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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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1answer
14 views

Constraints on $p$ and $\rho$ from cosmological metric and $T_{; \nu}^{\mu \nu} = 0$

I'm given the cosmological metric $$ds^2 = -dt^2 + a(t)^2dx^2 +a(t)^2dy^2 + a(t)^2dz^2$$ and perfect fluid with stress-energy tensor $$T^{\alpha \beta}=\left(\begin{array}{cccc}{\rho} & {0} &...
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29 views

Do black holes close at some point or do they always remain open, and if so, could they cause the universe to collapse? [on hold]

We know that black holes exist in different sizes. But size seems to be no problem for them to disappear.
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0answers
20 views

Temperature dependence of the Higgs VEV

Presumably, The Higgs field vacuum expectation value has a strong temperature dependence close to the electroweak symmetry breaking phase transition. Does this have known consequences for the ...
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1answer
55 views

Perturbed Ricci scalar in Modified Gravity

When getting the perturbed Ricci scalar in a Modified Gravity theory of the form $\mathcal{L}_{gr}=F\left(\phi,R\right)R$, $\phi$ being a scalar field, it is easy to arrive at an expression of it in ...
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3answers
1k views

Is dark matter inside galaxies different from dark matter in intergalactic space?

I just read a text about astronomy and when talking about dark matter the author says: [...], the dark matter responsible for the orbits of the stars in the Milky Way is probably different from the ...
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0answers
29 views

Geometry of Tegmark's Level I multiverse

Consider Tegmark's Level I spatially infinite multiverse: it has the usual gravity, and each local copy of some material configuration has the same gravity. But how does the geometry of the Level I ...
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1answer
45 views

Why don't galaxies have larger peculiar velocities?

I am trying to reconcile two facts: That galaxies' comoving velocities are generally close to zero, and That the universe has no preferred reference frame. Galaxies seem to move very little relative ...
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0answers
24 views

Experimental evidence of interaction out of thermal euilibrium?

Sakharov gave three conditions for the matter-antimatter asymmetry observed in the universe. (1) Baryon number violation: Don’t have any experimental confirmation yet. (2) C-symmetry and CP-symmetry ...
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0answers
23 views

Cosmology - Demonstration for equation of the evolution of the density contrast

In a context of cosmology, I need help about a differential equation that I can't get to demonstrate: The growth of density fluctuations obeys a second order differential equation. At early enough ...
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4answers
186 views

How much photon energy has already been destroyed?

We get taught in school that energy can neither be created, nor destroyed. The law of energy conservation is confirmed by many processes. It is an incredibly accurate assumption for every-day life. ...
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0answers
59 views

Lagrangian density of Electromagnetism

In this paper (journals.aps.org/prd/abstract/10.1103/PhysRevD.38.478) the author claims that if one uses FRW metric $$ds^2=\sigma^2[-N^2(t)dt^2+a^2(t)d\Omega_3^2]$$ With straightforward manipulation ...
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1answer
40 views

Doesn't universe violates law of conservation of linear momentum?

As mass of universe is not constant,it means that its linear momentum is not constant.But thinking in another way,is universe subjected to a net external force?But what is external for universe?...
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0answers
13 views

Does the homogeneity and isotropy of space imply that the expansion of the universe is uniform?

I have asked this question. Now I wonder what could happen if I take a step further. If space is assumed to be BOTH homogeneous AND isotropic, can I prove that the expansion of the universe is uniform?...
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0answers
20 views

With north and south poles fixed, do all geodesics have constant $\theta$ and $\phi$?

I was going thorough reading Kolb and Turner's The Early Universe where in Section 2.2 it starts by asking the following question. For a comoving observer with coordinates $(r_0,\theta_0,\phi_0)$, ...
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0answers
20 views

How has the size of the universe changed as a function of time? [duplicate]

Is there a plot or an equation that calculates/shows the size of the universe for each era? Preferably starting from the Big Bang, inflation and on up to today.
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57 views

Latest news about Cold Dark Matter [closed]

Can someone give me the most recent "generally accepted" vision on Cold Dark Matter cosmology ? A general overview of Cold Dark Matter cosmology can be found on Wikipedia https://en.wikipedia.org/...
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1answer
52 views

What caused the particles to move? [closed]

I wonder how the movement of the particles began? were they always on the move or was there a cause that set them in motion? Could it be that at the beginning of time, all the particles remained ...
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0answers
17 views

How the variation of Gauss Bonnet term is done?

