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.

705 questions with no upvoted or accepted answers
Filter by
Sorted by
Tagged with
2
votes
0answers
527 views

2nd law of thermodynamics in Cyclic Universe Model?

Standard view on Big Bang is that it has started with a perfect point, followed be faster-than-light inflation. This initial perfect point does not seem to agree with CPT symmetry, which is generally ...
2
votes
0answers
154 views

Why are all galaxies roughly the same size?

what determines the stable mass of galaxies, stars, star clusters or even galaxy clusters? Is there some obvious way to determine the classes of stable mass clusters from the initial conditions of the ...
2
votes
0answers
274 views

Conceptual understanding of silk damping

CMB anisotropies are washed out below the silk damping scale, because photons travel out of overdense regions. However does silk damping also damp matter perturbations (through scattering) on scales ...
2
votes
0answers
126 views

How does dark matter distribution help to identify it's composition

A question regarding this recently released data, and the paper https://arxiv.org/abs/1708.01538 that details it's distribution: Abstract: We use 26 million galaxies from the Dark Energy Survey (...
2
votes
0answers
106 views

Gravitational red shift compared to expansion red shift

From the gravitational red shift: $$\lambda(r)=\lambda(e)\left(1+\frac{gd}{c^2}\right)$$ where $r=received,\,e=emitted$ sources. Therefore light coming from a distant star to earth would be more red ...
2
votes
0answers
66 views

Inflationary model that predicts both gravitational waves and isocurvature perturbations

I 'm searching for inflationary models that predict both gravitational waves and isocurvature (isothermal) perturbations. If I understood well, de Sitter inflation predicts only curvature (adiabatic)...
2
votes
0answers
65 views

Statistical physics in cosmolgy

In https://ned.ipac.caltech.edu/level5/Sept03/Trodden/Trodden4_2.html and also in Mo et al. (Galaxy Formation and Evolution) the number density, energy density and pressure of the different particle ...
2
votes
0answers
153 views

Newtonian interpretation of the Friedman-equation

I'm a first year physics student so I cannot yet understand general relativity, yet the Friedman-Lemaître-Robertson-Walker (What's in a name?) equations are used in my introductory course in astronomy,...
2
votes
0answers
88 views

Galaxies moving in fixed background or fixed in expanding background

Question: what is the most direct experimental evidence to suggest that the galaxies in the universe are fixed in an expanding spacetime, as opposed to moving in a fixed spacetime? Comments: The ...
2
votes
1answer
191 views

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 ...
2
votes
1answer
62 views

Homogeny of the Universe far beyond the Observable Scale

Thought experiment: Suppose we have complete knowledge of the observable universe (as of 2016) within some margin of error $\varepsilon$ for arbitrary but final $\varepsilon > 0$. Call the radius ...
2
votes
1answer
52 views

Establishing true (ie present) positions of cosmic bodies

I may be missing some terminology to ask this question properly, but I've been looking for an explanation for how one might determine the actual current position of a star vs its apparent observable ...
2
votes
0answers
31 views

What is the plot function: number of galaxy-clusters ( gravitationally bound objects) within the cosmic event horizon vs. time?

I found this nice animation about the shrinking cosmic event horizon:http://yukterez.ist.org/lcdm/i.html#comoving But now I am interested about the number of galaxy clusters (biggest gravitationally ...
2
votes
0answers
274 views

Why would be axions be a candidate for warm/cold dark matter?

Axions are the current main favoured candidate for the identity of dark matter. While their masses are sub-$\mathrm{eV}$, why is it that they would be cold or possibly warm dark matter candidates ...
2
votes
0answers
191 views

Is there any primordial gravitational wave(form) that could explain dark energy?

First, I asked myself the question: Is there any gravitational waveform, however fantastical, that could explain dark energy. I suppose for a ubiquitous gravitational wave to be an explanation for ...
2
votes
0answers
76 views

Dark energy and gravitation force

This might be a silly question to ask but still I had to ask. we know that gravitational force between two galaxies are quite less as compared to the dark energy. so two galaxy are separated at speed ...
2
votes
0answers
55 views

How many times does baryogenesis happen during the history of the universe?

