Radio astrophysicist Nissim Kanekar’s work has put India’s radio astronomy capabilities on the world map. One of the six winners of the Infosys Prize 2022—he won it in the physical sciences category—Kanekar, professor, National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune, was recognised for his study of galaxies in an era (the so-called “cosmic high noon” period, dating back to around 10 billion years ago) during which star formation in galaxies was at its peak. Lounge speaks to Prof. Kanekar about his work, its importance and its relevance to modern dialogues on space, especially in the wake of advancements such as the James Webb Space Telescope (JWST), which bring the larger universe closer to us than ever before. Edited excerpts:
Q. Explain like I am five: What is radio astrophysics? What does it study? What does it aim to achieve? Why should we care?
Radio astronomy is a branch of astronomy in which one looks at distant objects in the universe, like stars, galaxies and quasars, using telescopes that are sensitive to light at radio wavelengths. The difference between radio astronomy and optical astronomy is simply the wavelength of the light that is used to study the astronomical objects. Optical astronomy uses light of short wavelengths (approximately 1 micron), while radio astronomy uses light with wavelengths of millimetres to metres.
Radio astronomy is an important branch of astronomy for two reasons: First, Earth’s atmosphere is transparent in only two wavelength ranges, optical and radio, so these are the only two wavelength ranges where one can carry out observations from the ground (one has to send telescopes to space to observe at ultraviolet or gamma-ray or far-infrared wavelengths). Second, there are certain phenomena that are either only observable at radio wavelengths or that are much brighter and easier to detect at radio wavelengths. This gives us a special “window” on the universe.
Radio astronomy aims to understand astronomical objects that emit or absorb at radio wavelengths. The broad goal is to understand how the universe works. Within this broad goal, there are more specific questions that one can ask in radio astronomy. For example, how do galaxies form and evolve? How does matter behave in and around the most compact objects, neutron stars and black holes? Do the fundamental constants of physics change with cosmic time? And many more...
Humans are the only species that we know of that understands a very large fraction of the universe. This is quite a remarkable achievement, and it includes how we ourselves came to be, and how our home, the Earth, and our bigger home, the Milky Way galaxy, came to be. Understanding the universe is a quest for knowledge which has value in and of itself. This, for me, is a good reason for why humans should care.
Q. In what way is your work linked to advances in the field that are expanding our understanding of the universe, like, say, the JWST?
The JWST operates at infrared wavelengths, roughly 1-50 microns, far shorter wavelengths than those at which radio telescopes operate. The questions that the JWST will be trying to answer are similar to those radio astronomers are trying to answer but the tools used are slightly different. For example, studying galaxy evolution and finding the earliest galaxies is one of the main science goals of the JWST, via studies of stars in these galaxies. In radio astronomy, too, we try to study galaxy evolution and try to observe the early galaxies, but we do this through observations of their interstellar gas. Gas and stars are what make up a galaxy, and so observations with the JWST and with radio telescopes are complementary to each other.
Q. “Stars are born out of the gas, and galaxies which are collections of stars, are held together by the gravity of matter. Working with colleagues Aditya Chowdhury and Prof. Jayaram Chengalur, Prof. Kanekar detected atomic hydrogen at a red shift of 1.3, corresponding to a look back in time of eight billion years.” If you were to, again, explain like I am five, what did your research essentially set out to do, how did it achieve this, where is it leading, and why is it important?
Our research aimed to measure the average atomic hydrogen gas mass in early galaxies, eight-nine billion years ago. Galaxies are made up mostly of stars and hydrogen gas, with stars forming from the gas. To understand galaxy evolution, one has to find out how much matter is in gas and in stars in galaxies at different epochs in the universe. Over the last three decades, studies at optical and infrared wavelengths have revealed some remarkable things about the star-formation activity in galaxies. We know that the star-formation activity in the universe is roughly 10 times lower today as compared to eight to 11 billion years ago. But why the star-formation activity slowed down was unknown, because we didn’t have observations of the gas, which is the fuel for star formation. We basically carried out the first measurements of the hydrogen gas content of galaxies during and towards the end of the period of peak star-formation activity of the universe. We found that the early galaxies contained much more hydrogen gas than stars (the opposite of today) but that the hydrogen gas is being consumed very rapidly by the process of star formation and that not enough gas is being added to the early galaxies from their surroundings to sustain the high star-formation activity. Our measurements thus answered a basic question in galaxy evolution, namely, why did the early galaxies slow down their star-formation activity around eight billion years ago.
Q. What excites you about astrophysics?
I love the idea of learning about the universe and astronomy provides us perhaps the biggest window for this. As J. B. S. Haldane put it: “The Universe is not only stranger than we imagine; it is stranger than we can imagine.” And astronomy lets us look at, and try to understand, the many strange features of the universe nearly all the way back to the Big Bang. And it’s great fun to use astronomical observations to probe fundamental physics, whether it be testing general relativity, looking for changes in fundamental constants, or searching for evidence for the nature of dark energy. Overall, astronomy definitely keeps me awake at night!
Q. Are more people excited about astrophysics today—maybe thanks to well-publicised inventions like the James Webb?
I think people have been excited about astronomy for a long time; basically, humans are curious about the universe. A good example is the observations of the solar eclipses in 1919 to test general relativity; the results, reported in the newspapers, caused an uproar. Over the last three decades, I think the wonderful images produced by the Hubble Space Telescope (HST) have made a huge impact on people and have increased the excitement about astronomy. The JWST is carrying on this tradition, and some of the JWST images are even more beautiful than the HST ones.
Q. Do you ever think about astrophysics at a fundamental, philosophical level? What are your predominant thoughts about the universe, our quest for knowing it, what this knowledge entails for humanity’s future as we battle climate change and many other challenges?
The time scales of these issues are very different: For example, the effects of climate change are likely to hit us within tens of years, while astronomical timescales are typically millions of years or longer. I am hence somewhat sceptical of the notion that astronomical knowledge has direct implications for humanity’s short-term future. However, I do think that no matter what challenges humanity is faced with in the future, there will always be excitement about the wider cosmos and a desire to understand how the universe got here and where it is headed.
This interview is part of a series of interviews with scientists Lounge will be publishing.