Nature of Microscopic Black Holes and Gravity in Theories with Particle Species

Relying solely on unitarity and the consistency with large-distance black hole physics, we derive model-independent properties of the microscopic black holes and of short-distance gravity in theories with N particle species. In this class of theories black holes can be as light as M_{Planck}/\sqrt{N} and be produced in particle collisions above this energy. We show, that the micro black holes must come in the same variety as the species do, although their label is not associated with any conserved charge measurable at large distances. In contrast with big Schwarzschildian ones, the evaporation of the smallest black holes is maximally undemocratic and is biased in favor of particular species. With an increasing mass the democracy characteristic to the usual macro black holes is gradually regained. The lowest possible mass above which black holes become Einsteinian is \sqrt{N} M_{Planck}. This fact uncovers the new fundamental scale (below the quantum gravity scale) above which gravity changes classically, and the properties of black holes and gravity are such as if some extra dimensions open up, although no such input exists in the theory. Our observations indicate that geometry of space may be an emergent concept following from large number of species and the consistency with macro black hole physics. We apply our findings to the phenomenological properties of the micro black holes that can be observed at LHC for large N. Extrapolating our findings to small N, one may ask whether the existence of quark and lepton flavors is already an evidence for emergent extra dimensions at short distances.Comment: 14 page

A supersymmetric matrix model: II. Exploring higher-fermion-number sectors

Continuing our previous analysis of a supersymmetric quantum-mechanical matrix model, we study in detail the properties of its sectors with fermion number F=2 and 3. We confirm all previous expectations, modulo the appearance, at strong coupling, of {\it two} new bosonic ground states causing a further jump in Witten's index across a previously identified critical 't Hooft coupling $\lambda_c$. We are able to elucidate the origin of these new SUSY vacua by considering the $\lambda \to \infty$ limit and a strong coupling expansion around it.Comment: 14 pages, 4 figure

String Cosmology: Basic Ideas and General Results

After recalling a few basic concepts from cosmology and string theory, I will outline the main ideas/assumptions underlying (our own group's approach to) string cosmology and show how these lead to the definition of a two-parameter family of ``minimal" models. I will then briefly explain how to compute, in terms of those parameters, the spectrum of scalar, tensor and electromagnetic perturbations, and mention their most relevant physical consequences. More details on the latter part of this talk can be found in Maurizio Gasperini's contribution to these proceedings.Comment: Latex file + 3 figures. Talk presented at the 3rd Colloque Cosmologie, Paris, 7-9 June 9

Pre-bangian origin of our entropy and time arrow

I argue that, in the chaotic version of string cosmology proposed recently, classical and quantum effects generate, at the time of exit to radiation, the correct amount of entropy to saturate a Hubble (or holography) entropy bound (HEB) and to identify, within our own Universe, the arrow of time. Demanding that the HEB be fulfilled at all times forces a crucial "branch change" to occur, and the so-called string phase to end at a critical value of the effective Planck mass, in agreement with previous conjectures.Comment: Latex file, 8 pages, 1 Figur

Inflating, Warming Up, and Probing the Pre-Bangian Universe

Classical and quantum gravitational instabilities, can, respectively, inflate and warm up a primordial Universe satisfying a superstring-motivated principle of "Asymptotic Past Triviality". A physically viable big bang is thus generated without invoking either large fine-tunings or a long period of post-big bang inflation. Properties of the pre-bangian Universe can be probed through its observable relics, which include: i) a (possibly observable) stochastic gravitational-wave background; ii) a (possible) new mechanism for seeding the galactic magnetic fields; iii) a (possible) new source of large-scale structure and CMB anisotropy.Comment: Latex file, 17 pages, 5 figures, Based on two recent talks, References update

String Cosmology: Concepts and Consequences

After recalling a few basic concepts from cosmology and string theory, I will discuss the main ideas/assumptions underlying string cosmology and show how these lead to a two-parameter family of ``minimal" models. I will then explain how to compute, in terms of those parameters, the spectrum of scalar, tensor and electromagnetic perturbations, point at their ($T$ and $S$-type) duality symmetries, and mention their most relevant physical consequences.Comment: 21p, latex, epsf, 3 figures in uuencoded fil

A Simple/Short Introduction to Pre-Big-Bang Physics/Cosmology

A simple, non-technical introduction to the pre-big bang scenario is given, emphasizing physical motivations, considerations, and consequences over formalism.Comment: 15 pages, macros included, 4 figures, lecture presented at the Erice School of Subnuclear Physics, 35th Course "Highlights: 50 Years Later", Erice, Italy, August 26-September 4, 1997 revised version, references updated, fig. 3 more precisely describe

Entropy Bounds and String Cosmology

After discussing some old (and not-so-old) entropy bounds both for isolated systems and in cosmology, I will argue in favour of a "Hubble entropy bound" holding in the latter context. I will then apply this bound to recent developments in string cosmology, show that it is naturally saturated throughout pre-big bang inflation, and claim that its fulfilment at later times has interesting implications for the exit problem of string cosmology.Comment: Latex file, 11 pages, 3 figures, Talk given in honour of F. Englert, Brussels, March 199

Isospin Mixing of Narrow Pentaquark States

Interpreting the recently discovered narrow exotic baryons as pentaquark states, we discuss, along an old argument of ours, the isospin mixing occurring within the two doublets of $Q = -1$ and Q=0 states lying inside the $S=-2$ ($\Xi$-cascade) sector. We argue that, at least within the Jaffe-Wilczek assignment, presently available data already indicate that mixing should occur at an observable level in both charge sectors, with mixing angles that can be predicted in terms of ratios of observable mass splittings.Comment: 11 pages, 2 figures, to be submitted to PL

From Super-Yang-Mills Theory to QCD: Planar Equivalence and its Implications

We review and extend our recent work on the planar (large N) equivalence between gauge theories with varying degree of supersymmetry. The main emphasis is made on the planar equivalence between N=1 gluodynamics (super-Yang-Mills theory) and a non-supersymmetric "orientifold field theory." We outline an "orientifold" large N expansion, analyze its possible phenomenological consequences in one-flavor massless QCD, and make a first attempt at extending the correspondence to three massless flavors. An analytic calculation of the quark condensate in one-flavor QCD starting from the gluino condensate in N=1 gluodynamics is thoroughly discussed. We also comment on a planar equivalence involving N=2 supersymmetry, on "chiral rings" in non-supersymmetric theories, and on the origin of planar equivalence from an underlying, non-tachyonic type-0 string theory. Finally, possible further directions of investigation, such as the gauge/gravity correspondence in large-N orientifold field theory, are briefly discussed.Comment: 106 pages, LaTex. 15 figures. v2:minor changes, refs. added. To be published in the Ian Kogan Memorial Collection "From Fields to Strings: Circumnavigating Theoretical Physics," World Scientific, 200