Abstract:
In this Ph.D. thesis, we study some aspects of strings and branes. We mainly focus on anomalous D-brane couplings and on non-supersymmetric branes. In Chapter 2, we illustrate three key concepts in a field theory context: duality, supersymmetry and anomaly inflow. Chapter 3 contains an introduction to string theory, emphasizing the concepts relevant to this thesis, like string scattering amplitudes and D-brane actions. In Chapter 4, we introduce the boundary state formalism and use it to check the presence of anomalous couplings in the D-brane Wess-Zumino action. In addition, we show that this action also contains non-anomalous terms. Chapter 5 gives a BPS analysis of D-branes in D-brane backgrounds, focusing on the baryon vertex in the AdS/CFT correspondence. Chapter 6 is devoted to branes in type 0 string theory. We derive the D-brane Wess-Zumino action and the massless spectra of NS-fivebranes, and combine these ingredients to comment on type 0B S-duality. In Chapter 7, we study non-BPS D-branes in type II string theory. We propose a Wess-Zumino action for these branes, check it by computing string scattering amplitudes and show that it is consistent with the interpretation of BPS D-branes as topologically non-trivial tachyon configurations on a non-BPS D-brane.

Abstract:
We compute scattering amplitudes involving both R-R and NS-NS fields in the presence of a D-brane or orientifold plane. These provide direct evidence for the anomalous couplings in the D-brane and orientifold actions. The D9-brane and O9-plane are found to couple to the first Pontrjagin class with the expected relative strength.

Abstract:
The direct string computation of anomalous D-brane and orientifold plane couplings is extended to include the curvature of the normal bundle. The normalization of these terms is fixed unambiguously. New, non-anomalous gravitational couplings are found.

Abstract:
The massless degrees of freedom of type 0 NS5-branes are derived. A non-chiral, purely bosonic spectrum is found in both type 0A and 0B. This non-chirality is confirmed by a one-loop computation in the bulk. Some puzzles concerning type 0B S-duality are pointed out in this context. An interpretation of the spectra in terms of ``type 0 little strings'' is proposed.

Abstract:
We briefly review the topic of AdS (in)stability, mainly focusing on a recently introduced analytic approach and its interplay with numerical results.

Abstract:
We study Big Crunch/Big Bang cosmologies that correspond to exact world-sheet superconformal field theories of type II strings. The string theory spacetime contains a Big Crunch and a Big Bang cosmology, as well as additional ``whisker'' asymptotic and intermediate regions. Within the context of free string theory, we compute, unambiguously, the scalar fluctuation spectrum in all regions of spacetime. Generically, the Big Crunch fluctuation spectrum is altered while passing through the bounce singularity. The change in the spectrum is characterized by a function $\Delta$, which is momentum and time-dependent. We compute $\Delta$ explicitly and demonstrate that it arises from the whisker regions. The whiskers are also shown to lead to ``entanglement'' entropy in the Big Bang region. Finally, in the Milne orbifold limit of our superconformal vacua, we show that $\Delta\to 1$ and, hence, the fluctuation spectrum is unaltered by the Big Crunch/Big Bang singularity. We comment on, but do not attempt to resolve, subtleties related to gravitational backreaction and light winding modes when interactions are taken into account.

Abstract:
The consistency of the orbifold action on open strings between D-branes in orbifold theories with and without discrete torsion is analysed carefully. For the example of the C^3/Z_2 x Z_2 theory, it is found that the consistency of the orbifold action requires that the D-brane spectrum contains branes that give rise to a conventional representation of the orbifold group as well as branes for which the representation is projective. It is also shown how the results generalise to the orbifolds C^3/Z_N x Z_N for which a number of novel features arise. In particular, the N>2 theories with minimal discrete torsion have non-BPS branes charged under twisted R-R potentials that couple to none of the (known) BPS branes.

Abstract:
We study loop corrections in boundary string field theory (BSFT). After commenting on problems with quantizing the tree level BSFT as an ordinary field theory, we discuss the tree level coupling to closed strings and define the loop corrections via factorization in the closed string channel. This description is weakly coupled in the vicinity of the closed string vacuum. Our proposal for the one-loop effective action differs in general from computing the annulus or cylinder partition functions. We also compute the decay rates and the loop corrections to the tensions of unstable branes in perturbative string theory.

Abstract:
We continue our analytic investigations of non-linear spherically symmetric perturbations around the anti-de Sitter background in gravity-scalar field systems, and focus on conservation laws restricting the (perturbatively) slow drift of energy between the different normal modes due to non-linearities. We discover two conservation laws in addition to the energy conservation previously discussed in relation to AdS instability. A similar set of three conservation laws was previously noted for a self-interacting scalar field in a non-dynamical AdS background, and we highlight the similarities of this system to the fully dynamical case of gravitational instability. The nature of these conservation laws is best understood through an appeal to averaging methods which allow one to derive an effective Lagrangian or Hamiltonian description of the slow energy transfer between the normal modes. The conservation laws in question then follow from explicit symmetries of this averaged effective theory.

Abstract:
We study how non-BPS type II D-branes couple to R-R potentials. Upon tachyon condensation the couplings we find give rise to the Wess-Zumino action of BPS D-branes.