Abstract: 

Astonishingly, human beings discovered that distant Type Ia supernovae (SN Ia) were accelerating away at an increasing pace. This led to the great discovery of late-time cosmic acceleration and brought us the concept of dark energy, responsible for such a phenomenon, but the underlying nature remains mysterious. Theoretically, modified gravity can be a framework to provide an alternative explanation for the the existence of dark energy. In this talk, we go into the theoretical perspectives of understanding the late-time cosmic acceleration. In particular, we will explore the effective field theory of dark energy and modified gravity. Using the latest Dark Energy Spectroscopic Instrument (DESI) data released in 2025 as well as other observational data, we are able to reconstruct the background evolution and find that the reconstructed dark-energy equation-of-state (EoS) parameter w(z) exhibits the so-called quintom behavior, crossing -1 from phantom to quintessence regime as the universe expands. We reconstruct the corresponding actions for f(R), f(T), and f(Q) gravity, respectively. We conclude that, certain modified gravity such as metric-affine gravity can exhibit the quintom dynamics and fit the recent DESI data efficiently, and for all cases the quadratic deviation from the ΛCDM scenario is mildly favored.

 

Bio: 

Yi-Fu Cai is a professor of physics and astronomy at the University of Science and Technology of China. He received the PhD degree in theoretical physics at the Institute of High Energy Physics, Chinese Academy of Sciences in 2010. He worked as a postdoc at Arizona State University and McGill University until 2015. He was selected into the China's National Youth Talents Program in 2015 and became a faculty at USTC. His particle cosmology group is responsible for the scientific goals of the AliCPT project associated with cosmological models. His research focuses on fundamental questions for physical cosmology including the big bang singularity, the origin of the universe, dark energy and cosmic acceleration, cosmological perturbation theory as well as the CMB sciences.

 

 

Time: 14:00-15:00, 6/May, Tuesday
Venue: Room 506 (Large seminar room), Department of Astronomy

You can also access the colloquium via :
https://www.koushare.com/space/329883/live