Super Data: Hail the cosmic revolution

With nicknames such as Gilgamesh, Aphrodite, and Athena–as well as Elvis–10 recently discovered supernovas are something special. Indeed, these supernovas provide what appears to be proof of one of the weirdest properties of the universe: Something is pushing objects in the cosmos apart at an ever-faster rate.

Adam G. Riess of the Space Telescope Science Institute in Baltimore is presenting the new findings this week at a cosmology symposium at Case Western Reserve University in Cleveland.

The data “will help us understand the nature of cosmic acceleration,” says cosmologist Michael S. Turner of the University of Chicago.

The notion of a universe speeding up its expansion rate has been in the spotlight since 1998, when two teams of astronomers measured the brightness of type 1a supernovas that were remote but considerably closer to Earth than the newfound crop (SN: 3/21/98, p. 185). Because type 1a supernovas all have about the same intrinsic brightness, like lightbulbs of the same wattage, it’s easy to predict how bright they ought to appear on the basis of their distance from Earth.

But the measured brightness of these stellar explosions proved puzzling: If the universe has expanded at a constant or diminishing rate since the Big Bang, then the supernovas are about 20 percent dimmer than expected. If the universe’s expansion has been speeding up over the past several billion years, however, the supernovas would be farther away than astronomers originally thought and therefore would have to appear dimmer.

But scientists worried that intergalactic dust, rather than distance, could account for the dimming. Or maybe these supernova explosions, which popped off at a time when the universe was about 10 billion years old, were intrinsically dimmer than younger ones.

The new findings sweep away those concerns, says Riess. The results rely on the relative densities in the early universe of matter and the mysterious entity believed to have revved up cosmic expansion.

That entity, which some cosmologists call dark energy, opposes gravity’s familiar tug by pushing bodies apart. Dark energy is believed to have been constant or nearly so for most of cosmic history. In contrast, the density of matter, which gives rise to gravitational attraction, was much larger in the past because the universe was much smaller. During the first several billion years of the cosmos, the density of matter was so great that its pull would have overwhelmed dark energy’s push.

Supernovas that hail from such an era, in which the cosmos’ expansion was slowing down, ought to appear brighter than supernovas in a universe that was always expanding at a constant or accelerated rate. In contrast, such confounding effects as dust could only make distant supernovas look dimmer.

Two years ago, Riess’ team reported that observations of a single type 1a supernova–the most distant ever found–revealed the brightening indicative of an early cosmic slowdown (SN: 3/31/01, p. 196: Available to subscribers at Starry Data Support Revved-Up Cosmos). Now, using the Hubble Space Telescope’s Advanced Camera for Surveys (SN: 9/6/03, p. 155:Getting the GOODS on Galaxies), the researchers have found 10 other remote supernovas that show the same brightening.

Riess’ data are “the first that provide direct and solid evidence for an earlier, decelerating phase” of the universe, says Turner.

That leaves theorists to puzzle over exactly what provides the cosmic push. If dark energy provides the impetus, researchers aren’t sure of its source. Alternatively, the accelerated expansion may be due to a new feature of gravity that makes its presence known only on the very large scales of the present-day universe.

The new supernova data are “very important to us” because they will help reveal whether new gravitational physics is at play, says one of the theory’s founders, Georgi Dvali of New York University.


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