Title: Modified Gravity on Compact Object and Ultra-compact Object
Supervisor: Prof. Dr. Anto Sulaksono and Handhika Satrio Ramadhan, Ph.D.
Abstract: In this dissertation, we investigate two kinds of modified gravity models and their impact on some properties of a neutron star. The first model is the semiclassical gravity (SCGrav) proposed by Carballo-Rubio [PRL 120, 061102 (2018)] and the second model is the Eddington-inspired Born Infeld gravity (EiBI) popularized by Banados and Ferreira [PRL 105, 011101 (2010)]. In the SCGrav model, there is a parameter lp which is a coupling constant for an additional term on the matter part of the Einstein field equation. In EiBI there are κ, which is the parameter that schematically set the strength of the nonlinear Ricci tensor terms in the Lagrangian (O(Rn+1), n ≥ 1), and λ, which is related to the usual cosmological constant Λc. In the SCGrav model, we focused on studying the effect of lp and we compare it with the standard Tolman-Oppenheimer-Volkoff equation (TOV) in general relativity (GR). From our analysis on SCGrav, we obtain that the effect of lp is not significant if compared to TOV GR. On the other hand, in the EiBI model we focus on the effect of cosmological constant Λc towards the moment of inertia I and tidal deformation parameter Λ. We use Λc from observation and compare the results with observational data from neutron stars with mass 1.4M⊙. From our analysis, the maximum mass which can be reached is only around 2.1M⊙.
Title: Gravitational Field of Noncanonical Global Monopole: A Study of Their Black Holes and Compactifications
Supervisor: Handhika Satrio Ramadhan, Ph.D.
Abstract: In this thesis we present some gravitational field solutions of global monopoles and its generalizations in higher dimensions. In general, we discuss the mathematical model in a higher dimensional manifold M with dim(M) = p+D. We discuss some blackhole solutions, whose horizons are what we focused on. We also discuss some compactification solutions (or more accurately factorized metric solutions) and list some possible compactification (factorization) channels from a (p + D)-dimensional space to a (p + 2) × (D − 2)-space of constant curvature.
Title: Instanton: Semiclassical Approach of Tunneling between Vacuum Spaces Phenomenon
Supervisor: Handhika Satrio Ramadhan, Ph.D
Abstract: Semiclassical approach could be used to explain quantum phenomena which can not be explained using perturbation technique. Using Euclidean space, tunneling effect between vacuums of a potential can be investigated. The solution is called instanton, whose probability is proportional to exp(−SE/¯h) with SE an action in Euclidean space. Using thin-wall approximation, the solution and its action can be derived analytically. In this research, we calculate instanton solution and its action without thin-wall approximation with numerical computation using shooting method.