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)

0
votes
0answers
28 views

Does matter shrink due to energy radiation and does it have anything to do with dark matter and dark energy?

Is matter shrinking? Is shrinking the cause of concentrated dark matter that makes up matter moving from concentration to unconcentration. Does dark energy and expansion of the universe come from this ...
-1
votes
2answers
141 views

Most stationary object in the universe

There is an object in the universe for which the Sun appears to be more stationary as it travels through the universe than a star on some other system. There is probably another object for which both ...
3
votes
2answers
252 views

Does the second law of thermodynamics imply a spacetime beginning of the universe?

Recently I have been studying thermodynamics and I noticed a article by a religious person which says that the second law of thermodynamics proves that the universe had a beginning. A spacetime one. ...
1
vote
0answers
35 views

Is the CMB Cold spot growing or shrinking?

What do we know about the nature of this region of the sky? is that region entirely devoid of galaxies? Could it be a bubble void, and we are simply unable to see the galaxies that are on the ...
0
votes
0answers
29 views

Observed flux from distant galaxies

If anyone is able to shed some light on this (pun intended). For an observed distant galaxy with known luminosity: L = 10^10 L(sun), the light is emitted at redshift z = 2 The Flux equation is F = ...
2
votes
0answers
85 views

Drawbacks of single-field inflationary models with $\phi^2$ or $\phi^4$ potential?

I am aware that existing models of inflation, especially the single field chaotic inflation with quadratic or quartic potential, e.g. $$ V = \frac12 m^2 \phi^2 \qquad \text{or} \qquad \lambda \phi^4 $$...
5
votes
1answer
244 views

An argument that massive particles don't redshift?

I start with the spatially flat FRW metric in conformal co-ordinates: $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)$$ This metric has the following non-zero Christoffel symbols: \begin{eqnarray} \Gamma^...
0
votes
3answers
109 views

Why is the universe expanding while its components actually attract each other?

First of all, what's the difference between gravity & gravitation? Anyway that's not the actual question. If every matter attracts each other toward itself, then wasn't the universe supposed to be ...
0
votes
0answers
10 views

DGP model, minus one dimension

Is there a name for a theory like the DGP model, but with one fewer dimension? That is, a theory whose action consists of the 4D Einstein Hilbert action at long distances, and the 3D one at small ...
0
votes
1answer
47 views

how is the state parameter determined?

For radiation, dark energy and dust the pressure and energy density are related, respectively, by: $p=\frac{1}{3}\rho$ $p=-\rho$ $p=0$ My question is why? How does one show that this is how ...
0
votes
0answers
34 views

Cosmology: Gravitational collapse, particle horizon

Looking for some insight on this. I know gravitational collapse happens when the size (lambda) of perturbation is greater than the Jeans length (lambda J) of the medium. How might I express (lambda J)...
0
votes
1answer
36 views

Cosmology: Partical horizons and structure formation

In the concept of 'particle horizons' and structure formation. How would I show that before the matter-radiation equality, the proper particle horizon during this era evolves as Dh=2ct, where t is ...
2
votes
0answers
44 views

What are the definitions and the differences between string “background” and string “vacuum”?

In cosmology one studies perturbations around FRW metric classically (pure GR, we say that we perturbe the FWR "background"). In QFT we have perturbation theory quantistically (we expand around a ...
0
votes
0answers
75 views

Does rest mass increase in the FRW metric?

The flat FRW metric can be written in conformal co-ordinates: $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)$$ where $\eta$ is conformal time. Let us assume that $a(\eta_0)=1$ when $\eta_0$ is the present ...
2
votes
1answer
348 views

Is there a relationship between the Cosmological constant and the Hubble constant?

I looked around to see if this precise question was asked before and it appears not to be. So is it just me or has anyone else noticed that, no matter what consistent set of units you use, $$ \...
0
votes
1answer
58 views

Can we really predict for distances 100s of light years away? [closed]

I read this article today http://timesofindia.indiatimes.com/home/science/8690kmph-winds-discovered-on-exoplanet/articleshow/49789687.cms?utm_source=facebook.com&utm_medium=referral&...
-1
votes
1answer
41 views
0
votes
2answers
126 views

Why doesn't the Cosmic Microwave Background heat my food like a Microwave?

