Liang Dai - Research

Research Overview

I have a wide range of research interests in astrophysics and phsyical cosmology. Working at the intersection between fundamental physics and astronomy, I aim to learn, from the phenomenon of gravitational lensing, about the physical nature of the dark matter, as well as the physics of star formation and evolution in the young Universe. I study opportunities to discover new physics from the early Universe from cosmological observables such as the Cosmic Microwave Background anisotropis. Moreover, I am interested in the astrophysics of gravitational wave sources and the extraction of physical information through data analysis.

Star Formation in Young Universe

Super star clusters (SSCs) are thought to be the progenitors of globular clusters and represent the densest star-forming environments in the Universe. Strong gravitational lensing offers us a natural microscope to study individual newborn SSCs during Cosmic Noon (z=2-3) when the Universe was a factor of few smaller than it is today.

In a latest work led by Berkeley gradudate student Massimo Pascale, our group analyzed and interpreted the stellar and nebular emissions from a ~3 Myr-old, very massive ( tens of millions solar masses) newborn SSC with escaping Lyman continuum radiation, in the Sunburst Arc (see Rivera-Thorsen et al). We found evidence for dense, radiatively-pressurized photoionized clouds surrounding the monster system at only several parsecs. We are surprised to find that these clouds have unusual chemical compositions such as a high level of nitrogen enrichment, which most likely originates from massive star ejecta.

Our findings have tantalizing implications for the long-standing puzzle of why globular clusters appear to consist of multiple stellar populations with distinct element abundances. We might be witnessing, in the first few million years of a compact massive starburst system, the making of self-enriched material which will later give birth to stars of unusual element abundances.

Gravitational Lensing

One focus of our latest research is distant gravitationally lensed sources with high magnifications in the field of galaxy cluster lenses, such as individual superluminous stars or star-forming clumps at redshift z=1-3. Examples were recently discovered by the Hubble Space Telescope. We study a number of interesting applications related to these sources:

Intracluster microlensing at high-optical depths and a probe of compact dark matter objects;
Probing the population of sub-galactic dark matter halos from astrometric and photometric signatures;
Micro-caustic transits of highly magnified stars as a probe of dark matter minihalos.

The study of these highly magnified sources can shed light on cosmic structure formation and assembly on sub-galactic scales, provide clues about the physical nature of the dark matter, and enable a magnified view of star formationa and galaxy evolution in the high-redshift Universe. Observations using the Hubble Space Telescope, the forthcoming James Webb Space Telescope, as well as an array of ground-based optical/infrared telescopes will help achieve these science goals.

In addition are works on gravitational lensing of non-optical sources:

Microlensing of extragalactic radio bursts as a probe of compact dark matter objects and cosmic strings.
Precision timing of lensed extragalactic radio bursts;
Binary black hole mergers as strongly lensed gravitational wave sources.
Probing non-luminous self-gravitating structures from wave diffractive lensing of gravitational waves and measuring lens structural parameters.

We look forward to the discovery of these phenomena in forthcoming observations.

Gravitational Wave Data Analysis

Ground-based laser interferometers such as LIGO and Virgo routinely detected gravitational wave signals of compact binary coalescence in their recent observing runs.

I am part of a collaboration based at the Institute for Advanced Study who have been developing an independent data reduction pipleline and are applying them on publicly release strain data in search of astrophysical gravitational wave sources. Our scientific results include new binary black hole candidate events found from LIGO/Virgo O1 and O2 (and some single detector candidates) observing runs.

We have developed new data analysis methodologies:

Relative binning for fast likelihood evaluation (application to GW170817, the first binary neutron star merger), and applied to spin-precessing waveforms.
Geometric approach to the construction of waveform template banks;
Modeling and mitigation of non-Gaussian detector noise.

Cosmology

I have worked on a variety of problems in cosmology. To name a few:

Search for cosmic birefringence signatures from axion strings;
General relativistic view of large-scale clustering of galaxies and application to primordial non-Gaussianity;
Imprints of primordial gravitational waves in CMB spectral distortion and in galaxy clustering.
Gravitational "Faraday rotation" of the CMB photon polarization states;
Cosmic inflation and reheating;
Transformation of the CMB anisotropies under relativistic aberration;
An elegant mathematical formalism to describe cosmological observables on the celestial sphere;