View Full Version : Expansion of the universe

2009-Aug-22, 05:36 PM
Space is and always has been expanding. OK. Why don't objects in space expand, too? After all, they consist mostly of space with some scattered particles in it. (I realize that my asking this question reveals the empty space between my ears.)

If the objects in space expanded along with the space of which they are part, then all yardsticks would expand, and space would seem always to be the same size, and nothing would ever get farther way from anything else. Red-shift wouldn't happen, because as wavelength increased, so would the size of our instruments, and no change could be detected.

I'd really like to hear from anyone who can straighten out on this.


2009-Aug-22, 06:35 PM
Objects don't expand for the same reason galaxies and galactic clusters don't expand - the effects of a force are greater than the effects of expansion. Thus, expansion's effects are overcome by other effects.

In the cae of galaxies and galactic clusters, the force is gravity. In the case of objects, it's the strong and electromagnetic forces. The strong force holds quarks and gluons together to form protons, neutrons, and other particles. The electromagnetic force holds electrons and protons together in atoms and molecules.

Jeff Root
2009-Aug-22, 08:21 PM
The reason that Mugaliens gave is certain. There is a second possible
reason that is not certain, but is more important if it is the case. Namely,
that the expansion of space has nothing to do with space itself expanding
somehow, but is simply the motion of clusters of galaxies away from each
other. We don't know why they are moving apart, or why their motion is
apparently accelerating. The only clue we have that space is expanding
at all is the relative motion of things tens of millions of light-years apart,
or more. There are no signs of any expansion at smaller distances.

That could be simply because the force causing the acceleration of the
expansion is extremely weak, and the forces holding things together on
small scales are vastly stronger, or it could be because there is no such
expansion force inside of galaxies at all.

-- Jeff, in Minneapolis

2009-Aug-22, 08:31 PM
Space is and always has been expanding. OK. Why don't objects in space expand, too? After all, they consist mostly of space with some scattered particles in it....

You've got a couple of answers already. Mugs gave a simple view of the mainstream idea, and Jeff pointed out that while the mainstream idea works, other ideas aren't strictly ruled out, though any specific idea given so far other than the mainstream idea has had problems explaining everything.

Another thing to think about with this is that space is expanding at about one part in fourteen billion per year. This is pretty small as far as particles and atoms are concerned, and the space that separates parts of atoms is driven by a balance of forces (in a classical sense), and so if there is a tiny amount of extra space that appears within an atomic nucleus, the nucleons will very quickly resettle to their old distances (this is an elaboration on mugs' description).

2009-Aug-22, 08:54 PM
and just adding further, expansion couldn't be measured locally since, as you stated in your OP, everything would expand at the same rate and would appear to be at a constant size. This is why we only see the effects out in deep space where the force of gravity is weaker than the expansion between distant galaxy clusters. What is puzzling is that it appears that the further out into space we look the faster this expansion seems to be, hence it is believed that the expansion is accelerating.

2009-Aug-22, 10:15 PM
Jeff's idea of other factors is a good one. Some points, though:

We know it to be true that the metric expansion of space (http://en.wikipedia.org/wiki/Metric_expansion_of_space), that is, that space itself is expanding, and it's not just due to galaxies moving in opposite directions, due to confirmation of the FLRW metric (http://en.wikipedia.org/wiki/FLRW_metric). This metric is an exact solution of Einsteein's field equations of general relativity.

The FLRW metric begins with the assumptions of homegeneity (confirmed by Hubble's deep space and ultra deep space telephotograpy) and isotropy (confirmed by CMBR observations). The mathematics behind it were rigorously proved by Howard Robertson in 1935, and later acknowledged by Einstein as correct.

In fact, this metric is further evidence of dark energy, as there is a replacement for the cosmological constant (http://en.wikipedia.org/wiki/FLRW_metric#The_cosmological_constant_term)in the metric that requires negative pressure, the so-called "dark energy" used to describe the fact that space expansion appears to be accelerating.

We know from the Lamda-CDM concordance model (http://en.wikipedia.org/wiki/Lambda-CDM_model), with a good degree of precision, that the universe is 13.75 billion years old. Thus, light has been travelling through the universe for that long.

However, due to expansion that has occurred over that time, the edge of the observable universe (http://en.wikipedia.org/wiki/Observable_universe) is determined by the fact that objects at that edge are receeding away from us at the speed of light. Furthermore, due to expansion, that edge is 46.5 billion light years distant from us. Finally, because of expansion, we appear to be in the exact center of the observable universe.

How large the universe is in reality is anyone's guess. Estimates have placed it as smaller than the observable universe, postulating matching-circle conditions based on the WMAP data, while Alan Guth, the founder of cosmic inflation, estimates the universe could be at least 10E23 to 10E26 times larger than the observable universe.

Thus, while matter may indeed have had initially high separation velocities, our current understanding of cosmology indicates that the expansion of spacetime itself is responsible for the vast majority of distance we see today throughout the observable universe.