The 2019 Nobel Prize in Physics recognizes research that helped explain the evolution of the Universe and reveal the prevalence of worlds like our own.
Where do we come from? Are we alone? The 2019 Nobel Prize in Physics honors researchers who have helped us find some answers to these cosmic questions. James Peebles of Princeton University received half of the prize for describing details of the evolution of our Universe from its hot dense past to its galaxy-filled present. Michel Mayor and Didier Queloz of the University of Geneva received the other half of the prize for their discovery of a planet orbiting a star like our Sun.
When Peebles began working on cosmology in the early 1960s, the field was fairly abstract, dominated by discussions over the mathematical consistency of an infinitely dense point at the very beginning of the Universe. In 1965 Peebles and his colleagues argued that—regardless of the initial state of the cosmos—matter and radiation should have been in thermal equilibrium in the early Universe, when the temperature was above 1010 K [1]. Running the cosmic clock forward, they calculated that the temperature of that radiation would cool to a present value of around 10 K, depending on the overall density of the Universe.
As luck would have it, a detection of background radiation in the microwave region of the spectrum was made in that same year by Arno Penzias and Robert Wilson at Bell Labs in New Jersey [2]. Penzias and Wilson approached Peebles and his Princeton colleagues to make sense of their signal, which corresponded to a radiation temperature of about 3.5 K. This first measurement of the cosmic microwave background (CMB) was a breakthrough moment for physical cosmology, the field that uses the laws of physics and astronomical observations to construct models of the Universe’s evolution. “One could almost say that Peebles invented modern physical cosmology,” says astrobiologist Charles Lineweaver from the Australian National University in Canberra.
The other half of this year’s physics prize focuses on the planetary scale. The ground-breaking discovery by Mayor and Queloz of a planet orbiting a Sun-like star came two decades before the Kepler and TESS satellites made planet detection routine. In the early 1990s, astronomers were still struggling to find hints of planets beyond our solar system. There were some detections of planets orbiting neutron stars called pulsars, but those environments were considered so unique that they gave little insight into the development of Earth-like planets.
The search strategy of Mayor and Queloz was based on the radial velocity technique, which involves monitoring the spectral lines in a star’s emission. If a planet is orbiting the star, those lines will shift slightly up and down in frequency as the star is tugged back and forth by the planet’s gravity. Previous radial velocity surveys had come up empty, but they had been limited to the brightest stars in the sky. Mayor, Queloz, and their colleagues designed a more sensitive spectrograph, called ELODIE, that allowed them to target a larger number of stars. In 1995, they reported a planet-induced wobble in 51 Pegasi, a Sun-like star located 50 light-years away [5].
The detected planet, named 51 Peg b, was not something that most astronomers would have predicted. The large amplitude of the radial velocity signal implied that the planet was massive, roughly half the size of Jupiter. But the period of the signal was just 4 days, so the planet’s distance to its star was only 5% of the Earth-Sun separation. Calculations suggested that the planet’s surface temperature would be 1300 K.
“Mayor and Queloz helped launch the exoplanet detection industry,” Lineweaver says. He explains that their discovery had an initially negative effect on searches for worlds like our own, as 51 Peg b—and other “hot Jupiters” that were subsequently discovered—gave the impression that Earth-like planets might be very rare. But once astrophysicists accounted for the high likelihood of detecting hot Jupiters, they realized that “our type of planetary system could be the most common kind,” Lineweaver says.
And if Earth-like worlds are ubiquitous, we might not be the only ones watching the cosmic evolution unfold.
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