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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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
273 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 ...
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3answers
185 views
Could some Red and Blue shifts be the result of light passing through “dark matter”?
As i see it, light behaves in certain ways, as the Double Slit experiement shows,
So when light comes into contact with dark matter, it becomes both a wave and a particle, the wave is bent around the ...
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2answers
413 views
Is 2.5x speed of light possible between two objects?
These news are in Finnish:
http://www.hs.fi/ulkomaat/Kaukaisin+havaittu+galaksi+et%C3%A4%C3%A4ntyy+maasta+valoa+nopeammin/a1305618680897
The main excerpt is:
"Distant galaxy moves away from us as ...
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0answers
83 views
Can decompactification explain the inflation of the early universe?
I've just reread chapter 11 of this book where it is explained among other things, that our four dimensional universe could be unstable concerning a decompactification transition, since potential ...
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2answers
190 views
Did the universe always have 4 space time dimensions?
Is the dimensionality of spacetime in all usual models constant?
3
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1answer
137 views
Cosmology constant as a negative pressure?
It seems a positive cosmology constant behaves like a negative pressure. However, the positive cosmology constant cause a expanding universe. So I am puzzling about this:
A negative pressure cause a ...
2
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2answers
258 views
Equation of the saddle-like surface with constant negative curvature?
What is the equation for the saddle-like 2d surface (embeded in 3d Euclidean space with cartesian coordinates x, y and z) with constant negative curvature frequently used to illustrate open universe ...
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1answer
74 views
Do atoms have lifespans?
From A Short History of Nearly Everything by Bill Bryson:
Atoms, however, go on practically forever. Nobody actually knows
how long an atom can survive, but according to Martin Rees it is
...
3
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2answers
180 views
How is it possible to come to a conclusion that Universe is a result of the Big Bang while we aren't able to observe the entire Universe?
-I'm not a religious person so this is not a denial. I'm just trying to understand the most fundamental topic about Universe.
-I know the Big Bang cosmological model is not a law but it's a theory.
...
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4answers
283 views
How was the universe created?
I do not know much beyond high school Physics. Thus, I am asking this question from almost layman's perspective:
What, as per the best of our existing knowledge and widely accepted among the ...
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0answers
92 views
QFT on fixed Robertson-Walker background spacetime [closed]
What's known about QFT (or even QM) on fixed, but non-Minkowski spacetimes such as the cosmological Robertson-Walker background defined by:
$ ds^2 = c^2 dt^2 - a(t)^2 * (dx^2 + dy^2 + dz^2) $
By ...
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2answers
128 views
Problem with Big Rip Theory
I have heard of the "Big Rip" as one theory for the eventual end of the universe. If the speed at which the universe is expanding, and all the matter inside it is moving away from all other matter at ...
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0answers
50 views
Energy can't be created or destoryed? [duplicate]
Possible Duplicate:
Conservation law of energy and Big Bang?
If energy can't be created or destroyed then how did the big bang create energy? and if energy can't be created then does that ...
4
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1answer
86 views
Experimental foundations of the “otherwise galaxies would not hold together” argument for dark matter
On the question of motivation and evidence for dark matter, there are some illuminating (so to speak) answers on this site. I'd like to understand in detail the foundations of one of the lines of ...
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3answers
192 views
Is the cosmic horizon related to the Big Bang event?
The Universe expands according to the Hubble's law: velocity is proportional to distance.
There must be some distance, therefore, at which the velocity reaches the speed of light. This defines the ...
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1answer
93 views
What would happen in the final days of the universe?
I would like to know the stages of how the universe would end and what would happen and what the possible scenarios are.
I understand that eventually all the stars would burn out and that would ...
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3answers
444 views
Is the total energy of the universe constant?
If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
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2answers
198 views
Einstein tensor in Friedmann equations : where is the missing $c^2$?
I would like to demonstrate the several forms of the Friedmann equations WITH the $c^2$ factors. Everything is fine ... apart that I have a missing $c^2$ factor somewhere.
In all the following $\rho$ ...
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1answer
124 views
Cosmology with a negative cosmological constant
Based on the Friedmann equation for a universe with only cosmological constant,
$$\left(\frac{\dot{a}}{a}\right)^2 \sim \Lambda$$
I would expect the scale factor $a(t) \sim e^{-it}$ if $\Lambda < ...
2
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1answer
62 views
Why is that the space-time associated with the Earth is not shrinking?
If we apply the Friedmann Lemaître equation to the Universe, we find a critical density $\rho_c$ : if the actual density $\rho$ is under it, the Universe will continue to expand, if it is higher than ...
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1answer
374 views
The Universe is a Hologram
First my background: no physics, but Ph.D. in math. I was reading this article on Slashdot. It links to this Wikipedia page. The content of the Slashdot article is that some physicists believe the ...
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9answers
740 views
Can a universe emerge from nothing?
If the Universe is flat and the total energy of the universe can be zero (we don't know if it is, but many theorists support the idea, i.e. at BB initial conditions:
t = 0, V = 0, E = 0) then is it ...
3
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1answer
140 views
Why are black hole singularities stable?
The Friedmann equations says that huge matter densities lead to huge expansion rates. In Newtonian gravity, two massive point particles separated by an infinitesimal distance will experience an ...
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1answer
73 views
What is meant by the term “mean rest frame”?
I found this part in the book of Bernard Schutz on GR. (Sec 12.2: General Relativistic Cosmological Models).
Robertson-Walker metrics: We shall adopt the following assumptions about the universe: ...
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0answers
289 views
Do intergalactic magnetic fields imply an Open Universe?
