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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43 views

Entropy increase after the Big Bang?

I'm trying to understand the increase of entropy just after the big bang. The way i see it going from a "soup" of particles to organized and ordered sytems like solar system, galaxies and clouster of ...
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65 views

Early Universe Flatness Problem

My question is about the curvature of spacetime in the early universe (Plank era) https://en.wikipedia.org/wiki/Flatness_problem Based on data from Wilkinson Microwave Anisotropy Probe, in order to ...
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Newton constant in scalar-tensor Brans-Dicke theories

I'm reading Valerio Faraoni's Cosmology in scalar tensor theories where he says From the action (1.25) or from the field equations (1.29), one reads the gravitational coupling of Brans-Dicke theory ...
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118 views

If time is relative, then how is the age of the universe calculated? [duplicate]

I am very new to the world of relativity, so the answer may be trivial, but to me measuring the age of the universe seems an impossibility due to there being no fixed frame of reference from which to ...
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37 views

Density parameters in cosmology and flatness of the universe

I was reading about the density parameter $\Omega=\frac{\rho}{\rho_{c}}$ in cosmology. I find that when cosmologists calculate the value of the density parameters for $\Omega_{m}$, $\Omega_{r}$ and $...
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14 views

Observers frame of reference for history / chronolgy of very early universe [duplicate]

What is the observers frame of reference when discussing t=0 the instant of the big bang, or the Plank epoch, or the Grand Unification Epoch, or the Inflation, or the Electroweak Epoch?
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45 views

Does the shape of the Higgs mexican hat depend on the temperature of the universe?

I am reading this paper and i wonder if it is mainstream. Does the VeV of the Higgs field evolve with time? What about the mass given to the fermions?
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48 views

Might local gaps in spacetime suffice to leave its infinity and eternality incomplete?

At https://arxiv.org/pdf/1403.1599.pdf, Vilenkin, in 2014, used the Borde-Guth-Vilenkin theorem's premise that the "bouncing" local universes of an expanding or contracting multiverse would ...
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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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42 views

What caused quadrupole anisotropy at the time of recombination?

The polarization of CMB requires a quadrupole anisotropy of the incident radiation field acting on the plasma. How did such a special pattern (i.e. hot and cold in two orthogonal directions) generate ...
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Cosmological lookback time equation and LCDM fits

When performing the lookback time integral and fiting the know Omega parameters, has someone tried to change not the DM but the DE for other w and see if we can find a best data fit?
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What does. DM- nu, sigma = 10^-33 m_DM GeV^-1 cm^2. mean?

I tried posting a screenshot above...... On Ethan Siegel's blog over at Forbes, he has repeatedly posted a graphic showing possible 'new physics' that could explain why the Planck satellite ...
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Uniform Comoving Density + Luminosity Test

Following this question here Cosmology - how to do the $V/V_{max}$ test? despite the ubiquity of this test in a lot of literature, I cannot find a proof anywhere that $\langle V/V_{max}\rangle = 1/2$ ...
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108 views

How to assign a temperature and a time to cosmic events in the universe?

Sorry if this question to too silly for the experts here. But how do cosmologists assign a temperature and a time to each important cosmic event in the history of evolution of the universe such as ...
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174 views

Physical meaning of the space-space components $T^{ij}$ of the stress-energy tensor $T^{\mu\nu}$

The time-time component $T^{00}$ and the time-space components $T^{0i}$ of the energy-momentum tensor $T^{\mu\nu}$ are respectively called the Hamiltonian (energy) density and the momentum density. ...
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Calculating the maximum total neutrino mass by using cosmological bound

In an article its written, $$\Omega_{\nu} = \frac{\rho_{\nu}}{\rho_{crit}}=\frac{\sum m_{i,\nu}n_{i,\nu}}{\rho_{crit}} = \frac{\sum m_{\nu}}{93.14h^2eV}$$ Now I am trying to derive this for myself ...
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78 views

Curvature of spacetime in Friedman cosmological models

Short version of my question is as follows: What is curvature of spacetime (rather than space) in Friedman cosmological models? Now the long version, including evolution of my thoughts and background ...
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197 views

What does it mean to say that the Universe is flat?

