Expanding Universe

[Bullgrit]

I heard an interview on the radio this weekend that discussed dark energy. It is responsible for the universe expanding. Listening, this question came to my mind: what is the "size limit" on things expanding? I know galaxies are getting farther apart, but what about the distance between stars? Is our solar system expanding? What about the atoms in everything?

I take it that in this situation, "the universe" doesn't mean "everything that exists." so what does "the universe" mean in this context?

Bullgrit

 

[JediSoth]

As I understand it, "the universe" in this context refers to the observer able universe--a sphere surrounding our solar system* extending outward to 14 billion light years, thereabouts. There is no known limit to the expansion of the universe because we do not have enough information on how dark matter/dark energy works and what its ultimate effect is. We don't even have an direct observational evidence of its existence.

Galaxies, solar systems, and you are not expanding because gravity (along with other weak forces of attraction in the case of our bodies) keeps things together. Every star in a galaxy is gravitationally bound to the supermassive black hole at its center. Every planet, rock, and ice particle in a solar system is gravitationally bound to the star(s) at the center of the solar system. Of course, gravity is considered a weak force because it if wasn't you wouldn't be able to pick up a rock.

This is all grossly simplified, of course, because I am not a cosmologist, astronomer, or physicist. But, I do read a lot of articles about this subject matter because I write sci-fi in which I try to get things right.

...or as right as I can without being a scientist.



* Not to be taken as an implication that the Earth is the center of the universe; it's just...we can only see 14 billion light years or so out, so anything beyond that is UNKNOWABLE because the information has not yet had time to travel to us.

 

[Umbran]

In general - the expansion of the universe holds out in the deeps between galaxies. In places where objects are bound together by gravitational attraction - within galaxies and solar systems, the expansion isn't notable. Stars in the Milky Way are not getting notably farther apart.

Mind you, the Universe is kind of like matter - mostly empty space. The overwhelming bulk of the universe is empty space, expanding, with a sprinkling of dust motes in that space.

 

[Morrus]

Yeah, that's how I understand it. It's space that's expanding. Everything's expanding, but some stuff is close enough (cosmologically speaking) that gravity and other forces presently totally override that expansion. So you and I and the Milky Way aren't expanding in themselves, but our galactic grouping is moving away from other galactic groupings. As billions and trillions of years pass, that accelerating expansion will eventually overpower these forces, so galaxies will be isolated (then stars, but I don't think there will be any stars left then), then atoms, with the eventual infinitely-long end being an infinitesimally non-dense sprinkling of fundamental particles.

 

[JediSoth]

Oh, of course, on a cosmologically smaller scale, some things are on collision courses, like the Milky Way and Andromeda galaxies. I think it's unlikely human civilization will be around to see that though, since it's expected to happen 3.75 billion years from now. But our two galaxies, along with many others are all gravitationally bound as The Local Group. (Don't you love the names astronomers give things?)

 

[Bullgrit]

Galaxy B is 3 units away from Galaxy A. Galaxy C is 3 more units beyond B. With the expansion of the universe, is C moving away from A faster?

A . . . B . . . C
--expansion--
A . . . . . . B . . . . . . C
B moved 3 units, and C moved 6 units. (From A.)

Is this correct?

Bullgrit

 

[Umbran]

Is this correct?

In basic concept, yes. The farther away it is, the faster it is moving away from you. It is Hubble's Law.

 

[Bullgrit]

Well then that brings up the speed of light limit.

Bullgrit

 

[Morrus]

Well then that brings up the speed of light limit.

Nope - that's the limit something can move through space. This is something different - it's space itself expanding.

 

[Umbran]

Morrus is correct. The objects themselves are not moving through space, so much as there is space developing between objects. You can have this sort of expansion yield apparent velocities greater than c. What happens then is these things recede *out* of the visible Universe.