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30 views

Formula for the bias of galaxies

From this article : https://arxiv.org/pdf/1611.09787.pdf , I try to deduce the equation that my teacher told me which links 2 quantities : 1) the global number density of galaxies 2) the local ...
1
vote
0answers
23 views

Demonstration about large-scale bias of galaxies

From this article : https://arxiv.org/pdf/1611.09787.pdf , I try to deduce the equation that my teacher told me which links 3 quantities : 1) the global number density of galaxies 2) the local ...
0
votes
0answers
11 views

How does the cosmological constant dominate cold matter perturbations?

I have been studying inhomogeneous universe in physical cosmology, whereupon I arrive at cold matter perturbations which are dominated by various background fluids. I understood the solutions for the ...
2
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2answers
120 views

Why physical / cosmological models that have been falsified weren't abandoned? [closed]

Before anything, I'd like to say that I'm a layman (non physicist) and english is not my main language; I apologize if my choice of words make me sound rude / arrogant, that is not my intent. I often ...
2
votes
1answer
66 views

How do they know the numbers of the energy pie chart of the universe?

They say that the total energy of the universe consists of 4.9\% ordinary matter, 26.8\% dark matter and 68.3\% dark energy. I hear this pie part in virtually every lecture in cosmology. How do we ...
5
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2answers
65 views

How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?

How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?
1
vote
1answer
64 views

Could six copies of the universe exist only interacting by gravity? [closed]

The ratio of dark matter to matter is roughly in proportions $1:5.4$ This suggests that if there were about 6 Universes placed in the same space that were completely invisible to each other, except ...
0
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1answer
47 views

Are pressureless, collisionless and self-interacting dark matter all synonymous?

Is there any distinction between pressureless, collisionless and self-interacting dark matter or does being one imply the other?
4
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2answers
3k views

Do we need dark matter and dark energy, if the behaviour of the universe in its initial stages was similar to that of the Sun? [closed]

According to CMBR the universe was a cloud of plasma and was a perfect black body, $380,\!000$ years after big bang. But the Sun in our solar system also is in the state of plasma, thus making it a ...
1
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1answer
58 views

Which decoupled first? The dark matter or the CMB photons?

According theories that work best for the structure formation which happened first in the history of the evolution of the Universe, the decoupling of dark matter or the decoupling of photons? How sure ...
2
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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
votes
1answer
78 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 ...
1
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3answers
84 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
40 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. ...
-3
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1answer
49 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 ...
0
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1answer
55 views

How do we know the quantity of dark matter in the universe?

I have been reading on dark matter and it is given specific numbers of its quantity, i.e, ~27%, in the universe. How we come up with that number as we have no strong opinion what it is made of? We ...
4
votes
1answer
57 views

Integral of Bolzmann distribution function for dark matter (modern cosmology)

In Dodelson's textbook modern cosmology ch4.5, I wonder if anyone knows how he did the integration by part over the phasespace on the distribution function of Eq(4.72) and Eq(4.78). Here is part of ...
-2
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1answer
74 views

Dark energy and dark matter difference

From the rotation curve of spiral galaxy it is found that for (>>r ) the total mass is not concentrated at the centre but varies as ~r for which the rotational velocities of the stars far from the ...
-1
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2answers
167 views

Could photons decay into Dark Matter? [closed]

So, we can "see" 13.7 billion (intentionally not including expansion) light years in all directions and we "see" a red shift. What if, the reason we can't observe photons beyond this limit is that ...
1
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1answer
157 views

Why is the heat death, rather than a Big Crunch, the most accepted theory of the ultimate fate of the Universe?

The Heat Death is accepted by most as the end of the Universe, but how can that be? Wouldn't the Big Crunch make a lot more sense? I mean, even if everything in the Universe is spread out uniformly ...
1
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2answers
175 views

Farnes, cosmology involving negative mass

I came across a paper recently which seems to be getting a lot of popular press, and a student also emailed me to ask about it: -- Farnes, "A Unifying Theory of Dark Energy and Dark Matter: Negative ...
5
votes
2answers
172 views

If neutrinos are disfavoured as DM candidates why aren't axions?

