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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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1answer
65 views

Why doesn't the centrifugal force cause rotating dark halos to flatten?

Consider a spherical cloud of dark matter like the spherical halo around our galaxy. Please see the diagram below Assuming that the halo is purely made up of dark matter which has only gravitational ...
2
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1answer
86 views

Dark matter should be nonrelativistic at $1$ KeV or earlier. Why?

If the dark matter (DM) is non-relativistic at freeze-out and if the kinetic decoupling occurs much later than freeze-out, it is called cold dark matter. More precisely, it is stated that if the DM ...
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2answers
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Where is the work done of adiabatically expanding universe going into?

According to this Wikipedia article, the universe is expanding adiabatically. However, any system that is expanding adiabatically must be losing energy $\left(\mathrm{d}E = -p\,\mathrm{d}V\right) .$ ...
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1answer
39 views

How does cosmological principle imply an expanding space?

My understanding of cosmological principle is this: Space looks same in all directions and is homogeneous. But this principle was under heading of Observational Evidence for expansion of space in this ...
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3answers
88 views

Can we measure dark matter's effect on spacetime?

I'm not a physicist or mathematician at all, just very curious, so sorry if this is a dumb question. Now I understand we can make astronomical observations and say, "Oh, 85% of gravity in the ...
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0answers
46 views

At what point is a shrinking black hole unable to feed itself due to gravity?

Let's suppose that Hawking Radiation does occur. According to my limited knowledge, as the radius of the event horizon decreases, the emitted Hawking Radiation can (must?) assume ever-smaller ...
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1answer
65 views

Friedmann vacuum solution linear rather than exponential?

Does the Friedmann vacuum equation have a linear solution rather than an exponential one? Using natural units one can write Friedmann's equation for the vacuum as $$ \begin{eqnarray} \left(\frac{\dot ...
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1answer
42 views

Does the scale factor evolution in a matter dominated universe allow for acceleration?

One probably very trivial issue which is bothering me. Standard cosmology says that in a FRW metric, in a matter dominated flat universe (Pressure $P = 0$ and $K = 0$), the scale factor, $a \propto t^{...
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1answer
39 views

Guessing the temperature dependence of a decay rate $\Gamma(A\to B+B)$

For a two-body decay of the form $$A\to B+B$$ if the interaction strength controlling the decay is $\lambda$, the Feynman amplitude $\mathcal{M}$ will contain a factor of $\lambda$ from the vertex ...
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4answers
152 views

Why hasn't anybody done a two-telescope test on the speed of starlight?

It seems that even though it wouldn't be too hard, nobody has taken the trouble to use a pair of telescopes to confirm that light originating from red-shifted / distant stars is indeed traveling at c. ...
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1answer
95 views

Evidence for zero curvature of universe (besides CMB)

There is a beautiful argument, based on the spherical harmonics of the cosmic microwave background, which calculates the curvature of the universe. Are there any other methods of computing the ...
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1answer
70 views

How much did the universe expand since recombination time? [closed]

Assuming adiabatic expansion, and a temperature at recombination time of $2700^{\circ} C$, how much has the universe expanded if the temperature today is $2.7K$? My idea is to use $\frac{dQ}{dt}=A\...
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0answers
63 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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2answers
78 views

On Distant Lanterns and Relative Speed [closed]

It is said, that by measuring distant sources, we can gather information about the past state of the universe. The further we measure, the faster things appear to recede, which brings me to my ...
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0answers
33 views

SNIa lightcurves

How do you explain the fact that in SNIa lightcurves the rate of reception of photons is less than the rate of emission by a factor of 1+z?
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0answers
34 views

Friedmann Expansion

I have a strange question that I can visualize but not find anything to really answer my query. The Newtonian law states that something in motion tends to remain in motion. We all know about ...
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0answers
42 views

Element creation in space

In my astronomy magazine I recently saw the periodic system revisited where the elememnts were color-coded according to their production site. For example, Hydrogen was created from the early Universe ...
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1answer
102 views

Why is the relation between luminosity distance $d_L$ and comoving distance $\chi$ $d_L=\chi/a$?

The textbook (Scott Dodelson, Modern Cosmology, Section 2.2 Distance, Page 35-36) states as follows: Another way of inferring distances in astronomy is to measure the flux from an object of known ...
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1answer
56 views

How do I know that I am observing a quasar?

When I find a faint object on the sky that looks like a star or a far away galaxy how do I know that it really is a quasar? I guess that I could first compare the apparent magnitude (how bright it ...
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1answer
44 views

Gravity and dark energy in space

I've been studying astrophysics and I read that our universe is still expanding because the gravity is not enough compared to the dark energy present. And right now we are not sure whether are ...
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0answers
42 views

What's the difference, if any, between Soft Hair & Quantum Hair

In the early 90s, John Preskill, Sidney Coleman, Frank Wilzcek and Lawrence Krauss presented a series of papers [1][2][3] on Quantum Hair on Black Holes due to Cosmic strings in a number of ways ...
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0answers
64 views

What is the co-moving distance to a point, at which the late-time acceleration of the universe began?

First of all sorry for my English - it is not my native language. According to the following wonderful diagram made by Pulsar, the late-time acceleration of the universe began at $t_{acc}=$ 7.7 ...
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1answer
61 views

Inverse proportional to cosmological scale factor

How can I show using calculation that the temperature of the universe is inversely proportional to the cosmological scale factor? I am just curious, as my textbook states this fact but does not show ...
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1answer
76 views

Is space expanding?

Is the observed stretching of the SNIa lightcurves, ie the delaying of the emitted photons at reception, evidence that space is expanding or should this be explained otherwise and how?
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0answers
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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
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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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0answers
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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0answers
35 views

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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1answer
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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0answers
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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0answers
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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
98 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
45 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
155 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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0answers
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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0answers
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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0answers
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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0answers
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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0answers
84 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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0answers
34 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^{...
3
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1answer
69 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 ...
3
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1answer
42 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
377 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 ...