I know the reason we have a CMB is because the photons don't react with any of the matter in the universe, or the mean distance between interactions for photons generated since decoupling is now the ...
-4
votes
0answers
96 views

Is Salah A. Mabkhout's hyperbolic universe theory convincing? [closed]

Did Salah A. Mabkhout develop a formula accounting for the flat rotation curve without a need for dark matter? Is his theory of the hyperbolic universe paradigm convenient? Did his theory connect both ...
4
votes
0answers
151 views

Can space move? [closed]

I have some questions; I hope you don't mind: $\bullet$ If the space between two distance galaxies is increasing, then is the volumes of space in which the galaxies find themselves also moving apart? ...
0
votes
2answers
49 views

Expanding universe via vaccuum?

I was wondering if anyone looked at the expanding model of the universe in terms of a vacuum pulling the universe out rather than an explosion pushing the universe outwards? Or explain why this model ...
1
vote
0answers
67 views

Is there a model of the universe with the transfinite (space)time?

In mathematics there is a concept of ordinal numbers where one can count to infinity and beyond. For example the least number that is greater than all the finite numbers is denoted by $\omega$. Such ...
0
votes
0answers
47 views

Definition of Energy in Friedmann equations?

The first Friedmann equation for a flat Universe is given by: $$\bigg(\frac{\dot{a}(t)}{a(t)}\bigg)^2 = \frac{8 \pi G}{3} \rho(t)$$ The energy density $\rho(t)$ is given by: $$\rho(t) \propto \frac{...
1
vote
1answer
59 views

Is the fluctuation pattern of the Cosmic Microwave Background (CMB) identical in every location of the universe?

I know that the Cosmic Microwave Background (CMB) is the leftover radiation from the "surface of last scattering". Let's say an alien civilisation lives on andromeda or further away. Would they see ...
0
votes
0answers
44 views

Direct vs indirect detection of dark matter

Background There are two categories of searches for dark matter. Direct Detection via reproducible experiments Indirect detection Detection of e.g. annihilation signal from astrophysical sources ...
1
vote
0answers
24 views

Do clouds of dark matter cool and contract?

If dark matter participates in the weak interaction, clouds could cool via thermal neutrino radiation...? And even if does not participate in the weak interaction, clouds should contract via ...
2
votes
1answer
155 views

Infalling light signals seen by a free falling observer

In this question/answer Does someone falling into a black hole see the end of the universe?, it is stated that an observer free falling toward/into a black hole will not see the end of the Universe ...
0
votes
1answer
60 views

Why should we expect dark matter with weak scale masses?

The WIMP "miracle" is often used to motivate WIMPs: that a WIMP with a weak-scale mass naturally freezes out of thermal equilibrium after the big bang with the right relic abundance. I understand the "...
1
vote
1answer
57 views

Describing the shape of a singularity

Hawking and Ellis write about the difficulty of describing the shape of a singularity when presented with a manifold that has curves of finite length that don't reach a point in the manifold. [...
7
votes
2answers
1k views

What is considered now over astronomical distances?

For the sake of discussion, let's say that Mars is exactly 5 light-minutes away and that Earth and Mars are moving with the exact same velocity so that special relativistic effects are irrelevant. Let'...
0
votes
1answer
41 views

Nomenclature “Material System”

This is just a quick question about nomenclature from General Relativity. Most textbooks refer to "material systems"; 1) Does a material system have to be a system with mass? or can it just be a ...
2
votes
2answers
322 views

How accurately could we theoretically see into far space?

Travels to different galaxies are strongly limited by the speed of light. Unless we find a way to travel through space with some wormholes, we will never reach planets in another galaxy. But what ...
0
votes
0answers
41 views

Is a terminal gravitational center to the universe possible?

The current consensus is that there is no evidence of, or no way to tell whether or not there is a center to the universe. Considering our observable universe is only a small portion of what we know ...
2
votes
2answers
111 views

Backwards in time?

