Eight
years ago Notre Dame astronomer Peter Garnavich and his colleagues
observed that certain distant exploding stars were fainter than
accepted formulas had predicted. What may seem like a "so what?"
fact turns out to have profound theoretical implications that
have fueled the speculation of cosmologists ever since. That these
Type Ia supernova stars were not as bright as they should have
been confirmed that not only is the universe expanding but the
expansion rate is accelerating, Garnavich explains.
"We had a very simple picture back then," the ND associate professor
of physics says. "It was thought that the universe was filled
with a certain density of matter. And so our question was: A)
Is that density enough to make the universe re-collapse to a single
point through gravity, or B) Is the density too little so it may
just expand forever?" While the observation answered one question
with "B," it posed another: Precisely what is causing the universe's
expansion to accelerate?
Garnavich and his collaborators were able to match their supernova
observations with a large amount of dark energy detected in the
universe. "But we don't know exactly what this dark energy is,
whether it is vacuum energy or something else," Garnavich says.
Cosmologists speculate that the acceleration might be caused by
dark energy or it could be related to some extra dimension to
the universe. Some have suggested that the clumping of matter
within the immense scale of the universe could set in motion a
reaction that might cause it to accelerate. In effect, the dark
energy could be the kinetic energy of all the motions of individual
galaxies with respect to the grand cosmic expansion.
"The theorists have really run away with this problem," Garnavich
says. "Nearly every day it seems there is a new theory to account
for how you could get the universe to accelerate. There are just
so many possibilities that it scares me."
Currently, the team of astronomers of which Garnavich is a member
is attempting to use these Type Ia supernova stars to make a more
precise measurement of the cosmic acceleration. For the past three
years, during fall break, Garnavich has worked in Chile at the
Cerro Tololo Inter-American Observatory. There he and his colleagues
use a four-meter optical telescope in the Andes Mountains to scan
the sky for stars that exploded about 7 billion years ago, halfway
back to the Big Bang. The team hopes to gather about 200 examples
in five years; this more precise data might allow them to winnow
down the number of possible acceleration theories.
One beneficiary of Garnavich's observations has been Grant Mathews,
a Notre Dame professor of physics. As a theoretical cosmologist,
Mathews tries to understand what makes the universe tick on the
grandest scale.
While Garnavich spends his time observing and calculating measurements,
Mathews and his colleagues work up elaborate mathematical models
to explain the observations and suggest new problems for the astronomers
to explore. "We're always looking for the next observation that
doesn't make sense, the next thing that needs to be explained,"
Mathews says. "That's when the fun begins. When something doesn't
fit, that's when it gets interesting."
One interesting observation cosmologists have found that has
implications for a host of things, including the acceleration
question, is a slight deviation in the elements that were produced
in the Big Bang. The ratio of helium to the isotope deuterium
doesn't fit. "Simply put, there is not enough helium produced
for the deuterium we see," Mathews says. For a number of complicated
theoretical reasons this suggests the existence of another dimension
of space. And this poses an interesting question for Mathews:
Why can't we just look out there and see things in this extra
dimension?
"One thing that could be happening," he says, "is that matter
might actually be leaking out into this extra-dimensional world.
In fact, what you would see, at least on the theories that I'm
working on, is that a proton or some matter would be moving along
and then literally disappear into this extra dimension. It doesn't
have any more gravity within this dimension. It's nothing. I always
make the joke that it is the final explanation of where socks
go in your dryer."
Mathews says that some work by Garnavich's group yielded a slight
hint that there may have been more dark matter in the past. "That
leads us to one of two possibilities," he says. "Either matter
is slowly leaking out into the extra dimension, or what we think
is the stable dark matter particle isn't actually stable; it's
decaying."
* * *
John Monczunski is an associate editor of this magazine.
(April 2006)
