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.

learn more… | top users | synonyms (1)

3
votes
1answer
101 views

Does inflation predict a multiverse?

Does inflation necessarily predict a multiverse? Or is it the case that "It all depends on the properties of the initial field responsible for inflation, about which we can only speculate"?
2
votes
1answer
120 views

Null Geodesics in Einstein Universe

I am currently taking a course in General Relativity, and I've hit a bit of a roadblock with a homework assignment. We are given the metric for Einstein's universe to be (forgive me, this is meant to ...
3
votes
1answer
139 views

The first $10^{-35}$ seconds [closed]

I am a rank amateur, so please forgive me if the answer to this is well-known. The following quote can in a weekly update for an EdX course I am following in astrophysics: "And what a week it's ...
6
votes
2answers
456 views

When did the first carbon atom in the Universe come into existence?

I am a chemist with a passion for astrophysics and particle physics, and one of the most marvellous things I have learned in my life is the process of stellar nucleosynthesis. It saddens me how my ...
4
votes
1answer
142 views

How does the Cosmic Microwave Background give us information about the Big Bang?

I was reading about CMB after this new breakthrough last week and I could not figure this out. The CMB did not exist before the epoch of Last Scattering. They were just photons which were formed at ...
2
votes
1answer
131 views

Immediately after the Big Bang, was the universe in a state of extremely low or extremely high entropy?

Phase space theory suggests that the largest course-graining region, $p$, in a phase space, $P$, is the point in the phase space with the highest entropy. As such, it is in thermal equilibrium with ...
5
votes
2answers
362 views

Light takes too long to get here

When looking at the night sky, we see lots of stars. Several places tell you that the light of those stars has traveled to many light years to reach Earth and there may be others where light has not ...
3
votes
1answer
77 views

What is $\phi$ as refered to in Guth 2007?

In this 2007 paper by Alan Guth discussing eternal cosmic inflation, he start's using a value $\phi$ on page 8. My understanding is that $\phi$ is the scalar field representing the dark energy of a ...
2
votes
0answers
45 views

Metric to describe an expanding spacetime from coordinates reflecting the perspective of a local observer

The FLRW metric describes the metric expansion of spacetime from the perspective of comoving coordinates. Given the way this metric is usually formulated, comoving distances stay constant, and the ...
0
votes
0answers
235 views

If nothing can travel faster than the speed of light, how can there be parts of the universe we can't see? [duplicate]

Assuming we originated from a single infinitely dense point in space time in the big bang, how can there be parts of the universe that we can't see as the light has not reached us yet, if nothing can ...
3
votes
1answer
139 views

Many times speed of light [duplicate]

http://www.huffingtonpost.com/2014/03/24/theory-of-everything-big-bang-discovery_n_5019126.html What does "many times speed of light" really mean in this context? For a layman it's easy to draw wrong ...
0
votes
1answer
133 views

What's the meaning of the age of the universe?

I'm not asking about how we worked backward from an expanding universe to the age of the big bang, but rather what is the meaning of time in a near infinitely dense point in the context of general ...
8
votes
1answer
137 views

In the B mode power spectrum, what is the relationship between the multipole number and the wavelength of the seed gravitational waves?

One of the key datasets of the recent BICEP2 results is the B mode power spectrum shown below. The existence of these B modes implies the existence of gravitational waves prior to inflation. My ...
4
votes
0answers
93 views

Current state of the existence of absolute space debate [closed]

Samir Okasha writes about the controversy concerning the existence of absolute space in Philosophy of Science: A Very Short Introduction (2002): One of the intriguing things about the ...
0
votes
1answer
147 views

What is the solution of general relativity for our universe? [duplicate]

So I just finished off learning quantum mechanics and special relativity. I just realized that in general relativity, there is Einstein field equation which must be solved in order to talk about ...
0
votes
1answer
48 views

Recession of galaxies

when you take such a given galaxy and it is said that you are moving away from us with a speed $v$, what do you mean really? Let me explain: is the galaxy itself which has an intrinsic velocity of ...
0
votes
2answers
123 views

Size of Universe after inflation

I read in some website that during the period of inflation, the expansion of the universe underwent incredibly fast, and its size increased by a factor of $10 ^{50}$, see this link In this field, I ...
3
votes
0answers
57 views
+50

Fate of largest scale structures?

