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.
3
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0answers
30 views
Time diffeomorphisms breaking in inflation
I am currently working on the topic of inflation.
It seems that at the stage of inflation, the universe can be described as a de Sitter space. In such a space, all spacetime diffeomorphisms are ...
2
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2answers
54 views
Cosmological models other than FRW
The FRW is a nice isotropic and symmetric metric but I think its assumptions are too many. I was wondering about alternative models. Specifically are there any prominent alternatives that have more ...
1
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0answers
28 views
Eddington-Weinberg relation
The numerical coincidence that triggered Dirac to postulate his Large Number Hypothesis can be summarized by expressing the proton-electron gravitational angular momentum in units $\hbar c$:
...
5
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2answers
100 views
Could spontaneous symmetry breaking happen again in our universe?
It is generally believed that $10^{-35}$ seconds after the Big Bang, the symmetry of a GUT was broken and after $10^{-12}$ seconds the electroweak force was broken:
\begin{equation}
\mathrm{SU(2)} ...
2
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3answers
87 views
Scale factor and age of the universe
Why is the cosmological scale factor (expansion rate of the universe) not simply the time $t$, i.e. the age of the universe?
2
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2answers
114 views
Dark age of universe when all fusion process ceases?
Some say we live in the golden age of the universe because there exits countless number of stars that shines in the dark universe. As the supply of gas for star formation is steadily being exhausted, ...
6
votes
1answer
361 views
What could explain the measurements that the Methuselah star is older than the universe?
So there has been talk in the news of a star named Methuselah that is "older than the universe". Moreover, this star happens to belong to our very own Milky Way.
The article mentions that Methuselah ...
0
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0answers
83 views
Maximum entropy Universe?
An attempt to understand the Friedmann equation describing a flat Universe using the principle of maximum entropy together with the laws of thermodynamics. This argument implies that the mass inside ...
4
votes
1answer
90 views
Fulvio Melia's linear Universe
I would be interested in what people think of Fulvio Melia's argument for a linearly expanding Universe.
I realize that the experimental evidence seems to be pointing to an accelerating Universe ...
1
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1answer
45 views
Newtonian approximation in cosmology
In Peeble's "Large scale structure of the Universe"
the Newtonian approximation in cosmology is examined by looking at a transformation that makes the metric locally Minkowski. On pg 38 it is stated ...
0
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5answers
138 views
What generates such a huge amount of gravitational attraction in a black hole?
I had been fascinated to hear that anything can be swallowed by a black hole. But what causes such a huge amount of gravitational force?
1
vote
1answer
71 views
Red shift and the expanding universe
Given that the universe is expanding because space itself is expanding, is that expansion occuring in all places and on all scales? Consider a photon emitted from a source billions of years ago. As it ...
2
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0answers
22 views
what is the radial extent of the last scattering “shell”?
At CMB recombination (z=1090), what is the radial extent of the last scattering "shell"?
a) Delta(z) = ....
b) Delta(comoving angular distance)= ....Mpc
The WMAP first-year parameters give
Delta(z) ...
1
vote
1answer
274 views
Is the total angular momentum of the universe zero? [duplicate]
I know that having a net angular momentum will contradict isotropy of the universe by preferring a specific direction. But is there any experimental data on the total angular momentum of the universe ...
0
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0answers
13 views
Binary star system - Revolution around Primary vs Companion
How likely is it in a binary star system for a non-star celestial body to revolve around the primary star rather than the companion star?
And as a bonus, do we know of any that work this way in our ...
1
vote
0answers
37 views
Photon temperature above Electroweak Symmetry Breaking (EWSB) transition
In discussions of the history of our Universe, photon temperature is substituted for time. As the Universe cools, phase transitions break symmetries, including electroweak symmetry.
Why does it make ...
1
vote
0answers
45 views
Do cosmologically redshifted photons violate energy conservation? [duplicate]
I understand that, due to the Doppler effect, different frames of reference moving at different velocities relative to each other will measure different photon frequencies and hence energy. The ...
2
votes
1answer
56 views
Relation between $\Lambda$ and $\Omega_\Lambda$ in $\Lambda\mathrm{CDM}$
In minimal $\Lambda\mathrm{CDM}$, there is a parameter labeled $\Omega_\Lambda$, and current fits place it at around $\left( \Omega_\Lambda \sim 0.73\right)$. Meanwhile, $\Lambda$ enters the Einstein ...
