arXiv subjects

Nick E. Mavromatos

Publications and source records attributed to Nick E. Mavromatos.

At least 145 records · Page 8Linked to original sources

arXiv2010

Stringy Space-Time Foam and High-Energy Cosmic Photons

In this review, I discuss briefly stringent tests of Lorentz-violating quantum space-time foam models inspired from String/Brane theories, provided by studies of high energy Photons from intense celestial sources, such as Active Galactic Nuclei or Gamma Ray Bursts. The theoretical models predict modifications to the radiation dispersion relations, which are quadratically suppressed by the string mass scale, and time delays in the arrival times of photons (assumed to be emitted more or less simultaneously from the source), which are proportional to the photon energy, so that the more energetic photons arrive later. Although the astrophysics at the source of these energetic photons is still not understood, and such non simultaneous arrivals, that have been observed recently, might well be due to non simultaneous emission as a result of conventional physics effects, nevertheless, rather surprisingly, the observed time delays can also fit excellently the stringy space-time foam scenarios, provided the space-time defect foam is inhomogeneous. The key features of the model, that allow it to evade a plethora of astrophysical constraints on Lorentz violation, in sharp contrast to other field-theoretic Lorentz-violating models of quantum gravity, are: (i) transparency of the foam to electrons and in general charged matter, (ii) absence of birefringence effects and (iii) a breakdown of the local effective lagrangian formalism.

gr-qc↗

arXiv2010

String Quantum Gravity, Lorentz-Invariance Violation and Gamma-Ray Astronomy

In the first part of the review, I discuss ways of obtaining Lorentz-Invariance-Violating (LIV) space-time foam in the modern context of string theory, involving brane world scenarios. The foamy structures are provided by lower-dimensional background brane defects in a D3-brane Universe, whose density is a free parameter to be constrained phenomenologically. Such constraining can be provided by high energy gamma-ray photon tests, including ultra-high energy/infrared photon-photon scattering. In the second part, I analyze the currently available data from MAGIC and FERMI Telescopes on delayed cosmic photon arrivals in this context. It is understood of course that conventional Astrophysics source effects, which currently are far from being understood, might be the dominant reason for the observed delayed arrivals. I also discuss how the stringent constraints from studies of synchrotron-radiation from distant Nebulae, absence of cosmic birefringence and non observation of ultra-high-energy cosmic photons can be accommodated within the aforementioned stringy space-time foam model. I argue that, at least within the currently available sets of astrophysical data, the stringy foam model can avoid all theses constraints in a natural range of the string coupling and mass scale. The key features are: (i) transparency of the foam to electrons and charged probes in general, (ii) absence of birefringence effects and (iii) a breakdown of the local effective lagrangian formalism. However, in order to accommodate, in this theoretical framework, the data of the FERMI satellite on the delayed arrival of photons from the short intense Gamma Ray Burst GRB 090510, in a way consistent with the findings of the MAGIC telescope, a non uniform density of brane foam defects must be invoked.

hep-th↗

arXiv2010

Stringy Space-Time Foam, Finsler-like Metrics and Dark Matter Relics

We discuss modifications of the thermal Dark Matter (DM) relic abundances in stringy cosmologies with D-particle space-time foamy backgrounds. As a result of back-reaction of massive DM on the background space-time, owing to its interaction with D-particle defects in the foam, quantum fluctuations are induced in the space-time metric. We demonstrate that these lead to the presence of extra source terms in the Boltzmann equation used to determine the thermal dark matter relic abundances. The source terms are determined by the specific form of the induced metric deformations; the latter depend on the momentum transfer of the DM particle during its interactions with the D-particle defects and so are akin to Finsler metrics. In the case of low string scales arising from large extra dimensions our results may have phenomenological implications for the search of viable supersymmetric models.

hep-th↗

arXiv2010

Comments on Ultra-High-Energy Photons and D-Foam models

We revisit the constraints that the non-observation of ultra-high-energy photons due to the GZK cutoff can impose on models of Lorentz violation in photon propagation, following recent work by Maccione, Liberati and Sigl [arXiv:1003.5468] that carries further an earlier analysis by the present authors (Phys. Rev. D 63,12402 (2001), [hep-th/0012216]). We argue that the GZK cutoff constraint is naturally evaded in the D-brane model of space-time foam presented recently by the present authors (Phys. Lett. B 665, 412 (2008) [arXiv:0804.3566]), in which Lorentz-violating effects on photon propagation are independent of possible effects during interactions. We also note a novel absorption mechanism that could provide a GZK-like cutoff for photons in low-scale string models.

