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.
5
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2answers
149 views
What's the significance of large-scale anomalies in CMB
What's the significance of large-scale anomalies in CMB that are confirmed by Planck? I've read somewhere that the cold spots can provide support for string theory or it may be due to a parallel ...
2
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1answer
176 views
Cosmological constant
I have always wondered about how cosmological constant is characterized. So since it is still a hypothesis you often read the “cosmological constant measured to be ….”. Shouldn't the statement read ...
3
votes
3answers
119 views
Looking out into the universe means looking back in time - how does that work?
This is a question that has been gnawing on me for many years now. Back a long time ago, as I recall in reference to a scene in a popular science show on TV, I was asked the following.
The claim is ...
1
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2answers
68 views
metric tensor of expanding universe
Why is the metric tensor of a expanding universe a function of time?
Why is it not a function of distance between the galaxies? I heard this from a lecture.
Can anyone help me understand?
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2answers
134 views
Advanced Heaviside-Feynman formula implies electromagnetic inertia?
The Heaviside-Feynman formula (see Feynman Lectures vol I Ch.28, vol II Ch. 21) gives the electric and magnetic fields measured at an observation point $P$ due to an arbitrarily moving charge $q$
$$ ...
3
votes
2answers
145 views
Universe Expansion and two tennis balls
Clear the universe of all matter except for two tennis balls. Place the two tennis balls in the same inertial frame 1 Mpc apart.
Are the tennis balls getting further apart?
Will the tennis balls ...
0
votes
1answer
87 views
Does our local time speed up as the Universe expands?
Starting from a simplified radial Freidman Walker metric we have
$$ds^2 = -c^2 dt^2 + a(t)^2 dr^2 $$
How does one measure one's proper time operationally?
One times a light beam along an element of ...
3
votes
1answer
102 views
Inflation and scalar spectral index
I've been reading that the results from the Planck satellite constrain a number called the "scalar spectral index" to be 0.96 rather than 1 at the 5-sigma level.
This is supposed to be big news, but ...
0
votes
1answer
175 views
Size of the Observable Universe [duplicate]
I wanted to know what the observable universe is so I was thinking and I thought, it must be age of the universe times 2.
Well I was wrong. I found on one website that it is 46B LY across in each ...
0
votes
1answer
78 views
Speed of Universe Expansion [duplicate]
How much is the universe expanding every second (Earth second)?
If you could be a bit simple in the answer that would be good.
7
votes
1answer
126 views
Gravitationally bound systems in an expanding universe
This isn't yet a complete question; rather, I'm looking for a qual-level question and answer describing a gravitationally bound system in an expanding universe. Since it's qual level, this needs a ...
1
vote
0answers
51 views
Second order action ADM formalism
I am trying to derive the second order action
$$S_{(2)}~=~\frac{m_{pl}^{2}}{8}\int a^{2}[(h_{ij}')^{2}-(\partial_{i}h_{ij})^{2}]d^{4}x, $$
used for tensor fluctuations derived from the ADM ...
2
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1answer
144 views
5
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0answers
53 views
Does the Standard Model plasma develop a spontaneous magnetisation at finite temperature?
Reference: arXiv:1204.3604v1 [hep-ph] Long-range magnetic fields in the ground state of the Standard Model plasma.
Alexey Boyarsky, Oleg Ruchayskiy, Mikhail Shaposhnikov.
The authors of this paper ...
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votes
1answer
106 views
maybe the universe has already 'ended'? [closed]
I aks you if this reasoning has a base in what we know of universe. There are many articles about so called "vacuum metastability event". As I understand this happens (can happen) with an enormous low ...
2
votes
1answer
57 views
Tensor perturbation inflation
During inflation the metric is de-Sitter so $dt^2-d\underline{X}^2 $.
I know that the eqn.motion governing GW's from inflation (tensor perturbations) is
$$2H\dot{h}+\ddot{h}-\nabla^{2}_{i}h~=~0,$$ ...
