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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Why does space expansion not expand matter?

REFORMULATED: I have looked at the other questions (ie "why does space expansion affect matter") but can't find the answer I am looking for. My question: There is always mention of space expanding ...
17
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5answers
3k 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 ...
21
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7answers
4k views

Why is the observable universe so big?

The observable universe is approximately 13.7 billion years old. But yet it is 80 billion light years across. Isn't this a contradiction?
12
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3answers
888 views

Can space expand with unlimited speed?

At the beginning, right after the Big Bang, the universe was the size of a coin. One millionth of a second after the universe was the size of the Solar System (acording to ...
9
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5answers
1k 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 ...
5
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2answers
861 views

on causality and The Big Bang Theory

With the notion of causality, firmly fixed by GR, we derived the concept of a singular point from where space-time begun. Causality alone gives us the possibility to talk about a known past (i.e. ...
16
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4answers
689 views

Does the universe have a center?

If the big bang was the birth of everything, and the big bang was an event in the sense that it had a location and a time (time 0), wouldn't that mean that our universe has a center? Where was the ...
8
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3answers
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What has been proved about the big bang, and what has not?

Ok so the universe is in constant expansion, that has been proven, right? And that means that it was smaller in the past.. But what's the smallest size we can be sure the universe has ever had? I ...
7
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3answers
859 views

How can a quasar be 29 billion light-years away from Earth if Big Bang happened only 13.8 billion years ago?

I was reading through the Wikipedia article on Quasars and came across the fact that the most distant Quasar is 29 Billion Light years. This is what the article exactly says The highest redshift ...
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5answers
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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 ...
10
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7answers
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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 ...
23
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5answers
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What if the universe is rotating as a whole?

Suppose in the milliseconds after the big bang the cosmic egg had aquired some large angular momentum. As it expanded, keeping the momentum constant (not external forces) the rate of rotation would ...
16
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6answers
1k views

Why isn't dark matter just matter?

There's more gravitational force in our galaxy (and others) than can be explained by counting stars. So why not lots of dark planetery systems (ie without stars) ? Why must we assume some undiscovered ...
14
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1answer
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How would we tell antimatter galaxies apart?

Given that antimatter galaxies are theoretically possible, how would they be distinguishable from regular matter galaxies? That is, antimatter is equal in atomic weight and all properties, except for ...
8
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4answers
1k 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 ...
11
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4answers
3k views

Did time exist before the creation of matter in the universe?

Does time stretch all the way back for infinity or was there a point when time appears to start in the universe? I remember reading long ago somewhere that according to one theory time began shortly ...
1
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1answer
389 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 ...
17
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1answer
340 views

Sympletic structure of General Relativity

Inspired by physics.SE: http://physics.stackexchange.com/questions/15571/does-the-dimensionality-of-phase-space-go-up-as-the-universe-expands/15613 It made me wonder about symplectic structures in ...
9
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2answers
662 views

Will the night sky eventually be bright?

I have read that the night sky should have been bright because every spot should end up pointing to a star in the infinite universe but this is not the case because the universe is expanding. I am ...
2
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5answers
2k 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?
0
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3answers
564 views

Whats left at the center of the Universe after Big bang?

If you consider big bang. According to that What's left at the center of universe where the Big bang occured?
2
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3answers
495 views

Does (it make sense to say that ) the universe has a center?

I was reading this page: http://www.guardian.co.uk/science/2011/oct/23/brian-cox-jeff-forshaw-answers and I found this sentence by Brian Cox: That seems to imply that everything is flying away ...
21
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3answers
762 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 ...
15
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6answers
1k views

How can it be that the beginning universe had a high temperature and a low entropy at the same time?

The Big Bang theory assumes that our universe started from a very/infinitely dense and extremely/infinitely hot state. But on the other side, it is often claimed that our universe must have been ...
6
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3answers
763 views

At what speed does our universe expand?

Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand?
5
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2answers
801 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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1answer
395 views

Superluminal expansion of the early universe how is this possible?

Is this a postulate? I get the expansion of the universe, the addition of discrete bits of space time between me and a distant galaxy, until very distant parts of the universe are moving relative to ...
12
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3answers
2k views

If the universe were compressed into a super massive black hole, how big would it be?

