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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3
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1answer
270 views

Has the cosmic alignment of Earth and the Solar System been explained (CMB anisotropy)

The Cosmic Microwave Background anisotropy is currently aligning Earth and the Solar System with the largest and earliest structure of the universe. Has this been explained yet? Or are the theories ...
-2
votes
0answers
28 views

A way to measure absolute velocity [on hold]

Why can't we take universe as a frame and measure absolute velocity with respect to boundaries of the universe? It seems to give absolute velocity as there is nothing outside this universe moving with ...
2
votes
0answers
14 views

Boötes Void numbers, 2016

As a general science (hence Wikipedia!) reader, the latest information I have is that 60 galaxies have been found and counted inside the Boötes void, this of 1997. (1) What is the latest count of ...
1
vote
1answer
15 views

Baryon Acoustic oscillations - simple confusion about length scales

I am confused about length scales and baryon acoustic oscillations. I am clearly missing something very simple. The length scale found in the $SDSS$ and $2dF$ data is quoted as being $150Mpc$. ...
1
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0answers
13 views

Measuring the growth of the Hubble sphere

I have a question which I believe will greatly help my understanding of the Hubble sphere and what it means physically. Say we have a flat ($k=0$) Universe with no dark energy ($\Lambda=0$) during ...
9
votes
3answers
508 views

What is the exact meaning of homogeneity in cosmology?

I understand that, in general, homogeneity is the physical attribute of being uniform in composition (" of the same form at every point"), but I'm slightly confused when it is used in cosmology as ...
0
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0answers
13 views

New approach to cyclic Big Bang [on hold]

I am trying to explain the Big Bang without the need to create a full-fledged universe from nothing in a fraction of a second. I would like to ask the readers of this page if they find any ...
4
votes
2answers
138 views

How does the inclusion of vacuum energy modify Newtonian gravity equation of motion?

The equation of motion (in the center of mass frame) due only to gravitational forces between two point masses is: $$\frac{d^2r}{dt^2} = -\frac{GM}{r^2}$$ How does the equation get modified when a ...
1
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0answers
33 views

Is it possible for dark energy density be decreasing? [on hold]

I have an equation relating dark energy density and temperature, actually they have a linear relationship, my question is how to choose the temperature? If I choose the temperature of universe, the ...
1
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0answers
50 views

The reason for the fading of galaxies?

Galaxies that are greater than a distance of c/H metres from Earth have recessional velocities exceeding the speed of light and begin to fade. Thus, theoretical astronomers 3 trillion years the future ...
1
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1answer
51 views

Cosmic Rest Frame

Cosmic Rest Frame is defined in the paper "Adventures in Friedmann cosmology :A detailed expansion of cosmological Friedmann equations" by R Nemiroff and B Patla as the frame at a point where average ...
1
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0answers
31 views

Are there characteristics that distinguish our universe?

Are there any defining characteristics of our universe that could be different for other universes (or "instances of our universe") that operate under the same laws of physics? And which, if any, are ...
0
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0answers
34 views

Gravitational entropy

In the beginning (big bang), mass was evenly distributed (if you consider an elementary particle as an extended object, like a circle on a very thin cylinder, with a radius near the Planck length, but ...
1
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0answers
29 views

Any method to determine the gravitational constant $G$ from observations in cosmological scale?

By the observations in cosmological scale, I mean observations by astrophysicists like motions of quasars in far distance from us, relative motions of our galaxy to others, etc. I know the famous ...
6
votes
3answers
201 views

Is the Boltzmann brain problem really clearly established as a problem?

Starting from the assumption of a cosmology in which Boltzmann brains dominate over evolved ones, it is not immediately obvious to me that there is a real problem, since only Boltzmann brains ...
2
votes
1answer
20 views

Two Robertson-Walker observers, at what time will a light signal be received?

