Why I think space is expanding

[dryson]

I wouldn't say that space is expanding. How can space expand? All space is is the area between two atoms that cannot be effected upon by atoms or energetic reactions but can effect upon atoms and energetic reactions within space. What is expanding are the billion's of solar system's within our (yet to be named Universe) Unviersal Domain. To think that the Universal Domain that the Sol System resides in as being the only Universe is the same as thinking that Earth is the center of all creation and that the planet is still flat.

 

[csmyth3025]

I wouldn't say that space is expanding. How can space expand? All space is is the area between two atoms that cannot be effected upon by atoms or energetic reactions but can effect upon atoms and energetic reactions within space. What is expanding are the billion's of solar system's within our (yet to be named Universe) Unviersal Domain...

Since the old "dots on a balloon" analogy just doesn't seem to do it for you, let's try to explain it this way:

You're one of billions of atoms floating around in a closed cylinder with a plunger on one end. The cylinder is so big that you can't see the end of it in any direction - even with your biggest atom-sized telescopes. The cylinder is your universe as far as you can tell. The Hand of God pulls on the plunger. All of your fellow atoms move farther apart. Space has expanded.

Chris

 

[SpeedFreek]

What is expanding are the billion's of solar system's within our (yet to be named Universe) Unviersal Domain. To think that the Universal Domain that the Sol System resides in as being the only Universe is the same as thinking that Earth is the center of all creation and that the planet is still flat.

Solar systems are not expanding. Galaxies are not expanding. Clusters of galaxies are not expanding. It is the gaps between the clusters of galaxies that increase in size.

 

[csmyth3025]

...Solar systems are not expanding. Galaxies are not expanding. Clusters of galaxies are not expanding. It is the gaps between the clusters of galaxies that increase in size.

On that note, I have a question about the "flatness problem", the Wikipedia article on which can be found here:

http://en.wikipedia.org/wiki/Flatness_problem

The article contains the following passages:

((Omega^-1)-1)(pa^2)=(3kc^2)/((8pi)(G))[3]
The right hand side of this expression contains only constants, and therefore the left hand side must remain constant throughout the evolution of the universe.

As the universe expands the scale factor 'a' increases, but the density 'ρ' decreases as matter (or energy) becomes spread out. For the standard model of the universe which contains mainly matter and radiation for most of its history, 'ρ' decreases more quickly than 'a^2' increases, and so the factor 'ρa^2' will decrease. Since the time of the Planck era, shortly after the Big Bang, this term has decreased by a factor of around 10^60,[3] and so ((Ω^− 1) − 1) must have increased by a similar amount to retain the constant value of their product.
and...
Data from the Wilkinson Microwave Anisotropy Probe (measuring CMB anisotropies) combined with that from the Sloan Digital Sky Survey and observations of type-Ia supernovae constrain Ω0 to be 1 within 1%.[7] In other words the term |Ω − 1| is currently less than 0.01, and therefore must have been less than 10^−62 at the Planck era.

Do these passages mean that despite the continued (and, apparently, accelerating) expansion of the universe, space will remain forever "flat" - as it has been for the past 13.7 billion years? That is - not "closed" (big crunch) and not "open" (big rip).

Chris

 

[ramparts]

The curvature of the Universe (measured on the aggregate) is highly unlikely to be exactly flat. We know from the CMB that Ω is somewhere between 0.99 and 1.01, and probably much closer to 1 than that (Ω=1 is, of course, the hallmark of a flat universe), but there's probably some distant decimal place at which it's not exactly 1. |Ω−1| is just the measurement of that deviation from true flatness, the absolute value of the difference between Ω and 1. Saying Ω is between 0.99 and 1.01 is the same as saying |Ω-1|<0.01.

The problem is that as the Universe expands, any deviation from flatness expands with it, and pretty severely. The calculation in the article is a standard one, showing that the difference |Ω−1|* must have grown by a factor of about 10^60 between the very early Universe and today. In order for Ω to be as close to 1 as it is today, back then it must have been ridiculously closer - in particular, |Ω−1|<0.01 today, so back then we can place an upper limit of |Ω−1|<10^-62, and it was quite probably even smaller. This is a serious fine-tuning problem, as we have no a priori reason to expect the Universe to start off so incredibly close to flatness. Solving this problem is one of the greatest triumphs of inflation.

*Math note: Obviously the term (Ω^−1)−1 (or 1/Ω−1) which is quoted in that formula on Wiki is different from |1−Ω|, but if Ω is very close to 1, then these two terms are almost exactly equal. The reason is that for very small x, (1+x)^n is approximately equal to 1+nx. Replace x with Ω-1, which we already know to be very small; then we have (Ω^−1)−1 = [(1+(Ω−1))^−1]−1 ~ 1−(Ω−1)−1 = 1−Ω.

 

[csmyth3025]

Thanks Ramparts.

I take it that the current "flatness" of omega=1 +0.01 is basically the limit of our ability to estimate it based on our observations. Is it generally believed that it will remain essentially the same after the (pressure X scale factor) term decreases by a factor of 10^60 again?

Chris

 

[ramparts]

Well, no, it will never remain the same if that pressure times scale factor squared term continues to change, because the ratio of those two has to be constant. So if that term increases by another 10^60, then all I can tell you is that the flatness will be no greater than |(Ω^-1)-1|=10^58 Luckily that moment is a long way off.

(Actually, a back of the envelope calculation suggests that moment is about a trillion years off, which is a long way but not too long - on the order of 10 or 100 times the age of the Universe. So it could certainly be interesting to get more constraints on the flatness!)

(Also actually, to get into more detail, we're dealing with (Ω^-1)-1 which is not the easy-to-interpret |Ω-1|. When Ω is close to 1, the two are pretty much the same, as I mentioned before, but that's not true here. So it would be wrong to say |Ω-1|<10^58. We would say (Ω^-1)-1<10^58, or Ω>10^-58. Which isn't very restrictive.)