12 research outputs found
The String Density of States from The Convolution Theorem
We study the microcanonical density of states and the thermal properties of a
bosonic string gas starting from a calculation of the Helmholtz free energy in
the S-representation. By adding more and more strings to the single string
system, we induce that, for infinite volume, there is no negative specific heat
region but a transition at a finite value of the energy per string from the low
energy regime to a region of infinite specific heat at the Hagedorn
temperature. Forcing the description of this phase in terms of strings gives a
picture in which there is a very fat string in a sea of low energetic ones. We
argue that the necessary changing of this description should not change the
fact that perturbatively is a maximum temperature of the system.Comment: 15 pages, LaTeX, epsf.sty and a4.sty, 5 figures in PostScrip
Counting Closed String States in a Box
The computation of the microcanonical density of states for a string gas in a
finite volume needs a one by one count because of the discrete nature of the
spectrum. We present a way to do it using geometrical arguments in phase space.
We take advantage of this result in order to obtain the thermodynamical
magnitudes of the system. We show that the results for an open universe exactly
coincide with the infinite volume limit of the expression obtained for the gas
in a box. For any finite volume the Hagedorn temperature is a maximum one, and
the specific heat is always positive. We also present a definition of pressure
compatible with R-duality seen as an exact symmetry, which allows us to make a
study on the physical phase space of the system. Besides a maximum temperature
the gas presents an asymptotic pressure.Comment: 16 pages, 5 Postscript figure
Quantum Corrections in Two-Dimensional Non-Supersymmetric Heterotic Strings
We study quantum corrections for a family of 24 non-supersymmetric heterotic
strings in two dimensions. We compute their genus two cosmological constant
using the hyperelliptic formalism and the genus one two-point functions for the
massless states. From here we get the mass corrections to the states in the
massless sector and discuss the role of the infrared divergences that appear in
the computation. We also study some tree-level aspects of these theories and
find that they are classified not only by the corresponding Niemeier lattice
but also by their {\it hidden} right-moving gauge symmetry.Comment: Minor changes. The discussion on the infrared divergences has been
expanded and typos have been corrected. Final version to be published in
Nucl. Phys. B. 40 pages. LaTeX, epsf. 4 Postscript figures in uuencoded fil
Thermodynamic nonextensivity in a closed string gas
Well known results in string thermodynamics show that there is always a
negative specific heat phase in the microcanonical description of a gas of
closed free strings whenever there are no winding modes present. We will
carefully compute the number of strings in the gas to show how this negative
specific heat is related to the fact that the system does not have
thermodynamic extensivity. We will also discuss the consequences for a system
of having a microcanonical negative specific heat versus the exact result that
such a thing cannot happen in any canonical (fixed temperature) description.Comment: Title + 13 pages, more typos correcte
Finite Temperature Matrix Theory
We present the way the Lorentz invariant canonical partition function for
Matrix Theory as a light-cone formulation of M-theory can be computed. We
explicitly show how when the eleventh dimension is decompactified, the N = 1
eleven dimensional SUGRA partition function appears. We also provide a high
temperature expansion which captures some structure of the canonical partition
function when interactions amongst D-particles are on. The connection with the
semi-classical computations thermalizing the open superstrings attached to a
D-particle is also clarified through a Born-Oppenheimer approximation. Some
ideas about how Matrix Theory would describe the complementary degrees of
freedom of the massless content of eleven dimensional SUGRA are discussed.
Comments about possible connections to black hole physics are also made.Comment: 32 pages. LaTeX, 1 epsf figure. Corrections and changes made in
Section 4. One reference adde
Unconventional Cosmology
I review two cosmological paradigms which are alternative to the current
inflationary scenario. The first alternative is the "matter bounce", a
non-singular bouncing cosmology with a matter-dominated phase of contraction.
The second is an "emergent" scenario, which can be implemented in the context
of "string gas cosmology". I will compare these scenarios with the inflationary
one and demonstrate that all three lead to an approximately scale-invariant
spectrum of cosmological perturbations.Comment: 45 pages, 10 figures; invited lectures at the 6th Aegean Summer
School "Quantum Gravity and Quantum Cosmology", Chora, Naxos, Greece, Sept.
12 - 17 2012, to be publ. in the proceedings; these lecture notes form an
updated version of arXiv:1003.1745 and arXiv:1103.227
A cosmological interpretation of duality
We study the cosmological meaning of duality symmetry by considering a two dimensional model of string cosmology. We find that as seen by an internal observer in this universe, the scale factor rebounds at the self-dual length. This rebound is a consequence of the adiabatic expansion. Furthermore, in this situation there are four mathematically different scenarios which describe physically equivalent universes which are in fact indistinguishable. We also stress that -duality suffices to prove that all possible evolutions present a maximum temperature.We study the cosmological meaning of duality symmetry by considering a two dimensional model of string cosmology. We find that as seen by an internal observer in this universe, the scale factor rebounds at the self-dual length. This rebound is a consequence of the adiabatic expansion. Furthermore, in this situation there are four mathematically different scenarios which describe physically equivalent universes which are in fact undistinguishable. We also stress that -duality suffices to prove that all the possible evolutions present a maximun temperature.We study the cosmological meaning of duality symmetry by considering a two dimensional model of string cosmology. We find that as seen by an internal observer in this universe, the scale factor rebounds at the self-dual length. This rebound is a consequence of the adiabatic expansion. Furthermore, in this situation there are four mathematically different scenarios which describe physically equivalent universes which are in fact undistinguishable. We also stress that R -duality suffices to prove that all the possible evolutions present a maximum temperature
Minimalist optical fiber design: capillary-like fibers
Microstructured optical fibers with ultra-simplified designs are investigated. Guiding mechanism can be achieved in an embedded or surface germanium-doped core or in the hollow part of capillaries. Fiber wall thickness and/or stress induced birefringence can be modified with external stimuli. Results showing this interesting platform for sensing of parameters such as pressure, temperature, refractive index and directional curvature will be presented.CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTĂFICO E TECNOLĂGICO - CNPQFUNDAĂĂO DE AMPARO Ă PESQUISA DO ESTADO DE SĂO PAULO - FAPESPSem informação2014/50632-6SBFoton International Optics and Photonics Conference2018-10-08Campinas, S