Thursday, July 14, 2016

Secret Science Club Post-Lecture Recap: Shedding Light on Dark Matter

Last night, I headed down to the beautiful Bell House, in the Gowanus section of Brooklyn, for this month's Secret Science Club lecture. This lecture featured the triumphant return of Dr Priyamvada Natarajan, of Yale University's departments of astronomy and physics, the Niels Bohr Institute in Copenhagen, and the University of Delhi, India, to the Secret Science Club event horizon. In 2014, Dr Natarajan kicked off the Secret Science Club North with a lecture about dark matter. Last night, Dr Natarajan's lecture was in support of her new book, Mapping the Heavens: The Radical Scientific Ideas That Reveal the Cosmos.

After a brief autobiographical introduction, detailing her childhood in India and her education at MIT and Cambridge University's Trinity College, Dr Natarajan gave a brief overview of science. Science is in the business of rethinking ideas- discarding or refining them as needed. She characterized the current time as a "golden age of cosmology", an amazing confluence of theory and technology, with astronomical discoveries occurring every day. Dr Natarajan described science as the arc of acceptance of radical ideas and stressed the need to demystify the techniques of science, which is the best way to understand nature and to make sense of the universe. Initially, there tends to be pushback towards new ideas, until a preponderance of data convinces skeptics. Fundamental to science is the interplay of ideas and instruments.

Dr Natarajan gave a brief overview of the history of astronomy, displaying images of the Nebra sky disc and the Venus tablet of Ammisaduqa as early astronomical artifacts. She then displayed an image from Riccioli's Almagestrum Novum depicting the muse Urania discarding the Ptolemaic cosmology and weighing the semi-geocentric model of Tycho Brahe (with the planets orbiting the sun and the whole orbiting the earth, which Riccioli favored) against the wholly heliocentric Copernican model:

Dr Natarajan then quickly pivoted to modern astronomy, specifically the mapping of the universe- citing the Hubble space telescope as being instrumental (HA). The main goals of astronomy are describing the contents, expansion, and eventual fate of the universe. To illustrate this combination of goals, she cited the example of the Cosmic Microwave Background which is a relic of the time shortly after the Big Bang.

Dr Natarajan's lecture then focused on two particular topics- dark matter and black holes. She contrasted the discovery of these two enigmatic phenomena- the existence of dark matter was determined through empirical observation (gravitational effects on other astronomical objects) and the existence of black holes was determined through theoretical modeling.

The topic shifted to dark matter, which was the subject of her fantastic Secret Science Club North lecture. The universe is composed of approximately 70% dark matter, approximately 25% dark matter, and approximately 5% baryonic matter. The existence of dark matter was first proposed by Fritz Zwicky in order to explain the observed behavior of galaxies in the Coma Cluster. In the 1970s, Vera Rubin and Kent Ford observed unexpectedly steady rates in the rotation of galaxies, evidence that there was a 'halo' of matter around these galaxies that balanced out the greater concentration of baryonic matter at their core. While dark matter has never been observed, it has an impact on dynamics- the motion of stars and galaxies and an impact on light rays. In a solar system, the dominant gravity is that of its star. In a galaxy, there is a lot of 'gravitating' dark matter at the edge. Dark matter is lumped and clumped and smeared all over galaxies, but has no interaction with other matter, except through its mass.

Light is both a wave and a particle, it can be bent through a process known as gravitational lensing. This gravitational lensing can be used to observe far distant astronomical features in galaxy clusters. The current model of the universe posits filaments of dark matter with galaxies at the intersections of filaments. The nature of dark matter is encapsulated in its smoothness- using its lensing effects on observable astronomical features, dark matter can be mapped with a high degree of resolution. It is thought that dark matter is cold, with few collisions between particles... Dr Natarajan noted that she was somewhat disappointed that this was so.

While dark matter has not been observed to interact with baryonic matter, except through gravitational forces, there are attempts to detect it- the Large Underground Xenon experiment is an attempt to detect WIMPS (weakly interacting massive particles), which are considered candidates for dark matter. So far, these haven't been found- we are stuck with cold dark matter, but we don't know what it is.

The topic then shifted to the second major focus of the lecture- how black holes became real. Black holes were predicted mathematically, the mathematical models were borne out observationally, to the extent that gravitational waves were recently detected. In science, mathematical models have to be squared with actual objects. Dr Natarajan wryly noted that the term 'black hole' entered the English lexicon in 1756, to describe the Black Hole of Calcutta, the proverbial place of no return. In 1783, John Michell proposed a dark star, and object so massive that light (which in the original Newtonian model was thought to have mass) could not escape its gravitational field. According to Einstein's Theory of General Relativity, mass bends spacetime, theoretically, an object could be so massive that it effectively 'punctured' spacetime. The term black hole was applied to this astronomical phenomenon by John Wheeler.

The first observational evidence of black holes came in the form of mysterious objects dubbed 'quasars' which have been determined to be X-ray emissions from black holes. Black holes are collapsed stars so massive that light cannot escape their gravity once past the event horizon, so dense that, were the Earth to collapse into a black hole, it would measure one cubic centimeter in volume. According to General Relativity, mass bends the curvature of spacetime into 'holes'- the more mass, the deeper the hole. Black holes are infinitely deep, the laws of physics that we know break down in the vicinity of a black hole. The curvature of spacetime due to gravity was described in Einstein's field equations- Karl Schwartzschild proposed a solution describing slowly rotating spherical objects and New Zealander Roy Kerr proposed a solution modeling gravitational fields around supermassive rotating objects.