In $\Lambda\mathrm{CDM}$, structures form "bottom up" with larger structures forming later. Structures are generally speaking supported by the velocity dispersion of their constituent objects (e.g. ...
7
votes
1answer
240 views

The BICEP2 data are evidence of gravitational waves and of inflation. Are they also the first observation that requires quantum gravity?

It strikes me that the recent announcement of data from BICEP2 contains two really Big Deals: the first evidence of gravitational waves the first evidence of inflation. Is there also a third? ...
6
votes
2answers
176 views

Which BICEP2 r value should be compared to Planck's r<0.11?

The BICEP2 paper reports a tensor/scalar ratio $r = 0.20_{-0.05}^{+0.07}$, but then says: Subtracting the various dust models and re-deriving the r constraint still results in high significance of ...
22
votes
2answers
2k views

The age of the universe

Many times I have read statements like, "the age of the universe is 14 billion years" . For example this wikipedia page Big Bang. Now, my question is, which observers' are these time intervals? ...
9
votes
2answers
73 views

CMB curly B-modes and dark matter

I raised a question a while ago regarding weak gravitational lensing of galaxies and the CMB. With all the fuzz with the BICEP2 data, I think it is time to raise even more questions about this amazing ...
3
votes
1answer
216 views

What is the minisuperspace Lagrangian for gravity plus a scalar field?

In this paper by Sean Carroll and Grant Remmen, in equation (11) they write a Lagrangian of the form $$\boxed{\mathcal{L}=3a\left(k-\dot{a}^2\right)+a^3\left[\frac{1}{2}\dot\phi^2-V(\phi)\right]}$$ ...
1
vote
1answer
42 views

Is Λ-CDM and no inflaton field compatible with the observed value of the scalar spectral index?

From the cosmic microwave background, one can extract the scalar spectral index $n_s$. It is measured to be smaller than 1 by several standard deviations. Wikipedia says that it is a parameter of the ...
6
votes
4answers
234 views

Do photons make the universe expand?

I have a problem understanding the ideas behind a basic assumption of cosmology. The Friedmann equations follow from Newtonian mechanics and conservation of Energy-momentum $(E_{kin}+E_{pot}=E_{tot})$ ...
-3
votes
3answers
174 views

Is Space conserved [closed]

Can space be created or destroyed? Is space conserved? I am not asking for matter,energy and time. Its just a question about conservation of space PS: I am asking for what was there before big bang. ...
8
votes
2answers
181 views

What do the names “E mode” and “B mode” mean? Where do they come from?

This has been bugging me a bit since the BICEP announcement, but if there are any resources that answer my question in a simple way, they've been buried in a slew of over-technical or over-popularized ...
9
votes
1answer
191 views

BICEP2 and e-foldings during inflation

After the BICEP2 results, we now know that $n_s = 0.96$ and $r = 0.2$. From what I understand, this fits extremely well with the basic chaotic inflation model given by $V(\Phi) = \lambda \Phi^4$. We ...
3
votes
0answers
62 views

Connection between BAO und CMB Spectrum

I have a problem understanding the connection between the accoustic peaks in the CMB spectrum and the baryon oscillation picture. On the one hand it is stated, that the odd accoustic peaks (1,3,5..) ...
3
votes
2answers
186 views

BICEP2 experiments

How was the polarization experimentally measured in the BICEP2 experiments and why did they look specifically at B-modes? Why is it implying the existence of gravitational waves and the need to ...
5
votes
1answer
266 views

What do the BICEP2 results mean for string gas cosmology and the ekpyrotic universe?

The imprint of gravitational waves created shortly after the big bang may offer direct evidence for inflation theory, according to a discovery by the BICEP2 experiment at the South Pole and released ...
6
votes
2answers
486 views

What experiments compete with BICEP 2, and when are their results expected?

