What are the Physical effects of the expansion of the Universe

[ashwinnarayan]

Homework Statement

I trying to learn general relativity and ran into a few questions while learning about the expansion of the universe.

I found a really good intuitive explanation of the expansion of the universe here: http://curious.astro.cornell.edu/question.php?number=274

However, I am still left with a few questions, particularly about this paragraph from the article:

Finally, I should point out that not everything in the universe is "stretching" or "expanding" in the way that the spaces between faraway galaxies stretch. For example, you and I aren't expanding, the Earth isn't expanding, the sun isn't expanding, even the entire Milky Way galaxy isn't expanding. That's because on these relatively small scales, the effect of the universe's stretching is completely overwhelmed by other forces (i.e. the galaxy's gravity, the sun's gravity, the Earth's gravity, and the atomic forces which hold people's bodies together). It is only when we look across far enough distances in the universe that the effect of the universe's stretching becomes noticeable above the effects of local gravity and other forces which tend to hold things together. (That is why, in the analogy of the tape measure I discussed above, the tape measure that you keep in your pocket does not get stretched, while the one that goes between two galaxies does get stretched. I bet some people were wondering about that!)

What exactly does it mean that gravity and atomic forces are stronger than the expansion of the universe? Does this mean that the expansion exerts a small constant force on every particle that forces them away from each other? And if such a small force does indeed exist then shouldn't we be able to find objects on the universe which are at a distance where the force of the expansion of the universe exactly cancels out the force of gravity?

Homework Equations

The Attempt at a Solution

 

[mathman]

I'm not sure what the exact distance would be. However, galaxies, galactic clusters, and super-clusters seem to hold together.

 

[phinds]

An analogy that I like is that the effect of "dark energy" on gravitationally bound systems is exactly the same as the effect of an ant pushing on a tank. It's not that the force exerted has a tiny effect, it's that it has no effect at all.

 

[ashwinnarayan]

I think I understand. So there is a small force. That's the reason one of the possible theories for the end of the universe is that the expansion of space overcomes the force of gravity and electromagnetism cause atoms to rip apart.

I think they call it "The Big Rip". They're quite fond of repeating that on all those shows about the universe on The History Channel

 

[ashwinnarayan]

The Force increases in the future because the expansion of the universe is accelerating.

 

[phinds]

I think I understand. So there is a small force. That's the reason one of the possible theories for the end of the universe is that the expansion of space overcomes the force of gravity and electromagnetism cause atoms to rip apart.

I think they call it "The Big Rip". They're quite fond of repeating that on all those shows about the universe on The History Channel

The force does NOT increase as far as is known. Its EFFECTS increase, outside of gravitationally bound systems, because it has a cummulative effect, but inside gravitationally bound systems, there is no evidence that it will ever have any effect. The big rip is total speculation.