The Lumpy Universe
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?
Astronomers generally agree 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, similar to a snowball rolling downhill,
picking up more snow as it goes.
But each snowball must have a beginning – a small "seed" around
which the other material will gather. Observations of the cosmic
microwave background (CMB) – which is as close as we can get to
seeing the Universe near its beginning – show that it has a
temperature that is very close to uniform. The temperature of the CMB
is a tracer of where matter was in the very early Universe. If the
temperature was completely uniform, there would be no seeds for
gravitational collapse no way to form the lumps we see today.
Upon closer examination of small areas of the CMB, very small
fluctuations are seen. Though these fluctuations are only at the
part-per-million level, they are enough to produce variations in
density, and thus determine where matter is more likely to coalesce due
to gravity, eventually producing larger and larger lumps of matter.
These slight variations were first discovered by NASA's Cosmic
Background Explorer (COBE) satellite in 1992, and studied in more detail
by the Wilkinson Microwave Anisotropy Probe (WMAP).
The variations in the cosmic microwave
background as seen by the COBE (top) and WMAP (bottom) missions.
COBE was the first mission to see the small variations in
temperature from one region to another in the CMB. WMAP later made
more-detailed observations of these temperature variations.
One of the more interesting observations in cosmology is that while
the matter contained in the Universe is indeed "lumpy," those lumps are
evenly distributed – "homogenous and isotropic," speaking in
scientific terms – within the Universe, on a large scale. This is
known as the cosmological principle: the fact that no matter where the
observer is located in space, the properties of the Universe will appear
to be the same, as long as you're looking at the Universe at a
sufficiently large scale. The average temperature and distribution of
galaxies on one side of the Universe will be the same as on the opposite
side, even though the two sides have been moving apart since the
beginning of time.