The curvature parameter $k$ in the FRW metric can take three possible values $k=0,\pm1$. The case $k=0$ is called a flat Universe. But observationally $k$ can take fractional value say k=0.001. What ...
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Deriving the correct expression for the action of the scalar field at the time of inflation

In its matrix form, the FRW metric is $$g^{\rm FRW}_{\mu\nu}=\begin{pmatrix}1 & 0 & 0 & 0\\ 0 & -\frac{a^2}{1-kr^2} & 0 & 0\\0 & 0 & -a^2r^2 & 0\\0 & 0 & 0 &...
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In the string theory multiverse, is every universe fated to collapse in a Big Crunch?

This is what I understand of the multiverse of the string landscape: the "laws of physics" reside in a 100-dimensional string landscape, which has ~$10^{500}$ minima. Each of the minima corresponds to ...
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1answer
102 views

Formula to Compute sigma8 for correction in non-linear regime

I need to apply a correction on $\sigma_{8}$ between linear and non-linear regime to keep it fixed (I make change the values of cosmological parameters at each iteration). I have to compute $\sigma_{8}...
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48 views

Lorentz invariance of the CMBR [duplicate]

From the Wikipedia entry on the CMBR dipole anisotropy (emphasis mine), From the CMB data it is seen that the earth appears to be moving at 368±2 km/s relative to the reference frame of the CMB (...
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1answer
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What does a universe look like where light cannot lose energy and is emitted all the time?

In a universe where light cannot lose energy, and is emitted all the time, what is the result?
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1answer
71 views

How far back can experiments confirm the LCDM model?

The LCDM is the standard model of cosmology and is well validated by experiments. If we go back to the Big Bang, the energy density diverges. Therefore, my question is "How far back can experiments ...
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75 views

Is the universe expanding at an increasing rate or is time slowing down or is $c$ changing? [closed]

If time slows down for an observer as velocity increases, then the earth is moving as fast as the speed of light away from objects at the edge of the observable universe. However when those objects ...
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29 views

Role of isotropy argument in fixing the form of $T^\mu_\nu$ of cosmology (Padmanabhan's book)

The following lines from Gravitation Foundation and Frontiers by T. Padmanabhan (he uses Latin indices for spacetime and Greek indices for space, which is unusual) The assumption of isotropy ...
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2answers
135 views

Voids and Hubble Constant

It has been proposed the Hubble tension can be solved if we assume our galaxy is located in a giant void (such as KBC). I am confused at this point. If we were living in a giant void, we should have ...
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2answers
122 views

Will atoms evaporate given enough time?

Thought experiment: someday in the distant future, space will be expanding faster than the speed of light. Imagine that trillions of years in the future, you have a single hydrogen atom. Will that ...
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1answer
261 views

Is general relativity compatible with an arrow of time?

The spacetime of Special Relativity is Minkowski's "Block Universe" where time is treated as a dimension. Does General Relativity allow for an arrow of time or "flow" of time? If yes, can this be ...
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Is the light escaping from a massive star held back a bit by the pull of gravity, or by the warping of spacetime near it? [duplicate]

In The Universe in a Nutshell chapter 4, Hawking explains the warping of spacetime according to general relativity. Near a massive but ordinary star, spacetime is warped such that the light emitted ...
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1answer
58 views

Formation of primordial black holes from density perturbations in the early universe

Recently I've been reading Carr and Hawking paper on the formation of primordial black holes "Primordial black holes in the universe" , but I couldn't understand part of it. It says that: A region ...
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1answer
58 views

What did Stephen Hawking mean with “not demanded by logic or physical principle”?

In his book, "The Grand Design", physicist Stephen Hawking said: "It appears that the fundamental numbers, and even the form, of the apparent laws of nature are not demanded by logic or physical ...
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In what frame of reference is time measured, just after the big bang?

