# What is meant by expanding space

1. Oct 1, 2004

### Gonzolo

Hi,

Cosmologists, starting perhaps with Hubble, say that our galaxy is expanding like a sponge, or a raisin bread. Now if I take only two raisins of this bread which are on its x-axis, am I to understand that :

1. the distance d = x2 - x1 is getting greater,

or

2. the x-axis itself is stretching, such that d = x2 - x1 remains constant

In other words, if I had a very long rigid stick from here to a red-shifted galaxy, would the galaxy then run away from the stick, or would the stick be stretching?

2. Oct 1, 2004

### cragwolf

The stick isn't expanding. The solar system isn't expanding. Our galaxy isn't expanding. Even our galaxy cluster isn't expanding. You need to consider the universe on the scale at which it appears (roughly) homogenous. So in your raisin analogy, just consider the raisins as galaxy clusters. The dough (space) between them expands.

3. Oct 1, 2004

### marcus

I agree with what cragwolf says. I also want to answer your questions explictly:
Yes, proposition #1 is right. Cosmologists use a distance function called the FRW metric to define the distance between two points. the formula has a time-dependent term a(t) which keeps increasing and making the distance between two points keep increasing.

such a long stick might be a physical impossibility and I do not see how such a very long stick could be rigid! but suppose.

then yes, the galaxy would run away from the stick, as long as the stick remained whole

if he was very long the stick might experience some slight tension. if he was too long he might snap in two. But no reasonable-size stick would stretch---molecular bonds and metallic crystal bonds are plenty strong to resist gentle expansion.

You are postulating an extreme case.

I personally think that if a stick were as long as the Hubble radius, namely 13.8 billion lightyears, then the expansion lf space would snap it in two, no matter how strong. Or steel cable, no matter what great tensile strength.

but such a length is ridiculous, nothing real is so long
even galaxies (a few hundred thousand lightyears wide) are not stretched by space expanding
because their own gravity keeps them together.

Last edited: Oct 1, 2004
4. Oct 1, 2004

### Gonzolo

Thanks. I know the stick is ridiculous, it was just a masseless rigid x-axis, so that I could eliminate the confusion that comes with "space is expanding". (If it can curve, than perhaps it could have "stretched", which I now see it does not.)