Numerical simulations of observed large-scale structure formation work best with Cold Dark Matter (CDM; see the answer here). Neutrinos are candidates for Hot Dark Matter (HDM), and hence they cannot ...
6
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1answer
641 views

Why do physicists think that the dark matter is cold?

Most of the dark matter in the Universe is cold or nonrelativistic which rules out neutrinos as dark matter candidate as they are hot or relativistic. But why do physicists think that the dark matter ...
2
votes
1answer
25 views

How can I understand the reaction rate of WIMP dark matter intuitively?

I'm reading a cosmology textbook(sorry it's Japanese) and stacked at statement that If the mass of WIMP is enough smaller than 100GeV, the reaction rate is represented as $\langle \sigma_a |\mathbf{\...
-1
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1answer
83 views

Chemical equilibrium for a $2-2$ scattering and Boltzmann equation in cosmology and astroparticle physics

Definition of chemical equilibrium According to the definition of chemical equilibrium in Wikipedia, it is a situation where the rate of the forward reaction is same as the rate of backward reaction ...
16
votes
2answers
4k views

How long has dark matter been around?

I've read that dark matter's existence is theorized because galaxies spin/clump in a way that would require much more mass than there appears to be. Also that there is gravitational lensing in places ...
1
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1answer
90 views

Could mass also experience a form of gravitational redshift?

Main question Gravitational redshift for photons are a wellknown phenomena where the electromagnetic radiation is reduced in frequency and thus the wavelenght is increased. The wave–particle ...
3
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1answer
90 views

What is the effect(s) of the kinetic decoupling of WIMPs after it has frozen out?

After freeze-out, the WIMPs departed from chemical equilibrium and till today, it maintains a constant number density. That appears enough to account for the observed relic density. But we also assume ...
1
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1answer
246 views

Why are there a bunch of cosmological coincidences?

There are several cosmological parameters that are more or less of the same order $10^{-60}$ (in Planck unit), namely, inverse of the age of the universe $$ \frac{1}{t_0}, $$ Hubble constant $$ H_0, $...
3
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1answer
275 views

Why is the dark matter freeze-out temperature $T\sim \frac{m}{20}$ so generic?

For a dark matter of mass $m$, the typical freeze-out temperature is $T$ is usually assumed to be between $\frac{m}{20}-\frac{m}{25}$. But the freeze-out temperature depends upon the annihilation ...
1
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0answers
26 views

What might be the equillibrium Distribution Function of Dark Matter?

Can we assign a thermal distribution function to Dark Matter(not necessarily known ones)? Is it possible that they won't even follow thermal distribution function and maybe a function of something ...
1
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0answers
41 views

Experimental constraints on dark matter pressure?

The $\Lambda$CDM model assumes collisionless dark matter, so that its pressure $P=0$. But what are the best experimental constraints on $P$? I guess on $P/\rho$ in fact…
0
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1answer
91 views

Why can't the Higgs vacuum energy clump to galaxies and act as dark matter (instead of dark energy)? [closed]

The Higgs field has a nonzero vacuum expectation value which contributes to the energy density of the Universe. Energies contribute to the curvature of spacetime and affected by gravitational ...
4
votes
1answer
75 views

Why are cosmological contents often modeled as fluids?

