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## Dr. Minxi He at KEK Theory Center got the Outstanding Presentation Award

February 22nd, 2022

In the 30th Workshop on General Relativity and Gravitation in Japan (JGRG30), Dr. Minxi He who is a researcher at KEK Theory Center got the Outstanding Presentation Award. JGRG is the workshop related to general theory of relativity and gravity and it has been held in Japan every year from 1991 (reference: https://www.tsujikawa.phys.waseda.ac.jp/jgrg30/). Outstanding Presentation Award is given to the researcher who made a great presentation.

“Reheating Process in Mixed Higgs-R^2 Model” is the title of Dr. He’s presentation. He researches the beginning of the Universe, especially the process from inflation to the hot big bang in an inflation model of “mixed Higgs-R^{2} inflation model”.

It is considered that the Universe was extremely hot and dense at early time, so-called the “hot big bang”. The Universe which was as if a fireball cooled down with expansion and became the present one. The big bang theory explains the fact that the Universe expands, we can observe the Cosmic Microwave Background (CMB) which is the light from about 3.8 hundred thousand years after the birth of the Universe, light elements like hydrogen and helium were made in the early Universe, like that. On the other hand, there are many facts which cannot be explained only with the big bang theory. For example, the observable Universe looks the same on the whole, but it includes difference locally such as the fact that there are various kinds of stars and galaxies. We can explain this Universe when inflation era existed before the big bang in Universe. The Universe expanded drastically (exponentially). However, the Universe should be empty after sudden expansion. This is contradictory to the Universe which was extremely hot and dense at the big bang, after inflation.

To connect inflation with big bang, the era of reheating appeared in. Reheating is the era between inflation and the big bang. The energy which caused inflation changed into matters in this reheating era, and the Universe became full of matters. Thus, this can solve the problem which was mentioned above. Dr. He researches this reheating process, in a word, how the hot big bang occurred after inflation.

It is very natural to consider that inflation occurred when the scalar particles (*1) of “inflaton” accumulated. In the Standard Model (this forms the basis of modern physics together with general relativity), the Higgs particle, which is the only scalar particle, is one of the most favored candidates of inflaton. So, the physics in the early Universe like inflation can be deeply related to the Higgs. There are many researches by other authors studying how to explain inflation with Higgs but their studies include a UV issue in reheating era. If the Higgs particle caused inflation, reheating energy exceeds the upper limit of energy below which we can use the usual perturbation theory, (*2) so the usual perturbative calculation becomes invalid. In other words, the theory that only the Higgs causes inflation (called “Higgs inflation”) is not suitable to explain the era from inflation to reheating.

To solve this problem, the new inflation model of “mixed Higgs-R^{2} inflation model” was born. The mixed Higgs-R^{2} inflation model is an inflation model which combines the R^{2} model with the Higgs model. The R^{2} model (or the Starobinsky model) is the first (in 1980) inflationary model. So far, R^{2} model is one of the most favored inflation models by observational studies.

As a result of previous researches, it is known that the Higgs inflation needs to be combined with R^{2} model by quantum effect. In addition, it is clear that the mixed Higgs-R^{2} inflation model does not encounter the UV issue. Then, we can explain the era from inflation to reheating only by two elements of the Higgs inflation and R^{2} term with the mixed Higgs-R^{2} inflation model. Dr. He analyses the reheating process of the Universe energetically using the mixed Higgs-R^{2} inflation model. This research of the reheating process is evaluated and he won the Outstanding Presentation Award.

An IPNS public relations officer interviewed Dr. He.

― Could you tell me your comments about winning this award?

**●Dr. He** “I am really honored to be awarded and I would like to thank all my collaborators. Also, I am grateful to the JGRG committee who recognized my research, which means great encouragement to me. I hope I can do more interesting and solid research in my scientific career.”

―What mystery or unsolved phenomena are you interested in?

**●Dr. He** “I am interested in the early Universe so I will continue to investigate inflation and reheating. Besides, many other interesting and unsettled problems are also worth studying, such as primordial black holes and gravitational waves which are also related to the early Universe. I hope I could contribute to these fields as well.”

# Footnotes

*1 scalar particles

Elementary particles have the property like rotation called spin. Scalar particles are particles which do not spin.

*2 perturbative theory

Method of calculation by assuming that interactions with particles (in this time, the Higgs and other particles) are small and adopting interactions one by one from main one.

# Link

## Gruop Link

(revised : 12/Dec/2022)