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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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44 views

Would other parts of the universe have a different rest frame for their own Cosmic Background Radiation?

The CMB, once the Doppler shift from the Earth's motion is subtracted, is fairly uniform, which seems to imply that all the matter that emitted it moved at more or less the same velocity when it did. ...
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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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How can the interior pressure of compact objects affect cosmology?

This paper suggests that dark energy concentrated in black hole interiors (they use an unconventional BH model) could act like a cosmological constant. Their claim is that to calculate the equation of ...
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What is the entropy of the cosmological horizon?

The Bekenstein-Hawking entropy for a black hole horizon is $S_{BH}=\frac{c^3 A}{4Gℏ}$. Is there a similar expression for the cosmological horizon?
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Could two atomic clocks really be used to detect gravitational waves from a distant source? If so, how?

Three articles report on the recent paper in Phys Rev. D: Flanagan, Éanna É. et al. 2019 Persistent gravitational wave observables: General framework (also ArXiv): Phys.org: Gravitational waves leave ...
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Is there any use of a quadratic equation of state in FLRW cosmology?

Consider standard FLRW cosmology. Usually, the relation between energy density $\rho$ and pressure $p$ of a cosmological fluid component is linear: \begin{equation}\tag{1} p = w \, \rho, \end{...
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Looking for a geometric cosmology model where time behaves like an orientation

This question is literally inspired from seeing the above scene unfold. Let the merging and splitting light spots you saw in the above gif be pairs of particles and anti particles, let the shape of ...
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Functional freedom in cosmology. Why does it matter?

Roger Penrose's book "Fashion, Faith and Fantasy in the New Physics of the Universe" is a fascinating (and challenging) read, but while I understand his mathematical points about how additional ...
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Candidates for cosmic strings

What are the observed candidates for cosmic strings (or other defects), that are not ruled out? With references if possible.
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Are trans-Planckian field values a problem for the relaxion?

The relaxion is a new model-building gadget that solves the hierarchy problem by allowing the Higgs mass term to dynamically relax to zero. To describe it as simply as possible, leaving a lot out: we ...
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77 views

Why should LTB dust be comoving?

In many research papers about inhomogeneous cosmology, one often considers spherically symmetric (LTB) spacetimes where in the co-ordinate frame $(t,r,\theta,\varphi)$ wherein the metric assumes the ...
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165 views

Can we use pressure analogy to understand inflation?

Classically, the expansion of a gas in a container requires the gas to apply outward pressure to the walls of the container. Is it also true for the Universe during inflation? How did the inflation ...
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Metric vs coframe energy-momentum tensor in metric-affine gravity

Conventions Latin indices represent components in the anholonomic frame and greek ones are for coordinate components. I will call $R_{\mu \nu} := R_{\mu \rho \nu}{}^{\rho}$ (Ricci tensor) and $\bar{R}...
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163 views

How much uncertainty has the relic graviton background?

In the paper [1], it is mentioned that inflation predicts that a relic graviton background is about 0.9 K (cf. cosmic neutrino background, 1.945 K, and cosmic microwave background, 2.73 K). How much ...
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53 views

Cosmological constant phase transition?

I recently watched at a talk by Cumrum Vafa in which he stated that the cosmological constant allows us to define a time-scale $T_\Lambda=1/\sqrt{E_\Lambda}$. The time scale of this time is about 10¹¹ ...
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Hawking radiatiom from our cosmic horizon

Does our Hubble radius/cosmic horizon emit Hawking radiation just like an empty de Sitter space would emit from its horizon?
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334 views

Adiabatic fluctuations

In Baumann's cosmology lecture,http://www.damtp.cam.ac.uk/user/db275/Cosmology/Chapter4.pdf , chapter 4, page 89, he defines adiabatic perturbation as: Adiabatic perturbations have the property ...
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Dynamics of the Inflaton field

The perturbations of the inflaton field $\phi$ are described by the following equation: $$\ddot\phi(x,t)+3H(t) \dot\phi(x,t)+\nabla^{2}\phi(x,t)/a(t)=-m^2\phi$$ where $a$ is the scale factor of ...
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303 views

Does any spacetime admit a global foliation in spacelike hypersurfaces?

In the comments of this question the following new questions came up: in general relativity, local coordinates can be found around any point, that single out a time coordinate and a three dimensional ...
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282 views

How to plot spectral indices $n_s$ and $r$ from Planck data?

From a certain inflationary model I have obtained the theoretical values for the spectral indices $n_s$ and $r$ as functions of $N$, the e-folds number of inflation. At this point it would be ...
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446 views

Mechanism for collapse of iron stars into black holes via quantum tunnelling

In the wikipedia page "Future of an expanding universe" it refers to the scenario of a future without proton decay. The page talks about how processes would lead to stellar-mass cold spheres of iron, ...
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Where does this data comparing the CMB with Eddington's 'temperature of space' come from?

Eddington estimated the temperature of space assuming that star light would be scattered by interstellar dust. He came pretty close to the temperature of the CMB. http://www.astro.ucla.edu/~wright/...
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342 views

How to understand that Inflation generate almost scale-invariant perturbations?

Since during the inflation, the Hubble rate is approximately constant, the perturbations generated by inflation are almost scale-invariant. How to understand that?
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Basic question on Cosmic Strings

What is the typical size of Cosmic String Loops? If a loop is formed today, what are the restrictions on its size? I would like to know if there is a distribution of cosmic strings depending on its ...
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86 views

What would we observe as background on the sky if the big bang had never occurred?

The data we've received so far from satellites such as WMAP paints a near uniform distribution in intensity of the background radiation that we take as evidence that our universe had an origin, and ...
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135 views

Is the fluctuation pattern of the Cosmic Microwave Background (CMB) identical in every location of the universe?

