CTPU Joint Tea Time

Friday 21 Jun 2024, 10:30 → 13:30 Asia/Seoul

B438

The purpose of these monthly events is for the two groups to exchange research and socialize.
We will have 30-minute presentations by one person from each research group in the CTPU seminar room at 10:30 am, followed by a 15 minute discussion, and then a light meal in the discussion area in front of the seminar room at 12:00 pm. We invite everyone to join us.

1 Early universe cosmology and modified gravity

My research interests are divided into two main categories: i) Early universe cosmology (inflation): stochastic gravitational waves, dark matter, and primordial black holes, ii) Modified gravity: scalar-tensor and vector-tensor theories, disformal transformations, and the effective field theory approach. I will briefly introduce my projects on these topics.

Speaker: Mohammad Ali Gorji

11:00 Discussion

2 Hairy Black Holes in EsGB theory

We study black hole solutions in Einstein-(Maxwell)-scalar-Gauss-Bonnet theory. We firstly show that the no-hair theorem is evaded for a general form of the scalar coupling function and so hairy black hole can exist. We then specifically employ the complex scalar field that includes quadratic and quartic terms. Thus the gravitational action has a global U(1) symmetry. We argued that when the effective mass of the scalar field is at the critical value, the black holes without hairs transform into hairy black holes in a symmetry-broken vacuum via spontaneous symmetry breaking. We found that the Goldstone bosons are trivial in this process. We further studied the scalar field perturbation on the hairy black hole backgrounds. The late-time behavior of the scalar field only differs from the Schwarzschild black hole by a mass term. The quasinormal modes (QNM) indicate that hairy black holes in the symmetric phase are unstable beyond the critical value of the coupling constant and hairy black holes in the symmetry-broken phase are stable in the parameter space that we concern. These numerical results provide strong evidence for a dynamical process that unstable black holes without hairs transition into stable hairy black holes in the symmetry-broken phase through the spontaneous symmetry breaking. Moreover, we found that the spontaneous symmetry breaking associated with local U(1) is unlikely to occur in this theory. (2205.00907, 2305.19814, 2405.08769)

Speaker: Miok Park

11:45 Discussion

3 Tea Time with light meal