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.
2
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0answers
78 views
Topics in particle cosmology [closed]
I am interested in learning more about this interdisciplinary approach.
1) What are some of the top questions in particle cosmology (e.g nature of dark matter, inflationary structure, topological ...
1
vote
0answers
67 views
Cosmic radiation cutoff at LOW energies?
The energy spectrum of the cosmic radiation (not CMB) is limited to both sides.
I know about the GZK-cutoff at high energies. Basically the interaction probability for photons of energies above 10^20 ...
0
votes
0answers
128 views
Stephen Hawkings and conscioussness [closed]
In the NaGeo docu 'Did God create the universe' Stephen Hawkings talks for an hour to ultimately come to the conclusion : Since time didn't exist before the big bang, God could not create the ...
16
votes
3answers
605 views
Are modified theories of gravity credible?
I'm a statistician with a little training in physics and would just like to know the general consensus on a few things.
I'm reading a book by John Moffat which basically tries to state how GR makes ...
9
votes
10answers
2k views
What is the most efficient way to destroy the universe? [closed]
Don't worry... I won't actually do it, just would like to know how.
9
votes
7answers
1k views
How many bits are needed to simulate the universe?
This is not the same as: How many bytes can the observable universe store?
The Bekenstein bound tells us how many bits of data can be stored in a space. Using this value, we can determine the ...
11
votes
5answers
735 views
How many times has the “stuff” in our solar system been recycled from previous stars?
Is there a cosmologist in the house? I've got a basic understanding (with some degree of error) of some simple facts:
The Universe is a little over 13 billion years old. Our galaxy is almost that ...
5
votes
6answers
670 views
Are many-worlds and the multiverse really the same thing?
Are many-worlds and the multiverse really the same thing?
Not too long ago, Susskind and Bousso uploaded the article "The Multiverse Interpretation of Quantum Mechanics" with the thesis that the ...
3
votes
2answers
497 views
Value of the Hubble constant over time
There is something I don't understand about the Hubble "constant" H, as it seems to clump two concepts together that I can't quite unify in my head. On the one side, we have
V = D * H
which means ...
3
votes
1answer
372 views
What is a maximal analytic extension?
Can someone explain (as rigorously as possible) what is involved in analytically continuing, say, the Schwarzschild solution to the Kruskal manifold? I understand the two metrics separately but I'm ...
1
vote
1answer
683 views
Zero energy universe
Regarding this thread:
http://www.physicsforums.com/showthread.php?t=506985
Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of ...
12
votes
9answers
169 views
In astronomy what phenomena have theory predicted before observations?
As far as I know, astronomy is generally an observational science. We see something and then try to explain why it is happening. The one exception that I know of is black holes: first it was thought ...
7
votes
6answers
541 views
Can (or How) Anthropic Principle be Made into a Scientific Theory?
Anthropic Principle says
physical Universe must be compatible
with the conscious life that observes
it.
I thought it is just philosophy mumbo-jumbo and it's not physics.
But I could be ...
6
votes
4answers
484 views
Is space unending?
Is the space unending, i.e it has no boundaries? If yes, how can a thing exist which is non-ending? Its impossible for me to imagine something like that. Secondly, if its not and has boundaries then ...
6
votes
4answers
450 views
de sitter cosmologic limit
It has been said that our universe is going to eventually become a de sitter universe. Expansion will accelerate until their relative speed become higher than the speed of light.
So i want to ...
6
votes
2answers
763 views
How many bytes can the observable universe store?
Is the number of states in the Universe countable?
What framework could be used to answer the question in the title?
4
votes
3answers
192 views
Is the cosmic horizon related to the Big Bang event?
The Universe expands according to the Hubble's law: velocity is proportional to distance.
There must be some distance, therefore, at which the velocity reaches the speed of light. This defines the ...
3
votes
2answers
608 views
Why do neutrino oscillations imply nonzero neutrino masses?
Neutrinos can pass from one family to another (that is, change in taste) in a process known as neutrino oscillation. The oscillation between the different families occurs randomly, and the likelihood ...
2
votes
1answer
73 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
$$S~=~\int d^{4}x \frac{a^{2}(t)}{2}(\dot{h}^{2}-(\nabla h)^2).$$
...
2
votes
1answer
107 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 ...
2
votes
1answer
189 views
2
votes
1answer
182 views
If Fine Structure Constant in not uniform in space what would that imply for cosmic background?
