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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11
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3answers
1k 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 ...
0
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2answers
21 views

Centre of Mass and tangential velocity star and planet

We have two objects - one massive star, and one planet which has a considerably smaller, but non-negligible mass, in a circular orbit about a common centre of mass. Using equation $F$=$GMm$/$r^2$ ...
0
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0answers
22 views

Cosmological constant theories

Are there any hypotheses on the expansion of the universe, which have a cosmological constant, or some other parameter, that results in a universe with an ever increasing rate of acceleration for the ...
7
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1answer
191 views

Pressure and density using a general Lagrangian

Given a lagrangian of a form: \begin{equation}\mathcal{L}=f(\phi,\partial_{\mu}\phi\partial^{\mu}\phi)\end{equation} where $f$ is a function, I need to derive pressure and density in a FLRW universe ...
4
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1answer
160 views

Probability of spontaneous Boltzmann brain formation

I was reading through: http://www.scottaaronson.com/papers/giqtm3.pdf But I can't make sense of page 61 discussing Boltzmann brains. Specifically the fact that it says: But the problem is worse. ...
0
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1answer
56 views

Is the center of the universe in another dimension?

If we live in a four dimensional world and when we try to find the center of the universe (where the big bang occured) we found that there is no apparent center, could that means that the big bang ...
0
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2answers
148 views

Tired light red shift hypothesis

Have there been any recent attempts to falsify the tired light hypothesis alternative to the Doppler shift explanation for the red shift, or is it simply ignored like the pilot wave quantum mechanical ...
1
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1answer
41 views

Redshift of supernova light curve

I am trying to understand how the width of a supernova light curve depends on the redshift of its component frequencies. Let us make the simple assumption that the light curve is Gaussian. The ...
2
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0answers
28 views

Using the boltzmann equation to find neutrino decoupling temperature

I'm an undergraduate student currently following a particle astrophysics lecture. I was reading some stuff about the neutrinos decoupling before the $e^+ e^-$ annihilations. It's possible to quickly ...
1
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0answers
88 views

Computations for Quantum Vacuum Fluctuations

For quite some time the notion of quantum vacuum fluctuations is bothering me. What exactly is the theoretical origin of this notion? This notion has become quite common in physics and is used to ...
25
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6answers
5k views

Total energy of the Universe

In popular science books and articles, I keep running into the claim that the total energy of the Universe is zero, "because the positive energy of matter is cancelled out by the negative energy of ...
23
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11answers
2k views

How can it be that the beginning universe had a high temperature and a low entropy at the same time?

The Big Bang theory assumes that our universe started from a very/infinitely dense and extremely/infinitely hot state. But on the other side, it is often claimed that our universe must have been ...
1
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0answers
54 views

Gauge invariant quantities

In the context of cosmological perturbation one write the most general perturbed metric as $$ ...
2
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5answers
307 views

Where does General Relativity (GR) fail experimentally? Experimental shortcomings of GR?

When speaking with researchers and professors at my physics department, I sometime catch this undertone in the conversation when speaking about GR: almost as if some are not entirely convinced about ...
1
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1answer
30 views

Definition of conformal time in de Sitter spacetime

I'm trying to follow the calculations in http://arxiv.org/abs/hep-th/0201158v2 The aim is to rederive the expressions (2.16), (2.17) for the power spectrum in de Sitter spacetime. In order to do so, ...
0
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0answers
20 views

Calculate Total Energy Of Universe [duplicate]

Physicist using $mc^2$ to calculate positive energy and $-MmG/R$ to calculate negative energy to calculate total nergy of the universe but I heard that one of them is relativly invariant and other ...
0
votes
1answer
60 views

Why is the universe 3D if the holographic principle says it's encoded in a its surface?

The way I understand the holographic principle is that everything in a 3D space can be thought of as living on the 2D boundary of that space. If that is the case, why does everything in the universe ...
-3
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1answer
70 views

How much consciousness can a stretched horizon have? [closed]

Giulio Tononi has a panpsychic theory which quantifies the amount of consciousness a system has by $\Phi$. The definition of $\Phi$ in his Integrated Information Theory depends upon dividing the ...
3
votes
2answers
135 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 ...
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0answers
8 views

Structure formation: growth of density perturbations in sub-horizon vs super-horizon scales

I've always had difficulty visualizing how sub-horizon scales work versus super-horizon scales. Inflation causes perturbations due to quantum fluctuations. These are under-densities and ...
-1
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1answer
114 views

Could there be a time dilation of 2,000,000x between Earth and other parts of the universe?

Don't ask me why, but you may be able to guess why I would ask such a question. The question is, what kind of conditions would have to be present in a universe in order for billions of years pass in ...
1
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1answer
103 views

Do Quantum Fluctuations happen evenly through universe?

I was wondering if Quantum Fluctuations are completely unpredictable, but do our observations tell us if these fluctuations happen evenly through space or are there regions where more quantum ...
10
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1answer
435 views

How does the Hubble parameter change with the age of the universe?

How does the Hubble parameter change with the age of the universe? This question was posted recently, and I had almost finished writing an answer when the question was deleted. Since it's a shame to ...
4
votes
2answers
122 views

How confident are we that mass is not being lost in the universe?

After reading about the latest super-massive black hole in Nature 518, 512–515 (26 February 2015), I couldn't help but wonder if the accelerating expansion is a result of mass being lost. Would a ...
4
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2answers
86 views

Is entropy absolute (as in absolute temperature)?

