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Uncovering the most compact objects in the Universe

The year 2016 has been the year of achievements for astrophysics. The discovery of Gravitational waves was a much celebrated event. This was followed by the recent news of discovery of thousands of exoplanets, exciting the scientists in this field. In our backyard at the Indian Institute of Science, Bangalore, Prof. Banibrata Mukhopadhyay and his team are making some path-breaking research in astrophysics.

Prof. Mukhopadhyay’s areas of research span across black holes to fluid mechanics to nuclear astrophysics. “Generally we are interested in physics related to black holes, white
dwarfs and neutron stars (called astrophysical compact objects) and
related fluid mechanics and nuclear astrophysics”, says Prof. Mukhopadhyay. Currently the group is working on the concepts of Einstein's theory of general relativity, basic nuclear astrophysics and fluid mechanics and engaged in resolving outstanding issues related to astrophysical compact objects.

In the recent past, the research group headed by Prof. Mukhopadhyay has taken strides in the field. The group proposed of a significant violation of the “Chandrasekhar-limit” and new mass-limit of white dwarfs. Chandrashekar-limit is the maximum mass a white dwarf (a dead star) can have to remain stable, beyond which it collapses because of gravity. The group’s discovery heralds a paradigm shift 80 years after the celebrated Chandrasekhar-limit was proposed. It also explains “over luminous” and “under luminous” type Ia supernovae. It was previously thought that the type Ia supernova occurs always when a white dwarf has a particular mass, resulting in same brightness every time it occurs. So scientists look for type Ia supernovae to find the distance of galaxies in space. But the group’s work points out that there can be type Ia supernovae occurring in space with different brightness. This may question the accuracy of previously measured distances, existence of dark energy and so on.

Another feather in the cap is the group’s proposal of a semi-empirical formula to correlate the spin and mass of black holes. This leads to the test of Einstein's theory of relativity for the Kerr metric (space-time curvature by the spinning black holes) and finally reduces the number of independent variables describing the black hole. This is very useful because now relatively lesser number of properties associated with a black hole vary independently and hence reduces the amount of work while observing a black hole.

The array of areas worked upon by the group speaks about the motivation behind their work. “The mission is to uncover certain fundamental issues lying with these topics - to measure the spin of astrophysical black holes (which is difficult due to the absence of their hard surface), to explain certain enigmatic peculiar supernovae related to limiting mass of white dwarfs, to resolve a severe mismatch between theoretical understanding and experimental/observational data of certain shear flows, etc. as a whole”, says Prof. Mukhopadhyay. 

The group focuses on theoretical physics where they make predictions about observations, rather than going from observation to some hypothesis. “We resolve the problems by introducing the respective theories of our own. While some set of parameter values explains the existing observation, some other set helps in predicting future observations”, explains Prof. Mukhopadhyay. At times, they also perform simulations and consider approaches lying primarily with observed data, as well as propose empirical laws.

The group is now expanding its areas of research to focus on specific topics. They are probing if Einstein's general relativity is the “ultimate theory” to explain observable universe or any modification needed. They doubt if Chandrasekhar's limit is unique and also trying to find the role of stochastic forcing in resolving the mismatch in theoretical findings and experimental/observational data. “Our future plan is to undertake resolving the issues in high energy, relativistic and nuclear astrophysics”, signs off Prof. Mukhopadhyay.

Contact Information:

Banibrata Mukhopadhyay is an Associate Professor in the Department of Physics, Indian Institute of Science.

Phone: +91-80-2360-7704 (preferred, for callers outside IISc), +91-80-2293-2799,

E-Mail: bm@physics.iisc,