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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2answers
497 views

How does universal inflation fit with the Planck length?

If the universe is undergoing inflation, and there is a minimum scale that things can exist at (the Planck length), does that mean that new Planck-sized domains have to be continuously popping into ...
1
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2answers
51 views

Locating a Comet, how is it done?

How are the comet-hunters able to precisely locate/know about the required comets ? How can they distinguish between comets from such a far distance ? How are they able to estimate the exact time ...
2
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2answers
2k views

Parabolic or Hyperbolic?

How can astronomers find the difference between a parabolic and a hyperbolic comet ? What are the criteria that helps them distinguish these ? Can a parabolic comet switch over to become a hyperbolic ...
1
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1answer
434 views

Density of universe during recombination

How can I calculate the density of the Universe at recombination?
4
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3answers
142 views

Proportion of dark matter/energy to other matters/energy at the beginning of the universe?

What was the proportion of dark matter/energy to other matters/energy at the moments after the beginning of the universe (standard Big Bang model)?
0
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0answers
26 views

Expansion of Universe [duplicate]

Possible Duplicate: Why space expansion affects matter? If the Expansion is prevalent, i.e. it is observable and true then shouldn't that result in the expansion of the Milky Way galaxy, ...
1
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1answer
116 views

Two definitions: 'semi-classical space-time' and 'supersymmetric Minkowski space'

By reading articles I ran several times into two terms, never being defined so I assume they must have well established definitions somewhere. The first is semi-classical space-time. If I where to ...
1
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2answers
193 views

A question about multiverse

I have a question that I would like to ask here since I'm not an expert or any near to that. Actually I just came up with this question when I was watching The fabric of cosmos. I couldn't find where ...
11
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2answers
492 views

Plotting the CMB power spectrum - Why $C_\ell \ell (\ell+1)$ rather than only $C_\ell$?

I can't find any convincing answer for the following question : Why do we always (or often) plot the CMB power spectrum in this way? I mean the y axis is $C_\ell \ell (\ell+1)$ and not only ...
2
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0answers
85 views

Does dark energy affect asymptotic freedom?

If the Hubble constant is extremely large, what will happen with quark confinement? I guess that quarks will remain confined because of asymptotic freedom. But can gravity or dark energy have any ...
-2
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3answers
576 views

Can the big bang be “light turning into space” at the edge of the universe? [closed]

The theory I've recently come to postulates that: The volume of space filling the universe is finite and is constantly growing, thus the boundaries of the universe are constantly expanding. The ...
0
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1answer
49 views

observation and implied time since creation

I read on a post Big Bang and Cosmic microwave background radiation? We detect light from another 13 billion years ago does this mean that one billion years ago we could only detect light from about ...
1
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1answer
358 views

How is it possible that we see light from shortly after the big bang?

How can astronomers see light from shortly after the big bang? How did we get "here" before the light that emanated from our "creation"?
2
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0answers
91 views

Temperature of the CMB when the Earth formed and the faint young Sun paradox

The cosmic microwave background (CMB) has a modern temperature of about 2.7 K. At the time of the origin of the CMB, about 13.6 billion years ago, it had a temperature of about 3000 K. ...
3
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2answers
869 views

Zero divergence of energy-momentum tensor and gravitational energy

Trying to teach myself general relativity and have just hit yet another confusion. I'm reading that in curved spacetime the energy-momentum tensor has zero divergence, ie ...
1
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1answer
79 views

If space is being doubled, how fast is it doubling? [duplicate]

Possible Duplicate: How long does it take for expanding space to double in size According to the standard concordence model, I heard that it's likely that space is doubled after 11.4 ...
0
votes
1answer
152 views

Proportion of energy compared to mass in universe

In Wikipedia (http://en.wikipedia.org/wiki/Dark_matter), there is a picture that shows us the proportion of matter and energy in the current universe - CMB. It seems to suggest there is nothing that ...
5
votes
1answer
416 views

Does the current acceleration of universe imply that our universe is open?

