The Fate of the Universe
Key points: effect of mass on behavior of
Universe; critical density
The confirmation of the Big Bang theory leaves more questions. For example: Will the
Universe expand forever?
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A Universe that is dominated by its mass will expand for a while, but will
be slowed by gravity and eventually will stop expanding and fall back in onto itself. This
case is a "closed" Universe.
One theory is that it implodes all the way back to its starting condition and then starts
the expansion all over -- producing an oscillating Universe.
| "Gravity is the force that rules the Universe. To understand its
workings, to the finest degree, is to understand the very nature of our celestial
home." - Marcia Bartusiak, in Einstein's Unfinished Symphony |
We show this case with an expanding ball analogy, and then with a graph that
plots the distance between objects (sort of a "size" of the Universe) as time
moves on and the Universe gets older. |
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A Universe that is dominated by the
expansion energy (that is, which has relatively little mass to slow the expansion and
reverse it) will expand forever. This case is called an
"open" Universe.
Again, we show it as an expanding ball and graph of a typical distance
between objects as it gets older. (animations by G. Rieke with an assist from
W. Hu http://background.uchicago.edu/~whu/physics/physics.html) |
In all the animations, the starting explosion is gross artistic
license, since the expansion is always self contained -- still, it marks the start better
than a tiny dot would. In fact, since we are contained within the expansion, it is
impossible to locate any center where it started. A very difficult question in all these
ideas is what happened before the Universe started
There has to be a case between closed
and open, where the amount of mass produces a gravitational energy that just matches the
expansion energy. This boundary case determines a
critical mass density, which we will call
densitycritical , or 
= 
,
that determines our eventual fate: > means the expansion will reverse and we will be crushed, < means
we will expand forever.
How can we determine where our Universe is relative to
?
Method 1: Measure how much matter there is by
counting how many stars, galaxies, etc. are observed. Be careful to make an allowance for
the unseen dark matter.
Result: Only about 10% of
can be accounted for.
Method 2: Since Ho is a measure of the rate of expansion of the Universe,
see whether Ho is increasing or decreasing with time.
Result: Not a practical experiment because only recently using the Hubble Space
Telescope have we been able to measure Ho to an accuracy of 10%; much too crude
to look for changes over time.
--------- We have to be much more clever!!! -----------------------
Method 3: Measure the structures in the 3K Universal
radiation. The advantage of these measurements is that these structures were created at
redshift, z, of about 1000, so we can look back to a very early stage in the expansion,
and the curvature of space is therefore a large effect. We will show soon (a few pages in
the future) how these measurements prove that we are just at the critical density.
Test your understanding before going on
Hindu legend: Brahma
opens his eyes and a world comes into being
. Brahma closes his eyes, and a world goes out of being. http://www.atributetohinduism.com/Hindu_Cosmology.htm |
|
 Chinese
mythology: In the beginning, the universe was nothing but swirling vapors and chaos.
From these vapors, the forces of Yin and Yang emerged. The two forces combined to create
all things, and all things created contained elements of both. http://tlc.discovery.com/convergence/eden/myths/myths_10.html |
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hypertext  G. H. Rieke |
Click to go to the Start of Everything |