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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6
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1answer
84 views

Could observed accelerating expansion be an illusion based on distance?

The thought occurred to me that if we just happened to be born into a time when the the universe started slowing down, it would take some time for us to observe this - especially in more distant ...
2
votes
2answers
69 views

During the “Dark Ages” of the Universe's evolution, how lumpy (anisotropic) and dynamic was the mass distribution?

In the dark ages between recombination (~0.4 Myr post-BB) and reionization (~300 Myr post-BB) of atoms, there was not any condensed-phase matter (except maybe some form of dark matter), nor radiation ...
2
votes
1answer
73 views

Convert from Relative Magnitude to Mass

I have data which gives me the magnitude density (${\rm mag}\,{\rm arcsec}^{-2}$) of M31 as a function of radius. How can I convert these data to the (enclosed) mass at a given radius (for velocity ...
2
votes
1answer
48 views

Why did formation of the first stars (population III) not happen earlier?

Was it the cooling of the universe? Naively, I would expect the lowering density to work against star formation.
-3
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1answer
74 views

If everything in the universe doubled in size overnight, would objects in space appear to move slower? [closed]

I've been struggling with this question for a while. Imagine an object moving from planet A to planet B at a constant speed of lets say 10 m/s. If everything suddenly doubled in size, I would imagine ...
0
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0answers
33 views

Connection between the energy scale when inflation starts, ends, the inflaton mass and the reheating temperature?

Let's assume inflation starts at some energy scale $E_s$ and ends at $E_e$. Is there anything general (I know there are many inflation models) one can say about the inflaton mass and the reheating ...
-1
votes
1answer
52 views

Singularity - Origin Point

How can it be said that the real beginning of the real universe involved a "singularity of zero size and infinite density of mass" when no space existed at all to define any density and when ...
1
vote
1answer
36 views

What are the best parameters for λCDM?

A man with one watch knows what time it is. A man with two watches is never sure. I've seen several combinations of 'standard' λCDM parameters. The NED search site has the three year WMAP as the ...
0
votes
0answers
22 views

How to calculate source counts in cosmology?

I can derive the equation for source count of galaxies as a function of magnitude or flux for a euclidean universe, but don't know how to derive the same for standard model of cosmology or tired light ...
5
votes
2answers
83 views

Velocity of Solar System relative to CMB

In the book 'An introduction to science of cosmology' by Raine and Thomas, they have two equations $$ \frac{\Delta T}{T} = \frac{v}{c}\cos\theta'\qquad (4.14) $$ $$ T = ...
0
votes
2answers
62 views

How to calculate the event horizon and the cosmological radius in a metric?

From reading about general relativity, the event horizon and the cosmological radius are the radius when $f(r)=0$, in the metric $$ ds^{2}=-f(r)dt^{2}+\frac{dr^{2}}{f(r)}+r^{2}d\Omega^{2} $$ However, ...
-1
votes
2answers
66 views

How would Boltzmann Brain function with all anti-matter in it? [closed]

Please cope with me, I have been thinking about Boltzmann Brain yesterday and if Quantum fluctuations always happen in pairs of particle and antiparticle since sum of all energy has to be 0. My brain ...
1
vote
1answer
57 views

How Is Universal Energy Conserved Here? (Cosmology)

There are three scenarios I will like to discuss here. 1- Jupiter and other "failed star" planets. These are gas giant planets which are much much bigger than earth (100 times or more, by mass) but ...
1
vote
4answers
205 views

Boltzmann brain vs Actual universe probability actual calculations [closed]

I have been reading lots of physics lately about how universe came into being. One idea is that universe is long gone (thermal death) and it might not exist the way I feel it and I am a sequence of ...
4
votes
1answer
65 views

How do symmetries break in cosmology?

The way I understand spontaneous symmetry breaking in thermodynamic systems is that the symmetry is actually explicitly broken by an infinitesimally small field. The system chooses one of the ...
0
votes
0answers
39 views

Do the entropy of the universe and third law of thermodynamics conflict? [duplicate]

We know that our universe was in low entropy state at the earlier time after the Big bang. But we also know that the temperature at that times was enormously high. But what the third law of ...
0
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0answers
49 views

Equilibrium temperature of the universe [duplicate]

At what temperature will the universe eventually equilibrate?
0
votes
1answer
91 views

How is the speed in Hubble's law determined, if not from the Doppler shift?

Hubble's law originally mentions the speed (i.e. proper motion) of a receding galaxy, as calculated via the Doppler effect from the observed redshift. However, the observed redshift is today explained ...
0
votes
1answer
124 views

Pre Big Bang knowledge

I have read that no one knows what preceded the Big Bang, but is this true? The Big Bang process (with or without inflation) followed known rules which “existed” prior to time zero. For example, some ...
5
votes
0answers
40 views

In a perturbative FRW cosmology, why do constant-density hypersurfaces define a good gauge?

It appears to be common in the discussion of perturbative FRW cosmologies to choose a gauge using hypersurfaces for special values of some quantity, like surfaces of constant density $\rho$, constant ...
4
votes
1answer
391 views

Is the cosmic microwave background radiation red shifted?

My question is. If the cosmic background radiation is red shifted how would you calculate the wavelength at the time it was emitted ?
4
votes
2answers
71 views

Are orbits possible in de sitter space?

Since the de sitter space has constant positive curvature does that mean that objects can't orbit around other objects?
0
votes
1answer
38 views

Clocks at rest in Kruskal coordinates

The proper time in Kruskal coordinates is given by $$d\tau^2=\frac{4}{e^rr}(dT^2-dX^2)$$ (where r is measured in units of Schwarzschild radii). Since at $T=+/-\infty$, $dT^2-dX^2$ goes to zero for an ...
0
votes
1answer
51 views

Does Schwarzschild metric have cosmological horizon?

Since the space is not expanding in the Schwarzschild metric does that mean that there is no cosmological horizon? Also, what if the Schwarzschild metric was not asymptotically flat and we replace ...
0
votes
2answers
67 views

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 ...
6
votes
4answers
223 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 ...
1
vote
0answers
23 views

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 ...
-1
votes
2answers
108 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). ...
1
vote
1answer
68 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 ...
0
votes
0answers
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 ...
1
vote
3answers
93 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 ...
0
votes
1answer
44 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). ...
3
votes
1answer
41 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?
0
votes
1answer
72 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, ...
0
votes
0answers
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 ...
3
votes
2answers
139 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 ...
1
vote
2answers
187 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 ...
-3
votes
1answer
121 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 ...
0
votes
0answers
31 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 ...
2
votes
1answer
120 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 ...
3
votes
1answer
119 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 ...
4
votes
1answer
86 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 ...
5
votes
2answers
64 views

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 ...
4
votes
2answers
267 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 ...
0
votes
1answer
41 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 ...
3
votes
2answers
167 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 ...
1
vote
2answers
147 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 ...
59
votes
3answers
8k views

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 ...
3
votes
2answers
68 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, ...