Does the current acceleration of universe imply that our universe is open? If the universe is closed, from the Friedmann's equation, the acceleration of universe wouldn't be possible, would it be? (Of ...
8
votes
1answer
1k views

In what sense does the universe have an outer edge?

I'm having trouble understanding something about Suskind's Holographic principle. Suskind speaks about the surface of the universe? In what sense does the universe have an outer surface? I'm a bit ...
2
votes
2answers
2k views

How is Big Bang related to theory of relativity?

I'm not someone with good scientific knowledge, so if my question are weird, correct me. I was reading about big bang and I came by the theory of relativity. Can someone explain the relation between ...
0
votes
1answer
104 views

What would be the effects of an inflating universe?

If our universe is inflating, which means it is getting bigger and bigger in size, and every thing it consists of is also enlarging, in a special way such that the mass of the universe remains ...
0
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1answer
323 views

If the unobservable universe is not “open” and will collapse, would we be able to tell?

Assuming hypothetically that it happens now while we are alive and analyzing our observable universe. If the unobservable universe collapses in a "Big Crunch", will we see its effects in our ...
5
votes
1answer
208 views

Do black holes play a role in quantum decoherence?

Sorry for such a vague question but I could have sworn I read somewhere that Hawking proposed the reason we might see a classically appearing universe is due to the possible role of black holes in ...
1
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1answer
490 views

What allows us to infer the existence of dark energy?

A recent Indian examination asked: how do we infer dark energy is there? It gave as options: rotation curves of galaxies, accelerated expansion of the universe, existence of microwave background ...
2
votes
1answer
572 views

spacetime expansion and universe expansion?

First of all, does the expansion of spacetime solely cause the expansion of universe? Secondly, if spacetime is the sole cause, do objects(matter with mass) themselves expand? Thirdly, by spacetime ...
1
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1answer
303 views

String theory landscape and false vacua

What is the multiverse of string theory? I always thought it would be from brane cosmology where there's chances for multiple collisions of the branes that can make a universe. So this "multiverse" ...
2
votes
0answers
37 views

Baryon asymmetry [duplicate]

Possible Duplicate: Baryon asymmetry Baryon asymmetry refers to the observation that apparently there is matter in the Universe but not much antimatter. We don't see galaxies made of ...
2
votes
1answer
253 views

Why is Hawking's No Boundary condition described in terms of an instanton if there's no tunneling?

Regarding his take about combining No Boundary proposal with inflation theory (Hawking and Turok), he talks about the "pea instanton". To my very limited understanding I thought instantons were only ...
6
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3answers
686 views

Baryon asymmetry

Baryon asymmetry refers to the observation that apparently there is matter in the Universe but not much antimatter. We don't see galaxies made of antimatter or observe gamma rays that would be ...
2
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1answer
183 views

Can someone explain probability flux in the tunneling boundary condition of Vilenkin?

This is what's leading to the notion of a quantum universe tunneling from nothing into existence, right? The idea is that probability flux flows out of superspace (configuration space) at ...
0
votes
0answers
231 views

Extrinsic curvature versus Intrinsic curvature (Euclidean versus Riemannian)

Do we believe the universe has any extrinsic curvature at all? As far as I'm aware extrinsic curvature is only used in geometry/math to model the intrinsic curvature, correct? From the answers to ...
10
votes
2answers
166 views

Why did the WMAP mission last so much longer than Planck?

NASA endorsed 9 years of data taken with the Wilkinson Microwave Anisotropy Probe (WMAP). The High Frequency Instrument aboard the Planck satellite ran out of coolant at the start of 2012, after about ...
2
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1answer
366 views

What is the fate of a 3-Torus universe?

Since it is flat, will it expand forever like a flat and open universe or collapse like a closed and curved universe?
0
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1answer
281 views

Is the FRW metric describing the shape of the universe or the shape of the expansion applied to the universe?

