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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WIMP dark matter annihilation rate equation (approximation)?

WIMP is a possible dark matter candidates, which self-annihilates. What is the WIMP annihilation rate, as a function of density? Most sources state this scales as $\rho^2$, (density squared), but ...
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1answer
42 views

Speed distribution of objects in the universe

If we pick a reference frame where, for example, our planet is standing still (although it should be irrelevant), what would the speed distribution of all objects in the universe look like? For the ...
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The universe is said to have begun as a singularity, but how do we know nothing else existed? [duplicate]

When I imagine the big bang, I first imagine a tiny ball surrounded by black void. Then the ball explodes and the universe begins. I assume most physicists have the same basic idea. But how do we ...
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time as consequence of hadronics [on hold]

it has occurred to me that time is solely consequence of non-electric fields, with latest work being reading about «anapole» cite: Simple theory may explain dark matter due to ...
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2answers
93 views

Can anyone tell me the name of this constant?

I was watching a TV program about cosmology (I can't remember which or how old it was). In it, it mentioned a fundamental number (which I'm assuming is a constant, but that's not necessarily true). ...
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1answer
48 views

What is Drake equation?

I was reading an article on aliens, where I found something called Drake equation. I tried to understand it, but just couldn't get it in my head, as I belong to class 11. So, can anyone explain me it ...
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27 views

Does the Big Bang Theory explain about the formation and destruction of matter? [duplicate]

I read here here that, The Big Bang theory explains how the universe was created. Even though it was unable to explain that how everything in the universe did came to existence from no matter state ...
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3answers
49 views

Is there any situation in which a primordial black hole-sized black hole form AFTER the “Big Bang?”

I have seen that primordial black holes may have formed at the start of the universe due to the density of matter. So, is there any way in which our current Universe could spawn a non-stellar mass ...
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1answer
37 views

Equations for Big Bang expansion, distance and energy

On the back page of the CPEP poster “The History and Fate of the Universe” a table gives distance scale (size), temperature, and energy (radiation) for seven expansion times (10-43 sec to now). ...
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36 views

How does space expansion affect cosmic rays?

As high energy protons travel through expanding space do we measure some physical difference depending on how far they travel before reaching us?
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1answer
64 views

Fractional number of particles in the universe?

Is it possible, that the metagalaxy contains a fractional (non whole) number of particles or fractional number of a particular kind of particles (such as electrons)? Sorry, if my question is stupid, ...
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49 views

Modeling of Big Bang [duplicate]

Well, I’m just an ordinary High School passed student. I had passion towards space-time- relativity and other things related with physics :D I have been watching theories of physics like Big Bang from ...
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2answers
115 views

How do we know that the CMBR is the oldest light?

How do we know that the CMBR is the oldest light which we can see? Is it based just on the facts 1.that waves redshift with expanding space, and 2.predictions of the big bang theory; Or is there a way ...
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2answers
110 views

Could we be on the inside of a concave hollow universe?

Recently I was discussing this theory again (a little drunk, I admit) and then tried to find answers, but couldn't find anything satisfying. There is a theory (or several theories) that we could be ...
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1answer
70 views

Are time and space interchangeable? [closed]

Mass and energy are interchangeable. Does that mean time and space are interchangeable too? Reason for question: The only difference I can comprehend between matter and energy is that energy has no ...
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0answers
22 views

Singular isothermal density distribution

Does anyone know where the Singular isothermal density distribution $$ \rho(r)=\frac{\sigma_{V}^{2}}{2\pi\,G\,r^{2}} $$ or how it was derived. Are there any good references on how this equation was ...
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1answer
103 views

How can the Hubble telescope see the light from galaxies that are millions of light years distant?

The Hubble Space Telescope is in the news every now and then. How can it photograph galaxies that are millions of light years from Earth? We can understand that light comes from galaxy and Hubble ...
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1answer
78 views

Do photons have six degrees of freedom?

Calculations involving pressure and volume relationships of photon gas during the cosmologic expansion of the universe posit an adiabatic cooling process with a heat capacity ration of 4/3. This ratio ...
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1answer
52 views

Since space expands (and can theoretically contract) faster than light, does that mean that in a big crunch information could travel FTL?

Layperson here. In a theoretical big crunch with space contracting faster than the speed of light, would information be travelling faster than speed of light? Because the points A and B themselves ...
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2answers
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Is there a proof that space expanding produces observers at all points that see what we see?

I know that galaxies are moving away from us, and so can see that it's intuitive that if space was expanding, then the astronomical observations from Earth would be the same as at all other points in ...
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2answers
93 views

Isn't the accelerating expansion of the universe intuitive?

I have a question about accelerating expansion of the universe. My understanding is that Hubble said the farther our we look into the universe, the higher the redshift, therefore the faster things ...
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1answer
24 views

Evolution of dark matter and gas temperature

So I have recently watched a simulation of dark matter density and gas temperature evolution in a universe. However I couldn't find description of under what assumptions it was was made and what it is ...
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2answers
136 views

How to understand the lifespan of large-scale cosmological structures?

When a star is said to live for 1 billion years, is that relative to the 'average cosmological time unit' or local time in the star's gravitational field? Also, if the sun is said to have 5 billion ...
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2answers
105 views

Is it plausible for spacetime to be shaped something like a torus? [duplicate]

I have heard three theories for how space-time is shaped, flat, sphere-like, or saddle-like. Flat is the most likely, as all our measurements implies that space time has curvature close to 0. Is it ...
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3answers
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Why didn't the Big Bang create heavy elements?

