# A universal conundrum.

1. Feb 7, 2009

### Peter Watkins

It is said that the universe began as a single tiny speck that suddenly expanded outward in all directions.
It is also said that the universe is not expanding away from a particular point in space.
These two statements are mutually incompatible. I await your reply with interest as you seem to have most bases covered, no matter how big a conundrum is offered.

2. Feb 7, 2009

### Staff: Mentor

They are not mutually exclusive. The problem is that the image you have in your head of what that looks like geometrically is incorrect. Think of the 2d analogy of an expanding balloon. It is impossible to actually visualize that in 3d, but geometrically, it works the same way.

3. Feb 8, 2009

### Chalnoth

The correct statement is that everything that is visible now was once contained within a minuscule volume. There is quite a bit of stuff outside what we can see.

There are many analogies for what the expansion looks like, but you have to bear in mind that these are just analogies. Let's imagine, for a moment, that the entire universe is one single rubber sheet. This sheet is infinite in all directions. It starts out in such a way that everything is close together: there are a bunch of dots drawn on the sheet, and they're all so close together that you can hardly distinguish between them. Now, the way that gravity acts, it causes the sheet to stretch in all directions. Everywhere on the sheet, the dots are moving away from one another: no matter where I go, at any point in time, the density of dots is the same. As time passes the sheet stretches more, and the dots get further and further apart.

This is a pretty accurate description of how our universe has expanded. We don't know that our universe is infinite, of course, but we do know it's significantly larger than the visible part of it which we can see. But if you paid close attention to the example above, you may notice that there is no apparent center: the entire sheet is stretching in the exact same way.

Blowing up a balloon, as russ_watters mentioned, is another way of looking at this, but the analogy is less accurate. To expand upon that analogy, you just draw a bunch of dots on the balloon before blowing it up, and then as you inflate the balloon, all the dots get further from one another by equal amounts. There is no point on the surface of the balloon that can be called a "center". Just as there is no point on the sheet above that can be called a center.

4. Feb 18, 2009

### Peter Watkins

Thank you for your replies, but surely the raisin bread analogy is a far more accurate way of describing the expansion of the universe, even though this is purely a description and not an explanation. Indeed, there never has been an explanation as to why galaxies are separating at a rate that increases with distance. The expanding balloon in no way resembles what we can see all around;- depth.

5. Feb 18, 2009

### Chalnoth

Sure it's been explained. There are only two possible behaviors for a universe that is, on large scales, homogeneous and isotropic (the same everywhere and in every direction). One is uniform expansion. The other is uniform collapse.

Uniform expansion behaves as you describe: recession velocity is proportional to distance.

So, why was our universe homogeneous and isotropic? And why is it expanding instead of collapsing? Well, inflation answers the first, and partially answers the second. Basically, inflation is a period of fast accelerated expansion driven by some high-energy field. This blows up a tiny patch into everything we see. So it doesn't actually matter what the beginnings of inflation were, as long as they were such that inflation could begin. And once it did begin, the inflating patch became almost completely uniform in the blink of an eye.

This also partially explains why we're in a period of expansion, because inflation is itself an expansionary phenomenon. A period of expansion is therefore necessary. How long this period of expansion lasts, however, depends upon the contents of the universe. If the contents of our universe were different, it would be conceivable that it could be collapsing now.

So, though inflation explains the expansion, it does leave some unanswered questions:
1. What, precisely, is inflation? Though we are fairly confident that inflation occurred, there are still some big questions as to its precise nature. These are being tested, however, and it is hoped that future CMB experiments will have quite a lot to say on this front.
2. How did inflation get started? We have even less idea here. If we're very lucky, nailing down the precise properties of inflation will allow us to determine how it started. Since inflation itself hides how it began, it is unlikely we will ever obtain direct evidence of how it began. Hopefully indirect evidence will be forthcoming.
3. Why are the contents of the universe in the ratios they are? This depends upon a lot of as-yet-unknown properties of high-energy physics, as well as the mysterious dark energy (or modified gravity, whichever it is).

To sum up, yes, we know the answer to this question, but our answer raises still more. Such is the nature of science.

6. Feb 19, 2009

### Peter Watkins

It seems that the inflation theory poses more questions than it resolves. Two little points; you say, along with others, that the universe is isotropic, but it isn't! Check the description of what can be seen in the H.U.D.F.view, ie., crowding and interaction, and compare this with what can be seen in the opposite direction. Huge voids between galaxies that are "strung out" resembling walls of up to a billion ly's long and tens of millions of light years thick. No great similarity there!

7. Feb 19, 2009

### Chalnoth

I'm not sure that's true. It solves quite a few issues. More than I've mentioned here.

It is very close to isotropic on large scales. That's what I meant. And by the way, it turns out that inflation also comes along with an explicit prediction for how the small deviations from isotropy are generated. And the way these are generated leaves a very specific signature in the cosmic microwave background as well as the distribution of galaxies in the universe. These predictions have been verified to startling precision.

If you break up the region you're looking at into boxes about 80Mpc on a side or so, you'll find that the stuff inside every box is distributed in about the same way.

8. Feb 20, 2009

### Naty1

Hi Peter,
While there ARE some theories about how this universe got started there are still many mysteries, in part because we have a divergence between quantum and relativity theories. For example, inflation solves many of the "strange" characteristics we observe experimentally, yet "space" expanded faster than light speed for a brief period of symmetry breaking when a transition from an unstable, high energy configuration moved to a lower, more stable environment we see today. (This resulted from a supercooled Higgs field.) That's because space, and time, did not exist yet, and perhaps did not exist before the big bang.

Well the superficial explanation of course was proposed by Einstein: the cosmological constant. This repulsive gravitational force arises from the energy and pressure of space itself which becomes important only over vast cosmological distances. A Higgs field caught onj a plateu, supercooled state, suffuses space with energy and negative pressure just like the cosmological constant.
There remains a related issue about why the "constant" is now causing an increasing rate of cosmological acceleration.

For another huge theoretical problem see Wikipedia at
http://en.wikipedia.org/wiki/Cosmological_constant#Cosmological_constant_problem.

"you say, along with others, that the universe is isotropic, but it isn't!"
It IS on large scales. Variations in cosmic microwave background radiation conform closely with theoretical variations resulting from quantum fluctuatations in the early universe.

Last edited: Feb 20, 2009
9. Feb 20, 2009

### Chalnoth

Just to add a bit to the isotropy of the universe: the CMB is isotropic to one part in 100,000. That's pretty isotropic.

10. Feb 20, 2009

### Peter Watkins

Thank you for your replies. It really is quite interesting how that which seems so simple, (the forces and photon production aside), can be made to seem, or perhaps actually is, so complicated. On the face of it, it seems that there are these gravity-formed clumps of matter, which are hurtling outward due to the kinetic energy imparted to them by the big bang, that are now being slowed by gravity, causing them to move apart.
Re isotropy; there is none to be seen in the Geller-Huchra Wedge.