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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Is there any evidence against the Cosmological Principle?

I have been slowly getting more into math, but haven't gone into differential geometry or anything like that yet, so this question might be basic. Trying to get a deeper understanding of the ...
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21 views

Contribution of gravitational waves to a maximum-entropy big bang

Our universe did not have a maximum-entropy big bang. If it did, then by the second law of thermodynamics we wouldn't see the universe today in a non-equilibrium state. I've seen this explained by ...
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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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35 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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71 views

Can I detect my own rotation by looking at the cosmic background radiation?

This is a simple question. Nevertheless it might be interesting to others. If I move with constant speed across the cosmic background radiation, I see a "dipole": a blue shift of the CBR looking ...
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55 views

What caused the first measurement to the universe?

I've been bumbling with a question recently. People postulate that with the give model of big ban, particles forms and breaks and eventually became what we saw as of today. However, if we put ...
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47 views

Exact solution for (simple) Reheating Boltzmann equations?

Let's say we have the Universe which is exiting Inflation, with the inflaton field $\phi$ decaying into relativistic particles (radiation $R$). Without any other assumption, the equations describing ...
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43 views

Increasing the view of observable universe

I have read that the maximum observable universe is about 92 billion light years in diameter by measuring the microwave background waves emitted after some few million years ago from the big bang and ...
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92 views

Does $G$ effectively change in an expanding metric?

Assume that we have a flat FRW metric expressed in conformal time $\eta$ so that the line element is $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where $a=1$ at the present time $\eta=0$ and the ...
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187 views

Is Gerard 't Hooft's Cellular Automaton Interpretation of Quantum Mechanics background independent?

In Gerard 't Hooft in his Cellular Automata Interpretation of Quantum Mechanics (https://www.researchgate.net/publication/...
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73 views

$c$ slowing down rather than universe expanding?

Can the Friedmann equations be reformulated with taking the universe as being rigid and instead have the vacuum-speed of light in the void decrease over time? So, not changing any equations, just ...
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2answers
175 views

How do Graviton-based theories of gravity explain the expansion of the universe?

In General Relativity, the expansion of the universe is modeled using the Friedmann–Lemaître–Robertson–Walker metric, and the expansion itself is a metric expansion by which the scale of space itself ...
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45 views

Horizon entry, Meszaros suppression and start of perturbation growth

I thought that the onset of perturbation growth was determined by horizon entry of the perturbation (because there won't be a gravitational collapse of an over dense region not causally connected to ...
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18 views

How to show no boundary wave function satisifies Wheeler-de-Witt equation?

I've seen various places it says the no boundary wave function (Hartle Hawking state) satisfies the Wheeler-de-Witt equation. But I don't think I've seen a proof. Is there a proof of this somewhere? ...
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1answer
33 views

Scale factor at surface of last scattering

Is there an exact theoretical scale factor associated with the CMB? More specifically, since the CMB is associated with a temperature of around 3000K (as I understand), is there an exact thermodynamic ...
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65 views

How is the entropy of the universe defined?

My understanding is that to define the entropy of a system what you have to do is as follows: Define the boundaries of your system. Define a set of "microstates" of the system. Define a partition of ...
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67 views

How could Proca equation be written in NP formalism

I am trying to do some calculation of fields in curved background. Is there anyone know the complete form of Proca equation $$\nabla_{\mu}F^{\mu\nu}+m^{2}A^{\nu}=0$$and$$F_{\mu\nu}=\nabla_{\mu}A_{\nu}...
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37 views

Particle identity in conformal cyclic cosmology

I have a few questions about Penrose's conformal cyclic cosmology from a lay perspective. What happens to each particle? The picture of the far future of an aeon in CCC is an exponentially expanding ...
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35 views

Long-term fate of hydrogen atoms in a rogue planet

Imagine a rogue planet drifting in intergalactic space. For the sake of specificity, suppose that it is the size of Jupiter and composed of pure hydrogen. What will happen to it over the next $10^{...
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51 views

Would Bekenstein bound disappear in some holographic models?

In Holographic principle models there's a limit to the information that the system can store known as the "Bekenstein bound". In physics, the Bekenstein bound is an upper limit on the entropy S, or ...
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46 views

Can I decompose a compact spacetime into conformal and $SL(2,C)$ transformations?

I've been thinking about compact spactimes lately. My understanding of the Yamabe problem is that one can always conformally transform a (compact) spacetime to one of constant scalar curvature, ...
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90 views

Meaning of time and distance in general relativity?

I have just started studying cosmology and I have realised that certain statements (that made perfect sense before I studied GR) no longer make sense: -The universe is 13.2 billion years old. What ...
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76 views

Antiparticles, CPT and leptogenesis

When people are being careful they'll tell you that antiparticles are the CPT conjugates of particles. You can't say that they are C conjugates or CP because these, while they do reverse the charge, ...
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84 views

Are deductions from CMB much less accurate than claimed, owing to approximations in the FLRW model?

My question concerns "timescape cosmology" and measurements of dark energy based on CMB radiation. The background to the question is Lawrence H. Dam, Asta Heinesen, David L. Wiltshire, "Apparent ...
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186 views

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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66 views

Is information conserved in an expanding universe?

