Blowing Bubbles from String Theory

• Since the discovery of the accelerated expansion of the universe at the end of the nineties, physicists have tried to describe this cosmological behaviour from theories that are complete in the ultraviolet regime, i.e. theories that are well defined at arbitrarily high energies. The most fundamental property that any proposed model must reproduce is that of a de Sitter space in the lower-dimensional description. In other words, an accelerated expanding four-dimensional cosmology driven by an unknown energy dubbed as dark energy.String theory seems to be the most promising candidate for a theory of quantum gravity, as it naturally accommodates the graviton, the quantum propagator of gravity and fulfills all the requirements to be an ultraviolet-complete theory. However, despite all the efforts of recent decades, no lower-dimensional effective field theory description with the desired positive cosmological constant has yet been obtained. This suspicious conspiracy has motivated the physics community to take a different approach; to investigate what criteria lower-dimensional effective field theories of gauge fields coupled to gravity should satisfy to be derived from string theory rather than aiming to derive four-dimensional effective field theories directly from string theory. One of the most striking conclusions that can be drawn from these criteria is that four-dimensional de Sitter cosmology may belong to the swampland, the set of effective field theories that cannot be obtained from string theory.In the first part of this monograph we will review both classical and quantum approaches to four-dimensional cosmology and then introduce the main aspects and issues faced by the aforementioned string cosmology constructions. The second part of this thesis presents an alternative and novel realisation of a four-dimensional expanding cosmology from string theory. This low-dimensional universe rides a three-dimensional brane that mediates the decay between two different five-dimensional and non-supersymmetric anti-de Sitter vacua. This proposal, known as the Dark Bubble model, turns into advantages some of the liabilities found within the swampland programme, granting an innovative way of realising dark energy from string theory. In this work, we will discuss the top-down construction of the Dark Bubble model from ten-dimensional supergravity, the higher-dimensional origin of four-dimensional dark energy and the new intrinsic hierarchy of energy scales that emerges from this stringy construction. Furthermore, we will also show how four-dimensional matter, radiation and both gravitational and electromagnetic waves will arise naturally from features of the higher-dimensional spacetime.

Ämnesord

NATURVETENSKAP  -- Fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences (hsv//eng)

Nyckelord

String theory; Supergravity; String Cosmology: String Phenomenology; Braneworlds; Dark Bubbles; Cosmology; Gravity in Higher Dimensions

Theoretical Physics

Teoretisk fysik

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