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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Temperature in space
Temperature is a measure of kinetic energy transferred to particles, henceforth, space being vacuum, temperature cannot be measured.
But then, there is cosmic background radiation. It is the leftover ...
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1answer
114 views
Cosmology: can something come from nothing? [closed]
It is often said that something CANNOT come out of nothing? Is this true? Is there a way to test the properties of nothing to make this assertion?
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0answers
49 views
$f_{NL}$ non-Gaussianity in cosmology
In the context of cosmology, what is meant by "..arbitrary quadratic non-Gaussianity i.e non-Gaussianity that is described to leading order by a 3-point function.."? (.."quadratic non-Gaussianity" ...
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1answer
200 views
Dirac Equation in General Relativity
Dirac equation for the massless fermions in curved spase time is $γ^ae^μ_aD_μΨ=0$, where $e^μ_a$ are the tetrads. I have to show that Dirac spinors obey the following equation:
...
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1answer
70 views
Time for Light to travel from most distant objects in universe
I understand that it is possible to detect the most distant objects in space over 13 billion light years away from us and that the universe is 13.75 billion years old.
Does this imply that there were ...
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0answers
55 views
How can I read density fluctuation from microwaves?
The Cosmic Microwave Background Radiation shows temperature differences. The red and yellow areas are warmer. The green and blue areas are cooler.
For example consider this picture of CMBR ...
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0answers
36 views
Vlasov equation in Liddle and Lyth or Dodelson's book
I am concerned about equations 2, 5 and 6 on page 2 of arXiv:astro-ph/0606028.
The paper refers them to the book by Peebles which I don't have access to at this point.
I believe that these ...
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1answer
75 views
The argument for cosmological natural selection
I am reading Smolin's book Life of the Cosmos. One part which I can hardly understand is the argument for cosmological natural selection as a scientific principle.
Smolin keeps on saying that ...
3
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3answers
220 views
Question on inflation
I have two particular questions regarding the inflationary scenario. They are:
1.) What is the physical origin of the inflaton field?
2.) Why has the potential of the inflation field its particular ...
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1answer
118 views
Is Dark Energy converted to Gravitational Potential Energy?
Dark Energy is basically doing work against gravity to accelerate the expansion of universe. So, for time-translational symmetry to be hold, dark energy must be converted to gravitational potential ...
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5answers
480 views
Does the amount of gravitational potential energy in the universe increase as it expands?
It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
3
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0answers
37 views
In the αβγ cosmology paper, how do the authors assume the integral of density over time in the early universe?
In the famous Alpher-Bethe-Gamow paper, the authors say: "it is necessary to assume the integral of $\sigma_n dt$ during the building-up period is equal to $5 \times 10^4 \frac{\text{g ...
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1answer
122 views
Question on inflation as a phase transition
I have just finished watching the following video http://www.youtube.com/watch?v=beQ9fZ0jVdE where Laughlin, Gross and some students discuss e.g. about inflation. The following question is risen:
Is ...
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1answer
69 views
How to understand movement in expanding universe?
I know that universe is expanding equally between every pair of points but it was a single point in it's very past... so I was wondering if we could locate this center point of universe. Now I know ...
4
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2answers
157 views
Total Energy of the Universe?
I've heard the total energy is zero, but I've also heard it cannot be said to be zero since there's so much unknown stuff in the universe. Is that true?
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2answers
73 views
Distance of universe's most distant objects in relation to expansion of the universe
Was reading this article about the Hubble XDF and it had the quote:
The most distant objects here are over 13 billion light years away, and we see them when they were only 500 million years old.
...
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0answers
183 views
Curiosity episode with Stephen Hawking. The Big-Bang
In an episode of Discovery's Curiosity with host Stephen Hawking, he claims the Big Bang event can be explained from physics alone, and does not require the intervention of a creator.
1) His ...