The recent results of the BICEP 2 experiment published on March 17th 2014, has generated a lot of media attention, with the general consensus being that "this is a major discovery" perhaps leading to ...
9
votes
1answer
686 views

What do the line segments on the BICEP2 B-mode polarization map mean?

The first image of BICEP2 visuals shows the "BICEP2 B-mode Signal", described as follows: Gravitational waves from inflation generate a faint but distinctive twisting pattern in the ...
74
votes
5answers
6k views

What was the major discovery on gravitational waves made March 17th, 2014, in the BICEP2 experiment?

The Harvard-Smithsonian Centre for Astrophysics held a press conference today to announce a major discovery relating to gravitational waves. What was their announcement, and what are the implications? ...
0
votes
2answers
487 views

If the universe didn't expand faster than light, would our nights brighter like day?

There's a common QnA which has amused and inspired many kids: There are billions of Stars in the sky. If we combine lights received from all stars, wouldn't it beat Sun? Why is night dark really? ...
0
votes
3answers
200 views

On Group Theory: Symmetry Groups and Our Interest

Over the past few years, I've been doing a lot of self education in the Quantum Mechanics and General Relativity, and of course, there are mathematical elements of both doctrines that are matrices. ...
2
votes
0answers
94 views

General formula to compute the redshift (first order perturbations)

Consider an expanding universe with the following metric in conformal time/co-moving coordinates: ...
0
votes
0answers
43 views

What is the perturbative vacuum?

Will someone please supply me with a good definition of the perturbative vacuum vis a vis quantum cosmology?
0
votes
2answers
291 views

Did the singularity really existed in the begining of the universe?

I read in Hawking's brief history of time the following: The final result was a joint paper by Penrose and myself in 1970, which at last proved that there must have been a big bang ...
-4
votes
1answer
204 views

Is the universe a perpetual motion machine? [closed]

Is the universe a perpetual motion machine?
-1
votes
1answer
182 views

Can we have a static point in the universe? [closed]

Big bang theory states that the universe began from a single point (which we may consider as a central frame of reference for the objects in the Universe). In 1929, Edwin Hubble confirmed that the ...
1
vote
1answer
134 views

Stress-Energy Tensor

As of recent, I've been doing a bit of self education in GR, equipped with a working knowledge of the key elements of the differential geometry in GR, and in looking at the Einstein-Rosen bridge, I ...
-3
votes
4answers
367 views

Was Einstein wrong when he said nothing can go faster than the speed of light? [duplicate]

If the universe is constantly expanding faster than the speed of light, how could Einstein be right?
2
votes
1answer
83 views

Object that causes spacetime to expand?

Is there any thing that will cause spacetime to expand, so that particles are pushed away from them rather than pulled towards it. I know things such as black holes and planets causes dips and curve ...
3
votes
2answers
50 views

Approximation of the energy for low $T$ in the early universe

In Perkins `Particle Physics', to compute the baryon-antibaryon-ratio, he uses that for $Mc^2\gg kT$: $$E= Mc^2 + \frac{p^2c^2}{2m}.$$ I realize that the approximation comes from $E^2=M^2c^4 + p^2c^2$ ...
1
vote
1answer
96 views

How many years of expansion occurred during inflation?

During inflation, expansion happens at a very rapid rate. How many years of expansion did it fast forward through? Meaning, if it weren't for inflation, how many years would it take for the ...
1
vote
1answer
47 views

How is the scale factor from the FLRW equation used with Volume?

I'm trying to put a spreadsheet together that shows the co-moving volume of the universe from the time soon after the Big Bang through the present and then as predicted into the future. I am pretty ...
1
vote
0answers
52 views

Entropy bounds and the size of the universe

The principle that the maximum amount of information or entropy a volume of space can hold is proportional to its surface area apparently applies to all space, not just black holes. Since volume grows ...
1
vote
1answer
68 views

Acceleration of the Expansion of Universe

Is the acceleration of the expansion of the universe (AEU) uniform in direction? Meaning that if a particular direction in the sky is chosen and the AEU is measured and then other directions are ...
2
votes
2answers
173 views

How can space expand?

How can space expand when it is only a perception of the separation between at least 2 objects. Isn't saying "space expands" implying it has properties?