In the case of a supernova explosion it is possible to create heavy elements through fusion. Supernovae have a tremendous amount of energy in a very small volume but not as much energy per volume as ...
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2answers
65 views

Restrictions on theories which describe particle which is the dark matter candidate

Lets have theory which describes (cold) dark matter candidate. I know two cosmological (not astrophysical) restrictions for particle: its lifetime has to be larger than the lifetime of the Universe, ...
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1answer
37 views

Can you compare curvatures in space, spacetime and hidden space?

Spacetime curvature is given by the cosmological constant, that produces a De-Sitter spacetime. It is non-zero. But space curvature is nearly zero (how close to zero, compared to the cosmological ...
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0answers
31 views

why galaxies are formed in spiral shape? [duplicate]

Why galaxies are formed in spiral shape. Do nature follow some specific rules creating universe? What could be in the center?
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1answer
30 views

Is Dark Matter accounted for in early universe inflationary models?

In the early universe there must have been an abundance of dark matter unless it formed by some unknown process later on. If it were present, is this significant and factored into our models?
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Is most of the matter in the observable universe within galaxies?

Do we know, either through observations or through theory driven computer simulations, the location of the majority of the visible matter in the observable universe? That is, is it located within ...
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2answers
63 views

Is it possible that one of the quasars we see is an ancient Milky Way?

As I understand it, even though our current measurements of the Riemann curvature of the universe overlaps with flat spacetime, this doesn't restrict the shape of the universe to being infinite. It's ...
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21 views

Backtracking position [duplicate]

Using a hypothetical photo from a hypothetical telescope, we were able to determine that the farthest galaxy present in the photo is 1 billion light years away. Because we are awesome we were able to ...
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0answers
36 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 ...
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1answer
65 views

Is it possible to directly test whether of not the vacuum gravitates?

According to GR, all sources of stress-energy (e.g. everything on the $T_{\mu\nu}$ side of the EFE) should gravitate (e.g. affect the curvature/$G_{\mu\nu}$ side of the EFE). We observe the expansion ...
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45 views

Any applications of kinetic physics on cosmological scales?

On galaxy cluster scales, one is usually forced to use approximate theories such as hydrodynamics or magnetohydrodynamics for modelling cosmological phenomena. I wanted to ask if kinetic theory is ...
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1answer
102 views

Misconceptions about the universe - Hubble Sphere

I recently watched Veritasium's (excellent) video "Misconceptions about the Universe". However, he said something (at 2min 25s) which I think is wrong. Can somebody please clarify this one way or the ...
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1answer
40 views

Dark age of the universe

during the early universe and just after the habitable epoch,when the universe was finally cooled down allowing the ions to recombine into atoms, comes the Dark Ages who are currently thought to have ...
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1answer
51 views

baryon acoustic oscillation

I have one question about baryon acoustic oscillation. I understand why we should have the baryon-photon fluid sound wave before recombination: Suppose we have a spherical overdense region. This ...
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1answer
95 views

Faster than speed of light [duplicate]

I was watching a Physics TV show, When someone called Alex Filippenko said that when there was the Big Bang, the Space extended at a speed faster than speed of light. He said that it wasn't against ...
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2answers
71 views

Is dark energy homogeneous and/or isotropic?

Shouldn't there be at least some fluctuations of dark energy in various regions, analogous to the fluctuations we observe in other forms of energy like matter?
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0answers
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How is translational symmetry related to Fourier decomposition?

The book (The Cosmic Microwave Background By Ruth Durrer) about cosmological perturbations says that because of translational symmetry of the background at a constant time, we can decompose our ...
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1answer
42 views

Quantum gravity- Vacuum catastrophe

Why does the predicted mass of the quantum vacuum have a very little effect on the expansion of the universe?
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1answer
54 views

Number density of CMB photons?

When we talk about the present-day number density of photons ($n_\gamma\approx 10^8 $ $m^{-3}$) in the universe do we mean the number density of CMB photons? I mean there are other sources of ...
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62 views

Do gamma ray bursts play a role in cosmic evolution?

If gamma ray bursts were to interact with gas clouds in the early universe, might it be a relevant factor in star production?
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1answer
54 views

What $f(R)$ models pass most of the known constraints? [closed]

In most papers and talks about $f(R)$ gravity authors repeatedly state that the model proposed by Starobinsky 2007 $$ f(R)=R+\lambda\,R_{0} \bigg[\bigg(1+\frac{R^{2}}{R_{0}^{2}}\bigg)^{-n}-1\bigg] ...
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3answers
97 views

Can a Dyson Sphere around a Black Hole be built so that it would not radiate significant IR?

This is related to this question If the sphere surrounded a BH and used it as a heat dump (as well as extracting energy from it by dropping in mass) could its exterior be engineered to match the ...
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1answer
45 views

Current constraints on lightest neutrino mass?

This paper from 2005 claims that the mass of the lightest neutrino is unconstrained. (see p9) Oscillations are only able to constrain the differences in squares as far as I know, but perhaps ...
3
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1answer
53 views

Distance between two galaxies of different redshift

Let $Q_1$ and $Q_2$ two different objects in the Universe (we can think to two galaxies or quasars), that we observe from the Earth at different angular position $(\alpha_1,\delta_1)$, ...
5
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1answer
165 views

Speed of light and current dimensions of the universe [duplicate]

I've seen several documentaries explaining that the diameter of the universe is currently estimated at over 90 billion light-years. And which that - in the face of the age of the universe being about ...
3
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1answer
396 views

Creation of a miniature universe?

I am not referring to the experiment being conducted at Lancaster University involving liquid helium and a magnetic field to build a marble sized representation of the early cosmos. My question is: ...