The Shape of the Universe
The shape of space and the nature of the universe are topics that
have been debated for centuries. As our scientific knowledge has
expanded and our tools of observation have become more capable, our
theories and beliefs concerning these topics have changed. From
Ptolemy to Stephen Hawking, the way we perceive the universe has changed
fundamentally while the universe itself had changed little.
The Ptolemaic picture of the universe is not a
scientifically accepted view of the universe today...
but for centuries this was what people believed the universe looked like.
In this universe the earth was the stationary center of the universe and
the planets, sun, and stars rotated about it in fixed spheres of movement.
This system represented a highly two dimensional picture of celestial
bodies because their movements were basically limited to the two
dimensional surface of larger and larger spheres.
Copernicus, however, did not accept the Ptolemaic model of the
universe and the concept of stars fixed in a spherical plane.
His observations led him to the conclusion that the sun rather than the
earth was the center and the planets were in orbit around it. Supported by
Galileo's observations, the Copernican view of space allowed for the
realization that distant stars were not two dimensionally fixed, but were
individual suns at varying distances. Space leapt into three dimensions
and man's picture of the universe was changed dramatically.
Isaac Newton contributed immeasurably to the science of cosmology and
introduced a new and important view of the universe. Newton reconciled his
theory of gravity with the idea of infinite space with stars more or less
equally spread out over that space. He believed that this model allowed
for a static universe in which the force of gravity would not cause a "big
crunch." The concept of the expansion or contraction of space had not yet
been introduced.
The Modern Picture of the Universe
In the 1920's, Edwin Hubble ushered in what Stephen Hawking calls the
"modern picture of
the universe." His work suggested that there were many more galaxies in
the universe than the "Milky Way" but they were separated by huge
distances. Using a sort of luminosity ladder of distance measurement,
Hubble found the distances to a number of galaxies. While attempting to
measure the luminosity and spectra of stars in other galaxies, Hubble
and others began to notice an interesting phenomenon: a slight but
distinct redshift in the spectra of almost every galaxy observed. Even
more surprising was the observation that the further away the galaxy the
more the redshift was pronounced. According the principles of the Doppler
effect, this discovery suggested that nearly all the galaxies were moving
away from us: and the further away from us they were, the faster they were
speeding away. It appeared as if we were the center of an expanding
universe. How could this be explained?
Actually, the answer had already been proposed by Alexander Friedmann, a
Russian physicist. He introduced two critical ideas to the field of
cosmology: one, the universe looks the same (on a very large scale)from
every point in it, and two, the universe is probably finite. When coupled
with
Hubble's observations, our apparent position as the center of the
universe turned out to be an illusion. In an expanding universe in
which all points experience the same large-scale view, very distant
objects would appear redshifted from all vantage points.
The model of a
finite universe which looks the same at every point has so far proven to
be empirically accurate (especially from the work in background radiation
by Penzias and Wilson) and yields some very interesting geometrical
pictures
of the universe.
One could never leave the boundary of the finite universe but might follow
the curvature of space back to where they began their journey. For this to
occur the traveller would live in a closed universe (see the origin and fate page). In this model, the
universe, pulled together by gravity, originates at a point, expands, and
collapses back to a singularity.
The shape of space is really dependent on the density and rate of
inflation of the universe. Friedmann believed that the universe was
closed, but if the density and therefore the gravity of the universe is
not enough to reverse its expansion, we may live in an infinite or a flat
universe (see origin and fate page).
These three models all begin with the notion that at some time in the past
all matter must have existed as a singularity. Almost all astronomers
today would say that this was the Big Bang: the point at which space and
time began and matter was created. To learn more about this, see the origin and fate page.
Different Pictures of the Universe
Steady State Model
The steady-state model of the universe, proposed in 1948 by Bondi, Hoyle,
and Gold, added one idea to the Friedmann model of the universe. They
proposed that that space not only looks the same from every spot, but at
every time. In an expanding universe, the steady-state proponents argued
that small very small amounts of matter were being continuosly created out
of nothing. Therefore, the universe would never disperse beyond
recognition at the largest scales. But this theory could not reconcile
cosmic background radiation and lost credence.
The Big Bang
Most astronomers today believe the universe began Big Bang style. How long
ago it may have occurred depends on the value of the Hubble constant
which is still being debated.
The Anthropic Model
A few scientists believe that the shape of the universe has a lot to do
with the existence of life. If the universe expanded too quickly matter
may not have had time to form the components necessary for life (top).
If it had expanded to slowly a "Big Crunch" may have occurred before life
could begin (bottom). Perhaps the current shape of the universe is
what it is because we exist?
The Inflationary Universe
Developed by American Alan Guth in 1980, this model differs from the Big
Bang model in that the expansion of the universe is not decreasing, but
forever expanding as a result of a huge burst of energy. This may explain
the uniformity of the temperatures of background radiation.
The No-Boundary Universe
This model was proposed by Stephen Hawking and Jim Hartle. It argues that
the universe did not begin with a singularity but is nevertheless finite.
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