1
vote
1answer
58 views
How is Doppler redshift of distant galaxies established?
Doppler redshift of distant galaxies gave first hint that the universe is expanding. I am curious to know how this redshift is actually measured and interpreted from observation. Suppose I observe ...
2
votes
1answer
76 views
Varying an action (cosmological perturbation theory)
I am stuck varying an action, trying to get an equation of motion. (Going from eq. 91 to eq. 92 in the image.)
This is the action
$$S~=~\int d^{4}x \frac{a^{2}(t)}{2}(\dot{h}^{2}-(\nabla h)^2).$$
...
1
vote
1answer
72 views
Antimatter in the universe [duplicate]
Is it possible that some parts of the universe contain antimatter whose atoms have nuclei made of antiprotons and antineutrons, sorrounded by antiprotons? If yes what can be the ways to detect this ...
5
votes
4answers
286 views
Why is it important that the equation of state parameter of dark energy is measured?
The equation of state for a perfect fluid is that $p=\omega \rho c^{2}$, where $p$ is the pressure, $\rho$ is the density, $c$ is the vacuum speed of light, and $\omega$ is called the equation of ...
2
votes
0answers
83 views
Our Universe in collision with another one
This question is related to the possibility that there are many other universes in the UNIVERSE,the multiverse, that were created during collisions between membranes, according to string theories. The ...
1
vote
0answers
100 views
Is there any way to prove/disprove we are in a computer simulation? [closed]
Is there any way to prove/disprove we are in a computer simulation in some transcendental reality?
Even if we are "really" not in a simulation, can we ever prove so? Even if some messages/evidence ...
2
votes
2answers
134 views
Why the temperature is getting lower when the universe is expanding
As we know, if an ideal gas expands in vacuum, as its energy is unchanged, the temperature remains the same. An ideal gas's energy does not depend on volume. In general, the energy is $kT$ times the ...
2
votes
1answer
37 views
Observational Cosmology, light polarisation
Can anyone explain the purpose of measuring light polarisation, as carried out by the upcoming planck satellite?
2
votes
1answer
110 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 ...
8
votes
2answers
366 views
Violation of Lorentz invariance (Lagrangian for particle)
I'm trying to get the relativistic action (or Lagrangian) for a free particle in the case of violation of Lorenz invariance.
Suppose we have the modified dispersion relation:
$$
...
-1
votes
1answer
103 views
Hubble Constant and Planck Constant
Has there been any effort to understand Hubble's limit as a quantum phenomenon?
It seems like they are suggesting the same thing.
1
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2answers
91 views
Does the distance to the cosmic horizon Lorentz-contract? Does the universe Lorentz-contract?
Our universe has a finite size. It is often called the "radius of the universe", or "distance of the cosmic horizon".
If we would fly with relativistic speed at the position of our Earth, would this ...
3
votes
0answers
55 views
How many orders of magnitude in energy spans the domain of validity of the Standard Model?
I am wondering if it makes sense to state that the upper limit is roughly 1012 eV (up to know the physics probed by the LHC seems to be pretty consistent with the SM) and the lower one is ... the ...
1
vote
2answers
72 views
The universe's lack of an 'edge', and how that relates to the multiverse?
So, we're fairly sure that the universe is infinitely large. We're also fairly sure that it is expanding. My question is how do these two facts relate to the multiverse? My favorite interpretation was ...
-3
votes
1answer
239 views
Is antigravity the source of accelerating expansion (dark energy)?
Is antigravity the source of accelerating expansion(dark energy)?
From the observation of 1998, we found that our universe has been continuing accelerating expansion, and the unknown cause for this ...
1
vote
1answer
78 views
In the big crunch theory, when the big crunch singularity forms, can the resulting black hole decay through hawking's radiation?
I've been pondering about this and I couldn't really find the answer for this. The big crunch theory postulates that the universe will eventually stop expanding and reverse back in on its self into a ...
