-Gaussianity i.e non-Gaussianity that is described to leading order by a 3-point function.."? (.."quadratic non-Gaussianity" ...
4
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0answers
130 views
Implications of Unruh-inertia to theories of gravity
If it turns out to be true that the galaxy rotation curves can be explained away by Unruh modes that become greater than the Hubble scale at accelerations around $10^{-10} m/s^2$ as proposed in here, ...
4
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0answers
34 views
Status of large-scale structure formation within cosmology today
Since the CMB results of the past decade, would it be fair to say that the consensus among cosmologists is that cosmic strings are no longer considered as a (major) source for density perturbations?
...
3
votes
0answers
54 views
How many orders of magnitude in energy spans the domain of validity of the Standard Model?
I am wondering if it makes sense to state that the upper limit is roughly 1012 eV (up to know the physics probed by the LHC seems to be pretty consistent with the SM) and the lower one is ... the ...
3
votes
0answers
147 views
Quantum Entanglement Versus Inflation in the Early Universe?
Quantum entanglement is one of the most fascinating and mysterious phenomena in nature. It needs no interactions, or any sort of exchange for it to take place. It is possible, not against any rules of ...
3
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0answers
37 views
In the αβγ cosmology paper, how do the authors assume the integral of density over time in the early universe?
In the famous Alpher-Bethe-Gamow paper, the authors say: "it is necessary to assume the integral of $\sigma_n dt$ during the building-up period is equal to $5 \times 10^4 \frac{\text{g ...
3
votes
0answers
49 views
squeezed radiation astronomy
Squeezed electromagnetic vacuum does have a renormalized energy density smaller than the vacuum. So it makes it in my opinion a inconspicuous candidate for a dark energy carrier.
Are there ...
2
votes
0answers
83 views
Our Universe in collision with another one
This question is related to the possibility that there are many other universes in the UNIVERSE,the multiverse, that were created during collisions between membranes, according to string theories. The ...
2
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0answers
44 views
Topological Solitons and the Higgs Condensate entanglement
While focusing on resolutions to the Firewall controversy, and the possible implications of the Higgs field as it relates to the issue, the possibility of using EPR correlations in the Higgs ...
2
votes
0answers
56 views
How can I read density fluctuation from microwaves?
The Cosmic Microwave Background Radiation shows temperature differences. The red and yellow areas are warmer. The green and blue areas are cooler.
For example consider this picture of CMBR ...
2
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0answers
83 views
How does one derive the 2 halo term in two-point correlation function
This question is in reference to the paper here. In Equation (86) on page 28, the authors have given the two point correlation function
\begin{equation*}
\xi(\mathbf{x}-\mathbf{x}^{\prime}) = ...
2
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0answers
54 views
Wick rotation for FRW in quantum gravity
There is no timelike Killing vector for FRW cosmologies. In the path integral formalism, is it possible to Wick rotate for quantum cosmology in quantum gravity? If yes, how? If no, how does one work ...
2
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0answers
85 views
Question on the Gell-Mann Low equation
Question on the Gell-Mann Low Equation.
In this paper, http://arxiv.org/abs/1205.3365, page 21, the author argues that if:
t →∞(1-iϵ), all the terms in equation (193) goes to zero, except the first ...
2
votes
0answers
83 views
Can decompactification explain the inflation of the early universe?
I've just reread chapter 11 of this book where it is explained among other things, that our four dimensional universe could be unstable concerning a decompactification transition, since potential ...
2
votes
0answers
74 views
Is it really true that slow-roll inflation is incompatible with string theory?
In this blog post, Motl claimed that slow-roll inflation is incompatible with string theory. Is that really true, and why do people think so?
2
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0answers
256 views
Is large-scale “time reversal” (Poincaré recurrence) possible given infinite time?
The following are some assumptions I'm basing my question on, from what (little) I understand of physics. I list them so an expert can (kindly) tell me where I'm going wrong.
There is a probability ...
2
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0answers
67 views
Is eternal inflation Lorentz invariant?
Start without general relativity. Consider a metastable vacuum over good ol'-fashioned Minkowski space. It decays. A bubble forms and the domain wall expands. The domain wall is timelike, and ...
2
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0answers
71 views
Does dark energy affect asymptotic freedom?
If the Hubble constant is extremely large, what will happen with quark confinement?
I guess that quarks will remain confined because of asymptotic freedom. But can gravity or dark energy have any ...
2
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0answers
300 views
Calculation of the non-Gaussity parameter for primordial cosmological perturbations by the ADM Formalism
Maldacena has used the ADM Formalism in one of his papers (http://arxiv.org/abs/astro-ph/0210603) in computing the the three point correlation function (i.e the non-Gaussianity) parameter for ...
2
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0answers
157 views
shear , viscosity and expansion of universe
what is the meaning of expansion ,shear and viscosity in context of universe? how can we conclude a result after getting a numerical value of above terms?
2
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0answers
210 views
Singularities in Bianchi models in general relativity ( physical science)
what are the conditions to check point type singularity in a bianchi type model ?
bianchi type model are of Type I,II,III,IX,IV or u can say we use different Bianchi type models having some specific ...
1
vote
0answers
17 views
Cosmic Inflation: Lower Expansion Rate with than without?
