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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84 views

Stringy corrections to Friedmann equation

Does anyone know a reference or a paper which discusses string theory correction to Friedmann equations?
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580 views

Dimensional analysis to estimate order of magnitude of quantities

In the Coursera course From the Big Bang to Dark Energy on several occasions dimensional analysis was used to estimate the scale of quantities. This almost seems like a contradiction in terms to me, ...
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65 views

Scaling in Vlasov equations

This question is in reference to the paper, http://arxiv.org/abs/1301.7182 What exactly is the argument being made on page 6 and 7? One deduces that the function $\Delta$ has to be such that, $\...
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Regular initial data

I have a very basic question. What exactly is meant by "regular" initial data in general relativity? Does it mean smooth? at least $C^{2}$? All literature on the subject just uses this term without ...
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854 views

Trouble with calculating Christoffel symbols of FLRW metric using Lagrangian method

The FLRW metric which I am using is $$ds^2 = dt^2 - \frac{a(t)^2}{c^2} \left( dx^2 + dy^2 + dz^2 \right)$$ where $a(t)$ is the so-called 'scale factor'. I did not want to calculate the Christoffel ...
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173 views

Cosmic Inflation: Lower Expansion Rate with than without?

When I read the Cosmic Inflation diagram at https://en.wikipedia.org/wiki/File:Horizonte_inflacionario.svg which I rotated to make it readable: Plot 1 http://yukterez.enabled.io/inflation.png where ...
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123 views

Topological Solitons and the Higgs Condensate entanglement

While focusing on resolutions to the Firewall controversy, and the possible implications of the Higgs field as it relates to the issue, the possibility of using EPR correlations in the Higgs ...
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94 views

How can I read density fluctuation from microwaves?

The Cosmic Microwave Background Radiation shows temperature differences. The red and yellow areas are warmer. The green and blue areas are cooler. For example consider this picture of CMBR ...
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403 views

How does one derive the 2 halo term in two-point correlation function

This question is in reference to the paper here. In Equation (86) on page 28, the authors have given the two point correlation function \begin{equation*} \xi(\mathbf{x}-\mathbf{x}^{\prime}) = \xi^{1h}(...
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99 views

Wick rotation for FRW in quantum gravity

There is no timelike Killing vector for FRW cosmologies. In the path integral formalism, is it possible to Wick rotate for quantum cosmology in quantum gravity? If yes, how? If no, how does one work ...
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178 views

Can decompactification explain the inflation of the early universe?

I've just reread chapter 11 of this book where it is explained among other things, that our four dimensional universe could be unstable concerning a decompactification transition, since potential ...
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195 views

Is it really true that slow-roll inflation is incompatible with string theory?

In this blog post, Motl claimed that slow-roll inflation is incompatible with string theory. Is that really true, and why do people think so?
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778 views

Is large-scale “time reversal” (Poincaré recurrence) possible given infinite time?

The following are some assumptions I'm basing my question on, from what (little) I understand of physics. I list them so an expert can (kindly) tell me where I'm going wrong. There is a probability ...
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175 views

Is eternal inflation Lorentz invariant?

Start without general relativity. Consider a metastable vacuum over good ol'-fashioned Minkowski space. It decays. A bubble forms and the domain wall expands. The domain wall is timelike, and ...
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118 views

Does dark energy affect asymptotic freedom?

If the Hubble constant is extremely large, what will happen with quark confinement? I guess that quarks will remain confined because of asymptotic freedom. But can gravity or dark energy have any ...
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400 views

Calculation of the non-Gaussity parameter for primordial cosmological perturbations by the ADM Formalism

Maldacena has used the ADM Formalism in one of his papers (http://arxiv.org/abs/astro-ph/0210603) in computing the the three point correlation function (i.e the non-Gaussianity) parameter for ...
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245 views

Singularities in Bianchi models in general relativity ( physical science)

what are the conditions to check point type singularity in a bianchi type model ? bianchi type model are of Type I,II,III,IX,IV or u can say we use different Bianchi type models having some specific ...
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1answer
146 views

Is there any way to tell if the Big Bang happened everywhere or just in some regions of the entire universe?

On large scales, matter seems to spread uniformly in our observable universe so we think the Big Bang happened everywhere in the observable universe. Is there any way to tell if the Big Bang happened ...
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2answers
59 views

Universe expansion from initial explosion

If the universe is expanding at an accelerated rate, is it because it hasn't reached its maximum velocity from initially exploding? I'm thinking in terms of a bullet being fired from a weapon. It ...
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1answer
153 views

Is it possible to directly test whether of not the vacuum gravitates?

According to GR, all sources of stress-energy (e.g. everything on the $T_{\mu\nu}$ side of the EFE) should gravitate (e.g. affect the curvature/$G_{\mu\nu}$ side of the EFE). We observe the expansion ...
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1answer
583 views

How come that an Infinite universe will collapse under gravity?

In this PSE post the issue is about the stability of an infinite universe under Newtonian gravity. Here I'will drop the Newtonian constraint because we know of the finite speed of interactions and I ...
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1answer
205 views

How to reconcile these two principles?

Quantum mechanics says that the entropy of an unobserved system remains constant. As such, the apparent growth of entropy is a subjective illusion. If we consider the wave function of the universe, ...
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1answer
357 views

Does a complete theory of quantum gravity require anthropic post-selection?

Does a complete theory of quantum gravity require anthropic post-selection? Certainly the black hole complimentarity and causal patch conjectures highlights the essential role of observers, at least ...
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2answers
108 views

What is the definition of a Collisional Fluid?

