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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64 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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1answer
114 views

The Cosmic Microwave Background Paradox

I was reading an article on Olbers' Paradox (why the universe isn't as bright as the sun) and the more I read on it, the more the same question came to mind... We know the observable universe is ...
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1answer
157 views

What's going on with the Hubble constant?

I'm doing a small project on LIGO for a third-year project at University. I want to write something about how LIGO can be used to measure $H_0$, which would be useful because the current state of the ...
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97 views

Isn't mass from pure topology in the absence of matter a contradiction?

Consider the 3 dimensional projective space minus the infinity point, and empty of matter. We see that it has ADM mass. In other words, a perfectly fine geometry, orientable and asymptomatically flat,...
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Does exponential expansion ensue locally if in a certain hubble patch vacuum dominates?

As a concrete example, consider a universe scattered with low density extremely massive objects (e.g. black holes) so that there is 1 of these per many many Hubble patches. (schematically in drawing) ...
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44 views

As space expands does the density of matter remain same or increase? [duplicate]

I learned that space is always expanding. Even between sun and earth, it's just gravity is stronger. Imagine space as graph paper and there is a square object that fills it. Space expands, 1x1 cell ...
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32 views

Is there a cosmological model where the observable universe is bidirectionally infinite and yet finitely bi-limited?

Is there a cosmological model where the observable universe continues to shrink as we go back in time, and continues to expand as we go forwards in time, and such that at both directions it never ...
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44 views

A doubt in Modified gravity

I am new to dark matter and modified gravity so excuse and inform me if I am wrong. If changes are made in the Friedman equations then there wouldn't always be an underlying action action principle. ...
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1answer
25 views

Hubble’s law distance and time dependence

Hubble’s law states that the recessional velocity is proportional to the distance. The further the observed distance, the further back in time one is observing. I want to know (generally) how this ...
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1answer
101 views

How to calculate the derivative of scale factor as a function of conformal time from the solution of Friedmann equation

For the flat geometry of lamda CDM model, the solution for Friedmann equation is $$ a(t) = \left\{ \frac{Ω_{m,0}}{Ω_{Λ,0}} \sinh^2 \left[\frac{3}{2} \sqrt{Ω_{Λ,0}} H_0(t - t_0)\right] \right\}^{1/3},...
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1answer
49 views

Could a large amount of antimatter accelerate the universe instead of dark energy? [closed]

It is not known whether antimatter is repulsed by or attracted to matter. For the purpose of this question, I assume that antimatter is repulsive to regular matter (otherwise this question makes no ...
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2answers
105 views

What happens when the wavelength of light becomes as large as the observable universe? [closed]

Say the wavelength of a photon became so large that it approached the size of the observational universe. Does something unexpected happen?
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2answers
74 views

Nature of motion between comoving observers; What is the common time that they agree on?

This is a set of follow-up questions regarding this post. The following four queries are very closely related and needed to asked at the same place. Question 1 Is it really possible to regard the ...
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1answer
46 views

Are there viable alternatives to the no boundary proposal?

As I understand it, quantum field theory can be described as the evolution of a wave function $\psi_t[\phi]$ depending on some fields, $\phi$. But when we include gravity and we admit that time is ...
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1answer
168 views

Solution of Friedmann equation in terms of conformal time

We know that for the flat geometry of lamda CDM model, the solution for Friedmann equation is $$ a(t) = \left\{ \frac{Ω_{m,0}}{Ω_{Λ,0}} \sinh^2 \left[\frac{3}{2} \sqrt{Ω_{Λ,0}} H_0(t - t_0)\right] \...
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34 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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1answer
127 views

Did the observable universe come from nothing?

Does the big bang theory hold that there is a possibility that the whole of the observable universe came out from NOTHING? I mean if all of this universe was occupying an extremely small size of a ...
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60 views

The fundamental transition problem [duplicate]

can anyone explain what is mentioned here https://arxiv.org/abs/1405.6091 (page 7 and 8) in simple terms: Finally, Krauss makes the very problematic claim that “the structures we can see, like ...
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29 views

Total mass-energy of a system in GR [duplicate]

the Gravitation book confirm that "for closed universe the total mass-energy and angular momentum is undefined and undefinable" why many cosmologists claim that the total energy of a closed universe ...
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30 views

What percentage of stars are giants/subgiants/main sequence etc

I am trying to find the percentage of main-sequence stars along with red-dwarf percentage for my research paper. I did find one article regarding, however it is very old and i require recent data ...
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3answers
117 views

Is there a contradiction between isotropy and the Big Bang? [duplicate]

Disclaimer: I'm not asking whether the Big Bang happened at a point. I'm asking whether the fact that the universe is isotropic and that the Big Bang happened contradict each other. To be honest I am ...
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85 views

How to relate CMB acoustic peak to BAO in galaxy surveys to better than a factor 2?

Question for a friendly cosmologist. Let's define the sound horizon as $s = \int_0^{t_{ls}} c_s (1+z) dt$ where $c_s$ is speed of sound in primordial plasma (roughly $c/\sqrt{3}$ but you can include ...
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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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1answer
75 views

Virial radius of a collection of point masses

I am experimenting with multi body simulations in astronomy ($N$-body algorithms). It is customary to give the results in dimensionless units. One of them is a length unit, and the virial radius is ...
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1answer
43 views

Is thermal coupling to a too cold thermal background impossible?