0
votes
1answer
97 views
Particles entangled after the big bang
Is that true that the big bang caused the quantum entanglement of all the particles of the universe so every particle is entangled to each other particle of the universe?
1
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0answers
44 views
Is the speed of light the ultimate speed limit? [duplicate]
As we all know nothing can go faster than the speed of light as mentioned by most of our pioneer's in physics. But as I was listening to one of the statements of Sir. Stephen Hawkins he stated that at ...
1
vote
2answers
136 views
Is cosmic background radiation dark-matter and/or dark-energy?
Dumb question alert: Is it possible that the cosmic background radiation might be the source of dark-matter and/or dark-energy? What is the mass of the background radiation in the known universe?
5
votes
0answers
87 views
Equation of state of cosmic strings and branes
I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ...
2
votes
1answer
109 views
acceleration of the universe
Moments after the Big Bang, the universe was expanding at an incredible rate, (I've heard) faster than the speed of light. Due to dark energy, scientists predict the rate of expansion will pick up ...
7
votes
3answers
282 views
Could a universe with purely Newtonian mechanics exist?
I've heard about the fine-tuning principle and how if the fundamental constants of the Universe were changed even a slight bit, life could not exist as we know it.
However, study on this subject ...
3
votes
0answers
150 views
Quantum Entanglement Versus Inflation in the Early Universe?
Quantum entanglement is one of the most fascinating and mysterious phenomena in nature. It needs no interactions, or any sort of exchange for it to take place. It is possible, not against any rules of ...
1
vote
1answer
188 views
Relation between multipole moment and angular scale of CMB
What is the relation between multipole moment $l$ and angular scale $\theta$ of the Cosmic Microwave Background? Somewhere on the web I found that $\theta\propto\frac{180^{\circ}}{l}$ but I need exact ...
0
votes
1answer
68 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 $c^4/G$,
where $c$ is the speed of light; $G$ is the gravitational constant. Thus (the speed ...
1
vote
2answers
77 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' ...
3
votes
1answer
105 views
Negative energy and large-scale spacetime structure
I was reading an essay from Stephen Hawking's on the Space and Time warps and I was trying to make sense on some statements referring to the Casimir effect such as:
The energy density of empty ...
2
votes
1answer
80 views
What was ticking just after the Big Bang?
When reading about the Big Bang, I see phrases like 3 trillionths of a second after... So, what was ticking to give a time scale like this? We define time now in terms of atomic oscillations, but ...
1
vote
2answers
81 views
How to comoving volumes depend on the evolution of the Universe?
I'm reading a paper which states that
Neutron star binary merger rate at redshift $z$ per unit observer time
interval per unit volume is $\dot{n}_{m} = \dot{n}_{0} (1+z)^{2}
(1+z)^{\beta}$, ...
-2
votes
1answer
109 views
Finite amount of matter and space
(Please bear with me.)
If the universe included multi-universes, and black holes transferred matter between them, AND yet there was a large, but FINITE amount of matter and space, How would that ...
0
votes
2answers
73 views
Why wouldn't Hubble's Law be directly in units of frequency?
Maybe using this as an example:
The energy of a particular color of yellow light is $3.44 \times 10^{-22}$ $kJ$
So if I want to find frequency of that light, I take that number, divide by $h$ ...
1
vote
1answer
77 views
Redshift and shapes of galaxies
How does peculiar velocities lead to the observed elongation of galaxy clusters in redshift space, otherwise known as the Fingers of God effect? I have read the relevant Wikipedia page, but cannot ...
4
votes
2answers
314 views
Is spacetime an illusion?
In consistent histories, for gauge theories, can the projection operators used in the chains be not gauge invariant?
In quantum gravity, for a projection operator to be gauge invariant means it has ...
1
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0answers
41 views
The definition of $f_{NL}$ and transfer function
To me there seems to be quite a few different definitions of $f_{NL}$ in cosmology and I would like to know if or how they are equivalent. Let me cite at least 3 such,
One can see the equation 6.71 ...
