Tag Archives: dark energy

Weighing the universe

Astronomers are about to take their best shot at weighing the universe. You might well ask how and why; University of Washington astronomy professor Andy Connolly recently tackled those questions in a lecture titled “Unraveling Our Own Cosmic History.” The talk was the first in a series dubbed The Big Bang and Beyond being sponsored by the UW Alumni Association as part of the celebration of the 50th anniversary of the university’s Department of Astronomy.

Connolly

Professor Andy Connolly spoke Oct. 21 to kick off the Big Bang and Beyond lecture series celebrating the 25th anniversary of the Department of Astronomy at the University of Washington. Photo: Greg Scheiderer.

The why is easy: to try to figure out dark matter and dark energy. The how, according to Connolly, is actually pretty simple, too: they’re going to weigh the universe by looking at it, and not in a carnival weight-guesser sort of way.

To explain the idea, Connolly used an example of a swimming pool with tiles on its bottom. Water refracts light, and as the surface of the water in the pool ripples the reflections of light on the bottom of the pool move. Similarly, if you watch the grid of tiles on the bottom of the pool, the view will change. Connolly noted that by taking precise measurements of the distortion, we could determine the size of the waves and the mass of the water in the pool. Blow that model up to astronomical scale, about six billion light years, and you can weigh the universe.

Connolly looked, and found no grid in the sky, but notes that there are galaxies everywhere which can serve the same purpose.

“If I can measure the shapes of galaxies, and measure how they’re distorted through gravitational lensing, in the same way that I could measure the mass of the waves on the surface of a pool, I can now measure the mass of the universe,” Connolly said. “More importantly, I can measure that structure as a function of the age of the universe.”

The challenge is that while the structures are huge, they’re also spread out and the distortion will be miniscule. Spotting it will take a better telescope, and that’s one of the research reasons that the Large Synoptic Survey Telescope (LSST) is under construction in Chile. The UW is a founding partner of the LSST, which will have an 8.4-meter mirror and a 3.2 billion pixel camera. Its images will cover 3.5 degrees of sky; the Hubble Space Telescope would have to shoot about 3,000 images to achieve the same results.

“This means that (the LSST) can survey half the sky every three nights,” Connolly said. By comparison, it took the wildly successful Sloan Digital Sky Survey ten years to image a fifth of the sky. In other words, we’re in for a big download of data. Connolly said that the LSST will produce a thousand times more data than did Sloan, which revolutionized astronomy by making so much data publicly available.

The possible discoveries from so much new data are staggering. Connolly noted that data on a mere handful supernovae led to the discovery of dark energy.

“It’s amazing that measuring the distances and the brightness of 42 supernovae could reveal a component of our universe that drives the expansion, a component of our universe that makes up 73 percent of the energy budget in the universe today,” Connolly said.

“With the LSST, in ten years we’ll have 1.2 million supernovae,” he added. “A few tens of thousands of galaxies led to the discovery of dark matter through gravitational lensing. With the LSST we get four billion galaxies.”

If it all works, Connolly said it would help us solve what it perhaps the greatest scientific riddle of our time.

“If we can understand dark energy, if we can understand dark matter, if we can understand how the universe formed in the earliest fractions of a second, then we may be able to unify two of the biggest discoveries in the last hundred years: the discovery of general relativity, which explains gravity and how structure forms; and quantum mechanics, how our universe might have come into being.”

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Panek speaks on dark matter and dark energy

“It’s 1610 all over again,” says author Richard Panek about the scientific revolution that is occurring in astronomy and physics over the universe’s dark mysteries. Panek, who wrote The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality, gave a talk Tuesday evening at Town Hall Seattle.

Panek is a storyteller, not a scientist, and the book is a history of how scientists came to believe in these two mysterious dark forces, and what they’re doing to figure it all out.

Four Percent UniverseHe related an amusing anecdote from a recent book signing, at which the cashier at the table read the front flap of the book jacket and exclaimed, “That’s crazy!”

“That’s just about the right response,” Panek said. “That’s the response that I had 10 years ago when I heard people talking about this dark matter and dark energy problem in the universe, and that most of what we thought we knew of the universe for thousands of years is only about four percent of the matter and energy that’s actually out there.”

He originally found the notion too wild to be true, but eventually became a believer.

“The more I looked at it, the more I saw that people in the sciences were trying to knock down these ideas, and they weren’t able to do so,” Panek said. “In fact, they were refining the information and the data in such a way that they were becoming more and more convinced. And as they were becoming convinced, of course I was becoming convinced, and I thought this is really something revolutionary.”

Panek read several sections of the book, including part of the story of the work of astronomer Vera Rubin on galaxy motion that ultimately suggested dark matter. More amusing was a battle between physicists and astronomers in the 1990s as they struggled to come up with a notion of dark energy. The physicists thought the astronomers were encroaching on their turf, and the astronomers felt the physicists didn’t know astronomy. Then a funny thing happened. Both sides reached the same conclusions at the same time and reported them to a skeptical scientific community.

“Part of the reason the community believes them is because these two teams that hated each other so much both came up with this counter-intuitive answer,” Panek said. “If only one of them had, it would have been dismissed, but because both of them had, they had to take it seriously.”

Panek said scientists came up with the percentages by running millions of computer simulations and comparing the results to the observed cosmic microwave background. A universe of 73 percent dark energy, 23 percent dark matter, and four percent for what we can actually see matches well with the observations.

“This is the evidence that astronomers and physicists have found compelling,” he said. “They have this computer simulation, they have this observation, and the two match. The problem, though, is that they don’t know what this 96 percent is.” Panek calls that “the biggest puzzle in physics.”

“It’s going to require reconciliation of the physics of the very small, the quantum level world, and physics of the very big, the general relativity view,” he said. “You have to figure out some way to make those two match.”

The talk, given to a full house at Town Hall Seattle, was well received. The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality, which grew out of a 2007 article published in the New York Times Magazine, should be an interesting read for those who love science, especially its history.

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