In (Krauss & Starkman 1999) the authors critique (Dyson 1979), and in one section argues that in the very far future cooling will become impossible because thermal contact with the cosmic ...
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1answer
383 views

Neutrino energy density vs photon energy density

So I'm currently following a course in Cosmology and we're covering the densities of different species in the universe right now. Starting from the photon density $\rho_{\gamma}$ we need to derive the ...
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1answer
53 views

Could dark matter be normal matter that is beyond the observational horizon?

As I understand it, matter that is too distant for its light to reach us would not be observable, and the CMB is the limit of what is observable from our location within the universe. Could what is ...
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1answer
86 views

Experimental status of the Big Bang theory

Take the most successful version of the Big Bang theory I'm aware that the Big Bang theory have unanswered questions related to the very first moments of the Big Bang. I also know that for our ...
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1answer
93 views

How much kinetic energy is there in the observable universe at any given time?

I know that it fluctuates, but, what would be a good estimate for how much kinetic energy is in the universe right now? I know that things cancel it out, but, how much is there?
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1answer
149 views

Can our universe be a true vacuum bubble?

The paper "Spontaneous creation of the universe from nothing" by Dongshan He, Dongfeng Gao and Qing-yu Cai claims that our universe was created by the quantum fluctuations in the metastable false ...
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49 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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1answer
124 views

Observable universe radius for distant observers

The radius of the observable universe is about 46 Gly. Is that figure true for all current observers in our universe? Is it true if the universe is finite or infinite, flat or curved?
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1answer
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Pridictions and Observational evidences of No Boundary Condition of S.Hawking

Reference: http://www.hawking.org.uk/the-beginning-of-time.html Predictions of No Boundary Condition proposal: 1) Irregularities in the current universe same as the Big Bang theory predicts and it ...
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1answer
67 views

Imaginary time concept in S.Hawking's No Boundary proposal, extra-time dimensions and the Big Bang

In this post I will be refering to S.Hawking's lecture: http://www.hawking.org.uk/the-beginning-of-time.html I have a couple of questions regarding the Imaginary time and the Big Bang. In the ...
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67 views

Volume expansion rate

In the paper Backreaction in late-time cosmology by Thomas Buchert and Syksy Rasanen, Annual Review of Nuclear and Particle Science 62 (2012) 57-79, in eq .2.2 the covariant divergence: $$\nabla_\...
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4answers
115 views

Present vs Past in Cosmological time

My question is in regard to “seeing back in time” when observing objects in space. I understand the concept that light takes a certain amount of time to reach the earth and we can determine the ...
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1answer
41 views

Lack of knowledge about ambient neutrinos below a certain energy?

A colleague and I were discussing the fact that beta decay can emit neutrinos with arbitrarily low energies. We have neutrino detectors that can detect solar neutrinos -- and maybe extrasolar ...
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Could I use this informal explanation for Hawking radiation?

I need to explain Hawking radiation to my classmates for a homework. Could I use this informal explanation? (unfortunately without math). Correct me if I need to change/add something: Quantum ...
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1answer
41 views

Hubble radius and communication between two observers

According to Dodelson, Modern Cosmology (p.146) There is a subtle distinction between the comoving horizon $\eta$ and the comoving Hubble radius $(aH)^{-1}$. If particles are separated by ...
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2answers
74 views

Could the cosmological constant be due to vacuum fluctuations in a box, i.e., in a finite universe?

Assumption: If the universe were a finite box whose boundary is the cosmological horizon, then there would be a zero-point energy inside that box. Consequence 1: This zero-point energy would be given ...
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Was the filament structure of the universe due to the same process as in the continuity equation?

In this long Twitter thread, Dr Phil Metzger suggests that the large-scale filament structure of the universe is due to the same process that determines the continuity equation. The entire tweet ...
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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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Vacuum energy in continuous phase transitions

In a first-order cosmological phase transition, the tunneling from the false to the true vacuum (see left picture below) releases false vacuum energy corresponding to the potential difference $\Delta ...
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1answer
236 views

Is MTW's argument in favor of Mach's principle valid?

Looking at older books, I was surprised to see that the general relativity "bible" by Misner, Thorne, and Wheeler is very strongly in favor of Mach's principle, which is treated in section 21.12. ...
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4answers
195 views

What would happen to de Sitter universe if the cosmological constant disappears?

De sitter universe is dominated by a cosmological constant (devoid of matter and radiation), corresponding to dark energy in the far future or the inflaton field in the early universe, leading to an ...
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1answer
60 views

Very Basic Hubble Constant Question

In an article titled "Gravitational Waves Could Solve Hubble Constant Conundrum" Data from the cosmic microwave background suggests the universe is currently expanding at a rate of about 41.6 miles (...
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54 views

Why the large-scale structure is hierarchical? [closed]

Reading a lot after asking my question, I understood that it is too broad. So, I asked only the second part of my question.
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5answers
92 views

Does the Universe itself move to any direction on top of expansion?

So the universe expands right, but does this expansion move to any direction in hyperspace as an expanding cluster of matter somewhere? It's like if you would explode a bucket of paint under the. ...
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2answers
144 views

Do free particles slow down in expanding universe?

My favorite toy model of cosmic microwave background in the expanding universe is a cube with totally reflective walls. If CMB is a black-body radiation, it must be isotropic and homogeneous - the ...
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1answer
65 views

What will happen with the cosmic microwave background radiation if the Big Rip theory is true?

There are three things I can imagine happening: It appears one light year further away every year, which would be a bit weird, because it would be behind the universal event horizon, but still ...