The Lumpy Universe
If It Starts Out Smooth, How Does It Become Lumpy?
The Universe that we see today is very lumpy. There are planets, stars,
galaxies, and clusters of galaxies. Yet when we look at the afterglow from
the Big Bang, we see an incredibly smooth glow across the sky. So how did
the matter in the Universe get to be so lumpy after starting out so smooth?
Most astronomers believe that gravity shaped the evolution of the lumps we
see in the Universe today. The force of gravity between different chunks of
matter caused the chunks to pull together into one body, and then that body
pulled in more material. However, it takes time for gravity to do this job
Universe is only about 15 billion years old. Has there been enough time? Only
if most of the matter in the Universe is some kind of strange material which
does not interact with light (so-called "dark matter").
The young Universe
was so hot that normal matter, i.e. matter as we know it here on Earth, would
not have been able to clump together until time passed and the
Universe expanded and cooled. The Universe is probably not old enough for the
gravitational attraction of ordinary matter to be responsible for the structures we see today.
The variations in the cosmic microwave
background as seen by the COBE mission.
Is Dark Matter Required to Make the Lumps?
The clumping discussed above could have started early on
only if there is a lot of material in the Universe known as dark matter,
which behaves differently. If the clumping could have started when the
Universe was still quite hot, there has probably been enough time for
structures such as stars and galaxies and clusters of galaxies to evolve.
However, if the young Universe started perfectly smoothly, then we would see
no clumping today. Things must have been at least a little tiny bit unsmooth in
the beginning. Such slight variations were first discovered by NASA's Cosmic
Background Explorer (COBE) satellite in 1992. Astronomers believe that the
Universe started out with very tiny lumps and that a type of dark matter
helped gravity along to develop much of the larger lumps we observe today.
The questions then remain: what caused the original tiny lumps? what is this
exotic dark matter? does this picture really hold together?