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1answer
162 views
Parallel universe and Infinite monkey theorem [closed]
Is the Infinite monkey theorem helpful for determining the existence of the very same our universe somewhere else?
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2answers
193 views
Dark matter and dark energy [duplicate]
Possible Duplicate:
Dark matter references
I have recently read about dark matter and dark energy, and why physicists think it must exist (dark matter: mass of galaxies are far bigger than ...
2
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0answers
83 views
How does one derive the 2 halo term in two-point correlation function
This question is in reference to the paper here. In Equation (86) on page 28, the authors have given the two point correlation function
\begin{equation*}
\xi(\mathbf{x}-\mathbf{x}^{\prime}) = ...
4
votes
6answers
189 views
Why are distant galaxies not actually tiny bits of matter?
Distant galaxies are said to be moving away from the Milky Way (and us) at speeds approaching the speed of light. Since Special Relativity tells us that any object moving away from us at a velocity of ...
2
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2answers
171 views
What is the current radius of cosmological event horizon?
Doing some crude calculations (using the value of $H_0$ at this point of time only, since it is time dependent but not distance dependent thanks to Johannes answer) what is the radius of cosmological ...
3
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0answers
74 views
Conservation of Energy in the Universe [duplicate]
Possible Duplicate:
Is energy really conserved?
Why can’t energy be created or destroyed?
One of the laws of the universe that dazzles me the most is the law of conservation of energy. I ...
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0answers
90 views
Theoretical early-universe cosmology [closed]
I wanted to know what are the cutting edge/recent papers to read w.r.t theoretical early universe cosmology.
I would like to know of recent review papers
I would like to know of areas which use ...
10
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2answers
271 views
Limitations on how far one can travel in the universe
Someone once incorrectly told me that, given the speed of light is the speed limit of the universe, aliens would have to live for hundreds of years if they are to travel distances of hundreds of light ...
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2answers
101 views
following up dark matter accretion in supermassive black holes
A while ago, there was some conspicuous evidence that supermassive black holes didn't seem to be eating dark matter at the expected rate of 70%-30%, in fact, only 10% of the black hole mass increase ...
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3answers
196 views
If everything in the universe doubled in size overnight, would it be noticeable?
By my understanding, if everything doubled in size, such as the Sun and the Earth, and because the space in between them (which is nothing) can't expand, would the gravities greatly change and the ...
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2answers
468 views
The meaning of multiverse
A) I am intrigued by the multiverse theory as mentioned in Stephen Hawking's new book, "The Grand Design". According to his theory, one can have different 'universes' in one ultimate existence, a ...
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0answers
37 views
Far stellar object going away from us faster than light [duplicate]
Possible Duplicate:
What does Brian Greene mean when he claims we wont be able to observe light from distant stars due to the universe’s expansion?
Since the galaxies are fleeing us faster ...
9
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3answers
434 views
The meaning of imaginary time
What is imaginary (or complex) time? I was reading about Hawking's wave function of the universe and this topic came up. If imaginary mass and similar imaginary quantities do not make sense in ...
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1answer
96 views
Polaris distance correction and implications for cosmological measurements
Polaris has been a guiding light of navigation for centuries. But Polaris also happens to be the closest cepheid variable to our sun. These, together with the type II-A supernovae constitute standard ...
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3answers
205 views
Is observable universe an explanation against Olbers' paradox?
First of all, let me tell you that I'm not a physicist but rather a computer scientist with a mere interest in physics at nowhere near a professional level so feel free to close this question if it ...
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1answer
200 views
Is it possible that the Big Bang was caused by virtual particle creation?
As far as I understand, it is understood that throughout the universe there exists, what is known as, a quantum field from which, due to its fluctuations, temporary (pairs of) virtual particles ...
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0answers
35 views
What's a compact scientific answer to question “(Why there is) / (what is before) the Big Bang?” [duplicate]
Possible Duplicate:
Did spacetime start with the Big bang?
on causality and The Big Bang Theory
Before the Big Bang
What's a compact scientific answer to question "(Why there is) / ...