Why is it that in cosmology, everything is treated as a "fluid"? What justifies modelling radiation, matter, etc. as "fluids" (even dark energy??)? When does it break down -- surely at small scales ...
4
votes
2answers
309 views

How do we know that the dark matter is cold or non-relativistic? [duplicate]

According to the $\Lambda$CDM parametrization of the Standard Model of Big Bang cosmology, the universe contains a cosmological constant $\Lambda$ associated with $73\%$ dark energy, $23\%$cold dark ...
15
votes
3answers
5k views

How did we 'discover' dark matter? [closed]

I'm an astrophysics student and I've been researching this topic and there is one point that keeps eluding me. How did the scientific community realize that there had to be dark matter in the ...
1
vote
2answers
53 views

Could dark matters massive gravity create areas of slowly released expanding space left over from the big bang? [closed]

I am not a physicist and apologize if this wastes anyone's time. I follow progress in most theoretical fields of physics and understand the premises but not how they are realised. The question I posed ...
2
votes
0answers
46 views

Dark matter and standard model of cosmology: how does it enter into the equation? [duplicate]

I am a physics student and don't know very much about dark matter nor cosmology. But today I went to a talk in which the speaker showed us this image (or a very similar one): showing how the ...
4
votes
1answer
161 views

New observation of a Galaxy without dark matter: what is(are) the implication(s)?

This recent paper in Nature (also see here$^1$) claimed to have discovered a galaxy without dark matter. However, in the Standard paradigm of cosmology dark matter played a crucial role in the ...
4
votes
1answer
142 views

How are the dark matter (DM) thermalization rates derived in singlet DM model?

This well-cited paper talks about a minimal renormalizable extension to the Standard Model (SM) to incorporate particle dark matter (DM) into it by adding a real scalar field $S$ which (unlike the ...
2
votes
2answers
439 views

Why do we think that the dark sector is simpler?

Matter makes up roughly $5\%$ of the Universe while dark matter makes up roughly $25\%$. For matter and radiation, we have a complicated Standard model with many generations of fermions and various ...
0
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0answers
24 views

Alternative to the explanation of large scales phenomena [duplicate]

I've been reading a lot trough different articles about dark-matter and dark-energy and how they could explain observations such as the acceleration of the expansion of the universe or the ...
0
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0answers
35 views

How have protons and neutrons been scientifically eliminated as candidates for Dark Matter? [duplicate]

I know most scientists, if not all scientists, believe Dark Matter cannot be accounted for by baryonic matter. And I don't mean baryonic matter that is unseen because it is not emitting light, I mean ...
4
votes
1answer
113 views

About the consequences of $\mathbb{Z}_2$ invariance in dark matter models

This well-cited paper talks about is a minimal renormalizable extension to the Standard Model (SM) to incorporate particle dark matter (DM) into it by adding a real scalar field $S$ which (unlike the ...
-1
votes
1answer
48 views

Regarding dark energy/matter calculations

Dark energy/matter is assumed to exist based on calculations that don't include terms for radiation energy in transit thru the system under consideration. Could the energy in transit thru a given ...
5
votes
2answers
121 views

Why did dark matter interact with the Standard model particles in the early Universe but not now?

In the thermal dark matter (DM) scenario, it is the interaction of the DM particles with the Standard Model (SM) particles that kept the DM particles in kinetic equilibrium with the thermal plasma. ...
5
votes
1answer
386 views

Why is the Standard model Higgs not a candidate of dark matter (in particular, a WIMP)?

Please consider me as a naive self-learner in this field. The Standard Model (SM) Higgs boson is electrically neutral and has a mass of around $125$ GeV (which lies in the WIMP window i.e., between ...
4
votes
1answer
94 views

Intergalactic dark matter [closed]

A relatively recent research brings evidence for the presence of the dark matter not only inside and near the galaxies, but also in intergalactic space: Matter Distribution around Galaxies, S. Masaki, ...
4
votes
2answers
108 views

What are the steps to identify a dark matter candidate?

By looking at the Lagrangian of a simple particle physics model, how do people identify the dark matter (DM) candidates, if any? One of the criteria is to check whether the electric charge of the ...
3
votes
1answer
118 views

CMB anisotropies without dark matter

One of the strongest hints of dark matter are the CMB anisotropies and the peaks. What should a non-dark matter theory do in order to reproduce that evidence? I mean, what is the feature that a pure ...