Gravitational-Wave Imprints of Disappearing Matter in the Early Universe

by Dr Xiaohan Ma（马潇汉） (University of Science and Technology of China & IPMU)

Friday 21 Aug 2026, 14:00 → 15:00 Asia/Shanghai

Tsung-Dao Lee Institute/N6F-N600 - Lecture Room (Tsung-Dao Lee Institute)

Abstract:

An early matter-dominated era can be generated by nonrelativistic populations such as ultralight primordial black holes, Q-balls, or oscillons. If they disappear before Big Bang nucleosynthesis, the Universe returns to radiation domination, and the associated transition can source a stochastic background of scalar-induced gravitational waves. We develop a general framework connecting the microscopic law governing this disappearance to the transition dynamics and its gravitational-wave signal.

Using evaporating ultralight primordial black holes as a concrete example, we show that a finite-width mass distribution leads to a universal transition qualitatively different from that of a monochromatic population. Near the evaporation endpoint, the mass loss of individual black holes is accompanied by the continuous depletion of the surviving population. This evolution is controlled by the evaporation law and is largely insensitive to the detailed initial mass distribution. It imprints a characteristic high-frequency tail on the gravitational-wave spectrum, whose asymptotic behavior directly encodes the underlying evaporation law and may therefore probe black-hole evaporation physics. At the same time, this tail is strongly suppressed relative to the monochromatic prediction, reducing the total gravitational-wave energy density and substantially relaxing observational constraints on primordial-black-hole-driven early matter domination.

Speaker Bio:

Xiao-Han Ma recently received his PhD in astrophysics from the University of Science and Technology of China and will soon join the Asia Pacific Center for Theoretical Physics as a postdoctoral researcher. During his doctoral studies, he was also a joint doctoral student at the Kavli Institute for the Physics and Mathematics of the Universe, University of Tokyo. His research focuses on theoretical cosmology and the early Universe, particularly non-perturbative primordial non-Gaussianity, primordial black-hole formation, and cosmological gravitational waves. He develops analytical and numerical approaches to connect small-scale primordial physics with observable signatures, including gravitational-wave signals associated with evaporating ultralight primordial black holes.

Alternative online link: https://meeting.tencent.com/dm/tKaMDGwuLtMF
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