I know that the Cosmic Microwave Background (CMB) is the leftover radiation from the "surface of last scattering". Let's say an alien civilisation lives on andromeda or further away. Would they see ...
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141 views

How is translational symmetry related to Fourier decomposition?

The book (The Cosmic Microwave Background By Ruth Durrer) about cosmological perturbations says that because of translational symmetry of the background at a constant time, we can decompose our ...
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515 views

Does the entropy of the universe change as expansion exceeds the speed of light?

The potential encoded information in a photon that is at the edge of the observable universe would seem to be lost as the universe expands. Does that loss of information contribute to the overall ...
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1answer
356 views

Cosmological fluctuations: what is gaussian?

When we are speaking about gaussianity and non-gaussianity in a cosmological context, what is gaussian or non-gaussian in the CMB? What would a non gaussian CMB look like compared to a gaussian one? ...
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195 views

Power spectrum calculation for simple inflaton potential

I am trying to obtain an expression for the power spectrum $\Delta (k)$ of cosmological perturbations, see for example eqn (148) from the following TASI lectures. Right now, I would like to use the ...
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70 views

Thomson scattering on the electrons does not produce any circular polarization?

All references on CMB polarization has this statement as if it is a trivial fact. But I have to admit that I completely don't understand what this sentence is telling us.
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What role did the Higgs boson play in the Big Bang?

Scientists say that inflation stopped when a very peculiar energy was mobilized, did Higgs boson play any role in this? What is it's relation to the Big Bang? Some physicists, such as Michio Kaku, ...
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When Did Structures Begin to Orbit in the Universe?

In Big Bang Cosmology, I am familiar with the radiation era, and the moment of last scattering, but I am just not familiar with the physics of orbits, or of the “falling outward,“ if you will, that ...
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70 views

Active-sterile mixing for KeVins

People sometimes talk about KeV mass sterile neutrinos as a warm dark matter candidate. I think they call them KeVins (horrible name btw). Now, In order for it to be a good dark matter candidate it ...
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151 views

Avoiding Pseudo-tensors when addressing global conservation of energy in GR

Discussions about global conservation of energy in GR often invoke the use of the stress-energy-momentum pseudo-tensor to offer up a sort of generalization of the concept of energy defined in a way ...
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122 views

Should a radiation-filled Universe be scale invariant?

Imagine a spatially flat Universe, without cosmological constant, filled only with EM radiation. As Maxwell's equations without charges or currents are scale invariant then should this Universe be ...
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129 views

Why can't Wetterich's “Universe without expansion” be tested compared to standard cosmological redshift?

I have read on several sites that this is a theory that cannot be tested. Why is this a theory that can not be tested? It seems to me that if the mass of particles is continually increasing, then the ...
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90 views

Are there any stable (not metastable) string theory compactifications with a positive cosmological constant?

Previously, on https://physics.stackexchange.com/q/86509/33806, I asked how we can be saved from false vacuum decay assuming we are living in a false vacuum. What if we are living in a stable vacuum ...
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140 views

Were fermions ever massless?

In a discussion of the Standard model and Higgs mechanism it was claimed that accordingly: "During an early phase of the cosmos all fermions were massless." I wonder whether this claim can be ...
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328 views

Never ending inflation

There are a lot of different models of inflation in cosmology, with plenty of different features and issues. I read that the problem with some models is that they introduce a never-ending inflation, ...
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151 views

what is the radial extent of the last scattering “shell”?

At CMB recombination (z=1090), what is the radial extent of the last scattering "shell"? a) Delta(z) = .... b) Delta(comoving angular distance)= ....Mpc The WMAP first-year parameters give Delta(z) =...
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110 views

Closed linear cosmology implies $\frac {GM} R = c^2$?

I have a question about a linear FRW cosmology with $k=+1$. Assuming zero cosmological constant the first Friedmann equation can be written: $$\left(\frac{\dot R}{R}\right)^2 + \frac{kc^2}{R^2}=\...
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Evolution of black holes ensemble

Background: I’ve read many times that arrow of time can be explained from extremely low entropy of the Universe at the Big Bang (http://preposterousuniverse.com/eternitytohere/faq.html). The argument ...
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246 views

Questions about classical and quantum scale invariance

This is kind of a continuation of this and this previous questions. Say one has a free "classical" field theory which is scale invariant and one develops a perturbative classical solution for an ...
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108 views

When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?

It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
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In the αβγ cosmology paper, how do the authors assume the integral of density over time in the early universe?

In the famous Alpher-Bethe-Gamow paper, the authors say: "it is necessary to assume the integral of $\sigma_n dt$ during the building-up period is equal to $5 \times 10^4 \frac{\text{g sec}}{\text{cm}^...
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How is the poincare conjecture(and perelman proof) helpful in studying the properties of the universe?

Can someone tell me how the poincare's famous conjecture or its proof by perelmen can be helpful in deciding some properties like the shape of the universe?
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111 views

Why is the relation between luminosity distance $d_L$ and comoving distance $\chi$ $d_L=\chi/a$?

The textbook (Scott Dodelson, Modern Cosmology, Section 2.2 Distance, Page 35-36) states as follows: Another way of inferring distances in astronomy is to measure the flux from an object of known ...
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46 views

Does the Bekenstein entropy bound present problems for unitarity in cosmological models that invovle a collapse or bounce?

If we expect the Bekenstein bound, or something like it, to hold in a collapsing universe, will that not eventually force us to accept some loss of information, or is there something I'm missing?
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148 views

How do we know the universe is expanding and wasn't already infinite?

From what I know, the decision that the universe was expanding came from the discovery of the redshift and that other celestial objects, such as galaxies, were moving away from us; the farther they ...