We often read about the expansion of the universe in the first seconds, or milliseconds after the big bang. What frame of reference is used for these estimates of time? If the entire universe was ...
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30 views

Regarding expansion of the Universe and effect on the visible Universe

Does the faster than light expansion of the Universe have the following consequence: The extent of our visible universe increasing by 1 light-day every day, but due to expansion the actual contents of ...
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1answer
73 views

Can we look further than the CMB?

As I understood it, the map of the cosmic microwave background (CMB) corresponds to the time when photons decoupled from matter, making the universe transparent. In this sense, we might expect the CMB ...
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33 views

How far can we look into space?

Ho distant can we look into space by using a telescope? With "telescope" I mean here any device able to detect photons of any wavelength. In terms of distance, is it correct to sat that observing the ...
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2answers
96 views

Why does inflationary expansion of the very early universe require time duration?

If the grand unified force symmetry breaking separation into the strong force and electroweak force leads to a change in scale of the metric that defines distance within space is the cause of the ...
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1answer
34 views

Limiting the mass of the neutrino for a relativistic case

I came across a question that states What mass would a neutrino need to still be relativistic today (T = 2.37K) ? So for a particle to be relativistic we need $pc \gg mc^2$ Well Neutrino was ...
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Is the Universe 11 billion years old?

This clip claims scientists found a star 200M years older than the Universe. However, I took another assertion more seriously: scientists estimated a faster expansion rate of the Universe, driving age ...
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1answer
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Understanding the recombination and decoupling periods in the early universe

I have a couple of questions about the recombination and photon decoupling. The problem is I have 2 references that give different time scales for these two periods. For recombination: In Roos, it'...
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60 views

Does this paper falsify the vacuum energy? [duplicate]

If Dark Energy is falsified by this paper, https://arxiv.org/abs/1808.04597, does that mean the vacuum energy is also falsified?
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171 views

At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

At what cosmological redshift $z$, does the recession speed equal the speed of light? What equations are used to calculate this number (since at large redshifts, $z=\dfrac{v}{c}$ won't apply)?
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69 views

How to pick what coordinates transformations to apply in general relativity

I've been challenged by coordinate transformations lately, as most of you know during solving any problem in GR we have to go through lots of transformations, my question is how we decide the best ...
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1answer
158 views

Is there any other explanation for the expansion of the universe other than dark energy? [closed]

Why did the universe expand from the Big Bang singularity? If gravity was a separate force and in its current form by 1 Plank time, would not have gravity prevented expansion, unless there was some ...
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1answer
59 views

Embedding manifold equipped with FLRW metric

The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is as follows, in natural units: $$\mathrm{d}s^2=-\mathrm{d}t^2+a(t)^2\left(\frac{\mathrm{d}r^2}{1-\kappa r^2}+r^2\mathrm{d}\theta^2+r^2\sin^2\...
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22 views

Friedman equation with non-negligible neutrino masses

I have heard that the Friedman equation (for lambda-cdm) with significant neutrino masses is modified from its simpler form (sum over various densities of c.c., curvature, matter and radiation) to a ...
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2answers
64 views

Why wasn't CMB radiation absorbed long ago, since when it was emitted the universe was much smaller?

When CMB radiation was emitted, the universe was only 350,000 years old or so, and so why wasn't it absorbed at that time? The universe is postulated to have been expanding at that point but I think ...
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78 views

Numerically solving for cosmological scale factor in mathematica

I would like to solve the scale factor $a(t)$ in a FRW metric. The Friedmann equation for the scale factor is: $H^2 \equiv \left( \frac{\dot{a}}{a} \right)^2 = H_0^2\left[\Omega_R \left(\frac{a(t)}{...
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1answer
92 views

As space expands, does this not imply that time also expands? [duplicate]

According to General Relativity, space and time are aspects / dimensions of a unified whole. If the speed at which wave energy travels through spacetime is constant, $c$, then as distance expands, ...
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1answer
63 views

When does the warping of space time peak during the very early universe?

If gravity is a separate force and has taken its current form by 1 Plank time, then the very small 1 Plank length space-time must be warped beyond description according to General Relativity, or is it?...

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