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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2answers
432 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?
2
votes
5answers
943 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 ...
7
votes
2answers
344 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
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0answers
103 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
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2answers
254 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
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2answers
106 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 ...
10
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2answers
1k 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
81 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 : $S$ ...
0
votes
1answer
66 views

Cosmology and TOE

I have read an answer on this site regarding the change of laws over time . However a physisct told me that the laws did evolve at planck era and then stopped evolving after it , is that true even in ...
3
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1answer
817 views

Conformal time or cosmological time?

Let us assume a flat FRW metric $$ds^2 = -dt^2 + a(t)^2(dx^2+dy^2+dz^2)$$ Imagine a lightbeam traveling in the x-direction. It travels on a null geodesic with $ds=0$ so that its path obeys the ...
4
votes
1answer
104 views

How can contracting dimensions lead to cosmological inflation?

Using the Kasner metric, given by $$ ds^2 = -dt^2 + \sum_{j=1}^D t^{2p_j}(dx^j) $$ it is possible to not only describe the cosmological expansion of some space directions (the ones with positive ...
2
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0answers
44 views

Scaling in Vlasov equations

This question is in reference to the paper, http://arxiv.org/abs/1301.7182 What exactly is the argument being made on page 6 and 7? One deduces that the function $\Delta$ has to be such that, ...
0
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1answer
622 views

Is time slowing down and disappearing from the universe instead of the expansion of universe? [duplicate]

Are we fooled into thinking that expansion of the universe is accelerating, when in fact, time itself is slowing? Or if dark energy does exist?
2
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0answers
42 views

Regular initial data

I have a very basic question. What exactly is meant by "regular" initial data in general relativity? Does it mean smooth? at least $C^{2}$? All literature on the subject just uses this term without ...
2
votes
1answer
125 views

Membrane-reversed black holes and their relationship to white-holes

We usually think of white holes as 'thermodynamically reversed black-holes', and this kind of membranes have not been observed in our universe. However, there is some other kind of 'topologically ...
16
votes
2answers
1k views

What is the theoretical limit for farthest we can see back in time and distance?

13.2 billion years ago the universe was rather small, having started only half a billion years ago. Today, with the help of Hubble Space Telescope, we are able to capture the light of galaxies emitted ...
18
votes
2answers
360 views

Before inflation, what sets the initial value of the inflaton field?

[This is a version of the question that I've revised based on helpful comments from Dan.] I haven't studied inflation at a technical level. My picture of the process is that we have an inflaton field ...
1
vote
0answers
40 views

Maximal development/Development of a solution

I'm having troubles to rigorously understand what a development (or maximal development) of a solution is in General Relativity. I was reading a paper by Burnett and Rendall and they write "By maximal ...
0
votes
1answer
64 views

Einstein-de Sitter - confusing constant of integration

I'm struggling to understand this derivation of the Einstein-de Sitter universe model It starts with $$\int R^{1/2}dR=\int H_{0}R_{0}^{3/2}dt.$$ Evaluating this integral ...
-1
votes
1answer
128 views

Why is the Big Bang the biggest explosion in the universe?

What makes the big bang the biggest explosion in the universe? Why even should such an explosion ever happens? Through what process and how does this amount of energy get released as a big bang? Are ...
0
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2answers
77 views

Spread and direction of the cosmic background radiation

Something I can never understand is that where the cosmic background radiation spreads? If I know well, the cosmic background radiation is actually the light of the Big Bang. If it happened exactly ...
1
vote
1answer
116 views

Usefulness of SUSY models when it cannot exist at any non-zero temperature

Unlike other symmetries (like electroweak symmetry), SUSY is spontaneously broken at any non-zero temperature due to some variation of the fact that the boundary conditions on bosons and fermions in ...
4
votes
0answers
62 views

Why didn't a black hole form right after the Big Bang? [duplicate]

Why didn't a black hole form right after the Big Bang and the universe contract towards a singularity?
4
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2answers
494 views

Recent Planck probe results: how unexpected?

The data from the Planck probe's observations are in, and according to the European Space Agency they show a "hemispheric asymmetry in the cosmic microwave background (CMB)". Quote: an asymmetry ...
2
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0answers
228 views

Trouble with calculating Christoffel symbols of FLRW metric using Lagrangian method

The FLRW metric which I am using is $$ds^2 = dt^2 - \frac{a(t)^2}{c^2} \left( dx^2 + dy^2 + dz^2 \right)$$ where $a(t)$ is the so-called 'scale factor'. I did not want to calculate the Christoffel ...
3
votes
2answers
127 views

Will the increasing rate of expansion of the universe eventually result in Hawking Radiation suddenly occurring everywhere?

The universe is expanding at an ever increasing rate. Virtual particles are being created and then destroyed everywhere at every moment. As space expands even faster, will a time come when these ...
2
votes
1answer
222 views

How (why!?) does one introduce an UV cut-off in dimensional regularization?

