Today’s Nobel Prize in physics was evenly split between two discoveries and, thus, unevenly split among the three honored. Typically when this happens, the two discoveries are at least somewhat related; that doesn’t seem to be the case here, as the Prize Committee has recognized James Peebles for his contributions to theoretical cosmology and Michel Mayor and Didier Queloz for the first clear discovery of an exoplanet orbiting a main sequence star.
The physics of the Big Bang
It’s hard to find anything related to our current understanding of the Big Bang that doesn’t have Peebles’ fingerprints on it. He was at Princeton University (where he remains) when Arnold Penzias and Robert Wilson at nearby Bell Labs had identified the cosmic microwave background (CMB) that was produced in the aftermath of the Big Bang. Not fully aware of the importance of their discovery, Penzias and Wilson talked to the scientists at Princeton. As a result, Peebles was on a paper that described the theoretical underpinnings of the CMB published in the same issue as the write-up describing the CMB’s discovery.
Peebles immediately went to work trying to understand the physics underlying different aspects of the Big Bang, showing how temperature and matter density influenced the production of helium in the Universe’s earliest moments. Not being satisfied with that, he started considering how these conditions could influence the production of the Universe’s first galaxies.
Obviously, that work was incomplete given that, at the time, we didn’t understand the role of dark matter. But as the evidence built, Peebles contributed a paper that indicated that our Milky Way’s halo wouldn’t look like it did without copious amounts of dark matter. His work later helped lay the foundation for our current understanding that dark matter is cold, or moving at speeds far less than that of light.
But Peebles’ most important contributions potentially came in helping to understand something called the power spectrum of the CMB. This is the product of a series of oscillations that would have occurred in the immediate aftermath of the Big Bang, produced by interactions among the Universe’s materials. In an example cited by the Nobel Committee, gravity would cause an initial contraction. Afterwards, radiation drove an ensuing bounce back, which dark matter ignored since it doesn’t interact with photons. So when there’s yet another contraction, it will take place with dark matter providing a gravity well, influencing the strength of the oscillation.
By carefully measuring these, we’ve gotten critical information regarding how much of the Universe is occupied by regular matter, how much is dark matter, and so on. Actually measuring the power spectrum and finding out what these percentages are took decades and orbiting instruments, but Peebles was critical to laying the theoretical foundations for it.
Amazingly, much of this work took place before the idea of inflation (which describes the rapid early expansion of the Universe) had been fully developed; Peebles’ incorporated newer information on inflation into his ideas as work in the area continued. He also anticipated the return of the cosmological constant, which is a possible explanation for at least some of what we now view as dark energy—even though the evidence for dark energy didn’t come until decades later. He and his collaborators were critical to developing our current model of the Universe, which includes dark matter and dark energy.
The Nobel Committee cites Peebles’ work as providing a physical foundation for the Big Bang. But it’s probably safer to say that Peebles helped enable a two-way conversation between physics and cosmology. In some cases, like dark energy, Peebles helped show that a cosmological constant can make the Universe work long before the experimental evidence was present. In other cases, he helped incorporate ideas like dark matter as they were developed from other evidence.
And now for something completely different
The other half of the prize skips entirely from theory to practical experimentation. Michel Mayor and Didier Queloz are at the University of Geneva, but the prize honors their work done at France’s Haute-Provence Observatory. A method that could allow the detection of exoplanets was first proposed back in the 1950s, but the technology for doing so lagged behind the idea. The concept is that planets orbiting will pull a star toward them slightly as they orbit. These subtle pulls will impart a small Doppler shift to the light coming from that star: blue when it’s pulled towards Earth, red when it’s dragged further from us.
But these shifts are incredibly small. To pick them up, we need a very precise standard to measure them against, an extremely stable instrument, and a star that doesn’t have much inherent variability. Starting in the 1980s, a handful of scientists thought the technical developments were in place, and a few campaigns started that focused on bright stars. In the decade and a half afterward, planets that formed in the aftermath of a supernova were discovered around a neutron star, and a handful of potential signals were identified. But no clear planets were detected.
Meanwhile, Mayor and Queloz built an instrument that could work on dimmer stars, and they set to work. By 1995, they had data from 51 Pegasi that indicated a Jupiter-mass exoplanet was completing orbits in only four days. While this was unexpected—planets that size can’t form that close to the star, so it must have formed elsewhere and migrated inwards—the short orbits allowed Mayor and Queleoz to gather convincing data quickly and allowed other researchers to confirm the observations just as quickly. Unlike the other candidates that had been discovered up to that point, the planet’s existence was accepted quickly.
It’s fair to say that this set off a revolution in astronomy. In about 25 years, we’ve gone through multiple generations of hardware (including several space-based observatories), identified thousands of exoplanets, and started to characterize their atmospheres. Rather than being limited to the Solar System’s eight planets, we can now test models of planet formation against thousands of exosolar systems. While the discovery of exoplanets would have happened regardless of Mayor and Queloz—competing groups published results within a few years of their 1995 paper—they had the right hardware and planet to compel the astronomy community to accept the results.