Samenvatting
In this Thesis, we investigate some fundamental aspect of nonlocal field theories, like causality, unitarity and renormalizability. We show how to define and compute scattering amplitudes for a nonlocal scalar quantum field theory, and how they behave for a large number of interacting particles. We discuss the possibility to enlarge the class of symmetries under which a local Lagrangian is invariant by means the introduction of nonpolynomial differential operators.
Subsequently, we move to the gravity sector. After showing how to construct a ghostfree higher derivative theory of gravity, we will find a linearized metric solution for a (neutral and charged) pointlike source, and show that it is nonsingular. By analysing all the curvature tensors one can capture and understand the physical implications due to the nonlocal nature of the gravitational interaction. In particular, the Kretschmann invariant turns out to be nonsingular, while all the Weyl tensor components vanish at the origin. Similar features can be also found in the case of a Delta Dirac distribution on a ring for which no Kerrlike singularity appears. Therefore, nonlocality can regularize singularities by smearing out pointlike objects. At the full nonlinear level, we show that the Schwarzschild metric cannot be a full metric solution valid in the entire spacetime, but it can be true only in some subregion, for instance in the large distance regime where there is vacuum.
Finally, we discuss phenomenological implications in the context of ultracompact objects (UCOs), in which ghostfree gravity theories can be put on test and constrained.
Subsequently, we move to the gravity sector. After showing how to construct a ghostfree higher derivative theory of gravity, we will find a linearized metric solution for a (neutral and charged) pointlike source, and show that it is nonsingular. By analysing all the curvature tensors one can capture and understand the physical implications due to the nonlocal nature of the gravitational interaction. In particular, the Kretschmann invariant turns out to be nonsingular, while all the Weyl tensor components vanish at the origin. Similar features can be also found in the case of a Delta Dirac distribution on a ring for which no Kerrlike singularity appears. Therefore, nonlocality can regularize singularities by smearing out pointlike objects. At the full nonlinear level, we show that the Schwarzschild metric cannot be a full metric solution valid in the entire spacetime, but it can be true only in some subregion, for instance in the large distance regime where there is vacuum.
Finally, we discuss phenomenological implications in the context of ultracompact objects (UCOs), in which ghostfree gravity theories can be put on test and constrained.
Originele taal2  English 

Kwalificatie  Doctor of Philosophy 
Toekennende instantie 

Begeleider(s)/adviseur 

Datum van toekenning  5nov.2019 
Plaats van publicatie  [Groningen] 
Uitgever  
Gedrukte ISBN's  9789403421087 
Elektronische ISBN's  9789403421070 
DOI's  
Status  Published  2019 
Extern gepubliceerd  Ja 