[ExI] Expansion of the Universe

[John Clark]

On Sat, Dec 29, 2012 at 3:19 PM, Ben Zaiboc <bbenzai at yahoo.com> wrote:

>> What always occurs to me when I read about the expanding universe, is
> the possibility of something analogous to sound waves in air, applied at a
> hugely huger scale.  Maybe the universe is reverberating to the big bang,
> or some similar long-ago event, and we are currently in the expansion phase
> of a universal 'sound-wave'.  Maybe in a few billion years the galaxies
> will start compressing together again, then in a few billion years after
> that, expand again, etc. Any theoretical objections to this idea?  I've
> never seen it discussed.
>

I think this idea would have made more sense before we discovered that the
universe is not only expanding but accelerating. The acceleration is caused
by Dark Energy and if that was changing its value the shape of space would
be changing too and there is no evidence of that.

> Note that this is different to the 'periodic big bang' idea.  It's much
> less dramatic, just a periodic change in the density of the universe, like
> sound waves in air
>

I don't see how periodic oscillations in the density of the universe would
be compatible with observation. The Friedmann equation says that the energy
density of the universe (and remember E=MC^2) is the sum of the
contribution from the expansion of the universe and the contribution from
the curvature of space itself. If you plug in numbers and want to make the
curvature of space term to be zero, that is make the universe be flat, then
there would have to be a lot of energy in the universe that we haven't
detected before, Dark Energy. And in just the last couple of years we've
learned that the universe is indeed flat.

There is about 5 times as much Dark Matter as regular matter but even with
Dark Matter and regular matter combined it only amounts to 30% of the
mass-energy the Friedmann equation tells us that is needed to produce a
flat universe, and yet it is flat, so 70% of the mass-energy of the
universe must be in the form of Dark Energy. The theory was invented to
explain the observed luminosity of very distant supernovas but we find that
Dark Energy also explains why the universe is flat and why the universe
didn't start to accelerate until  8.7 billion years after the Big Bang, 5
billion years ago.

Astronomers proved that the universe is flat by looking at the Cosmic
Microwave Background Radiation (CMBR), it is the most distant thing ever
seen and was formed just 300,000 years after the Big Bang, so if we look at
a map of that background radiation the largest structure we could see on it
would be 300,000 light years across, spots larger than this wouldn't have
had enough time to form because nothing, not even gravity can move faster
than light, a larger lump wouldn't even have enough time to know it was a
lump. So how large would a object 13.7 billion light years away appear to
us if it's size was 300,000 light years across? The answer is one degree of
arc, but ONLY if the universe is flat. If it's not flat and parallel lines
converge or diverge then the image of the largest structures we can see in
the CMBR could appear to be larger or smaller than one degree depending on
how the image was distorted, and that would depend on if the universe is
positively or negatively curved.  But we see no distortion at all, in this
way the WMAP satellite proved that the universe is flat, or at least isn't
curved much, over a distance of almost 13.7 billion light years if the
universe curves at all it is less than one part in 100,000.

  John K Clark
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