Our research focus is gravity – the first fundamental forces of Nature to be studied by humankind, from Galileo to Newton, Einstein and Hawking. However, unlike the other 3 fundamental forces (electromagnetic force, strong nuclear force, weak nuclear force), which have all been successfully quantized, gravity remains (relatively) poorly understood -- some have even doubt that it may be emergent, not fundamental. Not only all attempts to find a full working theory of quantum gravity have not been successful, we are not even sure whether general relativity is the correct theory at the classical level. Cosmological observations have shown that the Universe is comprised of dark matter and dark energy whose true nature remain elusive, leading some to suspect that we need to modify general relativity to explain these anomalies. In addition, despite advances in observations and detection of gravitational waves from black hole collisions, the properties of black holes remain elusive. Our main research interests include: black holes and holography, gravitational wave, cosmology (with focus on 21 cm cosmology, and the physics of dark sector) and modified gravity.
Black holes are spacetime regions where gravity becomes so strong that not even light can escape from. Even after more than a century since its mathematical discovery by Karl Schwarzschild in 1916, black holes remain mysterious. We are interested in the properties of various black holes in general relativity as well as modified theories of gravity. A particularly interesting problem concerns the nature of Hawking radiation, and whether information is lost once things fall into a black hole, and if not, how exactly is the information preserved. It has also become quite clear through decades of research that gravity is related to quantum field theory. Specifically, gravity in anti-de Sitter spacetime is equivalent to a quantum field theory that lives on the spacetime boundary, without gravity. This gauge/gravity duality, or holography, has opened up new approaches to understand gravity, as well as using gravity to probe properties of physical systems that at first sight, has nothing whatsoever to do with gravity. For example, one could use certain black holes to understand superconductors. What else could we learn about gravity via holography?
Ever since antiquity the night sky is a source of marvel. Legends and myths were told throughout early civilizations concerning how creation came to be. However it wasn't until Einstein's time that we have the right tool to study the entire cosmos. It took a couple more decades after that to turn cosmology into a scientific field of study with detailed measurements. There were surprises everywhere we looked. For example, galaxies seem to contain a lot more mass than all of its stars and planets combined. The missing mass has been coined “dark matter”. We see its effect, for example, from lensing of lights by galaxy clusters, and the motion of stars at the edge of galaxies. On the other hand, the expansion of the Universe continues to accelerate, the cause of which remains unknown and has been dubbed “dark energy”. Could there be interactions between dark matter and dark energy? What is the underlying physics that govern cosmic inflation? Why is there an arrow of time? How did the Universe begin? How would the Universe end? Big questions like this continue to intrigue our imagination, but incredibly, physics can now attempt to provide at least some explanations.
While Einstein's general relativity has been well tested by various observations, it is possible that at the cosmological scale it might require some modification or extension, which could in turn explain dark matter or dark energy. There exist a plethora of modified theories of gravity, from simple straighforward generalizations of the Hilbert-Einstein action (e.g. f(R) gravity), to utilizing other geometric structures (e.g. f(T) gravity that models gravity as spacetime torison instead of curvature), it is important to constrain these theories from both observations and theoretical grounds (a good theory should be mathematically self-consistent and free of pathologies). [Figure source: Tessa Baker]