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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1answer
174 views

Can someone explain probability flux in the tunneling boundary condition of Vilenkin?

This is what's leading to the notion of a quantum universe tunneling from nothing into existence, right? The idea is that probability flux flows out of superspace (configuration space) at ...
2
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5answers
436 views

Can computers survive bubble nucleations?

According to string landscape theory, our vacua with a cosmological constant of $10^{-123}$ is a metastable vacua which can decay to a supersymmetric vacua with either a zero or negative cosmological ...
2
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1answer
258 views

How does inflation drive Ω close to 1?

I'll keep it simple. How does inflation drive Ω close to 1?
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1answer
53 views

Increasing matter density in the Friedman equations?

From what I understand, the Friedman equation is $H^2=\frac{8\pi G}{3} \rho$ and the $H$, the Hubble constant, is a measure of the rate of expansion of the universe. What I am confused about is that ...
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1answer
76 views

Infinite space or finite space?

I have been reading about the nature of space (Brian Greene's and Max Tegmark's recent books) and have a question on the size of space. Both references acknowledge that general relativity equations ...
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1answer
98 views

Reconciling “The Big Crunch” with the 2nd Law of Thermodynamics

Assume "The Big Crunch" scenario (the universe will collapse to a singularity). In this case, I think of the entire universe as an isolated system; in the "Big Crunch" scenario, it seems to me gravity ...
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1answer
464 views

Can the Universe create itself?--Is Gott's Use of CTC in Planck time or less valid?

My question is the title of a 1991 paper by Richard Gott and Li-Xin Li: http://arxiv.org/abs/astro-ph/9712344 and is also a subject of his popular book, "Time Travel in Einstein's Universe" Ultimately ...
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3answers
119 views

How far has a 13.7 billion year old photon travelled

I've read that the size of the observable Universe is thought to be around ~46 billion light years, and that the light we see from the most distant galaxies were emitted ~13.7 billion years ago as a ...
11
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2answers
799 views

How is it possible the universe expanded faster than the speed of light during inflation?

In a documentary written in collaboration with Stephen Hawking, the narrator (supposedly Stephen Hawkings) says that by the time the cosmos was 10 minutes old, it had already expanded thousands of ...
8
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1answer
237 views

Will entropy continue to increase even if the universe begins to contract?

If the universe is heading for a big crunch, when the universe starts to collapse will entropy decrease and the arrow of time consequently reverse or not? I'm interested in the explanations, not just ...
8
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3answers
127 views

What's dark matter and who discovered it?

I have heard about dark matter that's called the Master Of The Universe. What's this and is the dark matter the reason galaxies exist?
8
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1answer
1k views

In what sense does the universe have an outer edge?

I'm having trouble understanding something about Suskind's Holographic principle. Suskind speaks about the surface of the universe? In what sense does the universe have an outer surface? I'm a bit ...
8
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3answers
366 views

Does MOND make good predictions?

Well, it does according to this preprint for certain scales. What would be a simple way to explain MOND to a layman? Does it ignore mainstream physics? How much?
7
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1answer
170 views

What are the different ways to measure the spatial curvature of the universe?

Just what the question asks. Assuming the Friedmann-Rovertson-Walker (FRW) metric, what measurements can be performed to determine the spatial curvature of the universe.
6
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2answers
470 views

What experiments compete with BICEP 2, and when are their results expected?

The recent results of the BICEP 2 experiment published on March 17th 2014, has generated a lot of media attention, with the general consensus being that "this is a major discovery" perhaps leading to ...
5
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1answer
406 views

Where is radiation density in the Planck 2013 results?

I've been looking at the Planck 2013 cosmological parameters paper, trying to update my toy cosmology simulator with the most recent data. Most of the interesting values such as $H_0$, $\Omega_m$, ...
5
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1answer
140 views

Why don't orbits expand with the Universe?

Consider two bodies orbiting each other. As the Universe expands would the distance between them increase? Most people say that a gravitationally bound system will not expand with the Universe. They ...
5
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1answer
398 views

Does the current acceleration of universe imply that our universe is open?

Does the current acceleration of universe imply that our universe is open? If the universe is closed, from the Friedmann's equation, the acceleration of universe wouldn't be possible, would it be? (Of ...
5
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1answer
329 views

How and why will the Milky way collide with the Andromeda?

Hubble's law says that the universe is expanding.How come the milky way and the andromeda are on a collision course?How will they end up colliding with each other?
4
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1answer
256 views

Why does the Hubble parameter keep getting smaller if the expansion of the universe accelerates?

Suppose a region of space at a distance D from Earth is escaping from us with the velocity v. Since it seems like the expansion of the universe is accelerating, things at D from Earth should be ...
4
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0answers
95 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: ...
4
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2answers
259 views

Total Energy of the Universe? [duplicate]

I've heard the total energy is zero, but I've also heard it cannot be said to be zero since there's so much unknown stuff in the universe. Is that true?
4
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3answers
131 views

Proportion of dark matter/energy to other matters/energy at the beginning of the universe?

What was the proportion of dark matter/energy to other matters/energy at the moments after the beginning of the universe (standard Big Bang model)?
4
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1answer
63 views

Is there any chance we could eventually observe the supernova of the first (Population III) stars

In an answer to my previous question about the first stars it was stated they probably formed at Z=20 to Z=60 and may have had a mass between tens to 100s to 1000s of times the mass of the Sun. Given ...
4
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1answer
193 views

Do black holes play a role in quantum decoherence?