I am trying to do the variation of Gauss Bonnet Invariant which is $G=R^2+R_{abcd}R^{abcd}-4R_{ab}R^{ab}$ and having problem in doing the variation of $δ(R_{abcd}R^{abcd})$. Can anyone please give ...
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0answers
29 views

Density fluctuations of the present Universe with Large scale structures

Disclaimer The question below is based on a vague knowledge, and hence, statements can be potentially wrong or misleading. An important quantity in Cosmology is the density fluctuation of matter ...
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1answer
57 views

Positive local spatial curvature of the universe implies that the universe is compact (i.e. finite)?

I quote from the Wikipedia page about the shape of the universe: If the spatial geometry [of the universe] is spherical, i.e., possess positive curvature, the topology is compact. I'm trying to ...
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1answer
52 views

Does the homogeneity of space imply that the expansion of the universe is uniform?

Obviously, homogeneity implies that the density is the same everywhere at any time. However, does this imply that the expansion is uniform? By uniformity, I mean that if I pick three galaxies to form ...
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0answers
45 views

Runge-Kutta fourth order method. Integrating backwards [closed]

I am using a Runge-Kutta fourth order method to solve numerically the usual equation of motion of a background scalar field in curved spacetime with a quartic potential: $\phi^{''}=-3\left(1+\frac{H^{...
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0answers
18 views

A Belinski-Khalatnikov-Lifshitz (BKL) singularity

In general, if the perturbed Friedman model behaved according to the second law, Sir Penrose speculated that numerous black holes would merge and collapse, resulting in one final singularity - Very ...
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0answers
23 views

Order of Magnitude of Cosmic Microwave Background Anisotropies

How does the statement about the anisotropies being of the order of $\frac{\Delta T}{T} \approx 10^-5$ which is often made in the literature (see, e.g. Description of CMB Anisotropies by Scott and ...
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1answer
62 views

Conservation of Charge vs. Conservation of Energy

Is conservation of charge ever violated like conservation of energy is violated during cosmological expansion? I am trying to understand this with respect to Noether's Theorem.
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0answers
71 views

Negative temperature of the de-Sitter horizon?

I'm considering the $4D$ de-Sitter spacetime, in static coordinates (I'm using $c = 1$ and $k_{\text{B}} = 1$): \begin{equation}\tag{1} ds^2 = (1 - \frac{\Lambda}{3} \, r^2) \, dt^2 - \frac{1}{1 - \...
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0answers
63 views

Entropy of de-Sitter spacetime and the $10^{120}$ vacuum discrepency

While doing some lazy calculations, I came across a curiosity that I'm unable to interpret. It is well known that the cosmological constant $\Lambda \sim 10^{-52}~\mathrm{m^{-2}}$ is usually ...
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0answers
34 views

In cosmology, what means the 'Fourier modes'?

I am reading papers of cosmology for my thesis and always I read 'Fourier modes' but I don't understand why is necessary that we work in the Fourier space and what means 'Fourier modes'? Maybe it is a ...
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0answers
12 views

The equilibrium era and energy density

In Weinberg's 'Cosmology', at page 109, he presents an era "of equilibrium" when "the temperature and density were sufficiently high so that radiation and matter were in thermal equilibrium. Then, ...
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0answers
31 views

Using Hubble parameter to compare sizes now and then

What if the Hubble parameter is constant both in time and space. How would the scale factor look like? For $H_0=70~kms^{-1}Mpc^{-1}$, how much smaller would the universe be 14 billion years ago? I'm ...
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1answer
80 views

The “Many Worlds” description of quantum mechamics and the “Multiverse”

I see a Phys.SE question, asking if the "Many Worlds" and the "Multiverse" ideas are essentially equivalent. This question was quickly squashed with a no answer. I would like to rephrase the question. ...
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0answers
162 views

An uncertainty principle modification of GR? [closed]

Can we modify the GR field equations to contain uncertainty and then use an assumption that the uncertainities are somehow related? Are there any known attempts that try to extend the established ...
2
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0answers
30 views

Why potential energy of a non-relativistic particle in an expanding universe is $ma^2\phi$?

