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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