Sorry for such a vague question but I could have sworn I read somewhere that Hawking proposed the reason we might see a classically appearing universe is due to the possible role of black holes in ...
4
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1answer
164 views

Fine Tuned Universe

Is the fine tuning that cosmologists talk about (that our Universe is fine tuned for intelligent life) is the same as the fine tuning of the squared mass parameter of the Higgs in the Standard Model? ...
3
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3answers
75 views

If empty space has energy, and space is expanding, is this energy equally distributed as space expands?

The cosmological constant (dark energy) is often described in terms of empty space having a non 0 energy value and this energy being the source of the accelerated expansion of the universe. If space ...
3
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2answers
173 views

Is energy conserved in general relativity? Does $\nabla_aT^{ab}=0$ represent the conservation of energy and momentum?

For example, the radiation dominated cosmology, the energy density of radiation is propotional to $a^{-4}$ and the volume is propotional to $a^3$, where $a$ is the scale factor. So the total energy ...
3
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1answer
64 views

'There shall be reheating' and other inflation-related questions [closed]

I've some basic questions about inflation: What is reheating exactly? What triggered the reheating? Is it something that one pulls out of a hat? Through which force did the decay of the inflation ...
3
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1answer
163 views

Baryon masses in Wetterich's new cosmology

Christoph Wetterich has put out a paper in which the universe isn't always expanding; it can be static or expanding just some of the time or even shrinking. And then there is an interaction which ...
3
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3answers
1k 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?
3
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1answer
323 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 ...
3
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2answers
493 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?
3
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2answers
353 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 ...
3
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3answers
237 views

Assuming SUSY is found to be incorrect, what would then be the most compelling candidates for dark matter?

From what I've read, the only remaining candidates appear to be either sterile neutrinos or MOND (MOdified Newtonian Dynamics -- it does seem to keep changing.) Did I miss anything else plausible?
3
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1answer
145 views

What would happen in the final days of the universe?

I would like to know the stages of how the universe would end and what would happen and what the possible scenarios are. I understand that eventually all the stars would burn out and that would ...
3
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0answers
51 views

squeezed radiation astronomy

Squeezed electromagnetic vacuum does have a renormalized energy density smaller than the vacuum. So it makes it in my opinion a inconspicuous candidate for a dark energy carrier. Are there ...
2
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1answer
53 views

Can the proportion of dark energy change?

This post generalizes the posts: - Proportion of dark matter/energy to other matters/energy at the beginning of the universe? - How do people calculate proportions of dark matter, dark energy ...
2
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1answer
75 views

If we were able to double the mass of the ordinary matter in the universe it will recollapse?

What will happen if we suppose to be able to add mass at the universe? For example doubling the mass of the ordinary matter?
2
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3answers
567 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
108 views

If the nucleation bubble radius is greater than the deSitter radius, does that make the de Sitter space stable?

In our de Sitter phase, the cosmological constant is tiny. $10^{-123}M_P^4$. Suppose there is another phase with a lower vacuum energy. Is de Sitter phase still stable? The tunneling bubble radius has ...
2
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3answers
225 views

Which universe had a beginning? The universe or the observable universe?

When we say the universe had a beginning, do we mean the entire universe or the observable universe? Or did both of them have a beginning?
2
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1answer
85 views

Cosmic bubble collision

I'm reading this review right now. The claim seems to be that when you have an expanding "false vacuum", finite size bubbles form due to phase transition (cause by finite action instantons) (which, in ...
2
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1answer
150 views

The range of light

It occurs to me that the empirical evidence shows that there is a point out in space where light stops coming from. Putting aside the expansion of the universe for a second, and focusing strictly on ...
2
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2answers
188 views

Can the implications of dark energy be used to bridge the gap between Quantum Mechanics and General Relativity?

Can the findings of the Physics Nobel Laureates of 2011, namely the overpowering existence of dark energy (vacuum energy) have any implications in the quest the combine Quantum Mechanics and General ...
2
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1answer
513 views

Does the heat death of the universe really imply a maximum entropy state *all* of the time? Or most of the time?

Statistically speaking, you're going to still encounter deviations from equilibrium, even though the expected value is equilibrium. But these rare deviations from equilibrium - which are inevitable - ...
2
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3answers
245 views

Distance in cosmology

How can we calculate the change in distance over time between (say) two galaxies that they are at rest in respect to each other? In other words, how do we modify the "classical" equation: $$ ...
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1answer
68 views

Horizon problem in the standard cosmological model

The horizon problem is usually stated as follows: Causally disconnected parts of the universe that we see today have the same temperature, so how could that be so if no information travels faster than ...
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3answers
186 views

Does the Universe have finite number of particles? [duplicate]

I read that the number of atoms in the entire observable universe is estimated to be within the range of $10^{78}$ to $10^{82}$. Does the Universe have finite number of particles? If so, how could it ...
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1answer
122 views

Can Hubble red shift be interpreted as time dilation?

Can we interpret the de Sitter universe as a spherical cosmic horizon null surface of finite radius, centered at Earth, and containing the Hubble volume of space where time is dilated and radial ...