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.
3
votes
3answers
206 views
How scientists estimated that our universe is 4% normal matter, 21% dark matter, and 75% dark energy?
Is there a simple way to understand how scientists estimated/calculated the following percentages?
5
votes
1answer
90 views
Is there more mass in stars or interstellar medium?
From what I gather the interstellar medium has about about 1 atom per cubic centimeter. But on the other hand, as they say, "Space is big, really really big" So if it is known (or at least theorized ...
6
votes
3answers
96 views
Universe is expanding at enormous speed
I'm not an expert but I've come to understand that the universe is expanding at enormous speed. That means that all of the visible galaxies are moving away from us at great speed.
I also came to ...
4
votes
1answer
41 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 ...
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 ...
3
votes
2answers
352 views
This Expansion-of-the-Universe-Diagram Confuses Me
The following blue-cone Wikipedia diagram confuses me.
At any point of cosmological time the encircling horizontal lines in the diagram are of finite circumference. That is indicative of a closed ...
2
votes
1answer
64 views
What is the best evidence/theory about the first generation of stars
In particular, during what range of years after the Big Bang did the stars form? What is the expected range of masses of these stars and what is the expected lifetime before they supernova? I assume ...
4
votes
2answers
747 views
Physical laws prior the big bang (quantum fluctuations)
A theory among scientists says that quantum fluctuations caused the big bang and created the universe. This seems plausible to me.
What I can't grasp yet is how a quantum fluctuation can even start ...
2
votes
1answer
128 views
Dark matter and mass-energy conservation
I have been thinking recently about dark matter, and it lead me to the following question.
Consider a hypothetical particle which, like the photon, is chargless and massless. However, unlike the ...
16
votes
3answers
717 views
How do we know Dark Matter isn't simply Neutrinos?
What evidence is there that dark matter isn't one of the known types of neutrinos?
If it were, how would this be measurable?
1
vote
1answer
116 views
Violation of Lorentz symmetry on cosmological distances
This question is about the domain of validity of Lorentz symmetry.
As far as I know, general relativity is a generalization of special relativity. Does that mean that Lorentz symmetry is violated on ...
3
votes
2answers
204 views
Big Bang and Cosmic microwave background radiation?
One of the experimental evidence that supports the theory of big bang is cosmic microwave background radiation (CMBR). From what I've read is that CMBR is the left over radiation from an early stage ...
1
vote
0answers
83 views
What is the meaning of colors in deep space pictures? [closed]
Are the telescopes really capturing these colors in deep space or are they just "artist's impression". What could make such vast space "red" or "green"?
8
votes
2answers
357 views
Is eternal inflation and the multiverse compatible with causal patch complementarity?
The argument for eternal inflation is we have some patch of metastable vacuum with positive cosmological constant, and so it expands exponentially a la de Sitter. Most of the patch decays to something ...
2
votes
3answers
233 views
Why every galaxy is moving away from every other galaxy on astronomical scale? because of an explosion or because of creation of space?
Why galaxies move away from each other in general on the astronomical scale?
Which answer is correct of the following?
It is because of the big bang theory, everything is just moving away from ...
5
votes
1answer
42 views
Are there stable string theory vacua with non-minimal cosmological constant?
Naive reasoning suggests that a string theory vacuum with cosmological constant Lambda1 is always unstable as long as there is a string theory vacuum with cosmological constant Lambda2 < Lambda1 ...
2
votes
3answers
352 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 ...
3
votes
1answer
177 views
Could a ship equipped with Alcubierre drive theoretically escape from a black hole?
Also could it reach parts of the universe that are receding faster than the speed of light from us?
3
votes
1answer
24 views
What happens when astronomical bodies made up of Dark Matter collapse or collide?
Does dark matter not produce heat or radiation after a collision etc?
Or, we detect radiations having no generators?
4
votes
4answers
90 views
Is dark matter around the Milky Way spread in a spiral shape (or, in a different shape)?
Dark matter doesn't interact with electromagnetic radiation, but it, at least, participates in gravitational interactions as known from the discovery of dark matter. But does dark matter exist in a ...
2
votes
1answer
129 views
The topology of a “closed” universe - is it really closed?
The spatial part of the positive curvature FRW metric has the form
\begin{equation}
ds^2=\frac{dr^2}{1-(r/R)^2}+r^2d\Omega^2
\end{equation}
or
\begin{equation}
ds^2=R^2(d\chi^2+\sin{\chi}^2d\Omega^2)
...
7
votes
8answers
587 views
Mathematical Universe Hypothesis
What is the current "consensus" on Max Tegmark's Mathematical Universe Hypothesis (MUH) which claims every concievable mathematical structure exists, including infinite different Universes etc.
I ...
13
votes
1answer
181 views
Sympletic structure of General Relativity
Inspired by physics.SE: http://physics.stackexchange.com/questions/15571/does-the-dimensionality-of-phase-space-go-up-as-the-universe-expands/15613
It made me wonder about symplectic structures in ...
8
votes
3answers
340 views
Does the dimensionality of phase space go up as the universe expands?
Ever since Hubble, it is well known that the universe is expanding from a Big Bang. The size of the universe had gone up by many many orders of magnitude as space expanded. If the dimensionality of ...
4
votes
2answers
264 views
Could the acceleration of universe expansion be caused by gravity itself?
