MODERN TIMES
by Art Hobson
ahobson@uark.edu
NWA Times 21 Jan 2006
THE ACCELERATING UNIVERSE AND THE
SCIENTIFIC PROCESS
The
golden age of cosmology--the study of the overall evolution of the
universe--began on April 23, 1992, with the publication of a microwave map of
the universe compiled by a NASA satellite during two years of data
collection. The results revealed
the large-scale structure of the entire early universe, a mere 400,000 years
after it birth.
Since
1992, an array of wierd and wonderful "telescopes" in space, on the
ground, and under the ground, has ferreted out data revealing quite a few
astonishing quirks in the universe.
This is the story of one of those quirks.
Edwin
Hubble discovered the expanding universe when, in 1929, he found that light
from more distant galaxies--huge collections of stars similar to our Milky Way
galaxy--was redder than the light from nearby galaxies. This "redshift" is evidence
that space is stretching due to the expansion of the universe during the travel
time of light from the other galaxy to our telescopes. The stretching of space causes the
light waves to stretch, which lengthens their waves, which happens to mean that
the light is redder than it would otherwise be. Furthermore, more distant galaxies had greater redshifts,
implying that more distant galaxies were moving away faster, just as though
they had all been blown apart by a giant explosion or "big bang"--a
theory amply confirmed since 1929 by several additional kinds of evidence.
Once
scientists realized that the universe is expanding, they began asking new
questions. Would the universe
expand forever, or would it eventually collapse back on itself in an ultimate
"big crunch"? This
question is similar to asking what happens to an object thrown upward from
EarthÕs surface. If you throw a ball upward, it slows as it comes to a
momentary stop at its maximum height and then gravity brings it back to the
ground. But if you could throw it upward faster than 25,000 mph, gravity would
slow it as it rose but it would never fall back down; it would continue rising
and escape from Earth.
To
determine whether the universe would eventually collapse back on itself,
astronomers needed to know not only the expansion rate, but also the rate at
which that expansion is slowing. But itÕs difficult enough to measure the
expansion rate, let alone the rate at which that rate is slowing, so the answer
remained open for decades.
Finally, in 1998, astronomers learned the answer.
This
measurement was one of observational cosmology's great accomplishments. Astronomers needed to accurately
measure both the speeds at which the galaxies are receding, and also the
distances to those galaxies. And
they needed to make these measurements for galaxies at distances so far away
that the light received from those galaxies would have been traveling during a
large fraction of the history of the universe. This long "look-back time" would give us a
comparison between the present expansion rate of nearby regions, and the
expansion rate of distant regions in the distant past.
But
what "markers" exist that can be seen clearly across much of the
observable universe, and whose distances from us could also be somehow
measured? The answer turned out to
be the very bright exploding stars known as "supernovas" that occur
only about once a century in a typical large galaxy such as our Milky Way.
Large modern telescopes can detect a particular type of explosion known as a
"type A supernova" in far-distant galaxies. These are bright enough
to be visible even across nearly the entire observable universe and hence
nearly all the way back in time to the universe's first stars, 13 billion years
ago. Furthermore, all Type 1a supernovas are nearly identical, so they all
shine with the same brightness during their one-month period of maximum
intensity following the explosion. Since they all have the same actual
brightness, more distant ones always appear dimmer from Earth, and from their
observed brightness one can deduce how far away they must be. Type 1a supernovas were the perfect
markers.
The
result was one of cosmology's great surprises. The essentially unanimous wisdom had been that the universe
was slowing down in its expansion and that the only issue was "by how much
is it slowing down?" But the
supernova data showed that it's speeding up!
I'll
have to save the fascinating implications of this for another column, but
there's an important lesson here about the scientific process. Scientific theories live or die by
observable evidence. An idea is
not scientific at all if it cannot possibly be disproved by observations of
nature. The ideas (also
called theories, laws, models, or
principles) of science all hang by the slender thread of observation. If observations ever conclusively contradict
a theory, then the theory must be eliminated or modified, regardless of how
certain it might have seemed. This
was the case with the notion that the universe's expansion must be slowing
down. It was soon clear that the
data showed quite conclusively that the conventional wisdom was wrong. So it wasn't long before all
knowledgeable scientists were convinced that, difficult though it may be to
believe, something was causing the expansion of the universe to actually speed up.
It's a sorely needed lesson for our time.
If the pseudoscientists who push such superstitions as "intelligent
design," the oil company flacks who deny global warming, the public officials
who warp science to further their own political goals, and the host of other
ideologues who pollute public discussion, could learn to follow the evidence--to
trust the universe rather than their own self-centered presuppositions--we
humans might at last learn to live at peace with each other and with our planet.