6,286 research outputs found
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
String Theory and Pre-big bang Cosmology
In string theory, the traditional picture of a Universe that emerges from the
inflation of a very small and highly curved space-time patch is a possibility,
not a necessity: quite different initial conditions are possible, and not
necessarily unlikely. In particular, the duality symmetries of string theory
suggest scenarios in which the Universe starts inflating from an initial state
characterized by very small curvature and interactions. Such a state, being
gravitationally unstable, will evolve towards higher curvature and coupling,
until string-size effects and loop corrections make the Universe "bounce" into
a standard, decreasing-curvature regime. In such a context, the hot big bang of
conventional cosmology is replaced by a "hot big bounce" in which the bouncing
and heating mechanisms originate from the quantum production of particles in
the high-curvature, large-coupling pre-bounce phase. Here we briefly summarize
the main features of this inflationary scenario, proposed a quarter century
ago. In its simplest version (where it represents an alternative and not a
complement to standard slow-roll inflation) it can produce a viable spectrum of
density perturbations, together with a tensor component characterized by a
"blue" spectral index with a peak in the GHz frequency range. That means,
phenomenologically, a very small contribution to a primordial B-mode in the CMB
polarization, and the possibility of a large enough stochastic background of
gravitational waves to be measurable by present or future gravitational wave
detectors.Comment: 25 pages, five figures. Contribution to the special issue of IL NUOVO
CIMENTO, published in honor of Gaetano Vilasi on the occasion of his 70-th
birthday (Il Nuovo Cimento C, Italian Physical Society, 2015
Squeezed Thermal Vacuum and the Maximum Scale for Inflation
We consider the stimulated emission of gravitons from an initial state of
thermal equilibrium, under the action of the cosmic gravitational background
field. We find that the low-energy graviton spectrum is enhanced if compared
with spontaneous creation from the vacuum; as a consequence, the scale of
inflation must be lowered, in order not to exceed the observed CMB quadrupole
anisotropy. This effect is particularly important for models based on a
symmetry-breaking transition which require, as initial condition, a state of
thermal equilibrium at temperatures of the order of the inflation scale.Comment: 13 pages, plain tex, three figures available upon request, to appear
in Phys.Rev.D, CERN-TH.6836/9
Primordial Magnetic Fields From String Cosmology
Sufficiently large seeds for generating the observed (inter)galactic magnetic
fields emerge naturally in string cosmology from the amplification of
electromagnetic vacuum fluctuations due to a dynamical dilaton background. The
success of the mechanism depends crucially on two features of the so-called
pre-big-bang scenario, an early epoch of dilaton-driven inflation at very small
coupling, and a sufficiently long intermediate stringy era preceding the
standard radiation-dominated evolution.Comment: 12 pages, latex, two figures available by fax upon reques
Exact Results in Non-Supersymmetric Large N Orientifold Field Theories
We consider non-supersymmetric large N orientifold field theories.
Specifically, we discuss a gauge theory with a Dirac fermion in the
anti-symmetric tensor representation. We argue that, at large N and in a large
part of its bosonic sector, this theory is non-perturbatively equivalent to N=1
SYM, so that exact results established in the latter (parent) theory also hold
in the daughter orientifold theory. In particular, the non-supersymmetric
theory has an exactly calculable bifermion condensate, exactly degenerate
parity doublets, and a vanishing cosmological constant (all this to leading
order in 1/N).Comment: 18 pages, Latex. 3 figures. v2,v3: minor changes, refs. added, to
appear in Nucl.Phys.
Quintessence as a run-away dilaton
We consider a late-time cosmological model based on a recent proposal that
the infinite-bare-coupling limit of superstring/M-theory exists and has good
phenomenological properties, including a vanishing cosmological constant, and a
massless, decoupled dilaton. As it runs away to , the dilaton can
play the role of the quintessence field recently advocated to drive the
late-time accelerated expansion of the Universe. If, as suggested by some
string theory examples, appreciable deviations from General Relativity persist
even today in the dark matter sector, the Universe may smoothly evolve from an
initial "focusing" stage, lasting untill radiation--matter equality, to a
"dragging" regime, which eventually gives rise to an accelerated expansion with
frozen .Comment: 31 pages, latex, 5 figures included using epsfig. New references
added and misprints corrected. To appear in Phys. Rev.
Assisting pre-big bang phenomenology through short-lived axions
We present the results of a detailed study of how isocurvature axion
fluctuations are converted into adiabatic metric perturbations through axion
decay, and discuss the constraints on the parameters of pre-big bang cosmology
needed for consistency with present CMB-anisotropy data. The large-scale
normalization of temperature fluctuations has a non-trivial dependence both on
the mass and on the initial value of the axion. In the simplest, minimal models
of pre-big bang inflation, consistency with the COBE normalization requires a
slightly tilted (blue) spectrum, while a strictly scale-invariant spectrum
requires mild modifications of the minimal backgrounds at large curvature
and/or string coupling.Comment: 14 pages, latex, 1 figure included using epsfig. A few typos
corrected, two references added, the figure slightly improved. To appear in
Phys. Lett.
Constraints on pre-big bang parameter space from CMBR anisotropies
The so-called curvaton mechanism --a way to convert isocurvature
perturbations into adiabatic ones-- is investigated both analytically and
numerically in a pre-big bang scenario where the role of the curvaton is played
by a sufficiently massive Kalb--Ramond axion of superstring theory. When
combined with observations of CMBR anisotropies at large and moderate angular
scales, the present analysis allows us to constrain quite considerably the
parameter space of the model: in particular, the initial displacement of the
axion from the minimum of its potential and the rate of evolution of the
compactification volume during pre-big bang inflation. The combination of
theoretical and experimental constraints favours a slightly blue spectrum of
scalar perturbations, and/or a value of the string scale in the vicinity of the
SUSY-GUT scale.Comment: 63 pages in Latex style with 14 figures include
Scalar fluctuations in dilatonic brane-worlds
We derive and solve the full set of scalar perturbation equations for a class
of five-dimensional brane--world solutions, with a dilaton scalar field coupled
to the bulk cosmological constant and to a 3-brane. The spectrum contains one
localized massless scalar mode, to be interpreted as an effective dilaton on
the brane, inducing long--range scalar interactions. Two massive scalar modes
yield corrections to Newton's law at short distances, which persist even in the
limit of vanishing dilaton (namely, in the standard Randall--Sundrum
configuration).Comment: 10 pages. Talk presented by V. Bozza at COSMO-01 conference,
Rovaniemi, 200
Gravitational Radiation from String Cosmology
A spectrum of relic stochastic gravitational radiation, strongly tilted
towards high frequencies, and characterized by two basic parameters is shown to
emerge in a class of string theory models. We estimate the required sensitivity
for detection of the predicted gravitational radiation and show that a region
of our parameter space is within reach for some of the
plannedgravitational-wave detectors.Comment: 4 pages, uuencoded p
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