## Tesi etd-11142018-235725 |

Thesis type

Tesi di laurea magistrale

Author

BOVA, CLAUDIA

URN

etd-11142018-235725

Title

Effective action for a cosmic string: corrections to gravitational wave emission

Struttura

FISICA

Corso di studi

FISICA

Supervisors

**relatore**Cella, Giancarlo

Parole chiave

- cosmic string

Data inizio appello

10/12/2018;

Consultabilità

Secretata d'ufficio

Riassunto analitico

Topological defects can be a consequence of phase transitions in which there is a symmetry breaking. This thesis analyzes, from a theoretical point of view, the particular case of one dimensional defects, named cosmic strings. These can be generated during the first stage of the Universe evolution.

Strings are astrophysical objects that after formation can compose a dynamical network and generate several observable effects. Their detection can give relevant information about fundamental physics and is an important issue in modern Cosmology.

Free cosmic string dynamics is described using a first approximation by Nambu-Goto action. The example examined in this thesis is the case of a symmetric circular cosmic string loop, a simplified case for the more general loop that can be created during the spontaneous symmetry breaking or during the process of crossing of two strings.

We found the equation of motion for the circular loop in Minkowsky space and we explicited the mode functions that characterize its evolution. After that we studied the same problem in a Friedmann Robertson Walker metric, finding the differential equation that link the string's radius with time. Finally we generalized the study to the general class of (p,q) harmonic solutions.

A string can emits gravitational radiation during its evolution, and we calculated the power of radiation in Minkowsky space in the circular loop case.

The original part of this thesis is the study of correction to the gravitational wave emission of a collapsed loop using perturbation theory, when the dynamic of the string is modified by adding corrections proportional to the curvature. After remembering some basic concepts about the mathematics of the hypersurfaces, a general treatment to the curvature corrections to Nambu–Goto action is described. The results of the dynamics corrections is compared with the basic Nambu–Goto case.

The natural continuation of this work is the study of the effects of a modified dynamics on solutions with cusps. These are singularities which radiate gravitational waves in a very efficient way. In this case the curvature corrections are expected to play a relevant role.

Strings are astrophysical objects that after formation can compose a dynamical network and generate several observable effects. Their detection can give relevant information about fundamental physics and is an important issue in modern Cosmology.

Free cosmic string dynamics is described using a first approximation by Nambu-Goto action. The example examined in this thesis is the case of a symmetric circular cosmic string loop, a simplified case for the more general loop that can be created during the spontaneous symmetry breaking or during the process of crossing of two strings.

We found the equation of motion for the circular loop in Minkowsky space and we explicited the mode functions that characterize its evolution. After that we studied the same problem in a Friedmann Robertson Walker metric, finding the differential equation that link the string's radius with time. Finally we generalized the study to the general class of (p,q) harmonic solutions.

A string can emits gravitational radiation during its evolution, and we calculated the power of radiation in Minkowsky space in the circular loop case.

The original part of this thesis is the study of correction to the gravitational wave emission of a collapsed loop using perturbation theory, when the dynamic of the string is modified by adding corrections proportional to the curvature. After remembering some basic concepts about the mathematics of the hypersurfaces, a general treatment to the curvature corrections to Nambu–Goto action is described. The results of the dynamics corrections is compared with the basic Nambu–Goto case.

The natural continuation of this work is the study of the effects of a modified dynamics on solutions with cusps. These are singularities which radiate gravitational waves in a very efficient way. In this case the curvature corrections are expected to play a relevant role.

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