56 votes

Is the Big Bang a theory or a model?

Back in the good old days, we used words like theory, model, hypothesis, law and so on, and their meanings arose from how the words were used, and dictionaries organised these senses - just like other ...
James K's user avatar
  • 118k
23 votes

Is the Big Bang a theory or a model?

My understanding is that the "big bang" (which I am taking to be a synonym for $\Lambda$CDM cosmology) is a model. It sets out a (relatively) simple framework, starting from a set of ...
ProfRob's user avatar
  • 149k
15 votes

Why do cosmologists assume that inflation began shortly after the Big Bang, rather than at the exact start?

The universe cannot have begun in an inflationary phase. Note that it is not necessarily the case that there was a phase that preceded inflation. However, if there was no phase preceding inflation, ...
Sten's user avatar
  • 4,500
12 votes

How can a black hole have a charge, or be charged?

The answer is reasonably simple: (nearly) all matter consists of protons, neutrons and electrons, thus of either positively, or negatively charged particles or those with no charge. The next step you ...
planetmaker's user avatar
  • 18.8k
10 votes
Accepted

How do comets' nuclei stick together?

Van-der-Waals forces and gravitation are the main answer. The main property keeping together bodies smaller than a few $100 {\rm km}$ are their constituents surface forces between the grains (van-der-...
planetmaker's user avatar
  • 18.8k
7 votes
Accepted

Why do some call the no-hair conjecture the no-hair theorem?

Why do some call the no-hair conjecture the no-hair theorem? Perhaps they mean something specific, or perhaps they don't know any better... you'd have to examine it on a case-by-case basis. Perhaps a ...
Daddy Kropotkin's user avatar
6 votes

How can Kerr black holes have a 'speed limit' to how fast they can spin?

Yes, general relativity does not contain anything preventing spacetime from expanding or moving superluminally (e.g. "warp drive spacetimes"): normally other constraints (e.g. the lack of ...
Anders Sandberg's user avatar
6 votes

What covers the outside of the Universe?

No. The universe is not a sphere with water ice surrounding it. Asking about "outside the universe" is akin to asking about what a candle flame is made of before it is lit. Just because you can put ...
James K's user avatar
  • 118k
6 votes

How can a black hole have a charge, or be charged?

Formally, black holes are a prediction of Einstein's theory of gravity. They have been observationally confirmed. Regarding theory: The field equations of Einsteinian gravity, the Einstein Field ...
Daddy Kropotkin's user avatar
5 votes

Do Einstein's ten field equations use 20 or 40 variables? (2 or 4 for each tensor equation?)

A single vector equation like $\vec{F} = m \frac{d^2 \vec{r}}{dt^2}$ is actually three equations one for each of the $x,y,z$ components of the 3-vector $\vec{r}$. In the spacetime of general ...
Paul T.'s user avatar
  • 687
4 votes
Accepted

What does 's' or 'ds' or $ds^2$ mean in the Schwarzschild equation(s)?

ds refers to a distance in spacetime (this includes time). Just as, in classical physics, you see $$ds^2=dx^2+dy^2+dz^2$$ for any given line element, in spacetime you get similar things going on. For ...
Justin T's user avatar
  • 3,404
4 votes

Black hole explosion due to rotation

There is in fact an upper-limit to the angular momentum of the black hole that is described by the Kerr solution. If a black hole existed with a higher angular momentum, you'd have a singularity ...
antlersoft's user avatar
  • 3,455
3 votes

How can Kerr black holes have a 'speed limit' to how fast they can spin?

As an addendum to @Anders Sandberg's excellent answer: The maximum limit on the spin is a consequence of two (not fully proven) conjectures that each imply the existence of this limit individually. ...
TimRias's user avatar
  • 2,531
3 votes
Accepted

Does gravity extend out infinitely?

Does gravity extend out infinitely? Does this technically mean that the gravity from celestial bodies extend out infinitely, even if it has a really tiny effect further out? Basically yes, you've ...
uhoh's user avatar
  • 31.3k
3 votes
Accepted

What happens if a black hole is produced inside another black hole?

From the no-hair theorem, black holes can have only three properties: mass, angular momentum, and electric charge. From this we'll see the original, supermassive black hole gain the mass, angular ...
Allure's user avatar
  • 4,245
2 votes
Accepted

Possible scenarios for life in the death of open universe possible?

Your scheme of creating energy from the tidal forces in a pair of orbiting planets cannot work because of the law of conservation of energy (aka the 1st law of thermodynamics), which states that ...
PM 2Ring's user avatar
  • 13.6k
2 votes

Can a star be massive enough to go 'supernova' yet not be massive enough to leave behind a neutron star? Or, perhaps vice versa?

This diagram from the Wikipedia article on supernovas mostly answers your question I think. There is in fact an interval that is expected to not produce a remnant, but it is not at the lower end of ...
Kristoffer Sjöö's user avatar
2 votes

In 'Modified gravity' theories like MOND, are galaxies and stars actually, slowly accelerating? In the normal sense?

As @PeterErwin's comment compassionately points out: "Acceleration" means changing the velocity vector, so an object in a circular orbit is always "accelerating" (because the ...
Kurt Hikes's user avatar
  • 4,927
2 votes

Black hole explosion due to rotation

No. Consider what "explode" would mean in this context: It means each piece of material tears away and exits the black hole. By definition, that simply can't happen. The material would need ...
Darth Pseudonym's user avatar
2 votes

How can Kerr black holes have a 'speed limit' to how fast they can spin?

Spin too fast and you have a "naked" singularity Basically, yeah, that's exactly what's limiting the rotational speed of a black hole. Rotating black holes, also known as Kerr black holes, ...
Furious Arcturus's user avatar
1 vote

Why are WIMP dark matter particles, if Majorana-like, expected to annihilate upon contact? But not neutrinos?

Gluons actually annihilate, so do neutrinos, just more rarely than Neutralinos. Neutrinos, Gluons, WIMPS, Majorana particles are always neutral being a Truly Neutral Particle. These Majorana particles ...
Arjun's user avatar
  • 1,256
1 vote

What is the relation between size (radius) of an astronomical object and the speed of oscillation of 'light' coming from it? (Strength/amplitude)?

tl;dr Size plays a role in this very specific scenario in kind of an indirect way, but generally speaking, variability in its enormous scope depends on a lot of other things more for other kinds of ...
Justin T's user avatar
  • 3,404
1 vote

What is the 'scale factor' equation for a dark-matter dominated universe?

You answered your question actually. You have said Note that this solution is not valid for domination of the cosmological constant, which corresponds to an w=−1 then you are saying But if 'w' is ...
seVenVo1d's user avatar
  • 576
1 vote

What covers the outside of the Universe?

It's very difficult to answer a question "Are there any theories ...." in the negative, since if nothing else, the questioner has probably just formulated such a theory. I can say with reasonable ...
Steve Linton's user avatar
  • 10.3k

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