| Why is our
Universe the way we see it to be? And what will ultimately become of
it? These are two fundamental questions which have long interested
humankind. In the more than 70 years since the discovery that that
the Universe is expanding, we have made some significant steps in
understanding how the Universe began and how it must have evolved to
be what it is today. We know this: galaxies and clusters of galaxies
formed from tiny fluctuations in the early Universe. We can measure
these fluctuations by mapping the cosmic background radiation and
relate them to the structures which we observe today. However, many
challenges remain such as:
What is the Age of the Universe? - or -
How Fast is the Universe Expanding?
In the 1920s,
Edwin Hubble
used the 100" telescope (2.5 meters) at the Mount Wilson Observatory
in California to detect variable
stars in
nebulae. He discovered that the stars he observed had the same
characteristic variations in their brightness as a class of stars
called
Cepheid Variables.
Earlier, astronomer Henrietta Levitt had shown there was a precise
correlation between the periodic change in brightness of a Cepheid
Variable and its
luminosity.
Hubble was able use this correlation to show that the
nebulae
containing the variable stars he observed were not within our own
Galaxy; they were external galaxies far beyond the edge of our Milky
Way.
Hubble determined the distances to the
galaxies and the velocities that they were moving at relative to
Earth. From this, he discovered an amazing thing - the farther
galaxies were away from us, the faster they were moving away from
us. In other words, the Universe was expanding!
Tell me more about Cepheid Variables and their
Use as Cosmic Yardsticks!
What is the Shape of the Universe?
The
density of
the Universe determines its geometry or shape. If the density of the
Universe exceeds the so-called "critical density", then the shape of
space is curved like the surface of a huge sphere. If the density of
the Universe is less than the "critical density", then the shape of
space is curved like the surface of a saddle. If the density of the
Universe exactly equals the "critical density", then the shape of
the Universe is flat like a sheet of paper.
Astronomers are still trying to accurately
measure the shape of the Universe. The most widely accepted theory
predicts that the density of the Universe is very close to the
critical density, and that the shape of the Universe should be flat,
like a sheet of paper.
Tell Me More about How We Measure the Size and
Age of the Universe!
What is the Fate of the Universe?
Cosmologists envision two possible fates
for the Universe: The Big Freeze or the Big Crunch. The evolution of
the Universe is determined by a struggle between the outward
momentum of expansion and the inward pull of
gravity. The
strength of gravity depends on the density of the Universe. If the
density of the Universe is less than the critical density, then the
Universe will expand forever. If the density of the Universe is
greater than the critical density, then gravity will eventually win
and the Universe will collapse back on itself.
Courtesy of NASA |
