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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34
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2answers
1k views

Are modified theories of gravity credible?

I'm a statistician with a little training in physics and would just like to know the general consensus on a few things. I'm reading a book by John Moffat which basically tries to state how GR makes ...
0
votes
2answers
144 views

Star trace in the sky

If the space of universe is expanding at fast rates..(even faster than light) Why we do not see the trace of the stars? A possible answer could be that the stars are only moving away but mantaining ...
2
votes
2answers
154 views

Mathematical Elegance in a Cosmological Theory Considered Necessary?

In general, it seems cosmological theories that encompass more and more of the phenomena of the universe are expected to be more and more mathematically elegant, in conception if not in detail. Our ...
4
votes
2answers
342 views

Is our universe a 2+1D superconformal field theory?

If our universe comes from a flux compactification of string theory over 6 dimensions with a nonzero flux, then it can't be continually deformed to another compactification with zero flux as the total ...
1
vote
1answer
272 views

Inflation and string theory in general relativity

How can we relate inflation and string theory? We study some inflationary cosmological model or some string cosmological model under some specific assumptions or conditions. Is their any relation or ...
11
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1answer
332 views

Is almost all entropy in our universe entanglement entropy?

Our observable universe, or a subregion of our universe many times larger than the observable universe, originated from inflating from a very tiny inflationary patch. Being so small, the initial ...
2
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2answers
135 views

Observation of cosmological topological defects

Are there any projects/experiments running for the observation of topological defects (domain walls, monopoles, etc)? Are there any past/canceled or future such projects?
9
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3answers
490 views

Does MOND make good predictions?

Well, it does according to this preprint for certain scales. What would be a simple way to explain MOND to a layman? Does it ignore mainstream physics? How much?
1
vote
1answer
195 views

Gravitational effect of the distant universe

When we observe at close to the cosmological horizon (as close as we currently can) we see the universe as a much younger, denser place. As we feel the gravitational forces from these objects distant, ...
11
votes
4answers
1k views

Dark matter and dark energy references

I've been looking for questions about dark matter, and I've read some very interesting answers. However, I desire too look into it deeply. This is not actually a question. I'm asking the community ...
3
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1answer
441 views

Quantum death like heat death possible?

Quantum decoherence is an irreversible process which is the result of interaction of the system with its environment. It prevents interference due to lack of coherence. Environment acts just like a ...
3
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4answers
2k views

Maximum size of the observable universe?

The size of our observable universe must have grown over time in the early universe. Conversely with the accelerated expansion, I have heard that eventually our observable universe will shrink down ...
7
votes
4answers
905 views

de sitter cosmologic limit

It has been said that our universe is going to eventually become a de sitter universe. Expansion will accelerate until their relative speed become higher than the speed of light. So i want to ...
-2
votes
1answer
565 views

History of Man Acquired knowledge of solar system [closed]

I know the basics of solar system like how earth moves around sun, and that we have so many planets, elliptical orbit of earth, and how far is sun from earth etc etc. I want to take a step back and ...
5
votes
4answers
1k views

Question regarding the validity of the big bounce

I have several questions regarding the "big bounce" theory. It appears to be popular among LQG researchers. My questions are as as follows. 1) How one reconciles it with the fact that it is now ...
13
votes
5answers
3k views

Did spacetime start with the Big bang?

Did spacetime start with the Big Bang? I mean, was there any presence of this spacetime we are experiencing now before big bang? And could there be a presence/existence of any other space-time before ...
6
votes
3answers
675 views

The Big Bang in an infinite universe

If the universe is spatially infinite (and assuming, if it makes a difference, that we don't have eternal inflation), what actually happened 13.7 billion years ago? Was the energy density infinite (or ...
2
votes
0answers
225 views

Singularities in Bianchi models in general relativity ( physical science)

what are the conditions to check point type singularity in a bianchi type model ? bianchi type model are of Type I,II,III,IX,IV or u can say we use different Bianchi type models having some specific ...
15
votes
2answers
1k views

Redshifted Photon Energy

A photon emitted from a receding source (Doppler redshift) has less energy when detected at an observer's location. Please explain the energy loss from the perspective of energy conservation.
9
votes
1answer
3k views

Where does CMB come/emit from?

Where exactly does CMB come from. I've seen it in documentaries as a huge sphere with Earth in the middle. But if all this radiation was ejected from the start of the universe some time after the big ...
1
vote
4answers
331 views

Cosmic background radiation

Where/what is emitting cosmic background radiation, and when did it come into existance, was immediately after the big bang? I know that the universe isn't 3D in the traditional sense, and I don't ...
3
votes
2answers
301 views

asymptotic curvature of the universe and correlation with local curvature

There is not-so-rough evidence that at very large scale the universe is flat. However we see everywhere that there are local lumps of matter with positive curvature. So i have several questions ...
2
votes
2answers
609 views

Can the Universe create itself?--Is Gott's Use of CTC in Planck time or less valid?

My question is the title of a 1991 paper by Richard Gott and Li-Xin Li: http://arxiv.org/abs/astro-ph/9712344 and is also a subject of his popular book, "Time Travel in Einstein's Universe" Ultimately ...
9
votes
3answers
3k views

Why does the low entropy at the big bang require an explanation? (cosmological arrow of time)

I have read Sean Carrol's book. I have listened to Roger Penrose talk on "Before the Big Bang". Both are offering to explain the mystery of low entropy, highly ordered state, at the Big Bang. Since ...
10
votes
3answers
2k views

How many bytes can the observable universe store?

Is the number of states in the Universe countable? What framework could be used to answer the question in the title?
4
votes
1answer
970 views

What is a maximal analytic extension?

