5,443 research outputs found

### 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

### 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

### Scalar perturbations in regular two-component bouncing cosmologies

We consider a two-component regular cosmology bouncing from contraction to
expansion, where, in order to include both scalar fields and perfect fluids as
particular cases, the dominant component is allowed to have an intrinsic
isocurvature mode. We show that the spectrum of the growing mode of the Bardeen
potential in the pre-bounce is never transferred to the dominant mode of the
post-bounce. The latter acquires at most a dominant isocurvature component,
depending on the relative properties of the two fluids. Our results imply that
several claims in the literature need substantial revision.Comment: 10 pages, 1 figur

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