We study the ultraviolet properties of quantum gravity and its consequences for black hole physics using the functional renormalisation group (RG). In particular we concentrate on the asymptotic safety scenario for quantum gravity put forward by S. Weinberg. This approach relies on the existence of an ultraviolet fixed point in the renormalisation group flow. In chapter 2 we review the functional renormalisation group formalism that is used in order to search for the existence of a fixed point with the properties required for asymptotic safety. Following this introduction, in chapter 3 we use these methods to find ultraviolet fixed points in four-dimensional quantum gravity to high order in a polynomial approximation in the Ricci scalar. In the following three chapters we concentrate on the implications of the renormalisation group for black hole physics. In chapter 4 we study quantum gravitational corrections to black holes in four and higher dimensions using a renormalisation group improvement of the metric. The quantum effects are worked out in detail for asymptotically safe gravity, where the short distance physics is characterised by a weakening of gravity due to the nontrivial fixed point. Furthermore, mini-black hole production in particle collisions, such as those at the Large Hadron Collider (LHC), is analysed within low-scale quantum gravity models. In chapter 5 we investigate the thermodynamical properties of the RG improved metrics in detail and study their evaporation process. In chapter 6 we study renormalisation group improved black hole thermodynamics in a metric free approach. Conditions are formulated under which the thermodynamic properties of four dimensional Kerr-Newman type black holes persist under the RG evolution of couplings. We show that the RG scale must be set by the horizon area of the black hole which acts as a diffeomorphism invariant cut-off for the underlying Wilsonian action.

The scale of quantum gravity could be as low as a few TeV in the existence of extra spatial dimensions or if the Planck scale runs fast due to a large number of particles in a hidden sector. One of the most striking features of low-scale quantum gravity models would be the creation of quantum black holes, i.e. non-thermal black holes with masses around a few TeV, in high energy collisions. This thesis deals with the production and decay mechanisms of quantum black holes at current colliders, such as the Large Hadron Collider (LHC). Firstly, a review of models with low-scale gravity is given. We will present an overview of production and decay mechanism of classical and semi-classical black holes, including the Hoop conjecture criterion, closed trapped surfaces and thermal decay via Hawking radiation. We will then introduce a phenomenological approach of black holes, very differently from the (semi-)classical counterparts, which covers a substantially model independent and specifically established field theory, describing the production of quantum black holes. This is done by matching the amplitude of the quantum black hole processes to the extrapolated semi-classical cross section. All possible decay channels and their probabilities are found for quantum black holes with a continuous and discrete mass spectrum, respectively, by considering different symmetry conservation restrictions for a quantum gravitational theory. In conjunction with these branching ratios, we developed a Monte Carlo integration algorithm to determine the cross sections of specific final states. We extended the algorithm to investigate the enhancement of supersymmetric particle production via quantum black hole processes. Studying such objects proves very important, since it provides new possible insights and restrictions on the quantum black hole model and likewise on the low-scale quantum gravity scenarios.

Inflation is the leading candidate to explain the initial conditions for the Universe we see today. It consists of an epoch of accelerated expansion, and regularly solves many problems with the Big Bang theory. Non-Gaussianity of the primordial curvature perturbation can potentially be used to discriminate between competing models and provide an understanding of the mechanism of inflation. Whilst inflation is believed to have lasted at least 50 - 60 e-folds, constraints from sources such as the cosmic microwave background (CMB) or large-scale structure of the Universe (LSS) only span the largest 6 - 10 e-folds inside today's Hubble horizon, limiting our ability to constrain the early universe. Strong constraints on the non-Gaussianity on smaller scales. Primordial black holes (PBHs) represent a unique probe to study the small-scale early Universe, placing an upper limit on the primordial power spectrum spanning around 40 e-folds smaller than those visible in the CMB. PBHs are also a viable dark matter candidate. In this thesis, the effect of non-Gaussianity upon the abundance of PBHs, and the implications of such an effect are considered. It is shown that even smaller non-Gaussianity parameters can have a large effect on the constraints that can be placed on the primordial curvature perturbation power spectrum - which can become stronger or weaker by an order of magnitude. The effects of super-horizon curvature perturbation modes at the time of PBH formation are considered, and it is shown that these have little effect on the formation of a PBH, but can have an indirect effect on the abundance of PBHs due to modal coupling to horizon-scale modes in the presence of non-Gaussianity. By taking into account the effect of modal coupling to CMB-scale modes, many models can be ruled out as a mechanism to produce enough PBHs to constitute dark matter.

The subject of this thesis is the description of quantum black holes as a way to probe quantum gravity. Scenarios of a lower Planck scale make quantum gravity experimentally approachable, therefore a theoretical framework is needed in order to be able to probe quantum gravitational effects. We present a field theoretical formalism for the treatment of quantum black holes and their interactions with particles of the Standard Model. We examine the properties and assumptions governing quantum black holes and develop a methodology to examine their behavior using quantum field theory language. We apply our formalism in several different cases and calculate the cross sections for the production of quantum black holes. We use our results to gain phenomenological insights to quantum gravity, such as the derivation of bounds for the Planck mass from Standard Model processes. The distinction between a continuous and a discrete mass spectrum, for a quantum black hole, is discussed and the relevant cross sections presented. Finally, we use quantum black holes as a gateway to supersymmetry and calculate the branching ratios for the decay of quantum black holes into supersymmetric particles.