Fundamental Physics Seminar Series - Phase-transitioning from the Primordial Universe: from Non-Gaussianity to Cold Dark Matter Based on an Analogy with Superconductivity

Abstract

This talk concerns two independent works. Inflation offers a simplistic yet compelling explanation of how the primordial universe evolved into the LambdaCDM paradigm. Measuring the non-Gaussianity of primordial density fluctuations will deliver crucial evidence for inflation and insight into its more fundamental mechanism. The speaker forecasts future constraints on local-type primordial non-Gaussianity (PNG) under the context of LSST and SPHEREx cross-correlated with Simons Observatory and ACT. This leverages the PNG-induced scale dependent bias on galaxy overdensity, cross-correlated with the CMB kinetic Sunyaev-Zeldovich (kSZ) reconstructed velocity for cosmic variance cancellation. The large-scale galaxy autospectrum suffers from intractable data systematics in observations, but the speaker emphasizes that using LSS x CMB cross correlations alone still yield promising constraints on local PNG. He will briefly discuss how this constraint depends on the uncertain amplitude of the kSZ velocity signal (i.e. the kSZ optical depth degeneracy). Finally, he will compare cross-correlations between galaxy and lensing with galaxy and kSZ velocity for constraining local PNG.

The speaker will then switch gears and consider the universe after inflationary preheating. LambdaCDM is an empirical model with the nature of the dark sector remains a mystery. He will present a novel candidate for cold dark matter consisting of condensed Cooper pairs in a theory of interacting fermions with broken chiral symmetry. Establishing the thermal history from the early radiation era to the present, the fermions are shown to behave like standard radiation at high temperatures, but then experience a critical era decaying faster than radiation, akin to freeze-out, which sets the relic abundance. Through a second-order phase transition, fermion-antifermion pairs condense and the system asymptotes toward zero temperature and pressure. By the present era, the nonrelativistic, massive condensate decays slightly faster than in the standard scenario–a unique prediction that may be tested by combined measurements of the cosmic microwave background and large scale structure. He will also show that in the case of massive fermions, the phase transition is frustrated, and instead leaves a residual, long-lived source of dark energy.


About the Speaker

Mr. LIANG Guanming is a PhD student with Prof. Blake SHERWIN at DAMTP and the Kavli Institute for Cosmology, University of Cambridge. Prior to that, he was an undergraduate student at Dartmouth College from 2021to 2025, where he worked with Prof. Robert CALDWELL. He is currently interested in CMB secondary effects like CMB lensing, the kinetic Sunyaev-Zeldovich effect, and how their cross-correlations with large scale structure (LSS) empower observations. He has sustained interests in particle cosmology, specifically the phenomenology of dark matter, dark energy and baryogenesis, as well as novel intersections of cosmology with field theory.


About the Center for Fundamental Physics

For more information, please refer to https://cfp.hkust.edu.hk/.


For Attendees' Attention

Seating is on a first come, first served basis.