The accelerating universe

The Nobel Prize for physics was awarded to the three Americans who, in 1998, showed that the expansion of the universe 14 billion years after the Big Bang has not slowed down. Instead, it speeds up. “A bizarre concept,” says professor of astrodynamics, Boudewijn Ambrosius.

Professor Boudewijn Ambrosius sees himself as a professional amateur in astronomy. His own field, space engineering at the faculty of Aerospace Engineering, merely provides astronomers with tools. Think of the Hubble telescope and its successor, the James Webb telescope. The developments of astronomical insights, Ambrosius has followed as an interested outsider. He remembers when he first heard about the Big Bang theory: it seemed logical that eventually gravity would slow down the expansion of the universe and pull everything together again. The idea of a cyclic universe attracted him. But some ten years ago, it became clear that instead the expansion speeds up. “It’s bizarre,” Ambrosius days, “that everything that once came into being will end up becoming thinner and thinner and thinner.”

The Nobel Prize was awarded on Tuesday to Saul Perlmutter (UC Berkeley), Brian Schmidt (Australia National University) and Adam Riess (Johns Hopkins University), who in the late 1990s worked with two competing teams that hunted for the most distant supernovas, in order to determine the speed at which the edge of the universe moves away from us. Perlmutter headed the Supernova Cosmology Project, while Schmidt and Riess worked with the High-z Supernova Search Team. After the analysis of about 50 distant supernovae, the researchers reached the same conclusion: not only is the expansion still going on, it even accelerates with distance.

“It’s a bit technical,” Ambrosius says, when asked to explain the findings. For measuring far distances in the universe, one needs what astronomers call ‘standard candles’. These are stars with known brightness. The teams selected a special type of supernovae (type Ia) as their standard, because other stars were not bright enough to be seen at distance of billions of light-years. In practice, these supernovae – however bright they may be – popped up as spots of only a few pixels in diameter in the images of the world’s largest telescopes. They were automatically detected by comparing photos from the same area taken with three weeks interval, since supernovae do not last that long.

Ambrosius explains: “From the brightness, you can calculate the distance to the supernova, and from its redshift you can calculate its speed. If you plot these on a graph, you don’t get a straight line as you would expect from a uniform expansion. No, the speed is higher at large distances than you would get by drawing a straight line. There is a ‘vacuum push’ that counteracts gravity.”

These findings have given rise to the concept of ‘dark energy’, which is calculated to be the form of an astounding 70 percent of the universe. “The findings didn’t fit in with standard physics,” says Ambrosius. “So something had to be found to make it fit, something that provides the pushing force. That has given rise to the concept of ‘dark energy’.”

Earlier, ‘dark matter’ was introduced to make up for the lack of mass in galaxies. So now we have 70 percent of dark energy, 20 percent of dark matter, and only 5 percent of known matter. A humbling thought? “We’re waiting for a brilliant mind to think up a new physical concept which can include dark matter and energy,” the professor concludes. “Perhaps we will then also be able to measure it.”