How Astronomers Deduced an "Extravagant" Universe
(A revised and expanded version of an article originally
published in the January 2003 issue of
Eyepiece)
He helped turn our understanding of the universe on end, and then he came to the Rose Center
to joke about it. On December 2, Robert Kirshner, Clowes Professor of Science at Harvard
and member of the High-Z Supernova Search Team, gave a talk on "The Extravagant Universe,”
which is also the title of his book, published by Princeton University Press. Injecting good
doses of humor and amusing anecdotes into a very heady and counterintuitive subject, Kirshner
told the story of how his team and a rival group, both observing supernovae in distant galaxies,
independently determined that the expansion of the universe is speeding up, the galaxies are flying
apart faster and faster. To account for this, the scientists concluded that there must be some unknown
force at work to counteract the pull of gravity that would otherwise slow or reverse the expansion of
the universe: a mysterious “dark energy” that comprises 2/3 of the universe. (Most of the remaining
third is “dark matter”: the unseen missing mass that is necessary to explain what holds galaxies
together. Only about 1 percent of our universe is composed of the familiar matter and energy we can
detect; the exact nature of the remaining 99 percent remains a mystery.) The accelerating universe
was named “Discovery of the Year” for 1998 by Science magazine. It has also produced more questions
than answers, and one of Albert Einstein’s most controversial concepts has been resurrected to try to
answer them.
Slipping items into his presentation that included as a graph comparing the price of a bicycle to the
urge to wear Spandex and a diagram showing how some students and professors at his institution believe
they are at the center of the universe, as well as anecdotes such as an account of the brilliant yet
quirky supernova researcher Fritz Zwicky, Kirshner gave an account of the past hundred years of cosmology
that was both informative and entertaining.
The story began with Einstein. in 1917, after he had created his theory of General Relativity (part of
which involves the idea that the presence of matter is responsible for bending space) he tried to apply
it to the universe as a whole, but found that the universe could either be expanding or contracting,
but not static and eternal, the way he believed it was. (At that time, our Milky Way galaxy was the extent
of the known universe.) In order to reconcile General Relativity with a static universe, he had to add
a factor to his equations: Lambda, the Cosmological Constant.
In the 1920s, Edwin Hubble at Mt. Wilson, by resolving individual stars within “spiral nebulae,”
he determined that they were actually galaxies resembling the Milky Way. By using the brightness
of supernovae and variable stars as “measuring sticks,” he was able to determine the distance
to a number of galaxies. He noted that the farther away from us that galaxies were, the more their light
was shifted to the red, and the faster they were moving away from us. Thus, an expanding universe, and one
quite different than what Einstein had imagined.e across the constellations.
Einstein is alleged to have called the Cosmological Constant his “greatest blunder.” What we know
he said, in 1932, is something more sensible: “An increase in the precision of data derived from
observation will enable us in the future to fix its sign and determine its value.” More than 65 years
later, the Cosmological Constant has once again become a factor in our understanding of the universe,
as a mathematical expression of this “dark energy,” also called clear energy or vacuum energy, that may
pervade the very fabric of space and push the universe apart at an ever-increasing rate.
Kirshner’s supernova-hunting team—led by Brian Schmidt, whose thesis Kirshner was the advisor on--as
well as the rival team from the Lawrence Berkeley Laboratories, led by Saul Perlmutter, thought they
would be measuring the “deceleration parameter”: the rate at which the expansion of the universe is slowing
down, as the gravity from the matter it contains acted as a braking force. They concentrated on type Ia
supernovae, formed by the explosion of a white dwarf in a close binary system after it has sucked in
enough matter from its companion star. These supernovae are all of similar brightness, and can be seen
across billions of light-years. Kirshner’s project primarily used the 4-meter reflecting telescope
at Cerro Tololo, coupled with a 112-megapixel CCD camera. Their strategy was to take images resembling
the Hubble Deep Field: very long exposures of small sections of the sky, each containing thousands of
galaxies, and then take another picture of the same regions in a month, and see if anything has changed.
By canceling out the light of all galaxies that remain unchanged over the course of the month, all that’s
left is what’s present in one image and not in the other; namely, distant supernovae. Astronomers believe
that there has been an average of about a supernova per century in our galaxy (many of them obscured by
intervening dust; there hasn’t been one observed in the Milky Way since 1604), about 5000 weeks per supernova.
Thus, a photo containing thousands of galaxies would stand a good chance of containing a supernova, and many
were found. Sometimes they would use the Hubble Space Telescope for sharper images, to measure brightness
and redshift.
The expected that these distant supernovae would appear unusually bright compared to closer ones,
indicating that the universe had slowed down, but instead they appeared fainter than expected,
and they were forced to conclude that the expansion of the universe is accelerating. But what could
the mechanism of that expansion be?
To address that, another constant makes its way into this story: Omega, which is the ratio of the
mass-energy of the universe to the critical mass-energy needed to stop cosmic expansion in an infinite
length of time. For an Omega of 1, the universe would be just on that knife-edge and would expand forever
gradually slowing down. For an omega less than 1, if matter was the only component, the universe would
expand forever, slowing down a little bit, and for an omega greater than 1, the expansion would turn
around and become a contraction. The more matter (as opposed to energy) in the universe, the greater
the effect gravity should have on slowing down its expansion. It raised the question, what would the
composition be of a universe whose rate of expansion is speeding up? Scientists had already postulated
a “dark matter” of unknown composition to explain the behavior of galaxies; to counter this invisible
mass they needed an as-yet unknown energy. Combining the supernova observations with data from the
microwave background radiation indicates a universe of 1/3 dark matter and 2/3 dark energy, with
precious little room for the matter and energy through which we perceive everything we know.
Kirshner likens our perspective on observing this mysterious, extravagant new universe to seeing
snow-covered mountains—the snow is rather insubstantial compared to the mountains it covers,
but it is only through the snow that we see the form of the mountains.
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