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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61 views

Why is the gravitational constant equal to cosmic radius times $c$ squared all over cosmic mass?

Actually, you don't even need the speed of light: $$\frac{\left(\text{cosmic radius}\right)^3}{\text{cosmic mass}\cdot\left(\text{cosmic age}\right)^2}$$ Or vice versa where you use the speed of light ...
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Spacetime and physicists' common sense [closed]

Are people knowledgeable in spacetime physics able to merge mathematics with their common sense or intuition, so as to genuinely achieve either of the following 2 deeds pertaining to understanding ...
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1answer
21 views

How to determine current expansion rate of universe from observing past objects?

How to determine the current expansion rate of the universe from observing past objects? When we look into the light from objects far away, it not only very very far away in the distance it is also ...
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Monopole problem: 't Hooft–Polyakov, Big Bang theory versus Cosmic inflation

I am not sure the validity of this claim from Wikipedia on 't Hooft–Polyakov_monopole The "monopole problem" refers to the cosmological implications of grand unification theories (GUT). ...
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107 views

Where am I going wrong in the integration of these distribution functions?

I don't understand how equation $(4)$ was derived in Sterile Neutrinos as Dark Matter. It isn't explained and I'm trying to fully understand the article. In it, we're presented with $(2)$: $$\left( \...
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47 views

The relation between the weak gravity conjecture and wormholes

Charged black holes are usually associated with the weak gravity conjecture at extremal limit, according to the well-known relation, i.e. charge to mass ratio. Now my question is that how the weak ...
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2answers
98 views

How is the curvature term of the Friedmann Equation calculated with the Newtonian derivation?

I'm trying to develop and intuitive understanding of the Friedmann equation. I'm afraid I get lost with the relativistic derivation as it's just a lot of crank-turning. When I derive it from the ...
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58 views

On the topic of imaginary-time

I apologize for my crude line of questioning, as I'm not well-versed in physics at all but it fascinates me. I was researching the concept of "imaginary-time" and the shuttlecock model of ...
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67 views

What is the speed of light derived from a supernova at the edge of the universe? [closed]

As the universe is expanding, a supernova at the edge of the universe is apparently moving with a speed of 3$c$ or even 4$c$. What is the speed of light derived from it?
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1answer
39 views

What would an observer in the most distant galaxy see?

Perhaps this has been asked before and perhaps it's rather trivial, but what would an observer on the most distant astronomical object see? Would they see only the CMB in one direction and us in the ...
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How smooth does reheating (after inflation) need to be?

Let's estimate the size of perturbations in the very early (GUT scale or earlier) universe required for consistency with CMB. The CMB fractional temperature (and hence density) perturbations are of ...
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Why is gravitational wave production in early Universe disfavoured for crossovers?

Production of gravitational waves in the early Universe (for example, in the electroweak phase transition) is favoured only if the Universe undergoes a first-order phase transition but not favoured if ...
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How is the first Friedmann equation derived from Einstein's field equations?

I see that Friedmann's first Equation (for flat space) is: $$\left(\frac{\dot{a}}{a}\right)^2=\frac{8\pi G}{3}\rho.$$ And I know that Einstein's equation, just considering the time-time component is: $...
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Sound speed in cosmology: non-adiabatic perturbations

The definition of sound speed is given by: $$c_{s}^{2}\equiv \frac{\partial P}{\partial \rho}.$$ In some books of cosmology to calculate the expresion for the sound speed in a baryon-photon fluid they ...
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Does Goldstone theorem have anything to do with Cosmic string

Cosmic strings are formed due to topological defects during symmetry breaking phase transition in early universe. While Goldstone theorem states whenever we have continuous symmetry and it is ...
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Confusion with comoving distance and physical distance

Suppose that you have a flat universe described by the FRW metric. $$ds^{2}=-dt^{2}+a^{2}(t)(dx^{2}+dy^{2}+dz^{2})$$ Then if you want to know the distance that a light ray travels from time $t_{e}$ to ...
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1answer
46 views

Sound Horizon in cosmology

I was trying to write the sound horizon in terms of the scale factor, however I don't understand all the steps in the derivation. I know that I should get: $$r_{s}=\int_{0}^{a_{d}}\frac{da}{a^{2}H(a)}$...
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Derivation of number density for fermions and bosons in relativistic limit

Kolb and Turner's 'Early Universe' states the the number density for relativistic bosons with $\mu < 0$ and $|\mu| < T$ is given by \begin{equation} n = \exp(\mu/T)(g/\pi^2)T^3 \end{equation} (...
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Does the Sachs-Wolfe effect depart from $1/3$ for large $k$?

