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.
21
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6answers
2k views
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 ...
5
votes
4answers
818 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 ...
8
votes
3answers
1k views
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 ...
9
votes
5answers
2k 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 ...
5
votes
1answer
567 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. ...
8
votes
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 ...
15
votes
4answers
614 views
Why does every thing spin?
The origin of spin is some what a puzzle to me, every thing spin from galaxies to planets to weather to electrons.
Where has all the angular momentum come from? Why is it so natural?
I was also ...
1
vote
1answer
210 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 ...
13
votes
6answers
835 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 ...
6
votes
4answers
913 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 ...
7
votes
4answers
1k 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 ...
5
votes
3answers
422 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 ...
13
votes
1answer
181 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 ...
16
votes
3answers
717 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?
6
votes
2answers
510 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 ...
6
votes
3answers
522 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?
1
vote
1answer
229 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 ...
14
votes
8answers
2k views
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 ...
12
votes
3answers
1k 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 ...
12
votes
6answers
661 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 ...
16
votes
5answers
2k views
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 ...
6
votes
2answers
742 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 ...
4
votes
4answers
552 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 ...
1
vote
1answer
336 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 ...
9
votes
1answer
726 views
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 ...
9
votes
3answers
52 views
Seeing cosmic activity now, really means it happens millions/billions of years ago?
A Recent report about a cosmic burst 3.8 billion light years away. It is written as though it is happening now. However, my question is, if the event is 3.8 billion light years away, doesn't that mean ...
9
votes
2answers
471 views
Why can we see the cosmic microwave background (CMB)?
I understand that we can never see much farther than the farthest galaxies we have observed. This is because, before the first galaxies formed, the universe was opaque--it was a soup of subatomic ...
7
votes
1answer
491 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
...
7
votes
1answer
458 views
On Flatness problem, Inflation etc
I have a couple of naive questions from the topic of the title.
We know
\begin{eqnarray}
\Omega-1=\frac{k}{a^2H^2}-\frac{\Lambda}{3H^2}
\end{eqnarray}
Now I read that from SBB model $\frac{1}{aH}$ ...
6
votes
4answers
307 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 ...
3
votes
2answers
164 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 ...
19
votes
5answers
1k views
Could gravity be an emergent property of nature?
Sorry if this question is naive. It is just a curiosity that I have.
Are there theoretical or experimental reasons why gravity should not be an emergent property of nature?
Assume a standard model ...
12
votes
5answers
391 views
What experiments, other than Hubble Expansion, support the Dark Energy theory?
Dark energy is introduced as a constant inside Einstein's equations. Its primary purpose, from what I understand, is to make Einstein's equations compatible with the accelerating expansion of the ...
8
votes
3answers
340 views
Does the dimensionality of phase space go up as the universe expands?
Ever since Hubble, it is well known that the universe is expanding from a Big Bang. The size of the universe had gone up by many many orders of magnitude as space expanded. If the dimensionality of ...
4
votes
1answer
556 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 ...
5
votes
1answer
964 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?
3
votes
1answer
325 views
What are the theoretical explanations behind the “no-boundary condition” in cosmology?
In his book "The Grand Design" on page 167-173 Stephen Hawking explains how one can get rid of the so called "time zero issue", meaning by this the very special initial state needed for inflation to ...
12
votes
3answers
104 views
What is meant when it is said that the universe is homogeneous and isotropic?
It is sometimes said that the universe is homogeneous and isotropic. What is meant by each of these descriptions? Are they mutually exclusive, or does one require the other? And what implications rise ...
10
votes
2answers
425 views
Can “big rip” rip apart an atomic nucleus?
Some scenarios describing the fate of the matter vs dark energy tug of war on the universe involve the acceleration of the universe increasing to the point that it ends up ripping apart even atoms. ...
8
votes
3answers
1k 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 ...
5
votes
5answers
2k views
What is the evidence for Inflation of the early universe?
The theory of Inflation explains the apparent consistency of the universe by proposing that the early universe grew exponentially for a 1E-36 seconds. Isn't a simpler explanation that the universe is ...
4
votes
5answers
496 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 ...
3
votes
3answers
157 views
What does Brian Greene mean when he claims we wont be able to observe light from distant stars due to the universe's expansion?
Brian Greene in this TED talk about possible multiverse, claims tomwards the end (At around 18:00 mark) this statement. 'Because the expansion is speeding up, in the very far future, those galaxies ...
3
votes
2answers
348 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
2answers
114 views
What is the curvature of the universe?
What is currently the most plausible model of the universe regarding curvature, positive, negative or flat?
(I'm sorry if the answer is already out there, but I just can't seem to find it...)
16
votes
3answers
2k views
Why is the universe so big?
The Universe is approximately 13.7 billion years old. But yet it is 80 billion light years across. Isn't this a contradiction?
4
votes
2answers
217 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 ...
4
votes
2answers
780 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 ...
3
votes
2answers
204 views
Big Bang and Cosmic microwave background radiation?
One of the experimental evidence that supports the theory of big bang is cosmic microwave background radiation (CMBR). From what I've read is that CMBR is the left over radiation from an early stage ...
2
votes
1answer
108 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 ...
