Research

Cosmic defects

Unified theories of particle interactions permit stable solutions referred to as "defects." If big bang evolution cooled the universe through a phase transition breaking such unified symmetries, then it is possible that cosmological defects were formed. These could leave signatures in the form of gravity waves, cosmic microwave background anisotropies, high-energy cosmic rays, etc., thus informing us of details of high-energy physics otherwise inaccessible to experiment. We study the evolution and observational effects of cosmic defects such as cosmic strings, superstring networks, and domain walls.

Cosmic strings are astronomically long, microphysically thin filaments which may exist in our universe. When strings intersect themselves, they can reconnect and so chop off moving loops, which may be observable sources of cosmic rays or gravitational waves.

The image to the left shows loops being emitted from a fast-moving piece of cosmic string. This picture won second place in the Illustrations category in the 2011 Visualizing Research@Tufts Awards.

Dark matter and dark energy

Cosmological and astrophysical observations have shown that dark matter exists, constituting close to a fourth of all energy density in our universe. However, the microphysical nature of dark matter is unknown. We approach this mystery from a variety of angles, including building models of dark matter, investigating dark matter production mechanisms, understanding cosmological signatures of dark matter interactions and astrophysical signatures from dark matter decay and annihilations.  We also explore models of dark energy, the material thought to be responsible for the current acceleration of the universe.

Keyi "Onyx" Li, NSF / NANOGrav

Gravitational waves

Gravitational waves are ripples in the shape of spacetime. They can be generated with a significant amplitude by a variety of mechanisms, such as by black holes or neutron stars orbiting each other, by cosmic defects such as cosmic strings and domain walls, by quantum vacuum fluctuations during inflation, by first-order cosmological phase transitions, or even by density perturbations in the early universe. They can be detected by ground-based interferometers such as LIGO/Virgo/KAGRA, by pulsar timing arrays such as NANOGrav, or by the upcoming space-based interferometer LISA.

We work on all of the above aspects, including analyzing pulsar timing array data, investigating possible early-universe sources of a stochastic gravitational wave background, and exploring new gravitational wave forms from effective field theory corrections to general relativity.

Cosmic inflation and early universe physics

The initial conditions for hot big bang evolution were probably established during a period of approximate vacuum energy domination in the very early universe, called "cosmic inflation". Aside from establishing the homogeneity and isotropy of the universe, inflation leaves tell-tale signatures in the cosmic microwave background, some of which have already been observed. However, pinning down the correct microphysical model of inflation remains elusive. We study cosmic inflation from a number of angles, including string theory and particle physics inspired models of inflation, developing strategies to uncover the fundamental interactions during inflation using primordial non-Gaussianity, and exploring connections with dark matter production.

Negative energies

Is it possible to create a stable wormhole, or to travel faster than light or backward in time? General relativity could produce spacetimes with such exotic possibilities, if given the right distribution of matter and energy to act as a source. But all such exotic phenomena depend on sources with negative energy density. Negative energies can arise in quantum mechanics, for example in the Casimir effect, but their properties are strongly constrained. We study quantum inequalities, which restrict the duration and magnitude of negative energy densities, and the validity of certain "energy conditions" which, if always obeyed, would imply that necessary sources for such exotic phenomena can never arise.

Connections to particle physics

An absence of a convincing detection of physics beyond the Standard Model indicates that new physics might be too heavy or too weakly coupled to be detected directly in collider experiments. At the same time, new physics is needed to explain several puzzles, ranging from the origin of the Higgs boson mass, the nature of dark matter and dark energy, and cosmic inflation. To understand these puzzles, we focus on new particle physics mechanisms and models, and study their signatures in terrestrial and cosmological observations. In particular, we study the capabilities of high-luminosity upgrades of the Large Hadron Collider, neutrino experiments, and future colliders in detecting new particles, and benefit from close interactions with our experimental high energy physics colleagues in the Department. We also explore models to understand cosmological tensions and test them using precision cosmology data.

Effective field theory

Effective field theories (EFT) provide a language to parametrize our ignorance of short distance physics in a model independent way and systematically compute their implications for long distance observables. This general principle has been applied widely in many physical contexts. We develop and use EFT methods in several contexts, including to understand the physics of inflation, the large-scale structure distribution of matter, a generalization of flat-spacetime EFT methods to curved spacetime, and to characterize the space of possible deviations from General Relativity.