In the Sachs-Wolfe effect (the one at last scattering, not the ISW), the standard result is $$ \frac{\Delta T}{T} = \left( 1 - \frac{2}{3} \right) \Delta \Phi = \frac{1}{3} \Delta \Phi . $$ To ...
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Isolated Neutron Stars

What is the difference between isolated neutron stars and neutron stars ? What is the meaning of isolated here ?
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2answers
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Why is it assumed that the Big Bang produced equal parts matter and antimatter?

It is well-known that a major open question in physics is why the Universe appears to be made almost entirely out of matter, with next to no antimatter, despite the two being strictly symmetrical ...
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1answer
60 views

What is the energy spectrum of all photons in the observable universe?

Does anyone know what the energy spectrum for the entire universe looks like? In other words, what would the graph look like if you plotted the number of photons on the $y$-axis and frequency on the $...
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1answer
44 views

Light could travel around the closed FLRW universe precisely once?

I seem to remember reading somewhere that within the lifetime of a closed, recollapsing Friedman-Lemaitre-Robertson-Walker universe light could travel around the space precisely once. Could someone ...
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How is the energy density of the universe determined? [duplicate]

According to Misner et al. "There is no such thing as the energy … of a closed universe, according to general relativity, and this for a simple reason. To weigh something one needs a platform on ...
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1answer
35 views

Derivation of Thermally averaged cross sections

In many sources discussing neutrino decoupling I find the following claim: "The thermally averaged rate of weak interactions is given by: $\Gamma = n \langle\sigma |v|\rangle$, where $\langle\...
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Mathematical coincidence of the Schwarzschild radius of the Universe?

I read here that a black hole with a mass of the observable universe, $M=8.8\times10^{52}kg$, would have a Schwarzschild radius of $r_s=13.7$ billion lightyears. I immediately noticed that at the ...
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62 views

A question about causality and philosophy of cosmology [closed]

So, I was watching a documentary about the philosophy of cosmology and there is a part in that that made me confused. This is the context: Robert Lawrence Kuhn: George, you've been dealing with ...
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1answer
46 views

Why can't primordial fluctuations be super-“horizon” without inflation?

I am trying to understand why, in cosmology, it is said that the presence of fluctuations at scales above the Hubble distance would not be expected in the absence of inflation or something like it. We ...
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1answer
43 views

Pivot scale in cosmology and CMB

What is a pivot scale, pivot frequency in general, and particularly in the cosmology? I don't get the idea. Almost everywhere I found the same the same sentence "we take the standard value of a ...
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1answer
44 views

How did Friedmann and Lemaitre based on Einstein's “original” GR theory find an expanding solution?

Wikipedia says, Einstein included the cosmological constant as a term in his field equations for general relativity because he was dissatisfied that otherwise his equations did not allow, apparently, ...
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1answer
81 views

Will we ever stop receiving CMB photons on earth? [duplicate]

If there is a lamp $1$ meter away from me which is switched on at time $t_1$ and switched off at $t_2$, I'll start receiving the light at time $t_1+\frac{1}{c}$ and stop receiving it at $t_2+\frac{1}{...
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Are the LIGO and VIRGO results consistent with the black hole statistics predicted by cosmology?

Have LIGO and VIRGO recorded enough gravity wave events to cross-check the black hole populations predicted by astrophysics and cosmology? Basically, we now have another experimental tool to verify ...
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50 views

What is the name of the constant in this formula for photon density?

I've found a formula for photon density which states: $$ \rho_\gamma = \frac{a_B\, T_0^4}{c^2}$$ What is the name of the $a_B$ constant? I'm going to guess the 'b' is Boltzmann, but I need the ...
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1answer
37 views

How astronomers estimate the redshift of galaxies clusters?