Here is a question I have that is inspired by this question here. The spacetime metric of a radiation-filled, spatially flat ($k = 0$) Robertson-Walker universe is given by$$ds^2 = - dT^2 + T[dx^2 + ...
0
votes
0answers
74 views

How can there be an infinite number of universes? [on hold]

Many proponents of the multiverse (Greene, Krauss, perhaps Hawking) have posited that there may be an infinite number of alternative universes. How could this be possible, or even understood? ...
4
votes
1answer
201 views

What is the difference between the Big Bang Model and the Λ-CDM Model?

If I'm going to write about "The Consensus Model of Cosmology" should I include Big Bang Model or should I go with just the $\Lambda$-CDM Model?
0
votes
1answer
164 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 ...
4
votes
5answers
518 views

What is the significance of Planck force?

I have been curious to find what could be the significance of Planck force? It is calculated by the formula $c^4/G = 1.21031359\times 10^{44} \, \mathrm{N}$, where $c$ is the speed of light and $G$ is ...
0
votes
2answers
50 views

Cosmic Scale Factor $R$ and redshift

I'm slightly confused by an idea of redshift: If we assume the distance between two points is given by: $$d = R Δx$$ we can assume that $$λ_0 = R_0 Δx$$ And when we receive the emission it will ...
1
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0answers
40 views

Could dark energy and Hawking radiation be the same thing? [closed]

Specifically, could black holes effectively emit some kind of negative mass or other form of dark energy, so that the mass of the black hole itself would actually increase rather than decrease ...
1
vote
2answers
366 views

Hamiltonian constraint in spherical Friedmann cosmology

I'm taking a GR course, in which the instructor discussed the 'Hamiltonian constraint' of spherical Friedmann cosmology action. I'm not quite clear about the definition of 'Hamiltonian constraint' ...
0
votes
1answer
87 views

How can Big Bang happen without time?

I had recently watched a video and he says big bang could have happen by the slow collision rather than a fast collision. But I know that there was no time before the big bang and fast and slow are ...
1
vote
1answer
50 views

Cosmic inflation and space flatness

I am new here, an avid Physics reader, but have lots of questions and doubts on which I'd like to get the help of a pro physicist. Here goes some about the theory of Inflation: a) after the first ...
20
votes
2answers
863 views

Do intergalactic magnetic fields imply an Open Universe?

According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty ...
105
votes
4answers
13k views

Did the Big Bang happen at a point?

TV documentaries invariably show the Big Bang as an exploding ball of fire expanding outwards. Did the Big Bang really explode outwards from a point like this? If not, what did happen?
0
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0answers
23 views

Weinberg Cosmology book Ch 5.2

I am working on Weinberg Cosmology book, and have one question about what contained in Ch 5.2 (page 229). Basically, this chapter is dealing with stochastic initial conditions. What he wrote is that ...
7
votes
4answers
813 views

Experimental evidence for parallel universes/multiverses

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
52 views

Many questions about expanding of the space

Sorry if my grammar is a bit off. But I have few questions about space and its expansion. If space expands faster and faster all the time, will there be a time when we cannot see further than "hubble ...
7
votes
2answers
742 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 ...
9
votes
1answer
393 views

Pressure and density using a general Lagrangian

Given a lagrangian of a form: \begin{equation}\mathcal{L}=f(\phi,\partial_{\mu}\phi\partial^{\mu}\phi)\end{equation} where $f$ is a function, I need to derive pressure and density in a FLRW universe ...
0
votes
1answer
51 views

Big Bang not really the beginning of a completely new universe? [closed]

From what I know about the origins of the universe and the big bang, it is stated that it all started from an intensely hot and dense mass. This sounds like a singularity to me, which means a black ...
-1
votes
0answers
28 views

What are the effects of relativity on the universe itself? [closed]

I'm obviously not anything like an expert on the subject, but it occurs to me that the relativity of objects or lack thereof would affect the environment around it. Time moves fastest when there is ...
0
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0answers
24 views

Question about errors, Hubble's constant

I am just looking through some old notes I have from for cosmology, and theres something cropped up that i can't seem to figure out: Say I have two (or more) values for $H_o$ each with errors such ...
3
votes
1answer
45 views

Will stars disappear from the sky when their particle horizon is outside the event horizon

Given that the universe is expanding at an accelerated rate, will there be a point where stars visible in the night sky will simply disappear from view when their particle horizon is outside the event ...
3
votes
1answer
56 views

How could the universe be hyperbolic if hyperbolic space isn't symmetrical?

