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.


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.”