Questions tagged [cosmology]
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.
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Is the FRW metric, based on spatial homogeneity and isotropy, rotationally and translationally invariant? If so, how?
The spatial part of the Minkowski metric, written in the Cartesian coordinates, $$d\vec{ x}^2=dx^2+dy^2+dz^2,$$ is invariant under spatial translations: $\vec{x}\to \vec{x}+\vec{a}$, where $\vec{a}$ ...
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Gravitational halos made of neutrinos
I have been recently interested in how halos made of standard model particles could be formed and behave.
After asking some questions in this site, I was told about how neutrinos could form such halos....
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What is the energy density of gravitational fields around neutron stars? [closed]
The field strength of gravitational fields around neutron stars is extremely high. This would lead to an extreme negative value for the energy density. But if an absolute negative energy density is ...
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Why can we use Bose-Einstein statistics in this expression for number density
In a system with $N$ particles in some volume $V$ in contact with a reservoir of temperature $T$, we find that
$$\bar{n_i}=\frac{g_i}{e^{\frac{{\epsilon}_i -\mu}{kT}} \pm 1}$$
depending on whether the ...
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The data file of the LISA Interferometer results
How to extract the data of the strain versus frequency plot of the Laser Interferometer Space Antenna (LISA): Figure (2) in this paper:
https://arxiv.org/abs/1702.00786
The paper doesn't contain any ...
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Magnetic monopole in CPT universe model
I've recently read this paper CPT universe, and a thought came into my mind.
Is it possible to discuss magnetic monopole based on this CPT universe model?
This paper points out that some mysterious ...
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Saddle Shaped Universe
The universe, as described by FLRW metric, if $k = -1$ is clearly a 2 sheet 3-hyperboloid described by $x^2+y^2+z^2-w^2=-R^2$. So where does the more common saddle shaped picture of the open universe ...
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How can I visualise a sphere with a negative radius? [closed]
I want to visualise the shape of the sphere , will having a negative radius turn the inside of the sphere outside or something other will happen ?
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Beginning of a dark energy (cosmological constant) dominated universe
Consider a flat universe with only dark energy (cosmological constant)
In such universe, the Friedmann Equation can be written as:
$$H(t)^2 = \left(\frac{\dot a}{a}\right)^2 = \frac{8\pi G \...
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Friedmann Equation and a contracting universe
Consider a universe with a nonzero curvature and matter.
One can write the Friedmann Equation in this universe as such:
$$\frac{H(t)^2}{H_0^2} = \frac{\Omega_0}{a^3}+\frac{1-\Omega_0}{a^2}$$
Where $H(...
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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?
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Absolutism of speed [closed]
If we know that any photon emission travels with light of speed in vacuum, what theoretically stops us to create big enough detecting machine, where photons (lasers probably) are emitted in different ...
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Are the optical medium of 10.1007/BF00758153 and the Lorentz Ether of 10.1007/s00006-011-0303-7 the same thing? [closed]
On the gravitational field acting as an optical medium
A Generalization of the Lorentz Ether to Gravity with General-Relativistic Limit
The first paper seems to be facing a question: whether there ...
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Free Neutron Decay
I've read that free neutrons can decay into hydrogen, but it's rare because the energy from the decay usually sends the electrons away, unable to bind with the protons. But if trillions of free ...
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Doppler Effect and the concept of relative velocity in GR
While reading Sean Carroll's book on General Relativity, I understood that the concept of velocity is ill-defined over large distances in arbitrarily curved manifolds, like the one used to describe ...
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