0
votes
0answers
39 views
Closed linear cosmology implies G M / R = c^2?
I have a question about a linear FRW cosmology with $k=+1$.
Assuming zero cosmological constant the first Friedmann equation can be written:
$$\left(\frac{\dot R}{R}\right)^2 + ...
2
votes
1answer
59 views
How to calculate mass of a closed Universe?
How does one integrate the mass density over a closed Universe (a 3-sphere?) to obtain the total mass of that Universe?
Is this the correct integral?
$$M = R(t)^3 \rho\int_0^1 4 \pi r^2 ...
2
votes
1answer
66 views
What are the general relativity equations for relativistic constant acceleration?
At constant acceleration in special relativity, the time differs for a stationary observer and the astronaut. see the following article for an in-depth explanation:
Relativistic Rocket
However, when ...
1
vote
3answers
234 views
Entropy - How it is possible for the universe to exist? [on hold]
[UPDATE]
There are some principles in entropy im having trouble understanding. If Entropy requires that the energy in the universe must always been constant, shouldnt in theory 'Heat Death' have ...
0
votes
1answer
78 views
The Faint young Sun paradox: an alternative solution?
The faint young Sun paradox describes the contradiction between the fact that there must have been liquid water 3.5 billion years ago for life to have begun then and the fact that the Sun's power ...
4
votes
1answer
230 views
Additional merits to Wetterich's “Universe without expansion” compared to standard cosmological redshift interpretation?
A recent news item in Nature promotes Wetterich's preprint "A Universe without expansion". All sounds very exciting but hard to judge for non-experts.
As I understand from the Nature's article, the ...
4
votes
0answers
85 views
Evidence that the Solar System is expanding like the Universe? [duplicate]
I've just come across Krasinsky and Brumberg's paper that claims, from an analysis of radiometric measurements, that the astronomical unit (earth-sun distance) is increasing at the rate:
...
3
votes
2answers
220 views
Why does $8\pi/3$ appear in the equations describing cosmological expansion?
What is the significance of $8\pi/3$ in the first Friedmann Equation, and in the question concerning the time independence of the Hubble Constant?
Is it the 'same' $8\pi/3$ that appears in the total ...
2
votes
0answers
21 views
Are there multiple ways to determine the redshift of the CMB experimentally?
I know that by using Boltzmann equation and all that one can calculate that photons become decoupled around T~3000K and because temperature scales as T~1/a we can determine that the redshift of the ...
4
votes
3answers
90 views
Hydrogen cloud at the universe's beginning?
What prevented all of the hydrogen at the universe's start from coalescing into one gigantic star?
8
votes
5answers
467 views
What evidence refutes a “pre-eternally existent cosmic egg”?
I've seen many Young-Earth Creationists attempt to discredit Big Bang cosmology. For example, an article by Cal Thomas in the 1990's stated that it meant there was a "pre-eternally existing 'cosmic ...
4
votes
0answers
103 views
Poincare recurrence and the multiverse
In this paper Susskind claims that a stable de Sitter universe is problematic (among other things) due to the existence of Poincare recurrence, which happen because of finite entropy. I disagree that ...
-1
votes
0answers
88 views
Are we travelling at a speed close to the light speed? [duplicate]
This is a question which has been puzzling me quite a while.
There was a news report saying that the farthest galaxy spotted so far is at 13.3 billion light-years away, and it is said that the galaxy ...
4
votes
4answers
172 views
Basic Question About General Relativity?
I'm a layman that loves Physics. I'm also horrible at math. Having said that I have many, many questions in regards to physics and General Relativity in-particular. I will try to keep my question(s) ...
6
votes
3answers
162 views
A Cosmological horizon at the Hubble radius?
I have calculated that if one extends a rigid ruler into space by a fixed proper distance $D$ then a clock at the end of the ruler, running on proper time $\tau$, will run more slowly than time $t$ at ...
1
vote
0answers
57 views
How often can virtual particle/antiparticle pairs form real particles in a space expanding sufficiently fast to do this?
And if the rate of expansion of the universe becomes sufficient for virtual particles to be ripped apart into real particles, what effect would the mass of these particles have on the rate of ...
