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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365 views

750 GeV diphoton resonance: KK graviton?

As everybody of you may know at LHC they found this probable resonance (https://cds.cern.ch/record/2114808, https://cds.cern.ch/record/2114853?ln=en). It may be a scalar or a KK graviton mode. Now, ...
4
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1answer
75 views

What is the average temperature of mass in the universe?

That is, ignoring speculative concepts like Dark Matter and just summing across stars and gas clouds etc
1
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1answer
36 views

About the ratio of the density of dark energy at the time of cosmic microwave background emission to the current density of dark energy [closed]

In a question, I am given the current densities of dark energy, dark matter and normal matter and am asked to find the ratio of density of dark energy at the time of CMB and now. The answer is 1. Is ...
-1
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0answers
43 views

Direction of Gravity [duplicate]

I ran into a pde that has a parameter in it that can be either $+1$ or $-1$. They say the $+1$ case corresponds to gravity pointing upwards, whereas the $-1$ case applies to gravity pointing downwards....
1
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1answer
146 views

How long ago, and how far away, could “A long time ago in a galaxy far, far away” have been? [closed]

Assume an event happened "A long time ago in a galaxy far, far away", and assume this event includes intelligent carbon-based life forms. What are bounds$^{a}$ on the time and distance (from us) of ...
4
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0answers
66 views

What would we observe as background on the sky if the big bang had never occurred?

The data we've received so far from satellites such as WMAP paints a near uniform distribution in intensity of the background radiation that we take as evidence that our universe had an origin, and ...
0
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0answers
37 views

I need Resources on CMB Neutrinos

I am doing a research paper for upper level undergrad class on CMB neutrinos (C$\nu$B). I need papers that explain CMB Neutrinos from the ground up like the main theory behind the CMB Neutrinos and ...
1
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1answer
33 views

Photon outside of Hubble sphere from perspective of comoving frame

I understand that there have been alot of questions on Hubble sphere, but I seek a more direct answer to this confusion that I have because I still can't resolve it after going through the posts. ...
0
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0answers
15 views

How is the Lyman alpha forest and Gunn-Peterson trough evidence for Reionization? [duplicate]

I'm trying to understand how the Lyman alpha forest and Gunn-Peterson trough are evidence for Reionization. From what I've read, the Lyman alpha forest tells us how much neutral hydrogen is in the ...
3
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1answer
108 views

Hawking in A Brief History of Time: No beginning or end of the Universe

I am reading Stephen Hawking's book - A Brief History of Time. I haven't studied cosmology and the related sciences. Nevertheless, I am interested to know few things regarding the extract taken from ...
3
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2answers
41 views

Were more stellar objects part of our observable universe in the past? [duplicate]

In the past, i.e. a billion years ago, if the universe is larger than our observable universe, would there have been more objects in our observable universe that have now accelerated beyond it?
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1answer
85 views

How can I best independently study physical cosmology? [duplicate]

My background : I'm currently in my junior year in college majoring in astrophysics. I have taken GR, and some intro cosmology here and there, but nothing in great depth. I had my heart set on ...
2
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3answers
110 views

How big bang could produce later collisions

The Milky Way and the Andromeda galaxies are supposed to "collide" in 4 billion years (collide in the sense of overlapping space, but nothing is really supposed to contact anything else). Assuming: ...
1
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1answer
84 views

Could our universe be destroyed within seconds? [closed]

I'm not that deep in physics. But some days ago I read, there could be different coexisting universes, which could even share "space" (or what ever it is) where each of them could have absolutely ...
1
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2answers
75 views

Relative motion of an arbitrary volume of space ( is space static or dynamic?) [closed]

Do physicists consider space as being "static" or "dynamic" ? Hi I asked a question about whether space can move, and from the answer, I realized I was not precise enough and did not actually ask ...
0
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0answers
28 views

Is there any reason (other than convenience) to assume the universe is paracompact?

In this discussion on MathOverflow, it is mentioned that the universe, being a Riemannian manifold, must be paracompact. But is there any reason to assume the universe is globally 'small enough'? In ...
4
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4answers
192 views

What allows us to assume spacetime is flat when no normal matter is present?

