New answers tagged cosmology
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Cosmology model with equation of state $P=\epsilon / (\epsilon+1)$
When the energy density $\epsilon\ll 1$, the pressure is
$$ P = \epsilon.$$
This describes "kination", a cosmological phase named by Joyce (1997), who considered how this phase could ...
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Cosmology model with equation of state $P=\epsilon / (\epsilon+1)$
Cosmological equation of state is $$p=\omega \rho \tag 1$$, where $\omega$ is
dimensionless equation parameter having form :
$$\omega = \frac {1} {3 (Kx^\alpha + 1)} \tag 2$$
Parameter $x=1$, ...
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Could it be possible that the Universe is expanding in some areas while contracting in other areas?
The universe is indeed expanding in some places and contracting in others. That's why we have galaxies: regions of the universe that were denser than average eventually stopped expanding, turned ...
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Could it be possible that the Universe is expanding in some areas while contracting in other areas?
If expansion is growing separation of comoving and force free worldlines and contraction is shrinking separation between them then yes, in the close field where matter collapses you can describe this ...
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How heavy is a 5 litre of neutron star
It might be in the order of 10^13 to 10^15 kg and depend on the neutron star itself.
You can find some more information e.g. on the Wiki article
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Do all FLRW models predict a finite age of the universe, and limit the lookback time?
A universe with a pure positive cosmological constant, $\Omega_{\Lambda}=1$, $
\Omega_m=\Omega_k=0$, has an infinite age. The scale factor grows as $\sim e^{Ht}$ so is never zero at any finite $t$ in ...
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Gondolo-Gelmini Change of Variables
I am not sure! But I doubt there is some mistake in eq. 3.2
In the paper, it is mentioned that $$ d^3p_1d^3p_2=4\pi p_1dE_1 4\pi p_2 dE_2 \frac{1}{2}dcos\theta $$
and the author is using relativistic ...
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Accepted
Is cosmological expansion paradoxical?
Say the three dots are blue, red and green, where red is within the cosmological horizon of both blue and green, but blue and green are not within each other's horizons.
In that case, in the time it ...
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Accepted
Why the Hubble parameter that is proportional to dark energy is squared in the Friedmann's equation?
It has a positive and a negative solution, the positive is for an expanding big bang universe and the negative for a collapsing big crunch universe, since the physics is time reversible both solutions ...
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Breakdown of quantum mechanical observables in cosmology
Why should the inability of an experiment to average infinitely long imply that the observed quantity is fundamentally ill-defined?
Experiments are carried out all the time, and the usefulness of ...
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2
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Accepted
How does neutrino free streaming length affect the growth of structures in the universe?
But a larger mass leads to a longer FSL (why?)
This is incorrect. A larger neutrino mass leads to a smaller free-streaming length. Neutrinos decouple at a known temperature $\sim 1~\text{MeV}$ and ...
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How does neutrino free streaming length affect the growth of structures in the universe?
I think I got it. On the second picture, when we look at the smaller scales (zooming in to the darker areas), we see less structure (therefore less clustering). This is because a larger FSL results in ...
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Confusion regarding the cosmological constant
To understand what the cosmological constant is consider it in the Einstein equation:
$$ \mathbf G + \Lambda \mathbf g = \frac{8\pi G}{c^4} \mathbf T $$
The stress energy tensor $\mathbf T$ has units ...
- 343k
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Continuity equation for the conservation of energy from the conservation of the energy-momentum tensor
Just in case somebody else comes here and is confused by the same things I explained in the edit to my post, I will leave here the wonderful explanation I got about this in Math Stack Exchange:
https:/...
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Accepted
What if everything doubled linearly?
The question is very vague to use it with mass , first case everything is doubled and the mass of an object remains the same and is more widely distributed decreasing density ( highly unlikely ...
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Could a fourth family of quarks and leptons account for dark matter?
Not really. These particles would still interact via the strong force, and the limit on the dark-matter-dark-matter-interaction cross-section from merging galaxy clusters rules that out. Also, as @...
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Why does the CMB have a spectrum like a black-body radiation?
There are lots of processes that serve to exchange energy between the photons and the particles. Thomson scattering results in no energy change for the light, so that isn't one of them, it just serves ...