According to a recent paper on the arXiv, they do. How credible is this result? The abstract says:
The detection of magnetic fields at high redshifts, and in empty
intergalactic space, support ...
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2answers
79 views
Causal reconnection
Can causally disconnected regions join up again? For example the universe is expanding faster than light creating cosmological horizon, but what if something causes the expansion to slow down and ...
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3answers
207 views
Why the red-shift of distant galaxies is considered to be the effect of expanding spacetime?
Why it's not explained just by Doppler redshift caused by faster movement of those galaxies billions of years ago when that light was emitted?
Would the speeds of the galaxies necessary for Doppler ...
3
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1answer
104 views
How do black holes in merging galaxies find each other to merge?
In many accounts of galaxy mergers, the prompt merging of their central black holes, if any, is stated seemingly as too obvious to need further explanation.
While I don't dispute that this may indeed ...
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3answers
119 views
Accelerating expansion of the universe and cyclic model
So I've heard that recent observations indicated that the rate of expansion of the universe is accelerating. Does this indicate that the universe can NOT be cyclical, with an infinite series of big ...
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8answers
2k views
What are cosmological “firewalls”?
Reading the funny title of this talk, Black Holes and Firewalls, just made me LOL because I have no idea what it is about but a lively imagination :-P (Sorry Raphael Bousso but the title is just too ...
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1answer
123 views
Total momentum of the Universe
What is the total momentum of the whole Universe in reference to the point in space where the Big Bang took place?
According to my reasoning (and a bit elementary knowledge) it should be exactly ...
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1answer
175 views
Does the Earth's revolution around the Sun affect radioactive decay?
Premises:
The radioactivity is either hastened or slowed inside a fast moving aircraft.
Speed of fastest aircraft: 3,529.6 km/h.
The earth's revolution is: 107278.87 km/h.
The earth's ...
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0answers
130 views
Implications of Unruh-inertia to theories of gravity
If it turns out to be true that the galaxy rotation curves can be explained away by Unruh modes that become greater than the Hubble scale at accelerations around $10^{-10} m/s^2$ as proposed in here, ...
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5answers
917 views
What is the easiest way to stop a star?
On long enough cosmological time scales, hydrogen and helium nucleii will become scarce in the Universe. It seems to me that any advanced civilisations that might exist in that epoch would have the ...
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2answers
97 views
Cosmological relativistic effects : misunderstanding between cosmological and relativistic communities?
I would like to clarify something that mixes cosmology and relativistic effects. Maybe I'm not understanding something or maybe there a difference of vocabulary between the cosmological and the ...
3
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1answer
137 views
Does cosmological horizon grow or decrease as it radiates?
Ron Maimon in many posts claimed that cosmological horizon is like a big black hole.
Black holes decrease as they evaporate and their radius decreases as well.
So what is with a cosmological ...
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1answer
123 views
Is a new universe created when dark energy rips open a singularity?
Dark energy will tear everything apart, even black holes. Is a new universe created when dark energy rips open the singularity of a black hole ? And does the topology of the universe depend on the ...
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1answer
171 views
Do atoms expand with universe? [duplicate]
Possible Duplicate:
Why space expansion affects matter?
Why does space expansion not expand matter?
As we know, the universe is expanding, galaxies are away from each other. But what ...
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2answers
122 views
Why is the critical density of the Universe non-zero?
I'm missing something and I wondered if someone could enlighten me. As I understand it, Einstein's equations say, among other things, that matter curves spacetime. I also understand, the Universe has ...
2
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2answers
542 views
Was the Universe's entropy equal to zero at the Big Bang? Is zero-entropy state unique?
It is postulated by many cosmologists that at the Big Bang time the universe was in an unusual low entropy state.
Does this claim specifically mean that the entropy of the initial universe was zero?
...
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1answer
137 views
easy or formal proof of Hawking effect … that anyone can understand?
where could i find an explanation (appart from Wikipedia) of how the Hawking's effect is obtained from quantum field theory GR and thermondynamic :D
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1answer
54 views
Observable universe does it account for farther away galaxies measure of the observable universe?
From our planet we can only see so far on any direction.
If a galaxy farther out than ours had a planet would its view extend beyond our measurements?
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3answers
125 views
If the universe started again with exactly the same conditions… would it be the same?
Sorry if this is a basic question but I can't find anywhere to ask else to ask it:
If the universe was created again and exactly the same conditions where in place at the beginning, would everything ...
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0answers
25 views
Reference for stellar figures
Could anybody suggest a reference for the present stellar models? In particular, I would appreciate references containing the core temperatures and pressures of neutron stars...
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5answers
802 views
Is the law of conservation of energy still valid?
Is the law of conservation of energy still valid or have there been experiments showing that energy could be created or lost?
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1answer
177 views
Light coming from distant galaxies
Some of the distant galaxies appear to be receding from us faster than the speed of light due to stretch of the space between us and those galaxies.
By an analogy with the ant on a rope paradox, the ...
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0answers
55 views
Space-time & solar mass
Does the space-time curvature described by Einstein have any affect on the accuracy of our determination in the age of a star or globular cluster? How does this affect our interpretion of how old we ...
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0answers
74 views
Is it really true that slow-roll inflation is incompatible with string theory?
In this blog post, Motl claimed that slow-roll inflation is incompatible with string theory. Is that really true, and why do people think so?
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1answer
135 views
What are the differences between dark energy and a cosmological constant?
My possibly mistaken understanding is that dark energy changes with time, whereas a cosmological constant is, well, constant. What about gravitational clumping? Detecting relative motion?
