The Stack Overflow podcast is back! Listen to an interview with our new CEO.

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.

Filter by
Sorted by
Tagged with
0
votes
1answer
50 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 ...
2
votes
1answer
183 views

Is Poplawski's “Cosmology with torsion” a “variable speed of light” cosmology?

In his 2010 paper "Cosmology with torsion", Poplawski mentions that, in the hypothesized formation of a new temporally-local universe within a black hole, its "expansion is not visible for observers ...
9
votes
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 ...
3
votes
3answers
129 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. ...
8
votes
4answers
3k views

Expansion of the Universe: is new space(time?) being created or does it just get stretched?

Is new space(time?) being created as the Universe expands, or does the existing spacetime just get stretched? If it just gets stretched, why do galaxies move along with the expansion instead of just ...
0
votes
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 ...
0
votes
0answers
27 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 ...
0
votes
0answers
23 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 ...
0
votes
0answers
21 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 ...
0
votes
0answers
58 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 ...
-4
votes
0answers
160 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 ...
0
votes
1answer
39 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?...
0
votes
0answers
12 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?...
0
votes
0answers
57 views

Latest news about Cold Dark Matter [on hold]

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/...
0
votes
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 $(r_0,\theta_0,\phi_0)$, ...
-1
votes
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.
-3
votes
1answer
51 views

What caused the particles to move? [on hold]

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 ...
0
votes
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 ...
0
votes
0answers
26 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 ...
2
votes
2answers
206 views

What is the interpretation of a wave function of the Universe in Hawking's no boundary proposal?

In the path integral formalism we have an in state $\Psi_{in}[\phi]$ and and out state and we find the amplitude for going from one to the other: $$\Delta[\Psi_{in},\Psi_{out}] = \int \Psi_{in}[\phi]...
1
vote
1answer
55 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 ...
2
votes
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 ...
-1
votes
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. ...
0
votes
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^{...
3
votes
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 - \...
-1
votes
1answer
105 views

Would eternal inflation be really eternal since quantum fluctuations are usually very rare and very small?

I’ve read somewhere that quantum fluctuations in the inflaton field are usually very rare and very small. If the effect of quantum fluctuations pushing the potential uphill is too rare and too small ...
1
vote
2answers
169 views

Do quantum fluctuations in the inflaton field lead to fluctuations in the potential energy density?

During inflation there are quantum fluctuations in the inflaton field. Do quantum fluctuations in the inflaton field lead to fluctuations in the energy density?
2
votes
4answers
194 views

What would happen to de Sitter universe if the cosmological constant disappears?

De sitter universe is dominated by a cosmological constant (devoid of matter and radiation), corresponding to dark energy in the far future or the inflaton field in the early universe, leading to an ...
0
votes
0answers
17 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 ...
1
vote
1answer
103 views

Does the potential $V(φ)$ of a scalar field decrease with the expansion of space?

If a scalar field (eg. inflaton field) starts with a high potential. Does the potential $V(φ)$ of the scalar field decrease with the expansion of space? If it doesn’t decrease, would it mean that ...
1
vote
0answers
83 views

Does Friedmann fluid equation prove that energy of the fluid is not conserved?

The Friedmann equation in question is:$$ \dot\rho + 3H \left(ρ + P\right) = 0 \,.$$ I’m a layperson so I don’t know why this equation is sometimes called fluid equation, energy conservation equation, ...
1
vote
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 ...
0
votes
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.
1
vote
1answer
209 views

Describing the shape of a singularity

Hawking and Ellis write about the difficulty of describing the shape of a singularity when presented with a manifold that has curves of finite length that don't reach a point in the manifold. [...
20
votes
3answers
12k views

Photons in expanding space: how is energy conserved? [duplicate]

If a photon (wave package) redshifts (stretches) travelling in our expanding universe, is its energy reduced? If so, where does that energy go?
0
votes
1answer
326 views

What does it mean by universe is expanding? I know that the universe is expanding because spacetime is expanding but again what does it mean?

I am aware about the balloon expanding analogue, where the galaxies are stuck on the balloon surface and here the balloon is our space time and as the balloon expand our galaxies also go apart. But ...
2
votes
0answers
62 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 ...
1
vote
2answers
567 views

Why are many things in the universe round?

Take a walk outside and you'll realise many things are round. Wheels are round, a cup is round and even that home button on your iPhone is round. But that's all man made. In nature, there are many ...
0
votes
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 ...
0
votes
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, ...
1
vote
1answer
118 views

What is the age of a universe with positive, negative, and zero curvature?

I am trying to calculate the age of universes with different curvatures using the Hubble constant and Friedmann equation. What does it mean when we say that the universe started out at equipartition ...
1
vote
1answer
333 views

Does the expansion of the Universe into a higher dimensional space imply that 4D objects are real?

It is my understanding that objects in the Universe are not just getting farther apart but space itself is expanding and so in some real sense, higher-dimensional geometry is "real" -- if so, on a ...
1
vote
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." (...
1
vote
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 ...
4
votes
2answers
6k views

What's the avarage temperature of the Universe?

I'm sorry if this was asked before but with all the stars and dark matter and all the other stuff, curently in the Universe, what's the avarage temperature of the Universe? Is it like extremely high ...
3
votes
1answer
207 views

What is the average temperature of mass in the universe? [duplicate]

That is, ignoring speculative concepts like Dark Matter and just summing across stars and gas clouds etc
1
vote
2answers
127 views

How do we measure distances in the FLRW metric?

We are in the flat FLRW metric in Cartesian comoving coordinates. The metric is expressed as: $$ds^2 = d\tau^2 + a(\tau)^2\big(dx^2 + dy^2 + dz^2\big)$$ The fact that the "universe is expanding" is ...
1
vote
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
votes
1answer
156 views

Does $\Lambda$CDM include inflation?

$\Lambda$CDM obviously includes dark energy (the $\Lambda$) as well as cold dark matter (CDM). Does it also include cosmic inflation? I've not seen a clear answer to this. Wikipedia's article on ...
2
votes
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 ...