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We study general classes and properties of extremal and non-extremal static black-hole solutions of N=2, d=5 supergravity coupled to vector multiplets using the recently proposed H-FGK formalism, which we also extend to static black…

In this Chapter we would like to review a "~phenomenological~" approach taking into account the most fundamental feature of string theory or, more in general, of quantum gravity, whatever its origin, which is the existence of a minimal…

We analyze the spacetime metric associated with a black string surrounded by a cloud of strings and an anisotropic fluid of quintessence in cylindrically symmetric AdS spacetime. We solve Einstein's equation to obtain the explicit form of…

In a previous paper we had proposed a specific route to relating the entropy of two charge black holes to the degeneracy of elementary string states in N=4 supersymmetric heterotic string theory in four dimensions. For toroidal…

We identify new, rather serious, physical and axiomatic inconsistencies of the current formulation of string theories due to the lack of invariant units necessary for measurements, lack of preservation in time of Hermiticity-observability,…

I solve the equations of the low-energy limit of string theory to obtain a solution corresponding to a microcanonical ensemble of highly-excited superstrings. This ``Superball of Strings'' is a static, spherically symmetric ``fuzzball'' of…

We show that polarization dependent string-string scattering provides new evidence for the identification of the Dabholkar-Harvey (DH) string solution with the heterotic string itself. First, we construct excited versions of the DH solution…

We consider nonuniform black strings inside their event horizon. We present numerical evidence, that the singularity touches the horizon as the horizon topology changing transition is reached.

We find a large class of new supersymmetric $AdS_5$ black strings from five-dimensional $N=4$ gauged supergravity coupled to five vector multiplets with $SO(2)_D\times SO(3)\times SO(3)$ gauge group. These solutions have near horizon…

We derive the solution for a spherically symmetric string cloud configuration in a d-dimensional spacetime in the framework of f(R) theories of gravity. We also analyze some thermodynamic properties of the joint black hole - cloud of…

We begin the study of a class of string-motivated effective supergravity theories in light of current data from the CERN Large Hadron Collider (LHC). The case of heterotic string theory, in which the dilaton is stabilized via…

The motion of massive particles in the background of a charged black hole in heterotic string theory, which is characterized by a parameter $\alpha$, is studied in detail in this paper. Since it is possible to write this space-time in the…

Cosmic strings are linear topological defects which are hypothesized to be produced during inflation. Most searches for strings have been relying on the string's lensing of background galaxies or CMB. In this paper I obtained the solution…

We discuss dark energy, dark matter and the hierarchy problem in the context of a general non-commutative formulation of string theory. In this framework dark energy is generated by the dynamical geometry of the dual spacetime while dark…

We show that the entropy of any black object in any dimension can be understood as the entropy of a highly excited string on the stretched horizon. The string has a gravitationally renormalized tension due to the large redshift near the…

The coupling of a string to gravity allows for Schwarzschild black holes whose entropy to area relation is $S=(A/4)(1-4\mu)$, where $\mu$ is the string tension. This departure from the A/4 universality results from a string instanton…

We find the general five-dimensional, supersymmetric black ring solutions in M-theory based upon a circular ring, but with arbitrary, fluctuating charge distributions around the ring. The solutions have three arbitrary charge distribution…

We construct the first rotating string solution in 6-dimensional Einstein-Gauss-Bonnet supergravity, carrying both electric and magnetic charges. By embedding the known rotating string solution of the 2-derivative theory into 6-dimensional…

The string propagation in the two-dimensional stringy black-hole is investigated from a new approach. We completely solve the classical and quantum string dynamics in the lorentzian and euclidean regimes. In the lorentzian case all the…

We explore the formation and the evolution of the string network in the Abelian Higgs model with two complex scalar fields. A special feature of the model is that it possesses a global $U(1)$ symmetry in addition to the $U(1)$ gauge…