Expansion of the Universe

[Kyrisch]

Just a simple question that occurred to me today: if the universe is expanding, why does the distance between the earth and the sun remain unchanged?

[pioneer]

The universe is expanding relative to the galaxies. Below that level things appear to be bound by gravity. This suggests maybe there is a practical limit to gravity. The theoretical limit is infinite but in the practical world it doesn't do very well beyond galaxy size. This could be due to dark energy with seems to truncate gravity beyond about the galaxy size.

[iNow]

Uhhmm... pioneer... what are you talking about?

 

 

Kyrisch, I'm pretty sure that the simple fact is that the distance between the earth and the sun is expanding along with the rest of the universe, it's just that the scale is so small relative to the rest that it's barely noticable.

 

When people talk of the universe expanding, it is on the scale of entire galaxies. Since our solar system is a barely notieable speck in just one single galaxy, you can see why the distance between two objects in that one tiny solar system don't seem to be changing.

[Klaynos]

Things which are bound strongly by gravity (or EM, weak or strong forces) are not expanding being because the bonds between them are stronger than the effect of the expansion. This is I'm sure what I've been told.

[doG]

Just a simple question that occurred to me today: if the universe is expanding, why does the distance between the earth and the sun remain unchanged?

 

For the same reason an atom of hydrogen remains the same. The binding force is strong enough to prevent it.

[john5746]

I think Andromeda and the Milky Way are in fact on a collision course. Is this explained by gravitational attraction between the two?

[thedarkshade]

I think Andromeda and the Milky Way are in fact on a collision course. Is this explained by gravitational attraction between the two?

Galaxies are moving away from each other. Everything is just moving away from everything. Our current model says that our universe appears to be infinite, and there's enough room at infinity for everything to move away from everything. And if this is happening, how would Andromeda possibly collide with Milky Way?!

[Klaynos]

Andromeda WILL collide with the Milky Way, and yes I believe they are gravitationally attracted.

[doG]

Galaxies are moving away from each other. Everything is just moving away from everything.

 

Ummmm......NO.

http://hubblesite.org/newscenter/archive/releases/2008/16/image/a/

[Kyrisch]

Nothing is 'moving away' from anything else. Space itself is expanding.

[doG]

Nothing is 'moving away' from anything else. Space itself is expanding.

 

It would seem that somethings are moving away from us or there wouldn't be any redshift.

[Jacques]

Galaxy cluster are gravitationnaly bounded, and Andromede is part of the local group cluster.

[Kyrisch]

It would seem that somethings are moving away from us or there wouldn't be any redshift.

 

Well of course there are relative velocities within the universe, but it is commonly said that, in reference to the expansion of the universe, as opposed to things moving away from each other, space itself is expanding. I am in no way suggesting that all relative motion is caused by expansion of space.

[Mr Skeptic]

It would seem that somethings are moving away from us or there wouldn't be any redshift.

 

Or, the space the photons are traveling through expands, increasing their wavelength (aka redshift). Not sure where the extra energy would go though.

[Royston]

Please excuse the crappy diagram, but this is just to illustrate to some of the responses on this thread, what is meant by redshift when the space between objects is expanding, and just to add, dark energy is a weaker force than gravity...otherwise there wouldn't be local effects, such as the distance between the Milky Way and Andromeda reducing...

 

The purple lines (I'm hopelessly colourblind BTW) signify the expansion between the observer and the object, (so imagine the circle's diameter increasing over time) whereas the red lines signify red shift. Whichever point I care to pick on the circle, to observe the central object in the diagram, it will display redshift.

 

This is a massive simplification, (e.g all objects are in motion, and the central object is assumed at rest) but for the case with distant objects, this has been observed. Needless to say this expansion is accelerating.

Edited June 16, 2008 by Snail