When I read the Cosmic Inflation diagram at
which I rotated to make it readable:
where the Hubble radius, and therefore also the Hubble parameter which is c/Hubble radius is constant over ...
1
vote
0answers
15 views
CMB anisotropies and tightly coupled limit
Sorry if this is a technical question. I am studying the origin of CMB anisotropies and the tightly coupled limit of the Boltzmann equations. We have a fluid composed of ionized electrons and photons.
...
1
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0answers
37 views
When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?
It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
1
vote
0answers
50 views
Second order action ADM formalism
I am trying to derive the second order action
$$S_{(2)}~=~\frac{m_{pl}^{2}}{8}\int a^{2}[(h_{ij}')^{2}-(\partial_{i}h_{ij})^{2}]d^{4}x, $$
used for tensor fluctuations derived from the ADM ...
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0answers
41 views
The definition of $f_{NL}$ and transfer function
To me there seems to be quite a few different definitions of $f_{NL}$ in cosmology and I would like to know if or how they are equivalent. Let me cite at least 3 such,
One can see the equation 6.71 ...
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0answers
183 views
Curiosity episode with Stephen Hawking. The Big-Bang
In an episode of Discovery's Curiosity with host Stephen Hawking, he claims the Big Bang event can be explained from physics alone, and does not require the intervention of a creator.
1) His ...
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0answers
57 views
Thermal gravitational radiation and its detection
To my poor knowledge on the topic, the gravitational waves that are most likely to be detected by LIGO or other experiments do not have thermal spectrum. But I'm not certain.
I know that Hawking's ...
1
vote
0answers
61 views
Reference request: FLRW with k>0, dust, and positive cosmological constant
The exact solution representing a FLRW universe with $k>0$ and dust (p=0), and $\Lambda=0$, is described by a cycloid. What is the exact solution for dust, in the presence of a positive ...
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0answers
115 views
How is the poincare conjecture(and perelman proof) helpful in studying the properties of the universe?
Can someone tell me how the poincare's famous conjecture or its proof by perelmen can be helpful in deciding some properties like the shape of the universe?
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0answers
54 views
is cosmic expansion related with IR divergencies?
This question is related to renormalization, but in the IR limit. It is assumed that unitarity does take care of IR divergencies in interacting theories like QED. But how would one interpret ...
1
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0answers
61 views
Unitarity and quantum cosmology
By studying quantum cosmology I was asking myself if the fact that the universe is expanding, so space is expanding and with it I would say that phase space is also expanding, so it's a non-unitary ...
1
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0answers
64 views
Temperature of the CMB when the Earth formed and the faint young Sun paradox
The cosmic microwave background (CMB) has a modern temperature of about 2.7 K. At the time of the origin of the CMB, about 13.6 billion years ago, it had a temperature of about 3000 K.
...
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0answers
67 views
Cosmic radiation cutoff at LOW energies?
The energy spectrum of the cosmic radiation (not CMB) is limited to both sides.
I know about the GZK-cutoff at high energies. Basically the interaction probability for photons of energies above 10^20 ...
1
vote
0answers
178 views
Why do galaxies collide? Reconciling dark energy and an expanding universe with the fact that some galaxies are on a collision course
My layman understanding of cosmology is:
galaxies are uniformly (more or less) spread throughout the universe, per the Big Bang and the fact that in a central explosion, all dispersed points are ...
0
votes
0answers
23 views
Perturbed stress-energy tensor in a cosmological context?
In the theory of cosmological pertubations, we can write the metric of a null-curvature expanding Universe as :
$ds^2 = -c^2\left(1+2\frac{\psi}{c^2}\right)dt^2 + a^2 ...
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0answers
50 views
How does cosmological horizon depend on position?
Suppose I am at a certain point in the universe, and the cosmological horizon is a distance, $d$ $km$, away from me.
Suppose just beyond the cosmological horizon, say at $(d+1)km$, is the edge of a ...
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0answers
105 views
Did force of gravity cause macroevolution?
Did big bang create gravity?
What role gravity is assumed to have played in the formation (starting from the big bang) of large structures of our universe and what other important physical mechanisms ...
0
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0answers
36 views
Vlasov equation in Liddle and Lyth or Dodelson's book
I am concerned about equations 2, 5 and 6 on page 2 of arXiv:astro-ph/0606028.
The paper refers them to the book by Peebles which I don't have access to at this point.
I believe that these ...
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0answers
25 views
Reference for stellar figures
Could anybody suggest a reference for the present stellar models? In particular, I would appreciate references containing the core temperatures and pressures of neutron stars...
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0answers
55 views
Space-time & solar mass
Does the space-time curvature described by Einstein have any affect on the accuracy of our determination in the age of a star or globular cluster? How does this affect our interpretion of how old we ...
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0answers
19 views
In the Hartle-Hawking boundary condition, what should we set as the initial values for the scalar fields?
In the Hartle-Hawking boundary condition, what should we set as the initial values for the scalar fields? Fields like the Higgs assuming it's fundamental, and the inflaton fields.
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0answers
170 views
Extrinsic curvature versus Intrinsic curvature (Euclidean versus Riemannian)
Do we believe the universe has any extrinsic curvature at all? As far as I'm aware extrinsic curvature is only used in geometry/math to model the intrinsic curvature, correct?
From the answers to ...
