Hunting for Order in the Cosmic Forest

“Astronomy is like a journey through the universe, which I often compare to a forest,” said 2020 Nobel Laureate in Physics Reinhard Genzel during his talk on the first day of #LINO70. “We see the beautiful trees and the enormous complexity, and on very rare occasions in this beauty we begin to see a certain order.”

Genzel’s ‘journey’ began in 1980 when he joined the group of Charles Townes (1964 Nobel Prize in Physics) as a postdoc. Just a few years earlier, Townes had taken gas measurements of the Milky Way that revealed there was a huge mass within a few light years of the centre – a strong hint of a supermassive black hole, but not proof.

From the 1990s onwards, Genzel’s team and that of his US contemporary (and Nobel co-recipient) Andrea Ghez began to peer into the heart of the Milky Way to look for more conclusive signatures of this black hole, dubbed Sagittarius A*. They reasoned that the stars near such an extreme object would have tell-tale orbits, accelerating as they fell closer and closer to the centre.

With Genzel using the four 8 metre telescopes of the European Southern Observatory in Chile, and Ghez the Keck telescopes on Mauna Kea in Hawaii, by 2008/9 they had their proof. The trajectories of several close-in stars indicated that Sagittarius A* measures less than 125 times the distance between Earth and the Sun, even though it contains 4 million solar masses. “Is it a black hole?,” asked Genzel. “I would say it’s very likely to be a black hole.”

Most satisfying for Genzel has been slowly uncovering order hidden in the universe’s complexity. By monitoring the motions and spectroscopy of the stars near the massive black hole over time, his team can compare the observations with what Einstein’s general relativity predicts would happen around such an object. Results have been unequivocal: “All of this is in exact agreement with what is expected from general relativity.”

Ripples in Spacetime

Similar extraordinary agreement with Einstein’s theory was announced to the world in 2016 when the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations presented the first direct observation of gravitational waves – cosmic ripples in the fabric of spacetime predicted by Einstein 100 years earlier.

In his lecture on day 2 of #LINO70, Kip Thorne (Nobel Prize in Physics 2017) showed the audience the now famous GW150914 gravitational wave signal perfectly matching an identical wave from theory.

This minute squiggle was a relic from the collision of two black holes 1.3 billion years ago, which for about 1/10 of a second produced ripples of distorted space with a power 50 times larger than that of all the stars in the universe combined before flowing out into the cosmos.

Given fellow Nobel Laureate Barry Barish had primed the #LINO70 audience just beforehand with an introduction to LIGO and that first gravitational wave detection, it was left to Thorne to dig into the weeds.

“When black holes collide, they create a storm in the shape of space and the rate of flow of time,” he explained. “And that storm generates the gravitational waves that LIGO and Virgo have observed from … colliding black holes.”

Describing our very latest understanding of how black hole collisions produce various types of gravitational waves and the many questions that remain unanswered, Thorne ended his talk on an optimistic note.

“Think back to the huge revolution in our understanding of the universe from electromagnetic astronomy by itself over the last several centuries,” he said. “I invite you to speculate what may happen with gravitational wave astronomy and multi-messenger astronomy over the next several decades and centuries, because it’s going to be very, very exciting.”

A Plenitude of Planets

Another Nobel Prize-winning astrophysicist speaking about a revolution in astronomy on day 2 of #LINO70 was Didier Queloz (2019 Nobel Prize in Physics). Alongside Nobel co-recipient Michael Mayor, Queloz’s 1995 discovery of the first planet outside the Solar System – 51 Pegasi b, roughly 50 light years away in the Pegasus constellation – changed our understanding of Earth’s place in the cosmos and gave birth to exoplanet research, a completely new field of physics.

In the intervening 25 years, scientists have discovered approximately 4,800 confirmed exoplanets, painting a very different picture of what a planetary system should be.

“You may have learned at school we have these three categories of planets,” said Queloz, referring to gas giants (Jupiter, Saturn), ice giants (Neptune, Uranus) and terrestrial planets (Mercury, Venus, Earth, Mars). “We can just stop learning that because… in the universe, there is a continuity between all these planets… you have a lot of categories.”

Like Thorne, he too ended his talk with optimism and excitement for the future. Describing techniques in development right now, he suggested we are on course for being able to test for hints of life on distant worlds without having to travel vast cosmic distances to get there.

“You can imagine that there are certainly 1,000 stars around us for which we can look at the detailed planet compositions, and we will learn quite a lot,” he said. “I think exploring other planetary systems… will have some kind of impact on our own civilization.”