Does the baryogenesis happen both in GUT epoch and electroweak Epoch during the history of the universe? If yes, are there any differences during this two times?
2
votes
0answers
56 views

Any method to determine the gravitational constant $G$ from observations in cosmological scale?

By the observations in cosmological scale, I mean observations by astrophysicists like motions of quasars in far distance from us, relative motions of our galaxy to others, etc. I know the famous ...
2
votes
0answers
74 views

What happens when we use the Virial Theorem iteratively?

Say I want to model the formation of structure in the Universe as a series of events whereby already virialised systems are brought together to create a larger virialised system. I will take the ...
2
votes
0answers
87 views

Could an antimatter-dominated end of universe fix CPT?

My question is about the apparent CPT violation of the observed universe, due to the imbalance of matter and antimatter, but first I have a motivating observation: General relativity respects time ...
2
votes
0answers
192 views

Why is Weinberg's prediction of omega sub lambda ($\Omega_{\Lambda}$) other than sophisiticated tautology?

I am not a physicist, but I do have a background in engineering (EE and computer science) and have followed physics and cosmology on a primarily conceptual basis for many years. I have looked at ...
2
votes
1answer
162 views

What are “the background equations” in cosmology?

We're currently working on perturbations within cosmology. There is something I have not heard before which has cropped up, that is: a reference to the term "the background equations". Are these just ...
2
votes
0answers
171 views

Quadrupole and Multipoles in Physics

I am a little confused over the notion of quadrupole and higher moments in physics in general. The first time I saw it was in electromagnetism, when we did multipole expansion to analyze higher ...
2
votes
0answers
49 views

What do the $T, E, B$ in polarization spectra mean?

I was reading about CMB Polarization here. I know that $E$ and $B$ stand for E-mode and B-mode, but what does the $T$ mean? The author states that there are 3 observables: $T, E, B$ and six spectra: $...
2
votes
1answer
82 views

Does the Entropy of the Visible Universe Decrease?

My understanding is that the entropy in a closed system increases. However, I would also assume that the number of particles in the open system of a visible universe decreases because of accelerated ...
2
votes
0answers
48 views

Comparing $X_\text{red}^2$ statistics for cosmological models

I'm evaluating a cosmological model which competes with ΛCDM. I have charted the $Χ_{red}^2$ and $R^2$ statistics for the sample of galaxies from McGaugh et al (2001) for my model and ΛCDM. Each ...
2
votes
0answers
51 views

Is the CMB Cold spot growing or shrinking?

What do we know about the nature of this region of the sky? is that region entirely devoid of galaxies? Could it be a bubble void, and we are simply unable to see the galaxies that are on the ...
2
votes
0answers
185 views

Drawbacks of single-field inflationary models with $\phi^2$ or $\phi^4$ potential?

I am aware that existing models of inflation, especially the single field chaotic inflation with quadratic or quartic potential, e.g. $$ V = \frac12 m^2 \phi^2 \qquad \text{or} \qquad \lambda \phi^4 $$...
2
votes
2answers
214 views

What is the interpretation of a wave function of the Universe in Hawking's no boundary proposal?

In the path integral formalism we have an in state $\Psi_{in}[\phi]$ and and out state and we find the amplitude for going from one to the other: $$\Delta[\Psi_{in},\Psi_{out}] = \int \Psi_{in}[\phi]...
2
votes
0answers
46 views

Equation of state of a universe full electrons

If I have a universe full of nothing but (slow moving) electrons, what would the equation of state ($w$, from $p=w\rho$) be? I think it would be incorrect to say that the electrons should be treated ...
2
votes
0answers
37 views

Correlation length during phase transitions in early Universe

During phase transitions of the second kind topological defects may form on the bounds of two areas separated by correlation length. In early Universe during phase transitions correlation length $l_{...
2
votes
0answers
361 views

Cosmological Bulk Viscosity - is the universe a fluid?

I was trying to make heads of tails of both this paper and the press release about it. The scientists have used bulk viscosity (as opposed to shear viscosity) and popped it into a model and somehow ...
2
votes
1answer
267 views

Momentum conservation in FRW spacetime

The spatially flat FRW metric in Cartesian co-ordinates is given by: $$ds^2 = -dt^2 + a^2(t)(dx^2 + dy^2 + dz^2)$$ As I understand it, since the metric does not depend on the spatial co-moving co-...
2
votes
0answers
38 views

Validity of Press-Schechter formalism at the end of recombination

I was wondering if Press-Schechter formalism on Spherical collapse is applicable sometime at the end of recombination, i.e. the photons have started to free stream but the free streaming length is ...
2
votes
0answers
83 views

Fluid approximation for inflaton?