I don't understand how astronomers estimate the redsfhit of a cluster. As far as I understand a cluster of galaxies is something really "big", so I expect that different galaxies in the ...
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30 views

Runaway potential in cosmology inflation

I'm looking for a review articles that explaining the issues with runaway potential in cosmology inflation, when a potential has a hilltop and then it goes to zero at infinity. I read somewhere that ...
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10 views

Boltzmann Equations (Modern Cosmology, Scott Dodelson)

I'm trying to understand the derivations of the Boltzmann equations for the different species in the universe from the book 'Modern Cosmology' by Scott Dodelson. So far, I feel very lost. For example, ...
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1answer
26 views

Hubble's Law Measurements

In measurements concerning distances, luminosity, etc. to evaluate the Hubble's parameter, do scientists take into considerations the fact that the light, emitted from a star, and received by a ...
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2answers
50 views

Why Andromeda is approaching the Milky Way in an expanding universe? [duplicate]

We know from Hubble law that the galaxy is receding from each other. Also, the general theory of relativity predicts the metric expansion of space ( voids are increasing). Then why Andromeda is ...
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23 views

Need help in deriving Boltzmann Equation for $3\to3$ scattering

This is the first time I am posting a question here, so please forgive me if I make any mistake. I am trying to derive the Boltzmann Equation for $3\to3$ scattering, which means there are 3 particles ...
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49 views

Could cosmological cold dark matter be a neutrino condensate?

What is wrong with the following? Neutrinos are dark matter. A neutrino condensate would be cold. (Often, neutrinos are dismissed as being automatically hot dark matter.) Cold neutrinos could be ...
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Cold neutrinos - how are they distributed?

Cold or slow neutrinos have non-relativistic velocities and hence very low energies. That makes them very difficult to detect. Answers to Where are all the slow neutrinos? make it clear that they are ...
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1answer
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I cant understand here why is receding acceleration is taken equal to gravitational acceleration?

Here is the full document link https://www.lapasserelle.com/cosmology/ Click on lecture 1 notes for the document Now my question is why the acceleration with galaxies is receding is taken equal to ...
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1answer
44 views

How do you calculate curvature from the density?

We know that this density results in a flat universe. $$\rho_c=\frac{3 H^2}{8 \pi G}$$ And we know that if the universe isn't flat, the density as a proportion of critical density can be expressed as ...
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43 views

Dark matter and velocity [duplicate]

We know that as the speed of an object increases, so does its relativistic mass. But can it be said that dark matter may be due to the increasing relativistic mass of nebulae, stars, galaxies, etc. ...
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Lose cosmological entropy in conformal cyclic cosmology?

In PBS Space Time "What Happens After the Universe Ends?", it is stated that black holes are needed to destroy entropy before a new aeon can start in a low-entropy configuration, which seems ...
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15 views

Derivation of Press–Schechter mass function for $P(k)\propto k^n$

From 'Galaxy Formation and Evolution' by Mo, van den Bosch and White eq. (7.45) we have that $$ n(M,t)\ dM = \sqrt{\frac{2}{\pi}}\frac{\bar{\rho}}{M^2} \frac{\delta_c}{\sigma}\exp\left(-\frac{\delta_c^...
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Understanding Verlinde: How to get from emergent gravity to MOND

Verlinde ( https://arxiv.org/abs/1611.02269 ) tries to deduce MOND from emergent gravity. Can you help? Emergent or entropic gravity goes back to Jacobson. He starts with the entropy-area connection $...
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Puzzle about co-moving coordinates distance

So I tried deriving the radial co-moving coordinate formula to an observable galaxy for which the light observed at time $t_o$ was emitted at time $t_{em}$. i.e $r_0=\int_{t_{em}}^{to} c/a_{(t)} dt$ ...
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37 views

Primordial perturbations, scale invariance and fundamental length

In cosmology, the initial perturbations to the metric can be described as $$\mathrm ds^2=-\mathrm dt^2+a(t)^2e^{2\zeta(\mathbf x)}\mathrm d^2x,\qquad(1)$$ where $\zeta(\mathbf x)$ is a gaussian random ...
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1answer
24 views

Is there any model in theoretical physics combining Wheeler's “Principle of mutability” and multiverses?

Physicist John A Wheeler proposed the "Principle of mutability" which said that it could be the case that the universe would eventually shrink in a "Big Crunch" and the be re-born ...

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