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#Science : Astrophysicist Says Experiments Might Soon Reveal Dark Matter's "True Nature"

Astrophysicist Says Experiments Might Soon Reveal Dark Matter's "True Nature"

Priyamvada Natarajan 
In a Q&A, Priyamvada Natarajan takes multiverses seriously but not cosmos-as-simulation idea

When assessing scientific claims, science writers rely more than they care to admit on character judgments. If you like a scientist, you are more likely to take seriously her assertions, even when they bump into your biases. Priyamvada Natarajan, an astrophysicist at Yale, is a case in point. In September I hung out with her at a conference in California, where we both lectured, and later at Stevens Institute of Technology, where she gave a talk about her terrific new book Mapping the Heavens: The Radical Scientific Ideas That Reveal the Cosmos. I found Natarajan, or “Priya,” so smart, funny and, well, sensible that she got me reconsidering my pessimism about physics and cosmology (see Further Reading). In the Q&A below, she contends that we are “poised to make an epistemic leap in terms of unraveling the true nature of dark matter.” She’s even upbeat about the prospects for detecting other universes! May her optimism prove prescient. –John Horgan

                                        

Horgan: Why did you become an astrophysicist?

Natarajan: Well I got seduced into becoming one! I was very inquisitive and curious as a child and it started with a fascination for maps – both terrestrial and celestial, followed by looking through a telescope for the first time and falling in love with the night sky. Carl Sagan’s Cosmos was a definite inspiration as well – it was the only time other than Star Trekthat I keenly watched TV when I was young. And of course, if there was ever a key moment, it was when I got a taste for research first hand when I was in high-school working with the then Director of the Nehru Planetarium in New Delhi, the late Dr. Nirupama Raghavan. And after my undergraduate studies at MIT, while I was enrolled in a PhD at the MIT Program in Science, Technology and Society, it was a meeting and long conversation with the astrophysicist Martin Schwarzschild that helped me make the decisive move to Cambridge, England, to pursue a PhD in astrophysics. So although I knew from a young age that I wanted to be a scientist, I gravitated slowly but surely toward the choice of astrophysics well after my undergraduate studies.

I have to say that despite it sounding a bit clichéd, the unreachable, the invisible and mysterious have always drawn me. Somehow the fact that you cannot touch the stars and cannot perform controlled experiments with the universe really intrigued me. Making sense of the cosmos scientifically, driven by data, was really where I eventually found my calling. Also, my work offers me the perfect escape from an imperfect world that I find noisy, unfair, complicated and disorderly in sharp contrast to the orderliness of the cosmos.

Horgan: What astrophysicist, living or dead, most inspires you?

Natarajan: There isn’t one single person that I am inspired by – there are different attributes that I admire in many of them. Albert Einstein for his intellectual audacity; Subrahmanyam Chandrashekhar for his depth of understanding and clarity; pioneering women like the Harvard astronomer Cecilia Payne-Gaposhkin for paving the way for future generations like mine; the astronomer Vera Rubin for her remarkable observational work discovering dark matter and personal encouragement of me in the early stages of my career….. Truth be told, I have found everyone that I have had the privilege to collaborate and work with to be inspiring in some way or another.

Horgan: Has sexism been eradicated from the physical sciences?

Natarajan: Seriously - is this a question? No, in fact, as with society as a whole it is going to be extremely difficult, if not impossible, to eradicate sexism. Alas! It is alive and well, even if not as overt as it was before. The problem is that sexism now manifests in subtle ways, and unconsciously often, so it is harder to pin down and mitigate. I fervently hope that the growing awareness will help us contain it and it will not impede the participation and progress of women in the field professionally. It would be a real shame if creative and talented women are deterred from choosing careers in the physical sciences due to sexism.

Horgan: Will we ever know why there is something rather than nothing?

Natarajan: We already know there is something rather than nothing. Even what we refer to as nothing – the vacuum - is replete with stuff, virtual particles that richly structure what we refer to as nothing-ness. For me, this is not the profound question, the deeper one is - why thisuniverse?

Horgan: Do you ever doubt whether dark matter or dark energy really exists?

Natarajan: No, I don’t doubt that they exist, as there is incontrovertible evidence, in my opinion, from many independent cosmological probes. Let me share why I have such a strong conviction about this. It really comes from observations of clusters of galaxies. Clusters are the most massive and recently assembled objects in the universe, and a lot of my own research is focused on using them as astrophysical laboratories. The fact that no matter what worldview one adopts, whichever conception of gravity one subscribes to – a Newtonian one or an Einsteinian view--we infer the presence of copious amounts (close to 90% of the total gravitating mass) of unseen matter in clusters. If we apply Newton’s tidy laws to the motions of galaxies that constitute a cluster and are gravitationally bound to it, we find that they are whizzing around too rapidly to be just under the gravitational grip of the matter that is seen. If, on the other hand, we embrace Einstein’s interpretation, then we can use the strength of the detected light bending – gravitational lensing, the distortion produced in the shapes of distant galaxies viewed through a cluster -- to infer the amount of matter needed to produce these effects. Incredibly, lensing also points to the existence of ~ 90% more matter that is invisible – more than what is seen in the stars in the cluster galaxies and X-ray emitting gas in a cluster. Remember, light bending is a tested prediction of Einstein’s Theory of General relativity. To me, the fact that these two disparate explanations point to the same mix of dark matter to visible matter in clusters is extremely compelling. And the only other alternate theory, one that dispenses with the idea of dark matter altogether and posits a modification to gravity, that has been proposed to date fails to account for the gravitational lensing that is seen. To me this is noteworthy. Clusters of galaxies offer a powerful test bed for the dark matter hypothesis.