In the 2-D projections of the shape of the universe shown here, we see that the flat universe and the spherical universe are perfectly symmetrical, so any triangle drawn anywhere on them will be the ...
2
votes
2answers
40 views

Is the metric expansion of space relatively uniform on different length scales?

Is the Metric expansion of space relatively uniform in space? In other words, loosely speaking, does expansion happens everywhere, and over a wide range of length scales. For example, the Hubble ...
0
votes
1answer
36 views

Expansion of the Universe and Conservation of Energy [duplicate]

How the expansion of the universe doesn't violate the principle of the conservation of energy? If we would put a spring between two objects, the distance would get bigger between them, so would the ...
4
votes
1answer
52 views

What is a slow-roll field?

I am studying inflation reading this article http://lanl.arxiv.org/abs/hep-ph/0406191 and in section 3 it states: This inflaton field may evolve slowly down its effective potential, or not. While ...
0
votes
2answers
194 views

Big Freeze or Heat Death?

I can't seem to find this on the Internet.... Which is most likely to occur to "end" the universe, the big freeze scenario? Or the heat death scenario? P.s. When, if it does at all, will the ...
0
votes
0answers
6 views

What is the relationship between the E mode polarisation of the CMB and the velocity of the primordial plasma precisely?

I understand that the CMB is polarised into E and B modes due to Thompson scattering in the primordial plasma of the early universe. Also, I understand that this polarisation is directly related to ...
0
votes
1answer
60 views

In a static bounded universe, what happens to particles that hit the edge?

Consider a flat universe. The metric tensor is $$g_{\mu\nu}=\left(\begin{array}{cccc}-1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & ...
3
votes
2answers
210 views

How do we know that the galaxies the farthest away from us are actually speeding up?

What i mean is Hubble proved that the galaxies the farthest away from us were moving away faster than the galaxies closer to us. I have a basic understanding of red shift. I've always wondered (and ...
0
votes
0answers
21 views

What is Phase Transition in Cosmology and where does the energy of it comes from?

There are different theories on how our universe will end. The Big Crunch, The Big Rip, The Big Freeze but I also heard about Phase Transition on a space science program on a science channel, what ...
1
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3answers
462 views

What is the speed of acceleration of the inflation of the universe?

Is the inflation speed of the universe accelerating or is it a constant speed of expansion proportional to distance between objects.
5
votes
1answer
65 views

What's the evidence, if any, for some local concentration(s) of dark matter in some region(s) smaller than a galaxy?

What's the evidence, if any, for some local concentration(s) of dark matter in some region(s) smaller than a galaxy? Galactic-sized - or larger - gravitating halos seem to get all the attention. I'm ...
-3
votes
0answers
38 views

General theory of relativity proves beginning of the universe? [duplicate]

As I'm not a physics student, i only have some basic idea about how the theory works. As Hawking quoted in his book - "A brief history of time " Roger Penrose and I showed that Einstein’s ...
-1
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1answer
66 views

What is a simple explanation to the fact that the universe is expanding? [closed]

How can we prove that the expansion of the universe is actually occurring to highschool children?
3
votes
1answer
126 views

Distance between two galaxies of different redshift

Let $Q_1$ and $Q_2$ two different objects in the Universe (we can think to two galaxies or quasars), that we observe from the Earth at different angular position $(\alpha_1,\delta_1)$, ...