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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3answers
425 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
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2answers
171 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 ...
2
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1answer
135 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 ...
4
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1answer
349 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 ...
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1answer
690 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 ...
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1answer
111 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.
9
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1answer
436 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 ...
2
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1answer
344 views

Does universal wave function exist?

Does universal wave function exist? What the science tells us?
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0answers
80 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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1answer
192 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
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1answer
66 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 $$2H\dot{h}+\ddot{h}-\nabla^{2}_{i}h~=~0,$$ ...
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1answer
67 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
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1answer
122 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 $$S~=~\int d^{4}x \frac{a^{2}(t)}{2}(\dot{h}^{2}-(\nabla h)^2).$$ ...
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1answer
99 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
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4answers
642 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
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0answers
154 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 ...
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0answers
153 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
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2answers
362 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
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1answer
44 views

Observational Cosmology, light polarisation

Can anyone explain the purpose of measuring light polarisation, as carried out by the upcoming planck satellite?
2
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1answer
196 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 ...
9
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2answers
477 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: $$ ...
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1answer
119 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.
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2answers
128 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 ...
5
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0answers
126 views

How many orders of magnitude in energy spans the Standard Model phenomenological spectrum?

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 ...
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2answers
90 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 ...
-4
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1answer
656 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 ...
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1answer
167 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 ...
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1answer
518 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?
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0answers
50 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 ...
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2answers
231 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?
9
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1answer
188 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
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1answer
177 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
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3answers
360 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 ...
4
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0answers
206 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 ...
3
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1answer
1k 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 ...
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1answer
129 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 ...
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2answers
148 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
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1answer
167 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
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1answer
96 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 ...
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2answers
152 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 $\dot{n}_{m} = \dot{n}_{0} (1+z)^{2} (1+z)^{\beta}$, ...
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votes
1answer
122 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 ...
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2answers
113 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 $h$ ...
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1answer
101 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
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1answer
493 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
56 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 ...
2
votes
2answers
191 views

What is the curvature of the universe?

What is currently the most plausible model of the universe regarding curvature, positive, negative or flat? (I'm sorry if the answer is already out there, but I just can't seem to find it...)
2
votes
1answer
76 views

Dark energy before radiation or after?

In the cosmic history of the universe, does the dark energy comes before the radiation epoch, or only now, in the 'matter' universe (matter dominated era)? Because, we now know that like 75% of our ...
2
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1answer
233 views

Did force of gravity cause evolution of large scale structures?

Did big bang create gravity? What role gravity is assumed to have played in the formation (starting from the big bang) of large structures of our universe and what other important physical mechanisms ...
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2answers
112 views

How exactly, or whats the process, rather, of energy changing into matter?

$E=mc^2$ this is the equation by Einstein claiming energy can change from energy to mass. this would happened at the big bang I assume, when electrons and protons were made to create hydrogen and some ...
2
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0answers
66 views

Topological Solitons and the Higgs Condensate entanglement

While focusing on resolutions to the Firewall controversy, and the possible implications of the Higgs field as it relates to the issue, the possibility of using EPR correlations in the Higgs ...