PhD defence by Pawan Gupta 'Probing compact objects with gravitational waves'
On October 9 2023, Pawan Gupta has successfully defended his thesis. He accomplished his PhD at the Institute for Gravitational and Subatomic Physics (GRASP), Gravitational Waves Group. The defense has taken place in the Academiegebouw in Utrecht.
Summary PhD thesis
Probing compact objects with gravitational waves
Bounding dark charges on black holes and exploring gravitomagnetic tides on neutron stars
In 2017, the first gravitational wave signal from a binary neutron star merger, GW170817, was detected by both Advanced LIGO and Advanced Virgo. From this merger, a gamma ray burst was detected by the Fermi Gamma-ray Space Telescope and the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) spacecraft which initiated a new era of multi-messenger astronomy.
Neutron star matter
Neutron stars are the most densest objects in our universe which are formed in supernova explosions. This makes neutron stars ideal astrophysical laboratories for testing theories of dense matter physics. A typical neutron star has a mass of the order of 1.4 solar masses and a radius of 12 km. The matter inside a neutron star is characterised by the relation between pressure and density, known as the neutron star equation of state. Equation of states of neutron stars are modelled by various theories in nuclear physics and quantum chromodynamics. A neutron star in a binary system is deformed due to tidal forces of its companion, either a neutron star or a black hole. This effect is similar to what happens here on Earth when the moon’s gravity raises the ocean tides. Internal oscillations of the neutron star arise when the companion’s tidal force varies at a frequency close to the star’s characteristic frequency. This is like a bridge oscillating when a band marches at a pace matching its frequency. The amount of tidal deformation and the characteristic frequency depend on the equation of state of the neutron star matter. Any tidal response of the star leaves a distinct imprint on the gravitational waves emitted by the binary. Thus, gravitational waves will reveal unique information about the exotic interior of the neutron stars.
There are two types of tidal effects: gravitoelectic and gravitomagnetic. Gravitoelectic are leading order tidal effects while gravitomagnetic are next to leading order tidal effects. In this thesis, we modelled the gravitomagnetic tides for a binary neutron stars and their effect on the gravitational waves signal. We used this model to understand the effect of gravitomagnetic tides on measuring the equation of state of neutron stars using third-generation gravitational wave observatories such as Einstein Telescope and Cosmic Explorer.
Dark Charges
Most of the matter in the Universe is made of dark matter which cannot be directly observed. It is called ”dark” because it does not interact with light or other forms of electromagnetic radiation. Its presence can be inferred from its gravitational effects on visible matter, such as galaxies and clusters of galaxies. Over the years, many candidates of dark matter have been proposed, one of them being dark charges. These are reminiscent of electrons, but their charge-to-mass ratio is smaller. They can accumulate on binary black holes through various astrophysical processes. In this thesis, we explored the possibility of detecting dark charges using gravitational waves from binary black holes.
PhD supervisors: prof. dr. C.F.F. Van den Broeck and dr. T.P. Hinderer