Space expands and gravity has a negative impact on the expanxsion?

At the event horizon of a black hole time stands still.

So in expanding space we have time flowing normally. In a gravity field time slows down.

Does that mean that space isn't expanding at the event horizon.

If that's true then is the rate at which space expands directly related to the speed of light?

Meaning that if space would expand slower,.... the speed of light would be different?


1 Answer 1


A lot of questions and some confusion. I'll try a few.

-time flowing normally in expansion: that's only because the time that is tied to the expansion flow, is the time in the rest frame of the bodies that are flowing with the flow. We are them. It's called cosmic time because anywhere in the universe it applies the same way. It is a result and artifact of the uniform and isotropic universe and flow.

-in a gravity field or to be more exact in a region where the gravity is stronger, like near the Sun, or near a Black Hole's horizon, it slows down, compared to where gravity is weaker. On the surface of the earth it goes slower than a few hundred or thousand miles up. For instance it is faster at the GPS satellite positions than on the surface of the earth, by a very small amount, but clocks need to be adjusted for it. See https://en.m.wikipedia.org/wiki/Time_dilation

-out in interstellar space there are regions where there are more galaxies (really a higher mass density), than in some other regions (called voids, they are no real voids just lower mass density). This happens statistically due to normal variations. In the voids the clocks go faster, by the corresponding amount, than near a lot of galaxies.

-how time flows depends on gravity. The speed of light does not. The way light is affected is it undergoes a redshift if it is coming out of a gravity well (high gravity), and blue shifted if going in. It can also be deflected (bent) by strong gravity (eg when it comes close to the Sun, it bends in, same going around as it skirts a Black Hole). But the speed is unchanged, measured by a local observer

-space can only expand slower if there was more mass density on the average in the universe. As it is, there is less than the dark energy, which makes it expand faster (very slightly now, but it is accelerating). Speed of light does not change.

-Oh, and the universe expansion basically does not affect the gravity or curvature near a Black Hole horizon. The Black Hole gravity is just too strong. Now, to be sure, that's now, anywhere in the universe, but as it expands faster it can have more of an effect. It'll about hundreds of billions of years tear up all the matter, probably only Black Holes will remain at late times, eventually they will all evaporate and as the universe continued accelerating and expanding it'll be mostly void.

-See the wiki article on the speed of light at https://en.m.wikipedia.org/wiki/Speed_of_light

See the wiki article on accelerated expansion at https://en.m.wikipedia.org/wiki/Accelerating_expansion_of_the_universe

Hope this clear up some confusion.

  • $\begingroup$ Thank you for your reply. That helped. What I'm really curious about is if there is anything known about why C is C. If there is some calculation based on other values that ends up with the speed of light being what it is. $\endgroup$
    – user144752
    Commented Feb 8, 2017 at 15:56
  • $\begingroup$ No, it is what iss measured. But the value is not as important as is the fact that it is the same in any local reference frame. That is what makes c unique. Btw, c is also obtained from the constants on electromagnetism, specifically c = 1/sqrt(vacuum permitivity x vacuum permeability) which came originally from measurements of electrical and magnetic fields. When they realized c was the same it served as evidence that light is an electromagnetic wave. Nowadays the permitivity is defined in terms of c and the permeability, but back then (1860's when Maxwell got his equations) they didn't know $\endgroup$
    – Bob Bee
    Commented Feb 9, 2017 at 1:55
  • $\begingroup$ C has over 100 years of having been established as the max speed of causation or any information transfer, and is the speed at which any massless particle has to travel. Nowadays lots of physics built around understanding c $\endgroup$
    – Bob Bee
    Commented Feb 9, 2017 at 1:57
  • $\begingroup$ Ok. I looked up how the vacuum permitivity and permeability are defined and if I understand it correctly, we can calculate C exactly without using any measured values. Is that correct? $\endgroup$
    – user144752
    Commented Feb 9, 2017 at 2:50
  • $\begingroup$ Measuring those two you get c without measuring c $\endgroup$
    – Bob Bee
    Commented Feb 9, 2017 at 4:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.