Dark matter causes a bend in spacetime. We see this through gravitational lensing. But what allows us to assume spacetime is flat when no normal matter is present? Why can't we say that dark matter ...
12
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4answers
1k views

How do we know that the cosmic background radiation comes from the early universe?

How do we know that the source of the CMB comes from the early universe, and we don't simply observe the rare interstellar or intergalactic dust of 3K temperature?
4
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1answer
63 views

Is dark matter (if any) the reason why we even exist?

If we were to model a universe without any dark matter in it, would stars and other large scale cosmic structures (galaxies or clusters) form in timescales similar to our universe? Or would they never ...
4
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1answer
75 views

Implications of observable Universe growing indefinitely [closed]

Why, if the observable Universe becomes infinitely large in proper units as time goes to infinity, the objects in space, that we can actually observe, become fewer? I don't know how to use ...
1
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1answer
39 views

When measuring distance how do we know when supernova comes from binary star and not single one?

The way I understand distance is measured for far out objects is by binary star supernova, since at the point of explosion there's is always same amount of energy released. But then supernova can ...
0
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1answer
62 views

How to calculate the scalar amplitude when two inflations happen successively?

Consider a single scalar field with some strange potential such that slow-roll inflation happens twice when it rolls down from the high potential point to the low potential point. In-between the two ...
2
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1answer
109 views

How did inflation lead to the expansion of space?

Big bang Theory gives a scientific explanation of the evolution of the Universe, where the scientific picture of it starts with Universe(space) uniformly filled with matter and energy, already ...
0
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0answers
21 views

Possible fate of our universe? A point in the future where the expansion remains a constant and the universe is just energy dominated.

What if The universe is ever expanding, but slows down at a continuous exponential rate, let's say (e), but continues as an asymptote. Is that liable? Would you say that, when it reached to an ...
1
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1answer
98 views

How are light years measured? [closed]

I know what a light year is but do not know how they are measured.I have tried several sites and can not find the answer. I thought you might have it.
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0answers
32 views

Cosmological inflation in LQG/LQC

I seek the state-of-the-art report on how cosmological inflation is described by loop quantum cosmology. I know that LQG/LQC predicts a big bounce to replace the classical big bang singularity of GR. ...
4
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1answer
61 views

Does the Entropy of the Visible Universe Decrease?

My understanding is that the entropy in a closed system increases. However, I would also assume that the number of particles in the open system of a visible universe decreases because of accelerated ...
4
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1answer
203 views

The CMB and the Hubble comoving radius

I have been thinking on this for a while and i don't get it. My point is, the comoving Hubble radius $(aH)^{-1}$ is the distance at wich the universe expansion begins to be superluminical. In an ...
0
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1answer
48 views

Is it possible for the universe to be already developed when the dark energy appeared and expansion started?

The main piece of evidence that supports Big Bang Theory is basically evidence that the expansion of the universe started long time ago (not necessarily from the begining of the universe). Is it ...
8
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6answers
362 views

If space is “expanding” in itself - why then is there redshift?

The "kid's" way of understanding the expanding universe is that: "space" is totally "ordinary", and all the galaxies are expanding through it (like an explosion). Of course, that's wrong. The usual ...
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0answers
28 views

Curved light cones converge at big bang

Space has expanded manyfold since the big bang, presumably even 1090x since the CMB. Thus our light cones which start out straight must curve inward to theoretically meet at the single point of the ...
0
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0answers
64 views

Size of the universe from CMB?

The CMB is almost 13.8 billion light years away in every direction so the diameter of the visible universe as we actually observe it is 27.6 billion light years, twice the distance light traveled over ...
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1answer
55 views

If there were infinite many stars [duplicate]

If there were infinite many stars, is the sky then always full of light so is there than even night?
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0answers
44 views

Cyclic model of the universe and second law of thermodynamics

Cyclic model from wikipedia The above link shows some theories on cyclic model. It states that a scientist named Richard C. Tolman showed that the cyclic model proposed by Einstein can not be ...
2
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0answers
36 views

Comparing $X_\text{red}^2$ statistics for cosmological models

I'm evaluating a cosmological model which competes with ΛCDM. I have charted the $Χ_{red}^2$ and $R^2$ statistics for the sample of galaxies from McGaugh et al (2001) for my model and ΛCDM. Each ...
0
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0answers
36 views

What is the form of Higgs potential, when written using higgs mass and quartic coupling.