What I think is entirely possible is that we are off in terms of our expected properties for the dark matter particle, which might account for why they remain elusive to date. For instance, we might be searching in a swathe of parameter space (mass and interaction cross-section) that is off. We have been beholden to the idea of weakly interacting massive particles, and the neutralino as the putative dark matter particle as it is permitted within the standard model. I am optimistic though that we are really close to solving this mystery of the missing particle. This is akin to a crime scene, where we have the motive, the weapon as evidence but the body is missing! To me there are several exciting upcoming developments that I keenly look forward to in the coming 3-5 years or so that might break this intellectual impasse. One is the on-going replications of the DAMA experiment that controversially claimed a detection of dark matter more than a decade ago. Five experiments using the same sodium iodide crystal detector fabricated by the same company have been commissioned around the globe and are currently collecting data to test the original claim. In a few years we will know definitely if the DAMA result holds up with data collection and analysis from these independent groups. Second, I am excited about the current renewed vigor in axionic dark matter searches, as I think it is very promising. These experiments allow us to probe beyond the WIMP scenario. And finally, there is the intriguing theoretical idea of superfluid dark matter, that stands to be tested with current astronomical data – the rotation curves of galaxies and gravitational lensing by clusters. I think we are poised to make an epistemic leap in terms of unraveling the true nature of dark matter very soon.

Horgan: Do you ever doubt whether black holes exist?

Natarajan: Nope – I do not doubt that they exist. But what I find intriguing is that there are two completely distinct ways to think about them and understand them. Observational astronomers and astrophysicists like me view black holes as material objects that are compact and dense and exert detectable influence in their vicinity. For us black holes are real as they correspond to the end states of stars – stars that we see in various stages of evolution around us. We view supermassive black holes, that lurk stealthily in the centers of most galaxies if not all, including our own, or that power quasars, the brightest beacons in the universe, to have simply grown from smaller-mass seed black holes over cosmic time. And for theoretical physicists and relativists, black holes are a mathematical construct, an exact solution of Einstein’s equations. These two apparently divergent views need to coalesce and will, once the gaps in our understanding have been bridged. In fact, I think this seemingly incommensurate view of black holes is an excellent example of what the historian of science Peter Galison has referred to as a trading zone – an intellectual space wherein disparate conceptual views converge and can be synthesized. And of course, the detection of gravitational waves from two merging black holes earlier this year by the LIGO collaboration confirms that they are not only real but also behave as predicted by Einstein’s Theory of General Relativity.

Horgan: Will multiverses always remain untestable speculation?

Natarajan: At the moment, it seems like the idea of the multiverse is untestable, perhaps even by construction. However, there is work underway on exploring potential signatures that might leave enigmatic fingerprints on the relic radiation from the hot and dense beginning of our universe – the cosmic microwave background – perhaps some echoes of a collision with another bubble universe?

Look, I have an extremely open mind about this idea, and most scientific ideas actually. One just has to look back in history to get a sense of how far we have come and how unpredictable our path of radical discoveries has been. When Copernicus re-ordered the cosmos (well, our solar system was the cosmos then) in 1543, he could have hardly imagined that in three hundred odd years, we humans would build spacecraft – the Voyagers – that could leave the sacred confines of the solar system. So, where our discoveries will lead us in the future is not something I can hazard a guess about. The human mind and its cognitive capacity is incredible, so who knows what more we might figure out about the cosmos in the coming 300 years!

Horgan: Do you take seriously the idea that the universe is a simulation?

Natarajan: No, I don’t take this idea seriously. I do not, as I do not see why it is intellectually stimulating or even useful.

Horgan: Some prominent physicists have called philosophy obsolete. Has philosophy been useful to your scientific work?

Natarajan: First, there is philosophy as an intellectual activity, which is somewhat different from how it is parceled into the academic discipline. Philosophy to me is an intellectual stance. It helps dissect how we frame and think about foundational questions about the creation of knowledge, so it inevitably informs my worldview and therefore my scientific work.

Horgan: Do you think our ideas about the universe might appear as wrong to future scientists as the ideas of Aristotle and Ptolemy seem to us?

Natarajan: Sure, they might seem naïve, and lacking in explanatory power to future scientists given their comprehension of the cosmos. But I think they would still see us as an essential part of the narrative of the arc of scientific discovery.

Horgan: Are you religious?

Natarajan: No, I am not – I do not practice any religion. I was born and brought up as a Hindu. Hinduism consists of several distinct philosophical schools, and one of them, Advaita, is a materialist, non-theist one that I find intriguing and compelling. My parents encouraged us to learn Sanskrit and read philosophical texts in the original. I love the Sanskrit language and have read the Bhagavad Gita and several of the Upanishads. They left it to us to figure out what role religious belief might or might not play in our lives.
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