A “Universe Today” article titled “Galaxies Are Moving Away From Us Faster than the Speed of Light” prompted the following question, which the author of the article was unable to answer: “Would such ...
3
votes
1answer
76 views

Could we recognize our galaxy by looking at it from a different place in the universe? [duplicate]

Suppose we're in an unknown place in the universe, only knowing that the Milky Way is somewhere is the sky (but of course, younger that it actually is, due to unknown distance light has to travel to ...
2
votes
1answer
63 views

Universe Expansion - Could galaxies be moving away from each other due to photon bombardment?

We know galaxies are moving apart from each other due to the ability to look at the color shift of those galaxies. The fact that galaxies are moving apart seems to be most-widely attributed to the ...
0
votes
1answer
192 views

Is the Universe infinite or it's just expanding to infinity?

According to the WMAP data in the past years we can say that our universe is considered flat or at least nearly flat and we also know that flat universe is allowed to be both infinite or finite in ...
6
votes
3answers
275 views

Why does the FLRW metric assume constant curvature?

So the FLRW metric takes the following form in reduced-circumference polar coordinates. $$\mathrm ds^2 = -c^2 \mathrm dt^2 + a^2(t) \left(\frac{\mathrm dr^2}{1 - k\, r^2} + r^2 (\mathrm d\theta^2+\...
1
vote
2answers
56 views

If the Universe is expanding, how is that galaxies run into each other?

As the title says. If everything is moving outward, thrown by the Big Bang, how could two Galaxies end up with velocity vectors that run them into each other?
1
vote
1answer
55 views

According to some theories space is constantly expanding but at what granularity? [duplicate]

One idea from cosmology is that galaxies are all moving away from each other. Naively this could be taken to mean that they are expanding into something. Popularly this could be seen as them moving ...
3
votes
2answers
153 views

Why is “gravitational” red-shift neglected in galaxy and galaxy cluster scales?

The red-shift of the light of a star in a galaxy or that of a galaxy in a cluster of galaxies is generally interpreted as how fast the star or the galaxy is moving, i.e. it is interpreted in a purely ...
2
votes
1answer
42 views

Redshift of mass-dominant universe

What was the value of redshift z when matter started to dominate the universe? Is there any way to calculate it without knowing the time?
1
vote
0answers
56 views

The steady state theory, can it now be falsified on particle physics grounds, in addition to CMB data?

The steady state theory is no longer taken seriously by most physicists and the Big Bang theory is supported by an enormous amount of evidence, especially the CMB data. But from a quick scan through ...
4
votes
2answers
239 views

Has neutrino redshift been observed?

I understand that neutrinos are emitted when supernova explode. I presume that there are quite accurate models that predict the energies of those neutrinos at the time of emission. Has the phenomenon ...
0
votes
0answers
27 views

de Sitter reviews?

I'm very interested in learning more about de Sitter and anti-de Sitter spaces and their applications in GR and cosmology. Can anyone recommend favorite review articles -- or, more likely, a series ...
1
vote
1answer
83 views

What's between the branes?

So I'm doing my best to explain M Theory to some friends... Mind you, I'm no expert... When we finally got past the obstacle of more than three dimensions... which can be difficult for folks who aren'...
1
vote
1answer
191 views

What is the temperature of the event horizon?

In a discussion with my son about absolute zero, we arrived at the conclusion that the event horizon might be the place to look, as it "absorbs?" all energy, including light. Found this in the ...
0
votes
1answer
40 views

Would those 'trapped' in a collapsing universe perceive space as expanding?

Let's say we have Uber Massif; a black hole so mindbogglingly huge, if you where to shrink the entire milkyway into a thimble Uber Massif would be the size of the milkyway. Those proportions need not ...
2
votes
2answers
89 views

Why can I always see a star? [duplicate]

Light moves along straight lines... Got it. So if the light from that faraway star is traveling in a straight line, and that beam is, considering the distaces involved, at best one or two photons "...
0
votes
1answer
67 views

Do sound waves travel before Big Bang?

So I was reading a Scientific American article about how the universe is spreading apart and how it is getting faster. But in the article there was something that baffled be and it was this "Sound ...