Is it (the output, i.e, spherical, flat, hyperbolic) describing the physical shape of the universe and its curvature at any given instance of time or is it describing the shape of the expansion ...
2
votes
1answer
351 views

Could the unobservable universe have curvature (be spherical)?

What prevents the unobservable universe from being a continuously enlarging sphere due to the inflating bubble universes? Our observable universe being close to flat could support such a scenario?
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5answers
315 views

Age of universe estimates

I was recently involved in a discussion on a sister site regarding how tightly coupled Physics is with the age of the Universe (and Earth). I believe that the Earth and the Universe are both billions ...
1
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1answer
451 views

Why is the universe described in terms of Euclidean space and not Minkowski spacetime?

The universe is described as an infinite Euclidean space in cosmology. Why isn't it treated as Minkowski spacetime?
0
votes
2answers
1k views

How did the inflaton field “add” energy to the universe?

How did inflation add energy to the universe? What mechanism did this occur by? In other words, where did that energy come from? Was it due to the quantum fluctuation (or that scalar field rolling ...
3
votes
2answers
369 views

Why does inflation (the inflaton field) push Omega down closer to zero (flatten the universe)?

I know that in our particular case the inflaton field expanded the volume of the universe while simultaneously maintaining a mass-energy density close to the critical density all the while, thus the ...
2
votes
2answers
2k views

Zero energy universe

Regarding this thread: http://www.physicsforums.com/showthread.php?t=506985 Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of ...
1
vote
1answer
543 views

How did the energy/entropy/volume/pressure/temperature relationship exist at the Big Bang and how did it evolve thereafter?

According to the current Big Bang with inflation cosmological model? I was under the mistaken impression that there was very low volume, very high temperature/pressure, very low entropy and the Big ...
6
votes
2answers
2k views

Hubble time, the age of the Universe and expansion rate

The Hubble time is about 14 billion years. The estimated current age of the Universe is about 13.7 billion years. Is the reason these two time are so close (a) a coincidence, or (b) a reflection that ...
4
votes
2answers
331 views

How does the critical density decide the fate of the Universe?

Trying to teach myself cosmology. Given the Friedmann equation $$\left[\frac{1}{R}\frac{dR}{dt}\right]^{2}=\frac{8\pi G}{3}\rho-\frac{kc^{2}}{R^{2}}$$ and the critical density ...
2
votes
1answer
131 views

Earth's beginnings and early years, Re radioactive decay or not

It is said that the Earth and solar system are 4.6 billion years old. Presumably this date is achieved from radioactive decay. If this is the case, since most of the radioactive elements would have, ...
4
votes
1answer
204 views

Very basic question about empty universe

I'm trying to understand the empty universe model. My first textbook (which doesn't cover the empty universe) gives the Friedmann equation as $$\left[\frac{1}{R}\frac{dR}{dt}\right]^{2}=\frac{8\pi ...
3
votes
3answers
485 views

Shouldn't stars disappear from our view due to expansion?

The currently accepted theory of the expansion of the universe would suggest that for any point in the universe, there is a 'boundary' (somewhat like the Schwarzschild radius) beyond which it cannot ...
21
votes
8answers
3k views

Are we inside a black hole?

I was surprised to only recently notice that An object of any density can be large enough to fall within its own Schwarzschild radius. Of course! It turns out that supermassive black holes at ...
8
votes
3answers
304 views

Why doesn't dark energy vary with time?

Trying to teach myself the basics of cosmology. I can see why, in an expanding universe, radiation and matter density, and radiation and matter pressure vary with time. But why is it assumed that dark ...
8
votes
3answers
1k views

How can we be so sure the universe has 3 dimensions?

I'm just a layman here so please bear with me if I don't get all the words or theories by correct name or whatever. I hope it will be clear enough what my question is. Here goes: When I read articles ...
1
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1answer
259 views

Are there any theories or suggestions for how the multiverse came into existence?

I've just seen a documentary about the multiverse. This provides an explanation for where the big bang came from. But it leaves me wondering: how did the multiverse come into existence? Because this ...