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We consider thorny spheres, that is 2-dimensional compact surfaces which are everywhere locally isometric to a round sphere $S^2$ except for a finite number of isolated points where they have conical singularities. We use thorny spheres to…
We consider solutions of six dimensional Einstein equations with two compact dimensions. It is shown that one can introduce 3-branes in this background in such a way that the effective four dimensional cosmological constant is completely…
We calculate the emission and absorption rates of fixed scalars by the near-extremal five-dimensional black holes that have recently been modeled using intersecting D-branes. We find agreement between the semi-classical and D-brane…
We propose a twistor--like formulation of N=1, D=3,4,6 and 10 null superstrings. The model possesses N=1 target space supersymmetry and n=D-2 local worldsheet supersymmetry, the latter replaces the kappa-symmetry of the conventional…
Four-dimensional string theories predict in general the existence of light exotic particles with fractional electric charges. Such particles could escape present observations if they are confined by a gauge group of the "hidden" sector into…
We develop ambitwistor string theories for 4 dimensions to obtain new formulae for tree-level gauge and gravity amplitudes with arbitrary amounts of supersymmetry. Ambitwistor space is the space of complex null geodesics in complexified…
We present Poincare invariant domain wall (``3-brane'') solutions to some 5-dimensional effective theories which can arise naturally in string theory. In particular, we find theories where Poincare invariant solutions exist for arbitrary…
Quantifying tensions -- inconsistencies amongst measurements of cosmological parameters by different experiments -- has emerged as a crucial part of modern cosmological data analysis. Statistically-significant tensions between two…
We study brane-world models and demonstrate that such models do not admit self-similar solutions through the matter collineation approach. By introducing the hypothesis of variable brane tension, $\lambda,$ we outline the new effective…
We argue for a relation between the supersymmetry breaking scale and the measured value of the dark energy density $\Lambda$. We derive it by combining two quantum gravity consistency swampland constraints, which tie the dark energy density…
We study the relationship between space-time-matter (STM) and brane theories. These two theories look very different at first sight, and have different motivation for the introduction of a large extra dimension. However, we show that they…
We study the dynamics of a coherent state of closed type II string gravitons within the framework of the Steepest Entropy Ascent Quantum Thermodynamics, an effective model where the quantum evolution is driven by a maximal increase of…
There is a tension between measurements of the amplitude of the power spectrum of density perturbations inferred using the Cosmic Microwave Background (CMB) and directly measured by Large-Scale Structure (LSS) on smaller scales. We show…
Motivated by the recent interest on models with varying constants and whether black hole physics can constrain such theories, two-dimensional charged stringy black holes are considered. We exploit the role of two-dimensional stringy black…
The spatial string tension of a SU(N) gauge theory without quarks is calculated using gauge/string duality for 1.2T_c<T<3T_c. The result is remarkably consistent with the available lattice data for N=2,3. Some evidence is found that to…
The effective string describing the large distance behaviour of the quark sources of gauge field theories in the confining phase in D=3 or D=4 space-time dimensions can be formulated, in the infrared limit, as a suitable 2D conformal field…
In Double Field Theory, the mass-squared of doubled fields associated with bosonic closed string states is proportional to $N_L+N_R-2$. Massless states are therefore not only the graviton, anti-symmetric, and dilaton fields with $(N_L=1,…
We consider 5D supersymmetric black holes in string theory compactifications that partially break supersymmetry. We compactify type IIB on $T^4$ and then further compactify on a circle with a duality twist to give Minkowski vacua preserving…
We review and elaborate on properties of the string tension in two-dimensional gauge theories. The first model we consider is massive QED in the $m\ll e$ limit. We evaluate the leading string tension both in the fermionic and bosonized…
We show how the classical string dynamics in $D$-dimensional gravity background can be reduced to the dynamics of a massless particle constrained on a certain surface whenever there exists at least one Killing vector for the background…