Recently J.Webb submitted paper which has extraordinary claim - that Fine Structure Constant is different in different directions in space! He (with others) measured $\alpha$ using quasar spectrum ...
2
votes
2answers
426 views
Big bang and time
I heard Carl Sagan talking about the Universe 15 Billion years ago, and the Big Bang. He made the statement that it was the biggest explosion of all time (at first I thought this a subtle pun). This ...
1
vote
1answer
200 views
Is it possible that the Big Bang was caused by virtual particle creation?
As far as I understand, it is understood that throughout the universe there exists, what is known as, a quantum field from which, due to its fluctuations, temporary (pairs of) virtual particles ...
1
vote
3answers
206 views
Mechanism for the gravitational field generated by photons
This question follows from a schooling I received in this thread.
I figured that photons do not interact with gravity, except when they've spontaneously converted into a particle-antiparticle pair. ...
1
vote
5answers
801 views
Is the law of conservation of energy still valid?
Is the law of conservation of energy still valid or have there been experiments showing that energy could be created or lost?
10
votes
5answers
105 views
How large is the universe?
We know that the age of the universe (or, at least the time since the Big Bang) is roughly 13.75 billion years. I have heard that the size of the universe is much larger than what we can see, in other ...
7
votes
3answers
72 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?
6
votes
3answers
443 views
Is the total energy of the universe constant?
If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
5
votes
3answers
375 views
Will the Big Rip tear black holes apart?
There seems to be an obvious contradiction between the predictions of the physics of black holes and the Big Rip, a predicted event about 16.7 Gyr in the future where local groups, galaxies, solar ...
5
votes
2answers
386 views
photons in expanding space: how is energy conserved?
If a photon (wave package) redshifts (streches) traveling in our expanding universe, is it's energy reduced? If so, where does it go?
5
votes
3answers
298 views
Does MOND make good predictions?
Well,it does according to this preprint: http://arxiv.org/PS_cache/arxiv/pdf/1102/1102.3913v1.pdf
for certain scales
How would be a simple way to explain MOND to a layman?
Does it ignore mainstream ...
4
votes
2answers
157 views
Total Energy of the Universe?
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
votes
1answer
170 views
Will acceleration rate of expansion of space become faster than speed of light?
From watching cosmology lectures, it seems that the space between galaxies is expanding at an accelerating rate, my question is since it is the space that is (acceleratingly expanding), the special ...
4
votes
1answer
140 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
votes
1answer
232 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
votes
1answer
729 views
Where does CMB come/emit from?
Where exactly does CMB come from. I've seen it in documentaries as a huge sphere with Earth in the middle. But if all this radiation was ejected from the start of the universe some time after the big ...
3
votes
1answer
111 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 + ...
3
votes
0answers
49 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 ...
3
votes
3answers
483 views
Do singularities have a “real” as opposed to mathematical or idealized existence?
I was thinking of, for example a Schwarzchild metric at r=0, i.e. the gravitational singularity, a point of infinite density. I realise that there are different types of singularities--timelike, ...
3
votes
3answers
323 views
References for ADM formalism and cosmological perturbation theory [closed]
What would you consider the best online resources for learning the 3+1 ADM formalism and gauge invariant perturbation theory in cosmology? (Assuming intermediate level GR and QFT familiarity)
2
votes
2answers
166 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?
2
votes
1answer
84 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
votes
1answer
842 views
Hubble time, the age of the Universe and expansion rate
The Hubble time is about 14 billion years. The estimated current age of the Universe is about 13.7 billion years. Is the reason these two time are so close (a) a coincidence, or (b) a reflection that ...
2
votes
1answer
118 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? ...
2
votes
2answers
164 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
votes
3answers
351 views
Does (it make sense to say that ) the universe has a center?
I was reading this page:
http://www.guardian.co.uk/science/2011/oct/23/brian-cox-jeff-forshaw-answers
and I found this sentence by Brian Cox:
That seems to imply that everything is flying away ...
2
votes
3answers
210 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:
$$ ...
2
votes
4answers
872 views
Maximum size of the observable universe?
The size of our observable universe must have grown over time in the early universe. Conversely with the accelerated expansion, I have heard that eventually our observable universe will shrink down ...
1
vote
1answer
80 views
Ricci scalars for space and spacetime, local and global curvature
If Ricci scalar describes the full spacetime curvature, then what do we mean by $k=0,+1,-1$ being flat, positive and negative curved space?
Is $k$ special version of a constant "3d-Ricci" scalar?
...