I have been reading a paper on neutrino clustering in an expanding universe: see section 3, particularly 3.3.2 of this paper. In this paper for non-relativistic neutrinos, which travel through a ...
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0answers
12 views

Proper distance and spherical clusters

In Weinberg's Cosmology, he defines page 79 $D_{\perp}$ and $D_{\parallel}$ with the following sentence: Suppose we observe a luminous object at a redshift $z$ that extends a proper distance $D_{\...
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0answers
19 views

Cross-sections at zero temperature and high temperature for a process and its reverse

If the Feynman amplitude for a $2-2$ forward scattering $ab\to cd$ is denoted by $\mathcal{M}_{ab\to cd}$ and that of the reverse scattering process, $cd\to ab$, is denoted by $\mathcal{M}_{cd\to ab}$...
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0answers
39 views

How to get this metric tensor perturbation equation? (Gravitational wave)

(I need some time to come back to re-edit this post) LHS of equation 7.37 is gauge invariant The Twisted H symbol is comoving hubble parameter, the h_ij is metric perturbation, I do not know how to ...
2
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0answers
41 views

How to justifiy that $\rho_{\text{rad}} \approx \rho_{\text{mat}}$ at recombination time?

In standard cosmology, the recombination time is estimated to be $t_{\text{rec}} \approx 380~000~\mathrm{years}$ after the Big Bang, when matter and electromagnetic radiation becomes decoupled. It's ...
2
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1answer
61 views

What is the expected size of primordial black holes?

I often see it implied that primordial black holes tend to be small compared to stellar masses; Since primordial black holes did not form from stellar gravitational collapse, their masses can be ...
1
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2answers
606 views

Why isn't this anomaly in the CMB taken more seriously?

Almost every time I try to find more information about the Axis of Evil, I end up with people trying to ignore the phenomena. The video The (Cosmological) Axis of Evil explains it. However, it's no ...
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1answer
20 views

Pressure of (cold matter)

The first pages of Weinberg's Cosmology state that matter, cold matter like dust has zero pressure. I find it very disturbing. Does 'cold matter' also include fluids since hot matter is radiation, ...
3
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1answer
80 views

Different versions of the Robertson-Walker Metric

One form of the Robertson-Walker metric is $$ds^2 = c^2dt^2 - a(t)^2[d\chi^2+ S_k(\chi)^2(d\theta^2 + \sin^2\theta ~d\phi^2)]\tag{1}$$ $$\\$$ Considering curvature, where k = 0 , +1, -1 for flat, ...
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1answer
65 views

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 ...
2
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1answer
109 views

What if there were only 1 generation of fermions?

The number of fermion generations has many effects beyond the mere existence of "3 copies of fermions," including determining scatterings, constraining CP violation, influencing electroweak symmetry ...
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1answer
115 views

Changes to a Length of Physical Ruler Caused by Gravity vs Caused by Cosmological Expansion of Space

I read here (Feynmann Lectures, Lecture 42) that "Just as time scales change from place to place in a gravitational field, so do also the length scales. Rulers change lengths as you move around." (...
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1answer
40 views

Is there cosmic background radiation in de Sitter space?

This is a simple question of understanding. Would a de Sitter universe show cosmic background radiation? (After all, it is often explained said that our universe is similar to a de Sitter universe.) ...
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3answers
184 views

Is it true that we are probably Boltzmann brains? [duplicate]

I read a post on here about boltzmann brains a few days ago, and got curious on the subject. From the articles I read, it seems that they are saying that we are almost 100 percent likely to be BBs ...
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1answer
44 views

Are high-frequency gravitational waves inherently stronger/higher energy? Like the way gamma photons have more energy than radio photons?

A simple yes or no would be helpful, but a brief explanation would be nice too..... What about sound waves?
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0answers
17 views

Is expansion an exception to the postulate that the relative velocity between two objects cannot exceed the speed of light? [duplicate]

If expansion is not an exception to this rule, then how to explain the idea that the speed of light limits the portion of the universe that we can observe? Both we and all of the rest of the matter ...
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0answers
49 views

If the observable universe's mass & energy was converted to white light, how many photons would there be?

Background: The idea is to get a large number that is relatable to humans. The number of atoms in the observable universe is estimated at around 10^80, but this number isn't satisfying as the ...
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2answers
109 views

A simple explanation of the Born rule?

The probability that an initial quantum state $|\psi_i\rangle$ becomes the final quantum state $|\psi_f\rangle$ is given by \begin{eqnarray} P(i \rightarrow f) &=& |\langle\psi_f|\psi_i\...