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1
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1answer
63 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 ...
-3
votes
1answer
48 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 ...
1
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1answer
65 views

Traveling to another galaxy impossible for ever?

If galaxies are moving apart from each other faster than the speed of light, does that mean that it's impossible to travel to another galaxy, regardless of what technology get discovered? Because you ...
0
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1answer
23 views

Relative size of galaxy calculation

So this is a past exam problem, however I am confused to a question which is related to this graph and the questions states: Question: On the graph, one galaxy is labelled A. Determine the size of ...
6
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2answers
313 views

Could the cosmic fluid be better described as a kind of van der Waals gas, or a kind of foam, instead of a dust gas?

In cosmology, it is usually assumed that the cosmological fluid made of galaxies could be described as a gas of "particles" without any pressure (the dust gas), of density $\rho_{\text{matter}} \...
2
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0answers
88 views

Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...
2
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0answers
41 views

Possible origins of the Antlia 2 galaxy?

At $32.3\,{\rm mag}\,{\rm arcsec}^{-2}$, the Antlia II dwarf galaxy is the most diffuse galaxy ever observed in terms of surface brightness. It was only discovered by pre-selecting stars based on ...
0
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2answers
192 views

What's wrong with the Big Spin Model?

A Dr. Serkan Zorba has a paper on arxiv in which he considers, what if the universe is actually slowly rotating? This gives rise to centrifugal and Coriolis forces on a galactic scale that seem to ...
0
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1answer
133 views

Cosmological redshift : “time of emission” vs “during travel”

Let’s consider a very distant galaxy, suppose it has no peculiar velocity, and let’s assume space expansion; If I understand correctly there would be a redshift due to the value of space expansion at ...
2
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0answers
62 views

Barkana & Loeb's Virial Radius Plot [closed]

Recently I have been trying to teach myself the basics of modern cosmology. To do so I have been using Barkana & Loeb. I've been working my way through and come to virial radius. They give the ...
0
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1answer
90 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 ...
2
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1answer
77 views

What causes structure formation: Baryon acoustic oscillations or inflation?

Formation of structures such as galaxies and galaxy clusters require anisotropies in the cosmic plasma which (as I understand) were due to the anisotropies or density inhomogeneities created by Baryon ...
3
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1answer
125 views

Why are there no more galaxies moving as relativistic speeds?

Introductory note: I am not discussing galaxies that are going away from us or that are at the border of the universe. If there are no preferred frames of reference, no galaxy or matter ensemble can ...
2
votes
1answer
125 views

What is Cosmic Downsizing?

I've had a quick look at a few lecture pdfs and papers as supplementals to my own given lecture notes, but I can't seem to get a proper explanation for what cosmic downsizing is. The closest ...
2
votes
1answer
79 views

Intuitive understanding of the virial radius/mass

I'm currently trying with no avail to understand the intricacies that define the virial radius or mass in a way that is different from the actual galactic radius/mass. I understand it is derived in ...
15
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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 ...
2
votes
2answers
81 views

Understanding a plot obtained from the 2dF redshift survey

The diagram below (taken from here) represents the distribution of Galaxies obtained from 2dF redshift survey. I have a few very naive questions about this. Which point does the centre correspond ...
4
votes
1answer
160 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 ...
0
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1answer
66 views

Original distance from us to galaxy at redshift of z=6 if we know the current distance and taking into account universe expansion?

So I'm given that a galaxy cluster is currently at redshift z = 6 and the question asks "what was its distance from us at the time the light left the cluster?" We only know the redshift, but the ...
1
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2answers
516 views

Is there an average distance between groups of galaxies such as our Local Group?

I believe our Local Group is $10$ million light years in diameter and, depending on which article I read, has $30-52$ galaxies. But what about other galaxy groups? How far would they be? I ...
3
votes
1answer
186 views

Why did the universe continue to expand from inertia after inflation stopped, if galaxies have no motion but space between them expands?

I’ve read many answers on this site about space expansion and I will write from what I understand so please correct me if I’m wrong. The space between galaxies expands but the galaxies themselves are ...
1
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5answers
253 views

Do the distances and velocities observed from galaxies other than Milky way really show that every point in the universe is the center?

Hubble's observation from redshift shows a pattern that the speed of the galaxies is proportional to their distance, by using those informations we can map the position of the galaxies. But these ...
-1
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1answer
205 views

Percent of spiral galaxies in universe

I want to know wich is the % of the galaxies with stable orbits (I think this are only Spiral ones). cheers.
0
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2answers
367 views

Why is dark matter called dark matter?

It has been observered that spiral arm formation galaxies exist. This is a problem because due to the laws of gravitation, stars close the galactic center should orbit with a much higher angular ...
1
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1answer
92 views

Rotation of our Galaxy's inertial frame

Suppose in the universe, there are inertial frames in the vicinity of galaxies. Suppose also that these frames rotate slightly with respect to each other - that the universe is not quite a 'mill pond'....
2
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3answers
125 views

Is it possible to estimate the number of galaxies in the universe?

According to this article the observable universe may contain 2 trillion galaxies. Assuming we know the large-scale curvature of the universe thanks to standard model of cosmology, can we estimate ...
2
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0answers
148 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
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0answers
124 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 (...
3
votes
3answers
474 views

Is there 'gravitational force of repulsion'?

According to hubble's law of universal expansion, the velocity of a galaxy moving away from ours is directly proportional to the distance between the two. Now velocity is increasing in direction away ...
2
votes
1answer
97 views

Does the Milky way have relativistic mass against galaxies which are moving away from it at high speeds?

