Cowsik's scientific contributions span over several decades and are in the fields of astronomy, astrophysics, cosmology and non-accelerator particle physics; these are recorded in his 175 research papers. He established the highest observatory in the world in Hanle, Ladakh, in the Himalayas, at an altitude of 15,000 ft, for astronomy in the optical and infrared wavelength bands. He has contributed significantly to the understanding of highly energetic phenomena in astrophysics such as cosmic rays, pulsars, supernova remnants, gamma ray bursts, active- galactic nuclei and other such sources powered by accretion flows. He has studied both the diffuse non-thermal radiations that permeate all space and also the emission of such radiations from discrete astronomical sources. The 'leaky-box' and the 'nested-leaky box' models invented by him are extensively used to interpret the observations of cosmic rays.
He has studied the virial discrepancy in the dynamics of clusters of galaxies and proposed the idea of weakly interacting particles as constituting the 'dark matter' that is responsible for the formation and the gravitational binding of galaxies and clusters of galaxies. Assuming thermodynamic equilibrium in the early hot and condensed state of a big-bang Universe, he calculated precisely the number density of the relict neutrinos, and thence derived a strict upper bound on the masses of the neutrinos - the 'Cowsik-McClelland bound'. His work, especially in the area of cosmology, is interdisciplinary in character and connects laboratory Physics with the large scale phenomena of the Universe. Accordingly, he has set useful bounds on the radiative instability of neutrinos and on baryon number, non-conserving proton-decay and neutron-antineutron oscillations. He developed a new method of deriving the age of the Universe from studies of the isotopic anomalies in pre-solar grains of aluminum oxide found in meteorites.
Non-accelerator particle physics refers to the study of fundamental particles and their interactions without using huge accelerators like FermiLab. Cowsik performed the first detailed calculations on the fluxes of neutrinos generated by cosmic-ray interactions in the atmosphere and discussed their observation in detectors placed deep underground--a study that later led to the discovery of neutrino oscillations at Kamiokande in Japan. At the Washington University, he and his colleagues measured the longest radioactive half-life ever, of the double beta decay of Te-128, as 7.7 x 1024 years. This long lifetime, in conjunction with the shorter lifetime of Te-130, implies an upper bound of ~1 eV on the Majorana mass of the neutrino, and strict bounds on majoran couplings. Most recently with a highly sensitive torsion balance he and his students have successfully observed the 'finite temperature' corrections to the Casimir forces which come into play at large separations.
His current efforts are primarily directed towards building an extremely sensitive torsion balance to probe possible violations of the inverse square law of gravity at sub-millimeter scales that are predicted by 'string-motivated' theories. This is a follow up of his longstanding interest in constructing sensitive torsion balances and using them to study Einstein's equivalence principle and to search for new fundamental forces. He is also interested in several problems in high energy astrophysics, dark matter and cosmology, and encourages students join him in this exciting research enterprise.