I am unsure whether this means the particles in the fluid must physically collide or does 'collisional' also apply to particles interacting in general, eg via gravity? I cannot find a good definition ...
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What are the key pieces of cosmological evidence if you want to compare the success of dark matter versus quantised inertia?

Quantised Inertia is a controversial fringe theory that is claimed to be an alternative explanation of, for example, galactic motion. The consensus explanation for why the outer stars in galaxies ...
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24 views

Book suggestion about Cosmological Density Perturbations

I am trying to find some nice explanatory books about cosmological density perturbation theory. I looked at Longair, Peacock, and Liddle. Even their level is nice, it seems to me that they are ...
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1answer
55 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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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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48 views

What is the motivation of 'minimal coupling' as applied to inflation theory?

The prescription of replacing the ordinary derivative $\partial_\mu$ by the covariant derivative $D_\mu=\partial_\mu+ieQA_\mu$ in QED is called minimal substitution. It is 'minimal' because it ...
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1answer
74 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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19 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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70 views

Why is the homogeneity problem actually problem?

I don't mean to be flippant, but have never understood why the homogeneity in the universe needs.explaining. I have searched other questions on stack exchange and they don't really address the ...
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30 views

Calculating $\nabla_\mu T^{\mu\nu}$ in linear cosmological perturbation theory

I am trying to calculate $\nabla_\mu T^{\mu\nu}$ for perfect fluid in the context of the linear cosmological perturbation theory. I'm working in the Newtonian gauge. I have successfully calculated $\...
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graph relativistic degrees of freedom

I'm trying to graph the relativistic degrees of freedom, which should look like the figure And I am trying to guide me with this Phys.SE answer: Number $g(T)$ of relativistic degrees of freedom as a ...
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66 views

Galactic Birkeland currents

Does Milky Way or any other galaxy produce Birkeland currents such as seen on Earth in auroras? If yes, how strong would they be? According to this questions answer, galaxies do have magnetic fields. ...
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1answer
31 views

If you accelerate every particle in the whole universe, would it slow down universe expansion?

Perhaps an odd question, but (Proposition): say I want to slow down everything that is happening in the universe, I believe this could be done by accelerating everything to a velocity close to the ...
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1answer
65 views

Question about quantum fields, elementary particles, and quantum numbers, and fundamental forces

As I understand it the allowable spin states of elementary particles is tied to the number of space and time dimensions, so for any two universes with the same number of space and time dimensions the ...
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50 views

What's the farthest away from us a photon could theoretically reach?

Assuming the expansion of the universe will be dominated by dark matter, is there a limit to how far a photon could travel in terms of co-moving distance, as the Universe will be expanding faster than ...
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Expanding space outside our galaxy

Some other Galaxy would have its own Hubble sphere. It is possible then that we could be on the edge of someone else’s Hubble sphere. Why is it then we are not experiencing this accelerated expansion ...
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44 views

How would physics change if the fine structure constant was slightly changed?

How would other physical constants and space be affected? Atoms? Chemistry? How would a patch of the vacuum with a, say, slightly higher value interact with the "normal"-value vacuum around it?
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AdS/CFT phenomenology and realistic FRW model building

Are there any examples of realistic holography (likely as a de Sitter type Universe: as it approximates FRW / is an FRW solution without baryonic and dark matter). I don’t see why one wouldn’t be ...
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Cosmology: Dimensionless density parameter calculation

The question: Given an FRW model dominated by a perfect cosmological fluid with eqn of state P=$\omega \rho$ for $\omega$ =constant. Along with the density parameter $\Omega (t) = \frac{ \rho (t) }{...
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Temperature dependence of the Higgs VEV

Presumably, The Higgs field vacuum expectation value has a strong temperature dependence close to the electroweak symmetry breaking phase transition. Does this have known consequences for the ...
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Order of Magnitude of Cosmic Microwave Background Anisotropies

How does the statement about the anisotropies being of the order of $\frac{\Delta T}{T} \approx 10^-5$ which is often made in the literature (see, e.g. Description of CMB Anisotropies by Scott and ...
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40 views

Using Hubble parameter to compare sizes now and then

What if the Hubble parameter is constant both in time and space. How would the scale factor look like? For $H_0=70~kms^{-1}Mpc^{-1}$, how much smaller would the universe be 14 billion years ago? I'm ...
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71 views

Cross-sections at zero temperature and high temperature for a process and its reverse

If the Feynman amplitude for a $2-2$ forward scattering $ab\to cd$ is denoted by $\mathcal{M}_{ab\to cd}$ and that of the reverse scattering process, $cd\to ab$, is denoted by $\mathcal{M}_{cd\to ab}$...
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45 views

How to get this metric tensor perturbation equation? (Gravitational wave)

(I need some time to come back to re-edit this post) LHS of equation 7.37 is gauge invariant The Twisted H symbol is comoving hubble parameter, the h_ij is metric perturbation, I do not know how to ...
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1answer
68 views

Dark matter in the time window between freeze-out and kinetic decoupling

Background After the freeze-out, when all annihilations have stopped, the abundance ($Y=\frac{n}{s}$) of thermal dark matter species no longer changes with time. However, it is still kept in kinetic ...
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1answer
131 views

Changes to a Length of Physical Ruler Caused by Gravity vs Caused by Cosmological Expansion of Space

I read here (Feynmann Lectures, Lecture 42) that "Just as time scales change from place to place in a gravitational field, so do also the length scales. Rulers change lengths as you move around." (...
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1answer
46 views

Is there cosmic background radiation in de Sitter space?

This is a simple question of understanding. Would a de Sitter universe show cosmic background radiation? (After all, it is often explained said that our universe is similar to a de Sitter universe.) ...