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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14
votes
2answers
826 views

Is the Boltzmann brain problem really clearly established as a problem?

Starting from the assumption of a cosmology in which Boltzmann brains dominate over evolved ones, it is not immediately obvious to me that there is a real problem, since only Boltzmann brains ...
11
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0answers
366 views

Equation of motion for cyclic model of the universe

I recently started to study about cyclic universe. I came across this article [1]. My question is about the action that used for describing the cyclic model: $$S = \int d^{4}x\sqrt{-g}(\frac{1}{16\pi ...
11
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1answer
312 views

Fate of largest scale structures?

In $\Lambda\mathrm{CDM}$, structures form "bottom up" with larger structures forming later. Structures are generally speaking supported by the velocity dispersion of their constituent objects (e.g. ...
10
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0answers
343 views

Time diffeomorphisms breaking in inflation

I am currently working on the topic of inflation. It seems that at the stage of inflation, the universe can be described as a de Sitter space. In such a space, all spacetime diffeomorphisms are ...
8
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0answers
176 views

Are there any experimental bounds on the ratio of neutrinos to antineutrinos in the universe?

In the Standard Model, both baryon number and lepton number are conserved quantities (excluding the theoretical possibility of sphaleron processes which are exceeding rare, at least at non-"near in ...
8
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0answers
253 views

The color of deep space background of an arbitrary universe

While writing my notes on cosmology in general relativity and the Olber's paradox, I was wondering about the color of the deep background of space. Our universe is mostly black because light didn't ...
8
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0answers
105 views

How do inflationary models predict the generation of gravitational waves during the inflationary period?

Recent results from the BICEP2 experiment have produced a lot of talk about the primordial gravitational waves produced during the inflationary period. I would like to have some explanation about how ...
8
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0answers
166 views

Does quark color contribute to “spin degeneracy” for QGP calculations?

Like the title say, does quark color matter in counting contributions in a early universe plasma (QGP), as when adding up the total plasma energy density, or is it just spin? The book I have (Pathria) ...
8
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0answers
129 views

Does the Standard Model plasma develop a spontaneous magnetisation at finite temperature?

Reference: arXiv:1204.3604v1 [hep-ph] Long-range magnetic fields in the ground state of the Standard Model plasma. Alexey Boyarsky, Oleg Ruchayskiy, Mikhail Shaposhnikov. The authors of this paper ...
8
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0answers
211 views

Implications of Unruh-inertia to theories of gravity

If it turns out to be true that the galaxy rotation curves can be explained away by Unruh modes that become greater than the Hubble scale at accelerations around $10^{-10} m/s^2$ as proposed in here, ...
7
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424 views

750 GeV diphoton resonance: KK graviton?

As everybody of you may know at LHC they found this probable resonance (https://cds.cern.ch/record/2114808, https://cds.cern.ch/record/2114853?ln=en). It may be a scalar or a KK graviton mode. Now, ...
6
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0answers
96 views

Is it a coincidence that Big Bang nucleosynthesis lasted about one free neutron half-life?

The free neutron half-life is about 10 minutes. Big Bang nucleosynthesis, which mostly involves protons and neutrons interacting, lasted about 20 minutes. Question: Is it a coincidence that these ...
6
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0answers
114 views

Meaning of the simplest potential of quintessence models. Fields in denominator?

I am reading Sec. 1.12 of the Cosmology book by Weinberg. In this section he explains the very simple model of quintessence which attempts to provide a dynamical explanation of the smallness of the ...
6
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127 views

Rotational axes of galaxies

The disc shape is a preferred form for galaxies due to angular momentum, and the axis of rotation indicates a locally preferred orientation for the angular momentum vector. Why do galaxies in Hubble ...
6
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1answer
107 views

What are the cosmological ramifications if we probabilise and continuify the order of differentiation in $F=\frac{d(mv)}{dt}$?

Newton's second law of motion states that $F=\frac{d(mv)}{dt}$. This is a first-order differential equation, in which the order of differentiation of momentum is 1. So we can write it $F=\frac{d^k(mv)}...
6
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0answers
74 views

In a perturbative FRW cosmology, why do constant-density hypersurfaces define a good gauge?