According to the article cited behind, and to a post in Astronomy SE , there are galaxies moving away from Milky Way faster than light, even at speeds of 2.3c . According to this article Can two ...
7
votes
1answer
115 views

Do galaxies have a halo of neutrinos and cosmic microwave background?

If virial arguments as in "How can dark matter collapse without collisions or radiation?" allow concluding that dark matter could collapse to galactic halos purely gravitationally, then is this true ...
4
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3answers
156 views

Current constraints on Dark Matter self-interaction from galactic profiles

The self-interaction of dark matter may be small but it cannot be negligible if it is able to dissipate energy to relax into galactic clumps (necessary to explain galaxy rotation curves). According ...
3
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1answer
104 views

Why sun has a velocity with respect to the dark matter halo?

I was reading that Sun has a velocity with respect to the local dark matter halo of about 244 km/s. Both sun and the dark matter halo revolve around the Milky way. Since the local dark matter halo ...
2
votes
1answer
267 views

Is the expansion of space universal or local?; does matter impede the expansion of space?

Michael Strauss in his recent book "Welcome to the Universe" with authors Tyson & Gott describes that space is NOT expanding within galaxies but rather between galaxies. So then the expansion is ...
2
votes
1answer
190 views

In large scale cosmological structure, are filaments, walls, sheets and nodes different terms for the same things?

As the question says - are these all essentially synonyms referring to the same phenomenon (described with different words depending on the specific superficial appearance)? Or do they convey a real ...
3
votes
1answer
186 views

A paradox about distant galaxies [duplicate]

When we observe a galaxy farther than 13 billion light years away, we see that galaxy as it was 13 billion years ago. But back then, that galaxy was much closer to us ,if indeed we live in an ...
0
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2answers
143 views

Is the Maffei 1 galaxy gravitationally bound to the Milkway Galaxy?

I watched a Kurzgesagt video explaining how the limit human exploration is the local group, because outside galaxies are accelerating away from us faster than the speed of light, (or faster than we ...
6
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2answers
207 views

How is it that we can detect CMB radiation but not the first stars and galaxies despite CMB originating from before the first stars were born?

The CMB originates from when the Universe became transparent, around 380,000 years after the Big Bang, and stars were born around 100 million years later. I know the first stars and galaxies aren't ...
5
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2answers
597 views

What is the evidence that distant galaxies are moving away from us with speeds greater than $c$, due to space expansion?

I came up with this query after @Rob Jeffries's answer to a previous question of mine. So, is there any evidence that distant galaxies are moving away from us with speeds greater than $c$, due to ...
4
votes
1answer
216 views

Is the gravitational effect of distant galaxies lost forever?

Hubble's law is usually expressed by the equation $$v = H_0D$$ According to this equation, the space between us and very distant galaxies, is expanding with a speed greater than the speed of light ...
2
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1answer
227 views

Cosmic Microwave Background seen from a hypothetical foreign Galaxy?

My basic understanding of the CMB tells me that this 'wall of radiation' is currently the furthest electromagnetic wave from our position that we can detect. Through relativity we know that objects ...
1
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1answer
244 views

Calculating velocity in particular reference frame given redshift

I am trying to do some analysis of galaxy velocities within cluster environments. This involves finding the velocity of that galaxy with respect to the cluster-frame, or the "proper velocity", $v_i$. ...
5
votes
1answer
116 views

Do very distant galaxies look systematically different from nearby galaxies?

According to big bang cosmology, we should see galaxies in an earlier evolutionary phase as they are more distant: is this indeed observed, do (very) distant galaxies or galaxy clusters systematically ...
1
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2answers
326 views

Is there an equation that can estimate chances of alien life in the universe?

Seeing how infinite the universe appears and out of all of those stars, planets, galaxies, there must be other life forms. Mathematically, the odds are very good. Is there a mathematical equation to ...
2
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2answers
216 views

Calculating orbital velocities of stars in disk galaxies

In Barbara Ryden's Intro to Cosmology, chapter 8.2, she derives and equation for the radial velocities of stars around the center of their disk galaxy. First, she states that when we look at disk ...
0
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3answers
296 views

How the speed of light is constant with the particle horizon moving toward us?

I asked a question earlier with the same subject in mind. However, I was rather upset at the time and it didn't make much sense. I'm taking an astronomy course and seem to be misunderstanding ...
0
votes
1answer
133 views

Why does most galaxies resemble a 2 dimensional plane? [duplicate]

Older galaxies mostly tend the revolve forming a disk. Why are the stars not revolving around the center of the galaxy like a sphere instead similar to electron in an atom?
1
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2answers
509 views

Why do galaxies in the Local Group not obey the Hubble law?

The way I understand it is that the galaxies in the Local Group have a gravitational attraction that causes a relative velocity inconsistent with the recessional velocity predicted in the Hubble law. ...
2
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0answers
53 views

Observationally distinguishing a galaxy of antimatter from a galaxy of matter [duplicate]

I was just wondering how, observationally, we would distinguish a distant galaxy of "normal" matter from one of antimatter. Maybe there is a simple answer but I don't see it. Once I started thinking ...
2
votes
1answer
130 views

Do satellite galaxies have the same proportion of dark matter as “ordinary” galaxies

My question is relatively straightforward: Do we know if satellite/ dwarf galaxies contain the same proportion of dark matter to ordinary matter as "regular" sized galaxies? The Milky Way, for ...