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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Energy of photons in perfect mirror box with FRW metric
In FRW metric, distance is described by
\begin{align}
ds^2 = dt^2 - a(t)^2[d\chi^2 + S_k(\chi)^2 d\Omega^2]\\
=dt^2 - a(t)^2\gamma_{ij}dx^i dx^j
\end{align}
where $a$ is the scale factor.
Now by using ...
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What happens to light that never hits anything?
Im a blue collar worker, just had a thought. What would happen to photons that never hit anything. It must be possible that at least one photon out there somewhere just happened to not ever hit ...
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Big-crunch divergence? (Not very serious) [closed]
Slightly whimsical question, inspired by Poul Anderson's story "To Outlive Eternity" and novel "Tau Zero."
If the Big Crunch model of cyclic cosmology actually applies in some ...
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Gondolo-Gelmini Change of Variables
In the article Cosmic abundances of stable particles: Improved analysis, P. Gondolo and G. Gelmini, Nucl. Phys. B 360 (1991), p. 145-179, they convert $\rm{d}^3p_1\rm{d}^3p_2=2\pi^2p_1p_2\rm{d}E_1\rm{...
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Dark matter in the time window between freeze-out and kinetic decoupling
Background
After the freeze-out, when all annihilations have stopped, the abundance ($Y=\frac{n}{s}$) of thermal dark matter species no longer changes with time. However, it is still kept in kinetic ...
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Fokker-Planck equation from Langevin equation in stochastic inflation
I'm reading this paper by Starobinsky and Yokoyama where they give the coarse-grained equation of motion,
$$
\dot{\bar{\phi}}({\bf x},t ) = -\frac{1}{3H}V'(\bar{\phi}) + f({\bf x},t)
$$
where $f({\bf ...
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Question coming from Cosmological Perturbation
We consider the following scalar perturbation on the FRW metric:
$$ ds^2 = -(1 + 2\phi)dt^2 +2a\partial_i B dx^i dt + a^2 \left( (1 - 2\psi)\delta_{ij} + 2\partial_{ij}E\right) dx^i dx^j $$
where $\...
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Distance and luminosity distance
In my cosmology lecture notes I read that a way to measure distances in cosmology is to use standard candles and the comparison between "absolute luminosity" of the candle and the apparent luminosity. ...
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Is the tensor product involved in the metric a symmetric product?
The expression of the FRW metric in Cosmology in usually written as:
$$ds^2=-dt^2+a^2(t)d\vec{x}^2$$
where $c=1$. However, $dt^2$ is a shortening of $dt\otimes dt$, that is, of the tensor product of $...
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Can Poincaré recurrences happen eventually in a universe with zero vacuum energy?
I am interested in the topic of possible vacuum up-tunneling and down-tunneling events in cosmology. One popular instance of this is a vacuum decay from a metastable vacuum energy level to a "...
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What is meant by "spontaneous creation" in this paper?
I have some questions in regard to the paper "Spontaneous creation of the universe from nothing". If I am not mistaken it is akin to Alexander Vilenkin's proposed cosmological model that has the ...
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Can self-indication assumption solve the fermi paradox? [closed]
Consider $P$ the probability that our civilization appears from beginning to the end. $P$ should also be the probability that at least 1 extraterestrial civilization exists. If we asses that $P$ is 50%...
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Understanding the CMB background as a reference frame
We say the Earth is in relative motion with respect to the cosmic microwave background (CMB), causing anisotropies in the CMB spectrum. I have four very simple questions about this.
How is it ...
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Clarification of Weinberg's cosmology book eqns 5.1.44-5.1.47 for scalar perturbation
Has anyone clarified the equations in Weinberg's cosmology book for scalar perturbation for nonzero $F$ and $B$, eqns 5.1.44-5.1.47. I am not sure why there are terms with $\nabla^2 \dot{B}$ and $\...
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Are vacuum "up-tunneling" phase transition in AdS and Minkowski spaces impossible?
I am interested in the topic of vacuum phase transitions in models of the universe. One popular instance of this is a vacuum decay from a metastable vacuum energy level to a "true" one (in ...
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Is gravitational particle production due to symmetry breaking?
A well-known fact about QFTs in curved spacetimes is that there is a phenomenon of particle production in expanding universes, these being described by the line element $$ds^2=-dt^2+b^2(t)d\vec x^2.$$
...
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Do maxima in the supernova redshift distribution correspond to generations of dying stars?
Within the Big Bang model, matter appears then coalesces to form stars that in turn die; some of them going boom in a supernova. Assuming the Universe is isotropic, one should observe maxima in the ...
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Is there a metric, a solution to Einstein's field equations, for a single body in a space of uniform non-zero density?
The Swarzschild metric describes a single body in an empty space with zero density, while the FLRW metric is presumably for a space with uniform non-zero density but no single body. But is there a ...
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Are there closed simply connected 2D manifolds that do not require a third dimension?
In the context of cosmology, space is commonly described as potentially having a global curvature that can be positive, zero, or negative. A common way that textbooks describe positive curvature is by ...
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Spatial Curvature of Universe at recombination vs now
From my understanding, we use the CMB data to measure the spatial curvature of the universe today. Why is it the value for today if the CMB data reflects the universe at recombination (380K years ...
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As per Friedmann Equations, is big-bang singularity necessary?
The Friedmann Equations do not directly require that the scale factor $a(t)$ was zero in the beginning. Since Einstein's static universe is still a valid solution, is it possible that before the Plank ...
Buzz♦
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Will expanding space rupture an empty box floating in outer space
Under the theory that space itself is expanding, but the space inside of atoms and molecules doesn't expand because nuclear and electromagnetic bonding forces exceed the forces that expand space, ...
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What is the gravitational field of a hole in an infinite perfect crystal?
Or equivalently and more interestingly: In the early universe when there was uniform H/He gas everywhere, gravitational field was close to 0 everywhere. Every test particle was pulled from all sides ...
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Can a light signal from Earth reach a galaxy outside the Hubble Horizon?
Is this video on the FLRW metric (timestamp 29:00 minutes) mistaken in its claim that a light signal from Earth cannot catch up with a galaxy outside the Hubble horizon, due to the horizon receding at ...
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How does the redshift - distance graph show the rate of expansion of the universe at every moment in time?
By plotting the graph of redshift against the distance of the object from earth, we are able to obtain a best-fit curve showing the relationship of redshit against the distance. How does this ...
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Viable values for the $K$ parameter in the FLRW metric
The FLWR metric is sometimes given as $$c^2 d\tau^2 = c^2 dt^2 - \frac{a(t)^2}{(1-KX^2)} dX^2. $$
I am not interested in the tangential motion so I set $d \Omega = 0$ although it is of interest in ...
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Dilaton field causes apparent universe expansion?
In string theory low-energy $n$-dimensional gravity is described by an action of the following form:
$$S^{(n)}=\frac{1}{2\kappa^{(n)}}\int d^nx\sqrt{-G}e^{-2\Phi}\Big(\mathcal{R}+4\partial_\mu\Phi\...
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Gravitational halos made of neutrinos...?
I have been recently interested in how halos made of standard model particles could be formed and behave.
After asking some questions in this site, I was told about how neutrinos could form such halos....
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Is the FRW metric, based on spatial homogeneity and isotropy, rotationally and translationally invariant? If so, how?
The spatial part of the Minkowski metric, written in the Cartesian coordinates, $$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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Cosmological numerical computations
I am unsure where to ask this question, whether here or in the Mathematica stackexchange, but either way, I was wondering what are some recommendations for cosmological computations specifically using ...
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