This question is in reference to the confusing equation 3.7 (page 14) of this paper. One sees the 1-loop answers in their theory as given in their A.7 and A.8 on page 20. Each of the terms is a ...
3
votes
0answers
93 views

Evolution of black holes ensemble

Background: I’ve read many times that arrow of time can be explained from extremely low entropy of the Universe at the Big Bang (http://preposterousuniverse.com/eternitytohere/faq.html). The argument ...
0
votes
1answer
79 views

The Univere's mass-energy and uncertainty [duplicate]

A virtual creation with total mass-energy = $E$ is allowed as long as that virtual creation doesn’t last longer than $E/h$. Can the uncertainty principle also be used to estimate the mass-energy in ...
6
votes
1answer
130 views

Causal structure of the inflationary multiverse

In the multiverse as it is described by eternal inflation, it is not clear to me what is its causal structure and in particular if the bubble-universes are causally connected. We start from a ...
3
votes
1answer
291 views

Diameter of the universe [duplicate]

Should the diameter of our universe always be more than its age in light years? As if the distance between any two points in the universe is equal to 13.5 billion light years then the light from the ...
2
votes
0answers
125 views

Cosmic Inflation: Lower Expansion Rate with than without?

When I read the Cosmic Inflation diagram at https://en.wikipedia.org/wiki/File:Horizonte_inflacionario.svg which I rotated to make it readable: where the Hubbleradius, and therefore also the ...
3
votes
3answers
92 views

Showing Hubble constant is time-independent

I have the following question for homework: Show that the Hubble constant $H$ is time-independent in a universe in which the only contribution to energy density comes from vacuum energy. So in ...
5
votes
2answers
243 views

Questions about the Jeans length

I have a couple of questions about the Jeans length. Suppose the universe has a homogeneous energy density, except there's a spherical region that is overdense. I understand that if the region is ...
1
vote
1answer
128 views

Assuming space is infinite can our observable universe be an island amongst an archipelego?

According to recent measurements our observable universe is roughly 93 billion light years in diameter; also it appears (according to WMAP measurements) that spacetime is flat. Supposing space is ...
3
votes
2answers
311 views

Excluding big bang itself, does spacetime have a boundary?

My understanding of big bang cosmology and General Relativity is that both matter and spacetime emerged together (I'm not considering time zero where there was a singularity). Does this mean that ...
1
vote
2answers
132 views

Is it possible that universe might not be speeding up expansion?

I'm not sure but I was thinking of galaxies shrinking with time while still moving apart from each other at almost a constant speed or less (i.e: uniform/slightly decelerating expansion). This may ...
4
votes
2answers
136 views

Cosmological triangle with PLANCK results

Is there an updated version of the cosmological triangle with recent PLANCK results included?
9
votes
2answers
296 views

Redshifted from what?

We need to know two of the following three to calculate the third: redshifted color, baseline color, and velocity. The velocity is related to the difference between the redshifted color and the ...
1
vote
3answers
124 views

If everything in existence were increasing in size at some rate, would we be able to detect it?

Would our eyes observe any changes? What about electronic measurement devices?
5
votes
1answer
364 views

Lookback Time & Age of the Universe Calculations

I try to calculate the age of the universe with the FLRW model: $$ H(a) = H_0 \sqrt{\Omega_{\mathrm{R},0} \left(\frac{a_0}{a}\right)^4 + \Omega_{\mathrm{M},0} \left(\frac{a_0}{a}\right)^3 + ...
4
votes
2answers
381 views

Is time going backwards beyond the event horizon of a black hole?

For an outside observer the time seems to stop at the event horizon. My intuition suggests, that if it stops there, then it must go backwards inside. Is this the case? This question is a followup ...
-3
votes
2answers
209 views

A question to the theory of multiverse

Guys I couldn't catch a point of multiverse theory.. Theory: If space-time goes on forever, then it must start repeating at some point, because there are a finite number of ways particles can be ...
2
votes
1answer
57 views

CMB anisotropies and tightly coupled limit

Sorry if this is a technical question. I am studying the origin of CMB anisotropies and the tightly coupled limit of the Boltzmann equations. We have a fluid composed of ionized electrons and photons. ...
0
votes
3answers
162 views

Acceleration in the rate of expanansion of the universe due to weakening gravity?

Could the acceleration in the rate of expansion of the universe be due to the weakening of gravitational forces, as the distance between objects continues to increase?
0
votes
1answer
150 views

Missing step while (classically) deriving Friedmann equation

I'm trying to understand the classical derivation of Friedmann equation but I'm missing one step. So, I start with accelerations, where $a$ is a scale factor $\ddot{a}=-\frac{GM}{a^{2}}$ ...
0
votes
3answers
226 views

Cosmological redshift interpretation

Can the cosmological redshift be interpreted as atomic frequencies increasing by the scale factor as the Universe expands? This explanation seems closer to the truth than the popular idea that a ...
3
votes
0answers
131 views

Questions about classical and quantum scale invariance

This is kind of a continuation of this and this previous questions. Say one has a free "classical" field theory which is scale invariant and one develops a perturbative classical solution for an ...
2
votes
1answer
255 views

Perfect fluids in cosmology?

In cosmology, it is often assumed that the equation of state of a cosmological fluid is of the form $p=w\rho$. Why is this? Is it the equation of a perfect fluid? Why does $w=0$ for matter $1/3$ for ...
1
vote
1answer
245 views

What is the curvature of an empty universe?

My calculations tell me an empty universe has hyperbolic curvature. Is this correct? If it is, can anyone help me understand why this is intuitively?