I understand only a little of general relativity, but that's why I'm here! :) Consider the hypothetical situation of some extra-terrestrial intelligence pushing all the mass in the universe, every ...
14
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2answers
981 views

What is the theoretical limit for farthest we can see back in time and distance?

13.2 billion years ago the universe was rather small, having started only half a billion years ago. Today, with the help of Hubble Space Telescope, we are able to capture the light of galaxies emitted ...
22
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4answers
1k views

How do we know Dark Matter isn't simply Neutrinos?

What evidence is there that dark matter isn't one of the known types of neutrinos? If it were, how would this be measurable?
7
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2answers
223 views

Assuming that the Cosmological Principle is correct, does this imply that the universe possess an empirically privileged reference frame?

OK...before everyone blasts this with references to the relativistic invariance of the physical laws, time dilation, etc let me add some context. Also, I am an amateur with an interest in physics, so ...
6
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2answers
1k views

What does it mean that the universe is “infinite”?

This question is about cosmology and general relativity. I understand the difference between the universe and the observable universe. What I am not really clear about is what is meant when I read ...
9
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3answers
2k 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 ...
7
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2answers
2k views

Has the speed of light changed over time?

Could someone judge my (stoner) hypothesis that the speed of light has changed over time -- ie. as the universe has expanded in volume light has slowed down, perhaps going so far as back to the big ...
7
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5answers
474 views

Dark matter 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 ...
5
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3answers
475 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 ...
4
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2answers
1k 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 ...
10
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6answers
256 views

How large is the universe?

We know that the age of the universe (or, at least the time since the Big Bang) is roughly 13.75 billion years. I have heard that the size of the universe is much larger than what we can see, in other ...
8
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1answer
630 views

Size of the universe

This is really a follow up to the Shape of the universe question. In the first answer to the question, Ted Bunn says: However, the best available data seem to indicate that the Universe is very ...
5
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1answer
303 views

Lookback Time & Age of the Universe Calculations

I try to calculate the age of the universe with the FLRW model: $$ H(a) = H_0 \sqrt{\Omega_{\mathrm{R},0} \left(\frac{a_0}{a}\right)^4 + \Omega_{\mathrm{M},0} \left(\frac{a_0}{a}\right)^3 + ...
2
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1answer
196 views

Observable (in principle) signal of a bubble collision in eternal inflation

Assuming a scenario of eternal inflation with a lot of "bubble universes" expanding, Lenny Susskind explains here that a potential signal of a collision of our universe with another bubble could be a ...
14
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8answers
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Reversing gravitational decoherence

[Update: Thanks, everyone, for the wonderful replies! I learned something extremely interesting and relevant (namely, the basic way decoherence works in QFT), even though it wasn't what I thought I ...
7
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2answers
2k views

How can something finite become infinite?

How can the universe become infinite in spatial extent if it started as a singularity, wouldn't it take infinite time to expand into an infinite universe?
4
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1answer
730 views

Supermassive black holes with the density of the Universe

This question was inspired by the answer to the question "If the universe were compressed into a super massive black hole, how big would it be" Assume that we have a matter with a uniform density ...
7
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2answers
997 views

photons in expanding space: how is energy conserved?

If a photon (wave package) redshifts (streches) traveling in our expanding universe, is it's energy reduced? If so, where does it go?
4
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5answers
988 views

What is the extent of the universe?

Is there any realistic, understandable, provable (even in some extent) explanation/model for the extent of the universe? What is its shape? and Why? I mean physical explanations not philosophical ...
2
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1answer
410 views

How is the expansion of space itself explained in LQG?

To explain the expansion of space I have often heard people saying that space is continuously created. This picture is usually applied to cosmological scales but I`m nevertheless curious if some ...
7
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3answers
2k views

Do the laws of physics evolve?

Hubble's constant $a(t)$ appears to be changing over time. The fine stucture constant $\alpha$, like many others in QFT, is a running constant that varies, proportional to energy being used to measure ...
6
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2answers
305 views

Will acceleration rate of expansion of space become faster than speed of light?

From watching cosmology lectures, it seems that the space between galaxies is expanding at an accelerating rate, my question is since it is the space that is (acceleratingly expanding), the special ...
4
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4answers
638 views

Experimental evidence for parallel universes

My idea of physics is that it is a collection of mathematical laws relating observables. And that one can perform alot of mathematical derivations on these laws to produce new laws between ...