HORNDESKI BRANEWORLD COSMOLOGY

• Main Author: FRANKLY SAMUEL PARDEDE (NIM: 20217017), KEVIN
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/28430
• Institution: Institut Teknologi Bandung

<p align="justify"> Einstein's theory of general relativity has been tested to high accuracy for various phenomena, such as precession of Mercury's orbit, gravitational lensing, gravitational waves, and others. Regardless of its success, general relativity has some limitation. Most notably is that the prediction of general relativity in cosmology does not fit current observational facts which suggest that our universe is undergoing an accelerated expansion phase. Simplest modification of standard cosmology to adhere for this fact is to add cosmological constant into Einstein field equations. Unfortunately, theoretical prediction of the value of cosmological constant using quantum field theory is off by 10120 order of magnitude from the observed value. Therefore, another alternative modification of general relativity (and hence standard cosmology) is needed. First part of this thesis is a review of two standard type of modified gravity, extra dimension (for example braneworld model) and scalar-tensor theory. An example of scalar tensor theory discussed in this thesis, Horndeski's theory which comprised of four Lagrangians P5 i=2Li is the most general scalar-tensor theory in four dimensions giving a second order field equations. Next, we combine these two theories. Firstly, we give the general formulation for Horndeski braneworld cosmology, where the brane is taken to be a constant scalar field phi hypersurface. Next, we investigate several cases, for example in the case of braneworld phi = phi(y) we consider L3 cosmology, nonminimal derivative coupling (NMDC), Horndeski without L4 and weak L5 coupling, and full Horndeski model with strong L5 coupling while we also considered the general NMDC braneworld model with phi(t). The resulting modified Friedmann equation contain several correction terms for the conventional equation. The correction terms are the t or y dependent linear, high energy quadratic and cubic term, dark radiation, dark radiation and matter interaction term, and low energy correction term provided by cosmological constant resulting from the scalar field or the Cardassian term with power n = &#8722;1/2,1/2. Cardassian term with n = &#8722;1/2 lies near the latest observational constraint. The high energy correction terms satifsy the big bang nucleosynthesis naturally. <br /> <p align="justify">