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Dechang Dai(戴德昌)

Dechang Dai

Distinguished Research Fellow

Email:
diedachung@sjtu.edu.cn

Research Area

Selected Publications

Introduction of research

My research focuses on particle physics beyond the Standard Model, gravity and cosmology. Below, I review the main topics of my present and future research interests, mostly on the interface of particle physics and cosmology:

Bulk Flow: The bulk flow is the last puzzle raised by the cosmological observational data. This is a coherent motion of a large part of our visible universe in a particular direction. Originally, it was claimed by Kashlinsky et al. by using the Suyaev-Zel’dovich effect in the Cosmic Microwave Background (CMB) radiation. This result was later reinforced by using a compilation of peculiar velocity redshift surveys. This analysis showed a large bulk flow which conflict with the Lambda-CDM expectation. Though the directions of the bulk flow are very consistent, the magnitudes of the bulk flow velocities are less consistent and vary from analysis to analysis. To verify independently if the bulk flow is real we studied the peculiar velocities of Type IA supernovae. If the bulk flow is real, then some imprint must be left on supernovae motion. Since the recession velocities of the high redshift supernovae are large and the magnitude of the bulk flow is moderate, one can expect the largest contribution to come from the low redshift supernovae. Our results are consistent with this expectation. Therefore, there is no clear conflict between peculiar velocity flow and Lambda-CDM.

Modified Gravity: The Lambda-CDM model is the current paradigm in cosmology. This model can explain a wide range of observations, but it is also not free of problems. Two of the most acute problems are the introduction of exotic forms of energy density – the dark matter and the dark energy. Since these forms of energy have never been observed in a laboratory, some theorists have begun looking for other options. For example, the MOND theory can replace dark matter in the galaxy, and the f(R) theory might explain the dark energy. These models modify either Newtonian gravity or General Relativity. Some of the important characteristics of GR or Newtonian theory are lost; most notably there is a violation of the conditions of Birkhoffs theorem. If Birkoff’s theorem is violated, one may need to know the distribution of all the mass in the universe, in order to calculate the gravitational force in each neighborhood. This would make a simple Newtonian system very difficult to treat.

Physics beyond standard model: The Standard Model of the strong, weak and electromagnetic interactions agrees well with experimental data. However it also leaves several problems unsolved; for example, there are at least 20 unexplained parameters some of which are beyond today’s experimental reach. There are many new models having been developed to replace or extend the Standard model, include SUSY, technicolor, extra dimensions and so on.