Can someone explain (as rigorously as possible) what is involved in analytically continuing, say, the Schwarzschild solution to the Kruskal manifold? I understand the two metrics separately but I'm ...
0
votes
1answer
323 views

Volume of Matter in the Universe [closed]

What would the size of the universe be if it were physically possible to remove all of the empty space, leaving only matter?
4
votes
3answers
765 views

Do singularities have a “real” as opposed to mathematical or idealized existence?

I was thinking of, for example a Schwarzchild metric at r=0, i.e. the gravitational singularity, a point of infinite density. I realise that there are different types of singularities--timelike, ...
15
votes
6answers
2k views

How many times has the “stuff” in our solar system been recycled from previous stars?

Is there a cosmologist in the house? I've got a basic understanding (with some degree of error) of some simple facts: The Universe is a little over 13 billion years old. Our galaxy is almost that ...
0
votes
1answer
159 views

Effect of Cosmological Expansion at the Atomic Level [duplicate]

Possible Duplicate: Why does space expansion not expand matter? Does cosmological expansion have any effects at the atomic level?
3
votes
1answer
237 views

If Fine Structure Constant in not uniform in space what would that imply for cosmic background?

Recently J.Webb submitted paper which has extraordinary claim - that Fine Structure Constant is different in different directions in space! He (with others) measured $\alpha$ using quasar spectrum ...
1
vote
2answers
447 views

Expansion of the space-time metric [duplicate]

If the space-time metric is expanding with the expansion of the universe, if I could travel back in time, would I be less dense than the matter in that previous era?
2
votes
3answers
305 views

Is it physically meaningful to talk about the arrow of time in other universes?

The arrow of time is a concept that has been around for a while. In modern cosmology, it has been argued that there may be different arrows of time in other universes. Since these are also ...
5
votes
3answers
591 views

References for ADM formalism and cosmological perturbation theory [closed]

What would you consider the best online resources for learning the 3+1 ADM formalism and gauge invariant perturbation theory in cosmology? (Assuming intermediate level GR and QFT familiarity)
13
votes
4answers
950 views

Big Bang snuffed by a black hole?

Wasn't the density of the universe at the moment after the Big Bang so great as to create a black hole? If the answer is that the universe/space-time can expand anyway what does it imply about what ...
2
votes
1answer
201 views

Size of our inflationary bubble

Once again, an attempt to connect popular and real science. I have read that inflation implies our universe should look flat, or really close to flat, because our observable universe is only a really ...
5
votes
2answers
466 views

Undergraduate-friendly reading material on the multiverse?

I'll be teaching a seminar for first-year undergraduates next year. The idea of my university's first-year seminar program is to expose students to exciting ideas and important texts in a somewhat ...
8
votes
6answers
592 views

Can (or How) Anthropic Principle be Made into a Scientific Theory?

Anthropic Principle says physical Universe must be compatible with the conscious life that observes it. I thought it is just philosophy mumbo-jumbo and it's not physics. But I could be ...
6
votes
1answer
1k views

Can/has string theory solved cosmological constant problem?

Can someone explain to me what the KKLT paper says, and what has and hasn't it achieved regarding the ability to construct solutions with a small positive or negative cosmological constant in string ...
0
votes
1answer
186 views

Is it a goal of modern physical theory to avoid big bang singularities and how do they approach the problem?

During discussion on another question, the question of singularties in modern physical theories arose. The big-bang is an obvious singularity in modern conceptions of the cosmos. Is it a goal of ...
11
votes
6answers
700 views

How Does Hubble's Expansion Affect Two Rope-Tied Galaxies?

Suppose we have two galaxies that are sufficiently far apart so that the distance between them increases due to Hubble's expansion. If I were to connect these two galaxies with a rope, would there be ...
5
votes
2answers
614 views

Is the observable universe enclosed by an infinitely dense shell?

In modern cosmology, it is commonly stated to the public that as we look deeper and deeper into space, we are seeing further into the past. CMB is the relic of the big bang after photon decoupling ...
2
votes
1answer
703 views

How to calculate the density of relic neutrinos?

May be not neutrinos, but antineutrinos? Or both types? In the last case, why they didn't annihilate and what is the ratio of relic neutrinos to relic antineutrinos? Is that ratio somehow related to ...
9
votes
1answer
382 views

Summing over topologies

Sometimes in quantum cosmology, when we are thinking about 'wave functions of the universe' we have in mind some sort of formal path integral, where we include not just the variations in the dynamical ...
1
vote
1answer
274 views

Why doesn't Brooklyn expand? (Or “Is the expansion of the universe kinematic?”) [duplicate]

Possible Duplicate: Why space expansion affects matter? Imagine two tiny spacecrafts that are moving with the Hubble flow and so are moving away from each other. Let's assume that they've ...
1
vote
0answers
173 views

Why is “dark matter” a more parsimonious explanation than “more normal matter than we thought”? [duplicate]

Possible Duplicate: Why isn't dark matter just matter ? The evidence usually presented in support of dark matter is larger-than-expected masses in galaxy clusters: the mass required for ...
16
votes
4answers
2k views

Shape of the universe?

What is the exact shape of the universe? I know of the balloon analogy, and the bread with raisins in it. These clarify some points, like how the universe can have no centre, and how it can expand ...
30
votes
6answers
7k views

Total energy of the Universe

In popular science books and articles, I keep running into the claim that the total energy of the Universe is zero, "because the positive energy of matter is cancelled out by the negative energy of ...
13
votes
6answers
5k views

What is our location relative to the Big Bang?

Given what we know about space, time and the movement of galaxies, have we or can we determine what our position is in relation to the projected location of the Big Bang? I've read some introductory ...
14
votes
3answers
3k views

How long will the Universe's hydrogen reserves last for?

I recently became really interested in learning about physics and cosmology, but I still know very little. Hopefully someone with more knowledge will be able to shed some light on my questions. Here ...