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Albert Einstein's General Theory of Relativity, which established the relationship between matter, space, time and gravity, governs modern cosmology's view of the universe. But when Einstein began to apply his theory to the structure of the universe, he was dismayed to find that it predicted either an expanding or contracting universe--something entirely incompatible with the prevailing notion of a static universe. In what he would later call "the greatest blunder of my life," Einstein added a term called the cosmological constant to his equations that would make his calculations consistent with a static universe.
Einstein admitted his mistake in 1929 when Edwin Hubble showed that distant galaxies were, indeed, receding from the earth, and the further away they were,the faster they were moving. That discovery changed cosmology.
Atoms emit or absorb light in characteristic wavelengths: hydrogen, helium, and all the other atomic elements have their own spectrum signatures. In the early part of this century, Vesto Slipher was studying the spectra of light emitted from nearby galaxies. He noticed that the light coming from many galaxies was shifted toward the red, or longer wavelength, end of the spectrum. The simplest interpretation of this "redshift" was that the galaxies were moving away from us.
Hubbles's Law
Hubble, who had been the first to establish that the universe included many
other galaxies outside of our own, noticed something else: the galaxies were
receding from us at a velocity proportional to their distance. The more
distant
the galaxy, the greater its redshift, and therefore the higher the velocity, a relation known as
Hubble's
Law.
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The velocity v could be determined by multiplying the distance R by H, the Hubble constant, given by the slope of the line in the above graph, in units of kilometers per second per million light years. The Hubble constant describes the universe's rate of expansion.
The apparent linearity of Hubble's Law implies that the universe is uniformly expanding. What does that actually mean?
For one thing, it means that no matter which galaxy we happen to be in, virtually all of the other galaxies are moving away from us (the exceptions are at the local level: gravitational attraction pulls neighboring galaxies, such as Andromeda and the Milky Way, closer together). In other words, it's not as though we here on earth are at the center of the universe and everything else is receding from us. The universe has no "edge" as such.
It also means that the galaxies are not moving away through space,
they
are moving away with space, as space itself expands. Think of a
loaf of
unbaked raisin bread you've set in a warm place to rise. The raisins are like
galaxies or clusters of galaxies, and the dough, space. As the dough
rises, the
raisins move farther apart, but they've moved with the dough, not
through the dough.
other words, a contracting universe instead of an expanding universe. Because
the Hubble Constant is a measure of how much space is expanding in units of
distance per second, it's possible to estimate how long it would take,
rolling
the movie backwards, for the most distant galaxies to collide with each other
and finally collapse in the Big Bang.
Unfortunately, it's not so easy to determine the Hubble Constant. While
cosmologists have mastered the trick of determining a galaxy's redshift, and
therefore its velocity, determining the distance to far-off objects is quite
another matter. We don't have any yardsticks that long.
Instead, cosmologists use standard candles, bright beacons that
serve as
reference points. One kind of standard candle are the Cepheid variables (the
North Star is one), so called because they blink at a rate that is precisely
related to their brightness. Because the brightness of individual stars is
proportional to their distance from us, cosmologists compare nearby Cepheids
(to which we know the precise distance) to those farther away. A Cepheid that
is four times fainter than a nearby Cepheid is estimated to be twice as far
away. Cosmologists use an entire ladder of distance indicators that are
calibrated using the lower (nearest) rungs.
Until just recently, most estimates of the Hubble Constant have hovered
around
50, which implies that the universe is about 20 billion years old. However,
this provides only an upper limit to the age of the universe, and is
based on
the present rate of expansion, as observed by the recession of distant
galaxies. It's likely that this rate was greater in earlier epochs of cosmic
evolution. As galaxies tugged at each other through their gravitation, the expansion
slowed down.
The Hubble Telescope was designed, in part, to find Cepheid variables and
other standard candles even farther away than those detectable by ground-based
telescopes. Cosmologists hoped that these objects, not influenced by the
gravitational pull of the Milky Way, would yield more accurate information
about the expansion of the universe.
One team using the Hubble Telescope
found a number of Cepheids in the Virgo cluster, which allowed them to estimate the
distance to the far-off Coma cluster.
The team estimated the Hubble
Constant to be 80, which would make the universe eight to twelve billion years old.
Separate, ground-based observations of another galaxy within Virgo yield an
even higher value of 87.
Other groups using another kind of standard candle called
supernovae --massive stars
that have collapsed
and exploded--come up with lower Hubble Constants, either 73 or 50.
On the other hand, astronomers who study the chemistry and life cycles of
stars
are quite certain that the oldest stars in the Milky Way are about 14 billion
years old. Clearly, cosmologists are facing a paradox: you can't have stars
that are older than the universe!
All of the galaxies studied are only in the region of 50 million light years
from Earth, too close to get a more truly "global" value for the Hubble
Constant. Studies are now underway at several observatories worldwide,
and with
the Hubble Telescope, to probe much further out and find redshifts
corresponding to times when the universe was one fourth or less than its
present size.
Clearly the pressure is on to find a correct value for the Hubble Constant.
Cosmologists hope that better instrumentation, earth-bound and
space-born, will
provide the means to do so.
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How old is the universe?
Determining the Hubble Constant is something of a Holy Grail for
cosmologists,
because it holds the key to the age of the universe. Imagine running a
film of
cosmic expansion backwards to the Big Bang--in
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