Usually we write Higgs potential as $V=-\frac{1}{2}m^2 \phi^2 + \frac{1}{4}\lambda \phi^4$ What are the present reliable values of parameters '$\lambda$' and '$m$'? Is '$m$' used here Higgs mass? If ...
3
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0answers
66 views

Would quantum fluctuations cause problems for scalar-field inflation?

Wheeler once said that spacetime would be highly curved at very small scales because of the uncertainty principle for energy-momentum. In which case the spacetime becomes very bumpy and not smooth ...
0
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1answer
54 views

Can you escape from a black hole in an expanding universe?

Suppose I have a mass $M$ crushed below the Schwarzschild limit. I'm assuming singularities are unphysical; the mass inside the black hole is a super dense and hot cloud of exotic matter, but it's ...
0
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2answers
114 views

Age of universe?

Well relativity teaches us that time interval between two events is a frame dependent quantity, then how can we say that our universe is 13.8 billion years old? Should it not depend on who is asking ...
0
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1answer
40 views

Space is expanding but the space around me isn't [duplicate]

Everyone says space is expanding and distances between galaxies growing. But space isn't limited to outside of the earth. Space is everywhere on earth and around me. However I don't notice any ...
1
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1answer
38 views

Frequency shift for accelerating source due to equivalence principle? Do cosmologists consider it alongside the Doppler shift?

It's a question that's been bugging me since I first read Einstein's paper on gravitational frequency shift. He derives it using the equivalence principle, and considers an accelerating source first. ...
2
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3answers
258 views

Confusion about the concept of cosmological horizon

This question was inspired by this one By definition we cannot see any event happening beyond the cosmological horizon. Let us assume that the expansion rate of the universe is such that the radius ...
0
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0answers
33 views

“How” infinite is the Universe? [duplicate]

They say the Universe may be infinite. Is it about matter (energy) or space? If it's about space, then would the "spatial infinity" likely imply that the amount of energy is also infinite? If energy,...
3
votes
4answers
208 views

Is it possible to learn about an event that occurred outside of your observable universe?

I have mixed intuitions about this. On one hand, nothing can travel faster than the speed of light and if there is enough space between you and an event then a signal will get stuck in the middle of ...
2
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3answers
86 views

Why didn't helium continue to generate after a few minutes post-Big Bang?

Helium stopped generating after a few minutes after the Big Bang, but what made it impossible for its generation to continue?
0
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0answers
20 views

How long would it take matter to double according to the steady-state theory?

So the universe would have to maintain an average constant density, right? If that's the case, then would I just have to calculate how long it would take for space to double in size? Would I just go ...
0
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1answer
36 views

What would a universe that expands at a constant rate, that has no pressure, and no cosmological constant be like?

Consider a universe that is expanding at a constant rate, or that a double dot is zero, and there is negligible pressure and no cosmological constant. What are some of its properties?
0
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0answers
28 views

Question of the scale factor $a(t)$ for the dark-energy, $\Lambda$, epoch

Just a quick question. I find a lot that the scale-factor $a(t)$ goes as $a(t)\propto{\exp(Ht)}$ in the $\Lambda$ dominated model, however, what I am specifically looking for is the exact scale factor ...
1
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1answer
69 views

What's the bubble's wall made up of in false vacuum decay? [closed]

It is well known that for some kind of double well potentials, there are two minima with one is unstable called the false vacuum while the other stable one called the true vacuum. The tunneling is ...
4
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1answer
91 views

In what manner does momentum of a particle with mass decrease due to spatial expansion?

I've read that the momentum of particles declines due to the universe's expansion. In particular, that $p \propto \frac{1}{a}$, where $a$ is the scale factor. For light, this momentum reduction ...