From the classical and quantum mechanical viewpoint, is information conserved in an expanding universe or not?
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65 views

Could the horizon problem be explained by a finite universe instead of inflation?

Suppose the universe is finite (either closed or open with a non-trivial topology), any point could be within the horizon of another.
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106 views

Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...
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42 views

Possible origins of the Antlia 2 galaxy?

At $32.3\,{\rm mag}\,{\rm arcsec}^{-2}$, the Antlia II dwarf galaxy is the most diffuse galaxy ever observed in terms of surface brightness. It was only discovered by pre-selecting stars based on ...
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36 views

How does the redshift - distance graph show the rate of expansion of the universe at every moment in time?

By plotting the graph of redshift against the distance of the object from earth, we are able to obtain a best-fit curve showing the relationship of redshit against the distance. How does this ...
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1answer
118 views

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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99 views

What was the informational content of the universe at the Planck time?

If the energy of $1 \, \mathrm{bit}$ of information is $k_{\small{\text{B}}}T \ln{2}$, then the energy of that bit increases with the temperature of the system. When I try to calculate how much energy ...
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98 views

Can a scalar field also has positive or zero pressure?

In inflation theory, the inflaton field produce negative pressure to drive inflation. But now I’m going to ask about scalar fields in general, not the inflaton field. Can a scalar field also has ...
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109 views

Does Higgs field exert pressure which contributes to the stress-energy tensor causing gravity?

First of all, I have never studied quantum field theory. Since Higgs field is a scalar field, does Higgs field also exert positive/negative pressure which contributes to the stress-energy tensor ...
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149 views

How could the universe's expansion “remove the mean gravitational potential”?

I'm citing W. C. Saslaw's The Distribution of the Galaxies: Gravitational Clustering in Cosmology, chapter 25, where he adresses (what seems to be a Newtonian approximation of) the thermodynamic ...
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103 views

Which specific smooth structure are we using in general relativity?

In this lecture by Fredric Schuller it is said that in the case of a non compact four dimensional manifold there is a non countable infinity of differentiable or smooth manifolds that are NOT ...
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40 views

What sense is there in entropy arguments when the system can't be known to be ergodic?

What sense does "entropy" make in non-equilibrium dynamics? What is its definition? How is the "entropy of the early universe" calculated, for example? And how can the "entropy of the universe" be ...
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1answer
53 views

Hadron contribution to effective degrees of freedom in early Universe

In transition from quark-gluon plasma to hadron gas in the early Universe, the value of the effective degrees of freedom $g_{\star}$ decreases abruptly. This seems to me like a sort of decoupling: ...
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44 views

How to use Belinsky-Zakharov transformation

I know it might be trivial. When using BZ transformation [1] to generate soliton solutions of Einstein’s field equations, one need a seed solution $g_{0}$ which gives $A_{0}$ and $B_{0}$. Taking them ...
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1answer
80 views

A question from cosmological perturbation theory

We consider the following scalar perturbation on the FRW metric $$ds^2=-(1+2\Phi)dt^2+2a(\partial_iB)dx^idt+a^2[(1-2\Psi)\delta_{ij}+2\partial_{ij}E]dx^idx^j,$$ where $\Phi$, $B$, $\Psi$ and $E$ are ...
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48 views

Isolated spacetime defects

I'm wondering if the possibility of existence of stable spacetime topological defects has ever been explored in the literature. As an example one can consider smth like propagation in time of a handle ...
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106 views

Parity violation

I've been reading on wikipedia about parity violation in the weak force and the conclusion is: This implies that parity is not a symmetry of our universe, unless a hidden mirror sector exists in ...
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145 views

Energy density of primordial gravitational waves- two different results for the same quantity?

I want to calculate the energy density of primordial gravitational waves, $Ω_{gw}$ in a specific time. The latter is in general given by the relation : $Ω_{gw}(k,τ)= \frac{1}{\rho_{c}(τ)} \frac{d\rho_{...
2
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1answer
170 views

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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151 views

Inflation and dark energy have constant energy density - energy is created out of nothing without any source or mechanism?

I can understand that a photon in an expanding universe can be redshifted and lose its energy by its wavelength being stretched from space expansion. But in the case of inflation and dark energy, it ...
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155 views

Raychaudhuri and Friedmann

Is it possible to derive the Friedmann equations from Raychaudhuri's equation, assuming only the Weak Energy Condition? The standard method for the derivation of Friedmann's equations is, if I am not ...
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1answer
155 views

Hints of pre Big-Bang physics and how to detect them?

The Big Bang model is the current large-scale accepted and tested theory. However, several new physics models or theories propose that there could be a previous Universe. To what extent can be test ...
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1answer
81 views

Hubble bubble and the shell theorem?

According to the Hubble bubble theory a local void could explain deviations of the Hubble constant for measurements in close vicinity to out local group compared to the global Hubble constant and ...
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138 views

How to derive number of nucleons in Universe?

I understand that there are roughly $N=10^{79}$ nucleons in the visible Universe. This number comes from adding up the nucleons of $100$ billion stars in $100$ billion galaxies in the visible Universe ...
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28 views

Splitting of Forces at Low Temperatures?

Since the electroweak force split from the strong force during the electroweak epoch then the electroweak force split into the electromagnetic and weak force during the quark epoch, is this an ...