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3answers
220 views
Do new universes form on the other side of black holes?
I have four questions about black holes and universe formations.
Do new universes form on the other side of black holes?
Was our own universe formed by this process?
Was our big bang a black hole ...
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2answers
217 views
Cosmology questions from a novice
These ideas/questions probably represent a lack of understanding on my part,
but here they are:
1) Cosmologists talk about the increasing speed of expansion of the universe and talk of dark energy as ...
2
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2answers
262 views
(Co)homology of the universe
In this post let $U$ be the universe considered as a manifold.
From what I gather we don't really have any firm evidence whether the universe is closed or open. The evidence seems to point towards it ...
2
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0answers
54 views
Wick rotation for FRW in quantum gravity
There is no timelike Killing vector for FRW cosmologies. In the path integral formalism, is it possible to Wick rotate for quantum cosmology in quantum gravity? If yes, how? If no, how does one work ...
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3answers
206 views
Mechanism for the gravitational field generated by photons
This question follows from a schooling I received in this thread.
I figured that photons do not interact with gravity, except when they've spontaneously converted into a particle-antiparticle pair. ...
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0answers
84 views
Question on the Gell-Mann Low equation
Question on the Gell-Mann Low Equation.
In this paper, http://arxiv.org/abs/1205.3365, page 21, the author argues that if:
t →∞(1-iϵ), all the terms in equation (193) goes to zero, except the first ...
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4answers
416 views
Why can't missing mass be photons?
After a star lives and dies, I assume virtually all of it's mass would be photons. If enough stars have already lived and died, couldn't there be enough photon energy out there to account for all the ...
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2answers
166 views
Assuming SUSY is found to be incorrect, what would then be the most compelling candidates for dark matter?
From what I've read, the only remaining candidates appear to be either sterile neutrinos or MOND (MOdified Newtonian Dynamics -- it does seem to keep changing.)
Did I miss anything else plausible?
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2answers
406 views
How was the Oh-My-God particle observed?
How exactly was the Oh-My-God particle (ultra-high energy cosmic ray) observed and its energy measured?
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5answers
227 views
How is distance measured to far away stars and galaxies?
What I need is an accurate description of the methods used to determine the distance to Andromeda. The Parallax method is for nearby objects as I presume. The red shift method applies, but how do you ...
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1answer
92 views
Fractal Cosmology and Misner's Chaotic Cosmology
I have a question pertaining to the ideas behind the considered homogeneity and isotropic nature of the universe (at a grand scale) versus the theory of a chaotic and anisotropy structure of the ...
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2answers
152 views
Could dark energy be heat? Could dark matter be stuff that is not seen because of its refraction limits?
Heat has to go somewhere. Would the light and heat of the stars in the universe amount up to being dark matter ...and dark energy ...
Dark matter would just be small pieces of stuff.
Dark energy ...
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1answer
295 views
How does an inflationary universe solve the Flatness Problem, Horizon Problem and Monopole Problem? [duplicate]
Possible Duplicate:
What is the evidence for Inflation of the early universe?
I am reading some public science books on inflationary universe, e.g. The Inflationary Universe by A. Guth.
...
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1answer
63 views
Böotes Void and Dark Galxies
Wouldn't it be possible that Böotes Void, a space we generally consider to be "empty" be filled with a number of undetectable dark galaxies?
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2answers
66 views
A thought about Quasars
If Quasars are "beams" of energy exiting a super-massive black hole, in order for them to get through the black-hole's event horizon, they'd have to be traveling faster than the speed of light. My ...
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1answer
271 views
Branes Collision -> Big Bang
Imagine universe occurred when two parallel branes collided, Momentum of Branes converted to big bang kinetic energy after Collision. Thus, high-energy quanta are high-Vibrating strings.
what ...
