Notes on Relativity and Cosmology - Physics Department, UCSB

300 CHAPTER 10. COSMOLOGY
these tiny inhomogeneities from the first experiment (a satellite called COBE)
to measure them.
This, by the way, illustrates an important point about the early universe. It
was not like what we would get if we simply took the universe now and made
all of the galaxies come together instead of rushing apart. If we pushed all of
the galaxies together we would, for example, end up with a lot of big clumps
(some related to galactic black holes, for example). While there would be a lot
of general mushing about, we would not expect the result to be anywhere near
as homogeneous as one part in one hundred thousand.
It appears then that the universe started in a very special, very uniform state
with only very tiny fluctuations in its density. So then, why are there such large
clumps of stuff today? Today, the universe is far from homogeneous on the small
scale. The reason for this is that gravity tends to cause matter to clump over
time. Places with a little higher density pull together gravitationally and become
even more dense, pulling in material from neighboring under-dense regions so
that they become less dense. It turns out that tiny variations of one part in
one hundred thousand back at decoupling are just the right size to grow into
roughly galaxy-style clumps today. This is an interesting fact by itself: Galaxies
do not require special ‘seeds’ to start up. They are the natural consequence of
gravity amplifying teeny tiny variations in density in an expanding universe.
Well, that’s the rough story anyway. Making all of this work in detail is a
little more complicated, and the details do depend on the values of Λ, k, and
so on. As a result, if one can measure the CMB with precision, this becomes
an independent measurement of the various cosmological parameters. The data
from COBE confirmed the whole general picture and put some constraints on
Λ. The results were consistent with the supernova observations, but by itself
COBE was not enough to measure Λ accurately. A number of recent balloonbased
CMB experiments have improved the situation somewhat, and in the next
few years two more satellite experiments (MAP and PLANCK) will measure the
CMB in great detail. Astrophysicists are eagerly awaiting the results.
10.4.3 A cosmological ‘Problem’
Actually, the extreme homogeneity of the CMB raises another issue: how could
the universe have ever been so homogeneous? For example, when we point our
radio dish at one direction in the sky, we measure a microwave signal at 2.7
Kelvin coming to us from ten billion light-years away. Now, when we point our
radio dish in the opposite direction, we measure a microwave signal at the same
temperature (to within one part in one hundred thousand) coming at us from
ten billion light-years away in the opposite direction! Now, how did those two
points so far apart know that they should be at exactly the same temperature?
Ah! You might say, “Didn’t the universe used to be a lot smaller, so that those
two points were a lot closer together?” This is true, but it turns out not to help.
The point is that all of the models we have been discussing have a singularity
where the universe shrinks to zero size at very early times. An important fact is