Early universe cosmology and modified gravity

• Mohammad Ali Gorji

Room: B438 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.

Hairy Black Holes in EsGB theory

• Miok Park

Room: B438 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)

Tea Time with light meal Room: B438