- 120k
5
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Accepted
How are black-hole evaporations possible?
Black holes do not abruptly explode when they reach a certain size. What happens is that the power of the emitted radiation is proportional to $1/M^2$. Specifically the equation is:
$$ P = \frac{\hbar ...
- 343k
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Why does the CMB have a spectrum like a black-body radiation?
The CMB we see today was generated when the hydrogen produced from the Big Bang changed from a plasma of protons and electrons to neutral hydrogen atoms. Neutral hydrogen is mostly transparent to ...
- 343k
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How are black-hole evaporations possible?
A simple reply
For a given mass one can always calculate a Schwarzschild radius, see the one for the earth here .
whereas Earth's is only about 9 mm (0.35 in) and the Moon's is about 0.1 mm (0.0039 ...
- 230k
6
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Why does the CMB have a spectrum like a black-body radiation?
Yes, you are missing the fact that at high enough energies, it's possible for a sufficiently energetic photon to create an electron-positron pair out of the vacuum; during the big bang, those ...
- 82.7k
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Lookback time resolves Hubble tension?
While benrg would seem to be on the right track with the answer above, just after this question was posted, there seems to have been professional interest as well, such that a Comment on: paper was ...
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Lookback time resolves Hubble tension?
Their equation (5) says
$$T_{lt} = T_0 − T(z) = T_0 − a(t)T_0$$
where $T_{lt}$ is the light travel time and $T_0$ is the present age of the universe. That implies they're assuming $T=a\,T_0$. In a ...
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Accepted
How is the universe becoming increasingly unreachable for things traveling at speed of light?
That being said, if we were to shoot a photon from A, would that photon not eventually be able to reach B in a finite time? And ...
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Accepted
Why is Big Bang so misunderstood?
The physical content of the Big Bang theory is that the Universe was much hotter and denser in the past than it is now. We see this directly in the Cosmic Background Radiation and infer it less ...
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What if the universe was not uniform...?
It sort of happens by definition. The wordings are a bit circular in this sense. Isotropy speaks to a symmetry in the universe which we can use to predict that things operate the same in all ...
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Lookback time resolves Hubble tension?
Look back time also known as light-travel distance isn't well defined, so it doesn't make sense to use it for astronomical calculations.
For more information please see Edward Wright's Article https://...
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Equation of state for matter, radiation and vacuum in Roberston-Walker Metric
In relativity, matter is a set of cold non-relativistic particles which behave like dust. There is essentially no pressure: $p_m = 0$. Stars and galaxies are considered matter because the pressure is ...
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Problem with the Fisher information matrix in case of $N$ measurements of two observables
If so far it is correct, knowing $F$ implies knowing the measurements of $x_i$ a priori of the experiment; how can this be possible?
If I understood the description correctly, $x_i$ are not ...
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Question on the relation between the Hubble constant and the absolute magnitude of Type Ia supernovae
Observation of Type Ia supernovae yield a redshift and an apparent magnitude at peak brightness. If you don't know the absolute magnitude at peak brightness then the apparent magnitude does not give ...
- 120k
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Is there anything truly "stationary" in the universe?
The word "stationary" in the modern (post-Newton) physics requires a reference frame: Object A is stationary relative to e.g. a non-rotating frame attached to the center of mass of the ...
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Is there anything truly "stationary" in the universe?
Your question is like asking whether there is a particular point that is the centre of the surface of the Earth. The answer is that the surface of a sphere has no unique centre, so it is meaningless ...
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Is there anything truly "stationary" in the universe?
As far as is understood, there is not absolute sense of "stationary" so the question you link is not even really well defined. All that could be asked is "do the galaxies move relative ...
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Is there anything truly "stationary" in the universe?
The laws of physics do not allow us to distinguish who in a pair is moving and who is stationary. As such, the question of whether or not there is anything in our world that is truly stationary is ...
- 82.7k
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Accepted
How is Hubble's constant (the expansion rate) predicted from LCDM and the CMB?
The CMB allows us to infer the value of $H_0$ because it provides us with a standard ruler, i.e. a feature which can be accurately measured and which has a physical size that can be computed from ...
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