Dark energy is suggested to be a repulsive force in the universe causing an accelerated expansion. If the amount of mass outside our observable universe is greater than inside (higher mass density), ...
2
votes
1answer
41 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 ...
1
vote
3answers
263 views
What was so surprising about accelerating cosmological expansion?
The 2011 Nobel Prize in Physics, as far as I understand, concerns the expanding universe -- galaxies moving away from each other at ever increasing speed (that's what I think I read in newspapers). ...
4
votes
5answers
496 views
What is the extent of the universe?
Is there any realistic, understandable, provable (even in some extent) explanation/model for the extent of the universe?
What is its shape? and Why? I mean physical explanations not philosophical ...
3
votes
2answers
622 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
195 views
The relation between the speed of light and the Big Bang Theory
I would like to know how much of the Big Bang theory is dependent on the constancy of the speed of light.
P.S.: It might be guessed that I am asking this because of the recent CERN news.
Yes, of ...
1
vote
0answers
178 views
Why do galaxies collide? Reconciling dark energy and an expanding universe with the fact that some galaxies are on a collision course
My layman understanding of cosmology is:
galaxies are uniformly (more or less) spread throughout the universe, per the Big Bang and the fact that in a central explosion, all dispersed points are ...
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?
1
vote
2answers
290 views
How long did inflation take to happen?
If inflation happened, it ended about 13.7 billion years ago. But how long was inflation going on before that? Was it just a short burst of time so that it's really okay to say that the age of the ...
1
vote
0answers
89 views
Confusion with infinity and time [closed]
I have some confusion between with resolving the following situation.
I know that no measurable quantity can have a value of infinity. For example, I just wrote something out, but clearly this ...
2
votes
1answer
146 views
Does anisotropic expansion of the universe imply quintaessence?
a Recent paper establishes under solid grounds anisotropy in the expansion acceleration rate in the universe. My question is very simple:
can this anomaly be explained entirely in terms of a ...
4
votes
4answers
552 views
Experimental evidence for parallel universes
My idea of physics is that it is a collection of mathematical laws relating observables. And that one can perform alot of mathematical derivations on these laws to produce new laws between ...
6
votes
1answer
166 views
Curvature of the Universe imaginary?
If the curvature of the universe is zero, then $$Ω = 1$$ and the Pythagorean Theorem is correct. If instead $$Ω> 1$$ there will be a positive curvature, and if $$Ω <1$$ there will be a negative ...
3
votes
2answers
205 views
Quantum Fluctuations as a model for the Big Bang?
I have quite often heard (and even used) the idea that quantum fluctuations are a way to explain the whole "something from nothing" intuitive leap. I am about to give a talk at a local school on ...
12
votes
6answers
661 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 ...
5
votes
1answer
125 views
How did enough material from other dying stars accumulate to start our sun and planets?
How far apart do scientists estimate was/were the dying star(s) that supplied the elements that comprise our sun, planet, and us? With stars so far apart and expansion of space (as I understand it) ...
5
votes
2answers
391 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?
7
votes
3answers
66 views
Are there any Earth-Meteorites on Earth?
A follow-on from this question, if meteorites can come from Mars, it seems they could have come from Earth and ended up back here (though I'm guessing that to be less likely). Do we have any evidence ...
3
votes
6answers
194 views
Are there free data available online from cosmology (or astrophysics) experiments that anyone can analyse?
One can understand a subject better in physics by trying to solve as many problems as one can from a textbook say. When it comes to experimental physics and data analysis, no book on experimental ...
6
votes
3answers
522 views
At what speed does our universe expand?
Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand?
5
votes
2answers
259 views
Can the distance of a quasar be determined accurately?
As noted in
A doubt about the age of the universe,
the wiki about quasars still contains the following misleading sentence:
"The highest redshift quasar known (as of June 2011) is ULAS_J1120+0641, ...
9
votes
1answer
259 views
evidence on the equation of state for dark energy?
If dark energy contributes mass-energy density $\rho$ and pressure $p$ to the stress-energy tensor, then you can define $w=p/\rho$, where $w=-1$ gives a cosmological constant, $w<-1$ gives a big ...
3
votes
1answer
122 views
Smoothed particle hydrodynamics in cosmological N-body simulations
What is the role of smoothed particle hydrodynamics (SPH) in cosmological N-body simulations like the Millenium Run (performed with Gadget-2)?
-3
votes
2answers
143 views
Could dark matter be expanded atoms, and dark energy expanded radiation?
After watching Lawrence Krauss' lecture 'life, universe, and nothing', an interesting idea lighted up in my mind. Is it possible that dark matter is just spread out matter waves and dark energy ...
4
votes
1answer
316 views
Which Friedmann equation is redundant?
For flat FLRW cosmology, we can write down two Friedman equations and one matter equation:
(1) $H^2=\frac{8 \pi G}{3} \rho$
(2) $\frac{\ddot{a}}{a} = -\frac{4 \pi G}{3} (\rho +3p)$
(3) $\dot{\rho} ...
8
votes
5answers
3k views
Stephen Hawking says universe can create itself from nothing, but how exactly?
Stephen Hawking says in his latest book The Grand Design that,
Because there is a law such as gravity, the universe can and will create itself from nothing.
Is it not circular logic? I mean, how ...
