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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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 ...
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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 ...
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Equation of motion for cyclic model of the universe
I recently started to study the cyclic universe. I came across this article [1]. My question is about the action used for describing the cyclic model:
$$S = \int d^{4}x\sqrt{-g}(\frac{1}{16\pi G}R-\...
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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 ...
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The emergence of space-time from entangled states
I recently read an article by Yasunori Nomura (https://arxiv.org/abs/1711.05263), in which he says that space-time is an emerging phenomenon. At the same time, space-time disappears when the ...
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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 ...
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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 ...
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Why is the Ricci tensor diagonal for isotropic spacetime?
I'm reading Zee's Einstein Gravity in a Nutshell and while calculating the Ricci tensor for FRW spacetime he claims that because the spacelike slices of constant $t$ are rotationally invariant, the ...
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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) ...
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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 ...
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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, ...
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How many apparent horizons could the universe have?
I was reading a paper written by Nobel laureate George Smoot, Go with the Flow, Average Holographic Universe, which assumes the holographic principle as true and conjectures that our universe would be ...
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Is GR the only theory in physics which cares about absolute energy?
In my QFT course, they justify dropping the vacuum energy as 'physics only cares about relative energies except for GR in the stress-energy tensor'.
Is this strictly true?
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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} = ...
user87745
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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, ...
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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 ...
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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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What is torsion physically in the Einstein-Cartan theory?
In Einsteins theory of gravity the metric gives a unique torsion free connection called the Levi-Civita connection.
In the Einstein-Cartan theory we allow any connection compatible with the metric ...
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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?
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Adiabatic fluctuations
In Baumann's cosmology lecture, chapter 4, page 89, he defines adiabatic perturbation as:
Adiabatic perturbations have the property that the local state of matter (determined, for example, by the ...
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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 ...
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$f_{NL}$ 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" ...
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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?
...
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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 ...
user84158
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Wheeler-deWitt equation in words?
Looking at the Wheeler-deWitt equation. My attempt to describe it in words is this:
"For each 3d-manifold given by metric tensor field $\gamma$, associate a complex number $\Psi$. The Wheeler-de-Witt ...
user84158
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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 the following:
Another way of inferring distances in astronomy is to measure the flux from
an object of known ...
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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 ...
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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 ...
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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 ...
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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 ...
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Breakdown of quantum mechanical observables in cosmology
In several different contexts, I've heard the claim that quantum gravity in an accelerating universe messes with our ability to define precise quantum observables.* One version of the argument goes ...
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What is the best way to describe a classical field in quantum field theory (coherent state)?
In quantum field theory, we have the following expansion on a scalar field (I follow the convention of Schwarz's book)
$$\phi(\vec{x},t)=\int d^3 p \frac{a_p exp(-ip_\mu x^\mu)+a_p^{\dagger}exp(ip_\mu ...
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Does dark energy violate causality?
Assume a perfect fluid with $p = w \rho$, approximately flat space-time and the existence of a hidrostatic equilibrium solution $\rho_0$ for $\rho$. With the definition of the speed of sound $c_s^2 = \...
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What kind of matter's energy density scales as the inverse of the scale factor
We know that radiation energy density scales as $a^{-4}$ with EoS parameter ($w=\frac{1}{3}$), matter as $a^{-3}$ with ($w=0$), curvature as $a^{-2}$ with ($w=-\frac{1}{3}$).
Then which kind of matter ...
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What is the current status or resolution of Greisen–Zatsepin–Kuzmin (GZK) cosmic-ray paradox?
The Greisen–Zatsepin–Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium to our galaxy.
A number of ...
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Bargmann–Wigner equations in NP formalism
Bargmann-Wigner equations describe free particles of arbitrary spin $j$, namely
$$(-\gamma^{\mu}\partial_{\mu}+m)_{\alpha_r \alpha_{r’}}\Psi_{\alpha_1,..,\alpha_{r’},...,\alpha_{2j}}=0$$
where we have ...
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Understanding the CMB power spectrum
We are able to expand the CMB temperature anisotropy map in terms of spherical harmonics.
My current understanding is that each l (on the x-axis of the power spectrum below) corresponds to a unique ...
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Understanding Verlinde: How to get from emergent gravity to MOND
Verlinde ( https://arxiv.org/abs/1611.02269 ) tries to deduce MOND from emergent gravity. Can you help?
Emergent or entropic gravity goes back to Jacobson. He starts with
the entropy-area connection $...
user85598
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Can one quantify the total luminosity of the cosmological horizon?
If I integrate all the power (radiation, matter particles, neutrinos) radiated from the cosmological horizon into the universe, what number do I get?
Is it true that the integral power/luminosity is ...
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What is the entropy of the cosmological horizon?
The Bekenstein-Hawking entropy for a black hole horizon is
$S_{BH}=\frac{c^3 A}{4Gℏ}$.
Is there a similar expression for the cosmological horizon?
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Robertson-Walker metric and cosmic homogeneity
The Robertson-Walker metric is of the form
$$\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 $a^2(t)$ ...
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Quantum fluctuations producing CMB fluctuations also cause inflation to be eternal?
It is believed that quantum fluctuations in the inflaton field caused inflation to end at different times in different places, which led to CMB fluctuations (1 part in 100,000).
Eternal inflation ...
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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 ...
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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 ...
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Entropy and the curvature of the Universe
Foreword
What I know (and please correct me if I'm stating malarkey):
The entropy of the Universe (its description) is contained in Weyl tensor. Einstein's field equations don't directly relate the ...
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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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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 ...
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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.
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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 ...
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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 ...
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