Doctoral defence: Débora Aguiar Gomes „Theoretical and astrophysical aspects of extended general relativity“

On 19 June 2025 at 14.00, Débora Aguiar Gomes will defend her doctoral thesis „Theoretical and astrophysical aspects of extended general relativity“ („Laiendatud üldrelatiivsusteooria teoreetilised ja astrofüüsikalised aspektid“).

Supervisors:

visiting professor Tomi Sebastian Koivisto, University of Tartu
prof. assistant Aneta Magdalena Wojnar, University of Wrocław (Poland)

Oponent:
Assistant prof. Sante Carloni, Charles University (Czechia)

Summary
Throughout the history of science, observations have often challenged existing knowledge. This type of situation can either lead to the adoption of new theories or the prediction of new objects. For instance, Newton’s laws defined our understanding of gravity for a long time, but they could not explain Mercury’s orbital precession. In 1915, Einstein’s General Relativity (GR) theory addressed this issue and has since become the standard theory of gravity, having Newtonian mechanics as its special case. However, GR cannot satisfactorily explain galaxy behaviour – indicating the existence of a new form of matter called Dark Matter – and the accelerated expansion of the Universe – suggesting the presence of an unusual form of energy called Dark Energy. Although Dark Matter and Dark Energy are believed to compose most of the Universe, their true nature remains unclear. These issues, among others, drive the exploration of Modified Gravity (MG) theories. My thesis explores the application of various formulations of GR and their extensions to cosmology and stellar physics. I investigated theories that introduce a scalar field – which represents, essentially, a number at each point in space – to describe gravitational interactions alongside spacetime curvature. In this context, I explored how these theories affect the evolution of stars before they reach the Main Sequence. Additionally, I worked on theories based on geometrical properties of spacetime, such as torsion – a “twisting” of spacetime – and non-metricity – a “stretching” of spacetime. In spite of reproducing GR’s predictions, these theories provide a novel perspective on gravity and are the starting point for some MG theories. In this context, I studied cosmological solutions in GR-equivalent theories and the viability of an MG theory based on non-metricity.