Mapping the heavens with Priya Natarajan

Priyamvada Natarajan, a theoretical astrophysicist at Yale University, is excited to be working in physics and astronomy at a time she and others call the “golden age of cosmology.”
“The maturity of our theoretical understanding, the sophistication of our instruments and tools that allow us to get the data—spacecraft, detectors—and the advanced computing are all aligned at the moment,” Natarajan said this week during a talk at Town Hall Seattle.

Theoretical astrophysicist Priyamvada
Natarajan spoke Nov. 14, 2016 at Town
Hall Seattle.

Natarajan has done a lot of work on mapping dark matter and dark energy, on gravitational lensing, and on figuring out how supermassive black holes are formed. It’s the latter that has her excited for the launch of the James Webb Space Telescope. She’s been a leader in pushing the idea that supermassive black holes could be formed by the direct collapse of matter. The physics pencils out, and Webb will peer back and possibly find the most distant, and therefore the first, black holes, and perhaps validate her ideas.

“The fact that you can come up with an idea as a scientist, for me, that’s the privilege,” she said.

Natarajan is the author of Mapping the Heavens: The Radical Scientific Ideas That Reveal the Cosmos (Yale University Press, 2016). She said she wrote the book not only to help us understand new discoveries about black holes and dark matter, but also to demystify the process of science.

“I believe very strongly that the current rampant disbelief in science stems from the contingent nature, the provisionality of science.” Natarajan said. “It’s something that’s very hard for the public at large to understand.”
The plus side is that cosmology and astronomy have the potential to win converts.

“Unlike many other fields in science, the night sky belongs to all of us,” she said. “We have to just look up and it’s there; the glory and the awe of the night sky.”

We know a lot

Natarajan finds it interesting that we know so much about the universe, with pretty solid evidence for much of what has happened since the tiniest fraction of a second after the Big Bang.

“It still stuns me that with a cantaloupe-sized gelatinous thing in our skull we’ve been able to figure all of this out,” she laughed. Yet despite all we do know, she said there is still a lot of mystery about our peculiar universe.

“We happen to live in one in which the total energy content of the universe is dominated by two components that we don’t know what they are,” she said.

Chart: NASA

What we call them are dark matter, which makes up 24 percent of the universe, and dark energy, which makes up 71 percent. We and all the stuff we see are less than five percent. Though we don’t know what dark matter is, Natarajan said there is solid evidence that it is indeed out there.

“The idea came out of an empirical need to explain an observation,” she said. Oddly enough, one of her other research interests, black holes, were conceived in exactly the opposite fashion.

“Black holes were actually proposed as a mathematical entity,” she noted. “They were a mathematical solution to Einstein’s equations, and they eventually became real.”

A little history

Dark matter was first suggested by Fritz Zwicky in 1933. Vera Rubin and others looking at galaxies in the 1970s proposed it as the reason rapidly spinning galaxies don’t fly apart. Natarajan said more than 80 years of research has left little doubt.

“We have incontrovertible evidence from many independent lines of investigation for the existence of dark matter because of the effects it produces, although it has not been directly detected yet,” she said. “We don’t know the particle.”

There are two lines of evidence, according to Natarajan, that make dark matter far more than just an inference.

“We can exquisitely map it at the moment, even though we can’t see it, because of the gravitational influence that it exerts,” she said. “The other way in which we can detect dark matter is the impact that matter has on the propagation of light in our universe.”

This is where her work on gravitational lensing fits in. Large galaxy clusters, with as many as a thousand galaxies, can act as a sort of gravitational lens on steroids. Such clusters would be held together by enormous amounts of dark matter. The relativity “pothole” created by the cluster could be strong enough to split a beam of light.

“You end up seeing multiple images of an object where in reality there is only one object,” Natarajan said, noting that this has been observed many times now. Interestingly, she points out that the physics of both Newton and of Einstein would predict the effect.

“You can apply both of these arguments to clusters and you infer the same amount of dark matter,” she said. “In my opinion that is really, really strong evidence, compelling evidence, because they’re completely different world views and they still converge. There’s no escaping the concept of dark matter.”

Search for the holy grail

Natarajan said this sort of research may help us get to the holy grail of physics: a quantum theory of gravity.

“The motivation is to look for gaps, look for disagreements, and look for anomalies where an observation is actually inconsistent with our theoretical expectation,” she said.

A couple of great examples of this came out of the 1800s. The orbit of Uranus didn’t agree with Newton’s Laws, so they did the math and figured another planet could cause the observed discrepancies. That led to the discovery of Neptune. At the same time, there were anomalies in Mercury’s orbit, which led to the proposal that another planet, called Vulcan, was the cause. Vulcan was never found, but years later general relativity explained the precession of Mercury’s orbit perfectly.

“In one case the theory remained intact and an anomaly refined our understanding,” Natarajan said. “In the other case it pointed the way to the existence of a more fundamental covering theory that was yet to come.”

We can’t wait for the next breakthroughs in this golden age of cosmology.

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