In cosmology there is a widely used fluid approximation which applies when the mean free path is very small compared to the scale of observation and then all the properties of stress energy tensor can ...
2
votes
0answers
183 views

Using the boltzmann equation to find neutrino decoupling temperature

I'm an undergraduate student currently following a particle astrophysics lecture. I was reading some stuff about the neutrinos decoupling before the $e^+ e^-$ annihilations. It's possible to quickly ...
2
votes
0answers
267 views

Computations for Quantum Vacuum Fluctuations

For quite some time the notion of quantum vacuum fluctuations is bothering me. What exactly is the theoretical origin of this notion? This notion has become quite common in physics and is used to ...
2
votes
0answers
127 views

How do we get the power spectrum through the CMB?

How do we get the power spectrum through observing the CMB?
2
votes
0answers
104 views

Dark Energy EoS: which models ruled out, which remain?

Dark energy density is mostly assumed to be a constant, where the equation of state for w=-1 (for radiation it would be w=1/3, for matter w=0 and for curvature -1/3), so in the simplest model the ...
2
votes
0answers
99 views

Statistics of Black Holes

I am hoping to collect information about the sampling distribution of black hole sizes, densities, and their relative spatial distribution within galaxies and other formations. By sampling ...
2
votes
0answers
187 views

How strong is the force of space expansion?

There are many questions about space expansion, its cause, or its effects. But I have the feeling we never get straight and simple answers. I do not expect answers to be simple in general, but I ...
2
votes
0answers
79 views

Units and missing constants in quintessence expressions?

In cosmology, quintessence is an alternative to the cosmological constant. In this approach (described here), we consider a scalar field $\phi$ and its self-interacting potential $V\left(\phi\right)$ ...
2
votes
0answers
160 views

Does cosmological time expand like space does?

The FRW metric is given by: $$ds^2=-dt^2+a^2(t)\ dr^2$$ where $ds$ is an interval of proper length, $dt$ is an interval of cosmic time, $dr$ is an interval of co-moving co-ordinate distance and $a(t)$ ...
2
votes
0answers
140 views

Dependence of the neutron-freeze-out temperature

What is the physical explanation for the dependence for the freeze-out temperature $T_\text{F0}$ of the neutrons as a function of the number of degrees of freedom $g^\star$? $$T_\text{F0}=\left(\frac{...
2
votes
0answers
245 views

Hawking radiation at the mouth of Schwarzschild wormholes

I was researching a question for another post and it occurred to me that you might expect to see Hawking radiation at the mouth of wormholes. Given the mechanism of Hawking radiation at the event ...
2
votes
0answers
91 views

Question on Equipartition at Reheating?

Apologies in advance if this is a silly question. I am fimiliar with the idea of Reheating in cosmology, as discussed briefly here. I have been reading several papers which make the assumption of '...
2
votes
0answers
139 views

General formula to compute the redshift (first order perturbations)

Consider an expanding universe with the following metric in conformal time/co-moving coordinates: $$ds^2=a^2\left[-c^2\left(1+\frac{2\phi}{c^2}\right)d\eta^2+\left(1-\frac{2\phi}{c^2}\right)\left(dx^...
2
votes
0answers
89 views

Entropy bounds and the size of the universe

The principle that the maximum amount of information or entropy a volume of space can hold is proportional to its surface area apparently applies to all space, not just black holes. Since volume grows ...
2
votes
0answers
144 views

Modelling a matter dominated universe collapsing into a black hole

With the FLRW equations we can get solutions for a matter dominated closed universe in which the finale is an ultimate collapse, but this is only in terms of $a$ (the scale factor) and $t$ (time) and ...
2
votes
0answers
112 views

Angles in cosmology

We have several definitions of distances in cosmology. Take angular diameter distance. $$d_A = a(t)\int \frac{dt}{a(t)}\;.$$ If an object has proper length $L$ then the angle $\theta$ it is seen at ...