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Thirty-Ninth George Gamow Memorial Lecture

Robert P. Kirshner
Clowes Professor of Science
Harvard-Smithsonian Center for Astrophysics

"A Blunder Undone: The Accelerating Universe"

March 30, 2004

Exploding stars halfway across the observable universe reveal a surprising fact. Judging the distances to distant supernovae from their apparent brightness, the rate of cosmic expansion has been speeding up in the last 5 billion years. While gravitation acts to slow cosmic expansion, these observations require something else to make the universe accelerate. We call this the “dark energy”, though, in truth, we do not know what it is. Perhaps it is the modern version of Einstein's notorious cosmological constant, famously described by George Gamow as Einstein's “greatest blunder.”

This result was a big surprise to people working on the problem. Early in 1998, I wrote an e-mail to the members of our “High-Z Supernova Team” saying, “In your heart you know this is wrong, though your head tells you [that] you don't care and you're just reporting the observations.”

One reason to be wary was Einstein's bad experience with this idea, invented to make a static universe. Did we think we were smarter than Einstein? Einstein never liked the cosmological constant, as he wrote, “I am unable to believe that such an ugly thing should be realized in nature.” It became a kind of theoretical poison ivy—touched only by the unwary for about 65 years. But the data were leading us to reconsider. As Adam Riess wrote to the rest of the team, “Approach these results not with your heart or your head, but with your eyes. We are observers, after all!”

Technology is a lot better now: we use fast computers to scan digital images of the sky and pick out the objects that change. We've developed a pipeline system that pops out the supernova candidates about an hour after we take the images. This is important because supernovae are like fish—after about 3 days, they begin to lose their freshness. If you want to see the peak of the light curve prompt action is essential.

In 1998, we saw the signature of cosmic acceleration in supernovae at redshift 0.5: light that had been en route for about 5 billion years. Now we're using the Hubble Space Telescope to search for even fainter and more distant supernovae at redshift 1.5, roughly 9 billion light years away when we expect deceleration from dense dark matter.

We would like to understand better the nature of the dark energy. Is it really Einstein's cosmological constant, retrieved from the dumpster of history, smoothed out and made new again? Or is it some more general “quintessence” whose energy density changes over time. Better measurements will show whether the dark energy comes from a source that is constant, or one that changes subtly as the universe expands. Either way will be very interesting.

Faint light from distant stellar catastrophes traces the history of cosmic expansion. It is not what we expected to see. The universe contains more parts than the simplest universe we could imagine: atoms that glow, atoms that don't, neutrinos with mass, and another dark matter particle with more mass, something that made the universe expand exponentially in the era of inflation and something more that is making the universe accelerate now. Perhaps some day in the future all of this will seem essential, but at the moment, it seems we live in a recklessly extravagant universe, with extra parts whose function we do not yet fathom.

Robert P. Kirshner is Clowes Professor of Science at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Born in Sudbury, MA in 1949, he obtained his A.B. from Harvard College in 1970 and his Ph.D from the California Institute of Technology in 1975. Kirshner was elected to the National Academy of Sciences in 1998 and elected President of the American Astronomical Society in 2003. At Harvard, he teaches a large course for students who are not science concentrators, “Matter in the Universe.” He is also Master of Quincy House, one of Harvard's undergraduate residences. In 2003, Princeton University Press published Kirshner's popular-level book, “The Extravagant Universe: exploding stars, dark energy, and the accelerating cosmos.”