Oldness/Flatness Problem

[otseng]

This has been mentioned a couple of times in different threads: Anthropic Principle and Intelligent Creation (God) as opposed to Evolution. But, I'd like to put this in its own thread.

So for debate. Why is the universe flat? That is, why does it have Euclidean geometry?

[otseng]

If two dots are on a balloon and the balloon is expanding, the two dots would be moving apart. Though the dots are stationary on the balloon, from the dots perspective, they are moving away from each other. And they could even measure how fast they are receding from each other.

Likewise, if objects on the space-time fabric are just "sitting there", but the fabric is being stretched, the objects would experience that they are moving away from each other.

So, if the fabric is stretching FTL, the objects would still be moving FTL. Further, by saying it is faster than light, it would only be meaningful from the perspective of the objects, since FTL would only make sense within space-time, not outside of it.

Sure it's an argument, but would it not persuade you on its merits alone?

I'd like to get back to this.

No, it would not persuade me. What I'm looking for is evidence that the universe experienced inflation. Simply assuming that inflation is true and then pointing to its conclusion does not show the assumption to be true.

[Goat]

Think of a string with an ant walking along it. As the ant walks from one side of the string to the other, the string keeps on getting pulled so it is longer. As the ant proceeds down the string, it walks a certain distance, but the length of the string is longer than the distance that the ant walked.

Analogy is not quite right because objects in the universe are not independent of the space-time fabric. Whereas the ant is independent of the string.

The typical description is dots painted on a balloon. Then as the balloon expands, the dots get farther apart. But, the dots are also expanding. If space-time stretches, then all objects would also experience stretching. I would guess even atoms would experience stretching.

The local strong forces at work overcome the stretching

[otseng]

The local strong forces at work overcome the stretching

Strong forces are only at the subatomic level.

What would keep atoms and molecules from flying apart?

[Goat]

The local strong forces at work overcome the stretching

Strong forces are only at the subatomic level.

What would keep atoms and molecules from flying apart?

The weak nuclear force.. and there is there is also (for stars and planets), gravity.

[otseng]

The weak nuclear force.. and there is there is also (for stars and planets), gravity.

Weak nuclear force is also only at the subatomic level.

The weak interaction affects:

neutrinos
charged leptons
quarks

http://nostalgia.wikipedia.org/wiki/Weak_nuclear_force

And gravity is too weak at the atomic level.

[Goat]

The weak nuclear force.. and there is there is also (for stars and planets), gravity.

Weak nuclear force is also only at the subatomic level.

The weak interaction affects:

neutrinos
charged leptons
quarks

http://nostalgia.wikipedia.org/wiki/Weak_nuclear_force

And gravity is too weak at the atomic level.

At the atomic level it is strong enough to overcome the current expansion of the universe.

And gravity, on the macro level, is strong enough to over come it on the local , non-atomic level.

This might not always be so. With the appearent increase of expansion, one way that the universe might 'end' is in the Big Rip

[otseng]

I have serious doubts that gravity can keep molecules together.

Additionally, do we have any evidence that the space-time fabric is currently stretching? How can this be tested?

And again, do we have any evidence that the unobservable universe is significantly larger than the observable universe? And how can this be tested?

[QED]

Additionally, do we have any evidence that the space-time fabric is currently stretching? How can this be tested?

And again, do we have any evidence that the unobservable universe is significantly larger than the observable universe? And how can this be tested?

otseng, I wonder if you could be persuaded to read this Scientific American article titled: Misconceptions about the Big Bang first. I think it would help considerably if we were all "on the same page" regarding the cosmology we're considering.

[otseng]

otseng, I wonder if you could be persuaded to read this Scientific American article titled: Misconceptions about the Big Bang first. I think it would help considerably if we were all "on the same page" regarding the cosmology we're considering.

OK, I read the article. And surprisingly, I don't think I'm too far off in my understanding of what cosmologists say. I think.

The expansion of the universe is like Darwinian evolution in another curious way: most scientists think they understand it, but few agree on what it really means.

Interesting statement.

James Peebles of Princeton University, wrote in 1993: "The full extent and richness of this picture [the hot big bang model] is not as well understood as I think it ought to be ... even among those making some of the most stimulating contributions to the flow of ideas."

I think this is related to the first statement above.

Renowned physicists, authors of astronomy textbooks and prominent popularizers of science have made incorrect, misleading or easily misinterpreted statements about the expansion of the universe.

This certainly doesn't help matters.

According to the usual Doppler formula, objects whose velocity through space approaches light speed have redshifts that approach infinity. Their wavelengths become too long to observe. If that were true for galaxies, the most distant visible objects in the sky would be receding at velocities just shy of the speed of light. But the cosmological redshift formula leads to a different conclusion. In the current standard model of cosmology, galaxies with a redshift of about 1.5--that is, whose light has a wavelength 150 percent longer than the laboratory reference value--are receding at the speed of light. Astronomers have observed about 1,000 galaxies with redshifts larger than 1.5. That is, they have observed about 1,000 objects receding from us faster than the speed of light.

Not too easy for me to accept, but, ok, I'll go along with it.

The idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place.

Yes, I am wondering about this.

One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us.

OK, you lost me here.

People often assume that as space expands, everything in it expands as well. But this is not true. Expansion by itself--that is, a coasting expansion neither accelerating nor decelerating--produces no force. Photon wavelengths expand with the universe because, unlike atoms and cities, photons are not coherent objects whose size has been set by a compromise among forces. A changing rate of expansion does add a new force to the mix, but even this new force does not make objects expand or contract.

I see no difference. If the space-time fabric is stretching, I still don't understand why objects that occupy space-time would not be stretching as well.

For example, if gravity got stronger, your spinal cord would compress until the electrons in your vertebrae reached a new equilibrium slightly closer together. You would be a shorter person, but you would not continue to shrink. In the same way, if we lived in a universe dominated by the attractive force of gravity, as most cosmologists thought until a few years ago, the expansion would decelerate, putting a gentle squeeze on bodies in the universe, making them reach a smaller equilibrium size. Having done so, they would not keep shrinking.

Still does not make it any clearer for me.

We hope, though, the confusion about the expansion will shrink.

If anything, it has probably expanded.

[QED]

OK, I read the article. And surprisingly, I don't think I'm too far off in my understanding of what cosmologists say. I think.

Well, thanks for taking the trouble and time anyway. I'm sure it has helped.

The expansion of the universe is like Darwinian evolution in another curious way: most scientists think they understand it, but few agree on what it really means.

Interesting statement.

It's a standard disclaimer in cosmology

The idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place.

Yes, I am wondering about this.

One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us.

OK, you lost me here.

That's fine. It's a very common difficulty when met for the first time, so much so that there is a paper specifically aimed at overcoming this difficulty for students. It explains how the Hubble sphere with radius c/H is not actually a horizon. Photons emitted towards us from within our light cone but outside this sphere are overtaken by the recession of the sphere while photons emitted from beyond the event horizon are never overtaken by the Hubble sphere and consequently never reach us.

People often assume that as space expands, everything in it expands as well. But this is not true. Expansion by itself--that is, a coasting expansion neither accelerating nor decelerating--produces no force. Photon wavelengths expand with the universe because, unlike atoms and cities, photons are not coherent objects whose size has been set by a compromise among forces. A changing rate of expansion does add a new force to the mix, but even this new force does not make objects expand or contract.

I see no difference. If the space-time fabric is stretching, I still don't understand why objects that occupy space-time would not be stretching as well.

I don't see the problem unless you somehow imagine things being woven into that "fabric"- which, by General Relativity, they are not.