It appears to be common in the discussion of perturbative FRW cosmologies to choose a gauge using hypersurfaces for special values of some quantity, like surfaces of constant density $\rho$, constant ...
6
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0answers
433 views

Poincare recurrence and the multiverse

In this paper Susskind claims that a stable de Sitter universe is problematic (among other things) due to the existence of Poincare recurrence, which happen because of finite entropy. I disagree that ...
6
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0answers
117 views

$f_{NL}$ non-Gaussianity in cosmology

In the context of cosmology, what is meant by "..arbitrary quadratic non-Gaussianity i.e non-Gaussianity that is described to leading order by a 3-point function.."? (.."quadratic non-Gaussianity" ...
6
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0answers
87 views

Status of large-scale structure formation within cosmology today

Since the CMB results of the past decade, would it be fair to say that the consensus among cosmologists is that cosmic strings are no longer considered as a (major) source for density perturbations? ...
6
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3answers
132 views

How do you explain cosmological red shifting in terms of gravitons?

We know that the photons from the big bang are continually being red shifted and losing more and more energy. In terms of the graviton view, how would you explain that? Where is the energy going? Are ...
5
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1answer
78 views

Robertson-Walker metric and cosmic homogeneity

The Robertson-Walker metric is of the form $$\tag{1} ds^2 = dt^2 - a(t)^2 \Big(\frac{dr^2}{1 - kr^2} + r^2 d\theta^2 + r^2 \sin^2\theta \, d\phi^2 \Big).$$ My question is related to the $a^2(t)$ ...
5
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0answers
130 views

Trying to reproduce curves with angle of CMB anisotropies as a function of distance and curvature parameter

I am looking for a way to get, by a simple numerical computation, the 3 curves on the following figure: For this, I don't know what considering as abcissa (comoving distance ?, i.e $$D_{comoving} = ...
5
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0answers
49 views

Sterile (4th) neutrino and cosmological bound

Recent results by MicroBoone seems to give support to old LSND experiment in favor of a fourth neutrino (sterile). How does it fit with the current cosmological bound for neutrinos?
5
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0answers
185 views

What are the applications of hyperbolic $3$-manifold theory to cosmology?

I am a pure mathematician specialized in hyperbolic $3$-manifold topology. That has been an incredibly active field of research in the past few decades due to the seminal work of Thurston, as many of ...
5
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0answers
157 views

Spatial dimension of galaxy filaments

Galaxy filaments are "amongst the largest known cosmic structures in the Universe. They [...] form the boundaries between large voids in the Universe." As their name suggests, filaments are ...
5
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0answers
144 views

At what expansion rate (H) would virtual particles be ripped apart into real particles, and what might be the density (temperature) of such an event?

Is it even possible for a virtual particle-antiparticle pair to be torn apart by cosmological expansion? Virtual particle-antiparticle pairs are ripped apart near black hole event horizons, creating ...
4
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0answers
61 views

Can one quantify the total luminosity of the cosmological horizon?

If I integrate all the power (radiation, matter particles, neutrinos) radiated from the cosmological horizon into the universe, what number do I get? Is it true that the integral power/luminosity is ...
4
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0answers
29 views

Energy conservation in cosmological phase transitions

Let us consider a cosmic phase transition, in which fermions $\psi_f$ condense and the vacuum expectation value $|\langle \bar{\psi}_f \psi_f\rangle |$ of the resulting fermion-bilinear field gives ...
4
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1answer
148 views

How are the dark matter (DM) thermalization rates derived in singlet DM model?

This well-cited paper talks about a minimal renormalizable extension to the Standard Model (SM) to incorporate particle dark matter (DM) into it by adding a real scalar field $S$ which (unlike the ...
4
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0answers
169 views

Why is a theory of quantum gravity expected to solve the BigBang singularity problem?

This a follow-up question related to this. A successful theory of Quantum gravity is expected to solve the BigBang singularity problem. Why (and how) is the quantization of gravity supposed to solve ...
4
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0answers
82 views

How is matter density derived from CMB?

