**Abstract**

It was discovered nearly tw o decades ago that we live in an accelerating universe that is dominated by dark energy. Understanding the origin of such an energy has turned o ut to be a very difficult open question in physics\, and calls on the ne ed for a fundamental theory like string theory. However\, despite decade s-long effort\, string theory has proven incredibly resilient to a satis factory construction of dark energy within its framework.

In the f irst part of this thesis and the included papers\, we examine this probl em and propose two possible solutions. The first is a construction withi n the framework of M-theory\, the eleven dimensional cousin of string th eory. Using only well-understood geometric ingredients and higher-deriva tive corrections to eleven dimensional supergravity\, we present a new c lass of four dimensional vacua that contain dark energy. In the process\ , we also construct a new class of non-supersymmetric Minkowski vacua th at were previously not known. Our second idea is a novel proposal that o ur universe could be embedded on the surface of an enormous spherical bu bble that is expanding in a five dimensional anti de Sitter spacetime. T he bubble is made of branes in string theory and its expansion is driven by the difference in the cosmological constants across it. We argued th at such a construction arises naturally in string theory\, and showed ho w four dimensional gravity arises in such a universe. We further showed that four dimensional matter and radiation arise from quantities that ar e innately five dimensional.

Another challenging problem in physic s concerns the nature of black holes &ndash\; the presence of an event h orizon in particular. This poses a paradox between well understood physi cal principles\, and requires a fundamental theory for its resolution. T owards this goal\, we constructed a novel class of horizonless objects t hat mimic black holes\, and proposed these objects as an alternative end point of gravitational collapse. Subsequently\, we constructed slowly r otating versions of these "\;black shells"\; and proposed an obs ervational signature that could distinguish them from black holes in cos mological experiments. This is discussed in the second part of the thesi s and in the included papers.

DESCRIPTION:Suvendu Giri försvarar sin avhandling New horizons in string theory: bubble babble in search of darkness. Disputationen ges på engels ka. SUMMARY:Disputation: New horizons in string theory: bubble babble in sear ch of darkness LOCATION:Ångströmlaboratoriet\, Häggsalen\, 10132 TZID:Europe/Stockholm DTSTART:20200917T130000 DTEND:20200917T235900 UID:20200917T130000-55264@uu.se END:VEVENT END:VCALENDAR