3
votes
0answers
76 views
At what expansion rate (H) would virtual particles be ripped apart into real particles, and what might be the density (temperature) of such an event?
Is it even possible for a virtual particle-antiparticle pair to be torn apart by cosmological expansion? Virtual particle-antiparticle pairs are ripped apart near black hole event horizons, creating ...
2
votes
1answer
102 views
Weyl Large Number Coincidence
I've just learned of the Weyl Large Number Coincidence on the Wikipedia. It looks interesting. Here is my interpretation of it.
What is the smallest "quantum" of energy in the Universe?
$$E_{min} = ...
1
vote
1answer
350 views
What is the geometry of our universe?
I have some questions about the notion of space in current cosmological theories. I have been trying to decipher a few papers written on this topic such as this one, but I keep running into some ...
7
votes
1answer
123 views
Baryogenesis only at the Planck scale, or none at all?
I can think of three general ways of explaining why the universe contains more matter than antimatter:
(1) Near the Planck time, the universe had zero baryon asymmetry, but at some later time, ...
1
vote
0answers
67 views
Spontaneous symmetry breaking by axions?
I am just reading at the beginnin of this nice article, that axions could be responsible for spontaneously breaking of a symmetry in the early universe.
Does anybody know which symmetry is alluded to ...
15
votes
4answers
602 views
Is space “real”, or is it some sort of accepted postulate?
sorry for this very naive (layman) question.
What is space?
It seems to be every where in the equations of physics (as some sort of postulate or hidden hypothesis). We also have a direct experience ...
16
votes
3answers
195 views
How do people calculate proportions of dark matter, dark energy and baryonic matter of the universe?
The Wikipedia page on dark matter mentions that the Planck mission had revealed that in our universe ordinary baryonic matter, dark matter and dark energy are present in the ratio: 4.9%, 26.8% and ...
4
votes
1answer
88 views
Why is there a line in the middle of the Planck full sky map?
Why there is a straight line (or perhaps a flat surface) in the middle of the Planck full sky map?
and zoom
(it makes it seem that the big bang was a collision between two unknown surfaces)
3
votes
2answers
223 views
Is Hubble's constant really constant? [duplicate]
How does Hubble's constant resembles age of universe? Isn't universe getting old each day? How can a constant be a reciprocal of age of universe? Hubble's value must be variable, isn't it?
1
vote
3answers
129 views
If the multiverse theory is true, can there be a Universe where there are different laws of physics?
This is probably a very difficult question. But my question is essentially this, if there are other Universes can different laws of physics exist in those Universes and if so, can't there be a ...
6
votes
2answers
121 views
How does dark matter halo outside a galaxy help to explain galaxy rotation curve?
How does a dark matter halo outside a galaxy help to explain a galaxy rotation curve?
Suppose for simplicity we use a model of a star rotating about a more massive star in a fixed circular orbit. For ...
1
vote
0answers
51 views
IR divergence and renormalization scale in dimensional regularization (part 2)
This is in continuation of my previous question,
IR divergence and renormalization scale in dimensional regularization.
Lubos gave a nice answer there but I want to get to a very specific example ...
0
votes
2answers
82 views
Effect of space time relativity on the age of the universe?
So we all heard about the twins paradox to explain einstein's time space relativity.
Wikipedia Quote :"
In physics, the twin paradox is a thought experiment in special relativity involving identical ...
1
vote
2answers
66 views
Evidence for expansion of space [duplicate]
If I understand correctly, Einstein's theory of General Relativity predicted the expansion of space itself, and Hubble confirmed this prediction by observing the red shift of receding galaxies.
I ...
7
votes
2answers
136 views
According to Hubble's Law, how can the expansion of the Universe be accelerating?
Scientists today think the expansion of the universe is accelerating.
According to Hubble's law, objects further away are moving faster than objects closer to us. The further away an object is, the ...
2
votes
1answer
54 views
General expression of the redshift: explanation?
In some papers, authors put the following formula for the cosmological redshift $z$ :
$1+z=\frac{\left(g_{\mu\nu}k^{\mu}u^{\nu}\right)_{S}}{\left(g_{\mu\nu}k^{\mu}u^{\nu}\right)_{O}}$
where :
$S$ ...