As the question says. I am looking for a technical but brief overview of how $\Omega_m$ can be derived from the CMB data to high accuracy for example as in the Planck papers. Thank you
4
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0answers
75 views

Are there any non-stationary asymptotically flat and geodesic complete solutions to the vacuum Einsteins's field equations

I read the book The global nonlinear stability of the minkowski space by Christodoulou, D., and Klainerman, S. In the Chapter I, Page 10 it says: “However, it remains questionable whether there ...
4
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1answer
218 views

Is a universe without massive particles scale-invariant?

In a popular talk by Roger Penrose about spacetime geometry, when introducing his conformal cyclic cosmology starting at 17:15 I think he says that as soon as there are no massive particles left in ...
4
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0answers
179 views

Distribution of wavelengths of monochromatic light sources in expanding FLRW universe

I'm looking for the distribution of all wavelengths (or frequencies) of light that a stationary observer would receive at his location (at $r = 0$ and time $t_0$), from light sources emitting a single ...
4
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0answers
56 views

LFRW Universe - equivalence principle and Hubble flow

After a suitable change of coordinates, the metric for the (flat) FLRW universe becomes $$ ds^2 = (1 + 2 \Phi(\vec{x},t))dt^2 - (1-2\Psi(\vec{x},t))d\vec{x}^2, $$ where $$ \Phi(\vec{x},t) = -\frac{1}...
4
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0answers
76 views

Which quantum gravity theories do predict a growing number of space dimensions?

In this 2013 Nature article there is a short presentation of the causal dynamical triangulations approach to quantum gravity, where it is said Intriguingly, the simulations also hint that soon ...
4
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0answers
86 views

Why do some stars actually produce “Gamma ray bursts”?

I looked it up but I haven't found any explanation as to why some stars produce them, I understand that collapsing and merging stars produce them, but my question is why is the energy concentrated in ...
4
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0answers
170 views

How to understand particle decoupling in the early universe?

We often say that when the rate of some interactions, say the beta decay and electron capture, are slower that the rate of universe's expansion, then the corresponding particles, say neutrinos, ...
4
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0answers
301 views

What is the theoretical geometry of bubble universes?

My research has led me to look into the idea of bubble universes which I don't know very much about. The first thing that I am looking for is understanding or visualising how could many bubbles co-...
4
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0answers
70 views

squeezed radiation astronomy

Squeezed electromagnetic vacuum does have a renormalized energy density smaller than the vacuum. So it makes it in my opinion a inconspicuous candidate for a dark energy carrier. Are there ...
4
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0answers
153 views

Temperature of the CMB when the Earth formed and the faint young Sun paradox

The cosmic microwave background (CMB) has a modern temperature of about 2.7 K. At the time of the origin of the CMB, about 13.6 billion years ago, it had a temperature of about 3000 K. ...
4
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2answers
757 views

Attractive gravity has negative energy, what about repulsive gravity in the inflation phase?

Alan Guth's cosmic inflation theory posits that about 10^-38 second before the Big Bang which led to our particular universe, a tiny patch of space doubled in size more than 100 times, from sub-atomic ...
4
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2answers
84 views

Physical differences between our universe and strongly negatively curved one

Interestingly, the idea of a negatively curved universe isn't entirely science fiction. In particular if decrease a single physical constant (known as the density parameter) to be less than one, the ...
4
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1answer
230 views

Why is Newtonian cosmology correct for curved space?

The Newtonian model of an expanding Universe gives Friedmann's equation exactly for non-zero spatial curvature $k$ (see http://hyperphysics.phy-astr.gsu.edu/hbase/astro/expuni.html). Instead of using ...
3
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0answers
69 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 - \...
3
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0answers
51 views

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 ...
3
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0answers
56 views

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?
3
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0answers
52 views

What do marginalised or marginalised error mean? Contours and posterior

I am curently working on Forecast in cosmology and I didn't grasp very well different details. Forecast allows, wiht Fisher's formalism, to compute constraints on cosmological parameters. I have 2 ...
3
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0answers
73 views

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 ...
3
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0answers
143 views

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{...