Subrahmanyan Chandrasekhar, pondering the fate of stars, formulated the Chandrasekhar limit, the maximum mass of a star which will form a white dwarf- according to the Chandrasekhar's theoretical model, more massive stars will collapse into neutron stars, even more massive ones will collapse into black holes. Jocelyn Bell Burnell, while a graduate student, discovered the first pulsar, which turned out to be a radiation-emitting rotating neutron star, empirical evidence for one of Chandrasekhar's theoretical end-term stars. The first empirical evidence of a stellar mass hole was the discovery of Cygnus X-1, a stellar mass hole which is pulling matter from a blue giant companion star.

More massive by far than stellar mass black holes are supermassive black holes. In 1963, Maarten Schmidt of Caltech discovered the first quasar, an extremely distant, extremely black object which was determined to be a scaled up supermassive feeding black hole... the brightness of the 'quasar' results from a 'flare' of matter ejected from the accretion disk of the black hole at high temperature and high velocity.

Major questions remain about black holes... boiling down to three 'F's'- formation, fueling, and feedback. How do they form? How do they grow? What do they do? Where do black holes reside? Does every galaxy harbor a supermassive black hole at its center? What are the 'seeds' of black holes? Do they result from direct collapse? How do they grow? The formation of a black hole would have to involve a lot of gas- everything would have to be right for one to form. Gravitational waves were discovered emanating from colliding black holes by LIGO, the Laser Interferometer Gravitational-Wave Observatory. LISA, the Laser Interferometer Space Antenna, is a project to develop a more sensitive gravitational wave detector.

As Dr Natarajan wrapped up her lecture, she displayed a NASA animation simulating a stellar mass black hole, a groovy visual accompaniment to a thoroughly groovy lecture:

The lecture was followed by a Q&A in which the Bastard was unable to get a question in. One of the most involved questions involved dark energy, which Dr Natarajan likened to the 'gas pedal of the universe', resulting in the increasing speed of the universe's expansion. She postulated that dark energy is a property of spacetime, but that more research was necessary. In the course of the Q&A, she uttered a line which cannot be stressed enough, so I'm putting in all-caps: NOTHING CAN SUPPLANT THE POWER OF DATA.

Dr Natarajan's return to the Secret Science Club, and her debut at the beautiful Bell House, was triumphant one. Kudos to Dr Natarajan, Dorian and Margaret, and the staff of the beautiful Bell House. For a taste of Dr Natarajan's scientific virtuosity, here's a video of the good doctor giving a brief lecture on the subject at hand:

Also, I'd like to give a hearty high-five to Dr Natarajan for the publication of Mapping the Heavens... congratulations! It's nice to see someone who can so perfectly articulate these cosmological concepts on a level that the layperson can understand. At the end of the lecture, Dr Natarajan addressed the packed house and quipped, "I thought I'd be speaking to ten people." I chided her afterwards by noting that her Symphony Space lecture was delivered to a full house. The good doctor has star power, which is entirely appropriate for an astrophysicist. Again, congratulations are in order.


ifthethunderdontgetya™³²®© said...

Congrats on successfully navigating the black hole of the NYC subway!

Here's Dark Star.

Also, Sugar Magnolia from New Haven 1978. I was there, first time I saw them.

mikey said...

Such an incredibly fascinating discussion. One of the more fascinating questions is whether the Standard Model gives the universe what it needs to include dark matter. Is it a massive Lepton? It almost certainly cannot be any kind of Hadron, because then it would interact with the strong force. But a massive ½ integer spin Lepton like the Tau is a possibility.

The challenge is that dark matter interacts with gravity - the force we know the least about. If we can create and measure the guage boson that mediates gravity - the so-called Graviton - we might learn more about where to look.

I love the fact that the answer will be found at the nexus of cosmology, high energy particle physics and QFT. A combination of observational research on the very large, the very small and theoretical work on the basic structure of matter in the universe.

Good stuff.

Big Bad Bald Bastard said...

Here's Dark Star.

Here's another Dark Star.

I love the fact that the answer will be found at the nexus of cosmology, high energy particle physics and QFT. A combination of observational research on the very large, the very small and theoretical work on the basic structure of matter in the universe.

That's the remarkable thing about it all- the infinitesmal and the cosmic scales both meet in this amazing field we call physics. Good stuff, indeed.

Big Bad Bald Bastard said...

I have to note, Dr Natarajan's joke about being a 'cosmic optometrist' is really hilarious.

mikey said...

Hey, Mr. Bastard, check this out.

It's a new theory about how neutrinos might bind to a universal scalar field, creating an effect identical to what we observe as Dark Energy. It's a really cool theory, and even includes some predictions that could confirm or falsify it observationally....

Big Bad Bald Bastard said...

Thanks,mikey, I'll download that to peruse. We live in amazing times, too bad there's so much BS getting in our way as a species.

mikey said...

Starting to look like the end of the line for WIMPs. And SUSY's not looking real healthy either...