Following this question on the Entropy at the Big Bang where I asked: Since Entropy always increases (in general); its expected that the entropy at the beginning of the universe should be the ...
3
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1answer
73 views

Number $g(T)$ of relativistic degrees of freedom as a function of temperature $T$

Let us consider the total number of relativistic degrees of freedom $g(T)$ for particle species in our universe: $$g(T)=\left(\sum_Bg_B\right)+\frac{7}{8}\left(\sum_Fg_F\right)$$ Where the sums are ...
2
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2answers
52 views

How does a particular galaxy's apparent recessional velocity CHANGE as the universe expands?

By this I mean will the light from distant galaxies continue to show red shift (i.e. if you look at light from a particular galaxy now and in 10000 years time, the spectra will have been shifted ...
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0answers
45 views

Friedmann Equations Derivation [closed]

I was trying to understand how one goes from the following system of Friedmann equations \begin{cases} \frac{\dot{a}^2}{a^2}+\frac{k}{a^2} = \frac{8\pi\rho(t)}{3} + \frac{\Lambda}{3}\\ ...
8
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3answers
2k views

Do the laws of physics evolve?

Hubble's constant $a(t)$ appears to be changing over time. The fine stucture constant $\alpha$, like many others in QFT, is a running constant that varies, proportional to energy being used to measure ...
3
votes
1answer
72 views

Do we need Dark Energy to explain the acceleration of the expansion of the universe?

As far as I know is the discovery that galaxies that are farther away are moving faster from us than galaxies the are closer by. This led to the theory of Dark Energy. The acceleration was speeding ...
0
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0answers
35 views

How one can solve Friedman equations of General relativity numerically?

How one can solve Friedman equations numerically subjected to any initial condition?
10
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3answers
459 views

Space is expanding so what is time doing? [duplicate]

Space is expanding and as we know space and time are intrinsically linked to be now known as spacetime. What is happening to time during expansion? Is there more time, longer time or is the time part ...
0
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2answers
44 views

Can I see stars in my light-cone that you can never see?

The light cone of our galaxy is a lot different to the light-cone of a galaxy 5 bly away. Our Hubble volumes are much different. Everyone is born in/at a different space/time. Can I see stars in my ...
0
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2answers
56 views

In a dust cloud around a star, what causes bodies to form?

I couldn't find an answer to this question. Suppose a dust ring around a new star contains relatively uniform dust particles. What's the process by which they aggregate into larger bodies? I can ...
2
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2answers
91 views

Why the CMB has not been dispersed so far?

Imagine you have a box of black body radiation. What happens if you open the box for a long time? It becomes dispersed and no radiation remains in the box. Now, apply this example to the Cosmic ...
1
vote
1answer
98 views

Spiral Galaxies and Gravity Lenses

Spiral Galaxies must have a great deal more mass than elliptical galaxies of the same size in order to account for the flat velocity curve. I've seen references of eight to ten times the visible ...
1
vote
0answers
30 views

How much has the universe expanded in total from the onset of inflation until today?

Starting from the onset of inflation, through inflation, reheating, radiation domination, matter domination, the transition toward dark energy domination all the way to today, how many e-folds of ...
0
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0answers
41 views

Total Mass in Universe [duplicate]

Currently (2015), what is the total mass of the universe? How can we say with mathematical certainty that 69% is dark energy; 25% is dark matter; 5% is atomic matter; and 1% is neutrino, photon, and ...
2
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2answers
76 views

Are physical constants determined by their observation?

In common interpretations of quantum mechanics, it can only be said that objects exist once I observe them - it is not legitimate to ask where an object was before I observed it. Does this point of ...
6
votes
1answer
177 views

Fate of largest scale structures?

In $\Lambda\mathrm{CDM}$, structures form "bottom up" with larger structures forming later. Structures are generally speaking supported by the velocity dispersion of their constituent objects (e.g. ...
3
votes
3answers
211 views

Are electrons held together by vacuum energy?

If one models the electron as a hollow spherical conductor with charge $e$ and radius $a$ then its electrostatic energy is given by: $$E_{em}=\frac{1}{2}\frac{e^2}{4\pi\epsilon_0a}$$ However if one ...
1
vote
1answer
120 views

What happens to black holes if/when the universe “ends”

I've heard several ideas about what happens to the universe when it dies, and they range from infinite expansion and heat death, to a "big crunch". In the event that something like a "big crunch" ...
0
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2answers
44 views
9
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2answers
650 views

Does everything orbit around some universal “center of mass”?

I have read many questions about the center of the Universe. I even have some favorite ones. I understand the fact that every point in the universe experiences inflation the same way. Also that it ...
1
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0answers
51 views

How do we get the power spectrum through the CMB?

How do we get the power spectrum through observing the CMB?
2
votes
1answer
88 views

Question about universe expansion

In general relativity, we cannot determine the global structure of the universe (since it is not flat), therefore all measurements and observations are only meaningful locally. In particular, we can ...
1
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2answers
68 views

Calculating energy density in the early Universe

I've been told to calculate the energy density in the early Universe. It states that it is completely dominated by neutrinos (3 species), photons, electrons, and positrons. Now, I've found an ...
0
votes
2answers
66 views

If space is expanding, how can I see a galaxy 12 billion light years away where it is today?

Assuming space is expanding (and that seems plausible), photons from a galaxy that have been traveling for 12 billion years must have been traveling on a curve to get to us due to the fact that where ...
0
votes
0answers
17 views

Olbers' paradox and singularity resolution

As Im sure many fo you there are many attempts at singularity resolution in quantum gravity approaches such in LQC and string theory. In the last few week there was a lot of press for a new approach ...
7
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3answers
332 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 ...