Perpustakaan Digital ITB

The cause of cosmic acceleration is still a mystery. As the dark energy standard model, the cosmological constant has complicated physical problems regarding its energy value. Therefore, various alternative models have been put forward, such as quintessence and k-essence. Having not finished with the challenge, currently, there are conflicts in the H0 and ?8 parameters in which one of the viable solutions is modifying or introducing another dark energy model, most of which are remarkably phenomenological. These tensions push the fundamental problem of dark energy to become overlooked. Formal theoretical investigations are urgently needed to explain the causes of cosmic acceleration. Hence, we propose a domain wall model with a noncanonical Lagrangian as a dark energy model. This idea is the main (primary) novelty of the dissertation because it has never been proposed previously by any researchers. The significance of this work is spreading a path of holistic knowledge that connects the dark energy concept in cosmology and the grand unified theory (GUT), considering that a domain wall is one of the topological defects predicted by the GUT. We investigated the physical and cosmological properties of noncanonical domain walls. We found that noncanonical domain walls could act as dark energy in late epochs and behave like dark matter in early epochs. This means the noncanonical domain wall unifies dark energy and dark matter models on large scales. The domain wall dynamics are independent of the potential form but are determined by their velocity according to the observer. However, not all solutions are stable. Based on the Hamiltonian perturbation stability test, the domain wall with v = 0 (freezing) is the only solution that satisfies all stability conditions. We study the cosmological background dynamics of the DWCDM (freezing noncanonical domain wall-cold dark matter) cosmolog model. Generally, our model gives a not different universe’s history compared to the ?CDM model, i.e., evolution’s path is the radiation-matter-dark energy era. However, in particular, there are some significant differences. During the matter-dominated regime, there are two types of dark matter, i.e., noncanonical domain walls and other dark matter (in the model we use CDM). According to the DWCDM model, the transition from the matter to the cosmic acceleration era occurred later than that of the ?CDM model. In addition, we also investigate the perturbation dynamics of the universe following the DWCDM model. Considering the natural property of the topological defects, i.e., they are nonperturbative, we assume that the domain wall does not cluster on a large scale. Using this assumption, we obtain that the structure’s growth following the DWCDM model is slightly slower than that of the ?CDM model. Then, we confirm this analytical result through numerical simulations. Our simulation shows that the dark matter and matter (dark matter + baryons) power spectra following the DWCDM model are slightly smaller at various scales than that of the ?CDM model. Extending the calculation to a smaller scale, we obtain that the ?8 predicted by DWCDM (? 0.76) is smaller than that of the ?CDM (? 0.83). To explain the nature of the unification, we also examine the forces resulting from the self-interaction noncanonical domain walls for the double-well or X potential. We discovered that the interaction between domain walls is attractive on a small scale and repulsive on a large scale. If we extrapolate this to a domain walls-dominated universe, it is associated with accelerated expansion dynamics. This result is consistent as well as explains the physical mechanism of the large-scale-repulsion that generates the cosmic acceleration of the universe. Not only that, but this result also refines the nature of the unification of dark energy and dark matter of noncanonical domain walls. The characteristic scale where the interaction vanishes, i.e., the transition of the matter-cosmic acceleration era, depends on a potential parameter that characterizes the domain walls’ energy density. Therefore, we may examine the fine-tuning problem by observing the cosmological distance in which the repulsive effect starts working effectively. In addition to proposing conceptual ideas of dark energy, we offer a new method to study the history of structures in the universe, considering that LSS is a prospective and significant field for testing cosmological models. The existing methods for studying LSS are the hydrodynamic N-body simulations and numerical simulations (Boltzmann solver), both of which have limitations. The N-body simulation is magnificent in providing a picture of the structure’s evolution, but the computational cost is expensive. On the other hand, the Boltzmann solver has much cheaper computational energy but is not for studying the history of LSS since its primary purpose is computing observables. Therefore, In this dissertation, we propose a new alternative method, namely modeling the inhomogeneous universe as a linear control system for formulating matter transfer function, computing matter power spectrum, and examining the history of LSS. We employ linear time-variant (LTV) control theory to calculate the matter transfer function over time. We execute this using block diagram algebra. To give meaning to these mathematical objects, we compare the mathematical properties of the transfer function, such as poles and zeros, with the physical properties of gravitational collapse. Afterward, we apply this approach to the simple ?CDM and DWCDM models. The openness property of this approach brings the structure formations scenario to a more intuitive understanding. Its modular property (plug-and-play) makes the inhomogeneous universe model easier to modify for various cosmological models. The LTV approach can extract pieces of information about structure formation, such as the scale solution of gravitational collapse and its existence over time, and determine a hierarchical mechanism, namely top-down or bottom-up, with lower computational costs..