astro-ph.HE↗

arXiv2010

Cosmological Constant and Local Gravity

We discuss the linearization of Einstein equations in the presence of a cosmological constant, by expanding the solution for the metric around a flat Minkowski space-time. We demonstrate that one can find consistent solutions to the linearized set of equations for the metric perturbations, in the Lorentz gauge, which are not spherically symmetric, but they rather exhibit a cylindrical symmetry. We find that the components of the gravitational field satisfying the appropriate Poisson equations have the property of ensuring that a scalar potential can be constructed, in which both contributions, from ordinary matter and $Λ> 0$, are attractive. In addition, there is a novel tensor potential, induced by the pressure density, in which the effect of the cosmological constant is repulsive. We also linearize the Schwarzschild-de Sitter exact solution of Einstein's equations (due to a generalization of Birkhoff's theorem) in the domain between the two horizons. We manage to transform it first to a gauge in which the 3-space metric is conformally flat and, then, make an additional coordinate transformation leading to the Lorentz gauge conditions. We compare our non-spherically symmetric solution with the linearized Schwarzschild-de Sitter metric, when the latter is transformed to the Lorentz gauge, and we find agreement. The resulting metric, however, does not acquire a proper Newtonian form in terms of the unique scalar potential that solves the corresponding Poisson equation. Nevertheless, our solution is stable, in the sense that the physical energy density is positive.

gr-qc↗

arXiv2009

D-Foam Phenomenology: Dark Energy, the Velocity of Light and a Possible D-Void

In a D-brane model of space-time foam, there are contributions to the dark energy that depend on the D-brane velocities and on the density of D-particle defects. The latter may also reduce the speeds of photons linearly with their energies, establishing a phenomenological connection with astrophysical probes of the universality of the velocity of light. Specifically, the cosmological dark energy density measured at the present epoch may be linked to the apparent retardation of energetic photons propagating from nearby AGNs. However, this nascent field of `D-foam phenomenology' may be complicated by a dependence of the D-particle density on the cosmological epoch. A reduced density of D-particles at redshifts z ~ 1 - a `D-void' - would increase the dark energy while suppressing the vacuum refractive index, and thereby might reconcile the AGN measurements with the relatively small retardation seen for the energetic photons propagating from GRB 090510, as measured by the Fermi satellite.

astro-ph.CO↗

arXiv2009

Probing Lorentz Violating (Stringy) Quantum Space-Time Foam

Quantum Space Time may be characterized by a plethora of novel phenomena, such as Lorentz violations and non-trivial refractive indices, stochastic metric fluctuation effects leading to decoherence of quantum matter and non-commutativity of space-time coordinates. In string theory, which is one of the major approaches to quantum gravity, such coordinate non-commutativities arise naturally in many instances. In the talk I review one such instance, which arises in the modern context of D-brane defects in the background space time, over which string matter propagates. This serves as a prototype of space-time foam in this context. I chose this model, over many others, because it may actually have some unique features that can be falsified experimentally either by means of high-energy astrophysical observations or in some particle-interferometers, such as neutral meson factories. In particular, the model may explain the recent observations of the FERMI Gamma-Ray Telescope on delayed emission of 30 GeV photons from a distant Gamma-Ray-Burst 090510, in agreement with previous observations from the MAGIC and HESS Telescopes, but can also lead to falsifiable predictions for quantum foam effects in forthcoming upgrades of certain "particle interferometers", such as neutral meson factories.

hep-th↗

arXiv2009

Flavour Condensates in Brane Models and Dark Energy

In the context of a microscopic model of string-inspired foam, in which foamy structures are provided by brany point-like defects (D-particles) in space-time, we discuss flavour mixing as a result of flavour non-preserving interactions of (low-energy) fermionic stringy matter excitations with the defects. Such interactions involve splitting and capture of the matter string state by the defect, and subsequent re-emission. Quantum fluctuations of the D-particles induce a non-trivial space-time background; in some circumstances this could be akin to a cosmological Friedman-Robertson Walker expanding-Universe, with weak (but non-zero) particle production. Furthermore the D-particle medium can induce an MSW type effect. We have argued previously, in the context of bosons, that the so-called flavour vacuum is the appropriate state to be used, at least for low-energy excitations, with energies/momenta up to a dynamically determined cutoff scale. In this work we evaluate the flavour-vacuum expectation value (condensate) of the stress-energy tensor of the (1/2)-spin fields with mixing in an effective low-energy Quantum Field Theory in this foam-induced curved space-time. We demonstrate, at late epochs of the Universe, that the fermionic vacuum condensate behaves as a fluid with negative pressure and positive energy, but alone it cannot lead to present-day accelerating Universes. One needs flavoured boson contributions for this purpose.

hep-th↗

arXiv2009

Incompatibility of Rotation Curves with Gravitational Lensing for TeVeS

We constrain the one-parameter class of TeVeS models by testing the theory against both rotation curve and strong gravitational lensing data on galactic scales, remaining fully relativistic in our formalism. The upshot of our analysis is that -- at least in its simplest original form, which is the only one studied in the literature so far -- TeVeS is ruled out, in the sense that the models cannot consistently fit simultaneously the two sets of data without including a significant dark matter component. It is also shown that the details of the underlying cosmological model are not relevant for our analysis, which pertains to galactic scales. The choice of the stellar Initial Mass Function -- which affects the estimates of baryonic mass -- is found not to change our conclusions.

astro-ph.GA↗

arXiv2009

Time Delays of Strings in D-particle Backgrounds and Vacuum Refractive Indices

Using standard techniques in string/D-brane scattering amplitude computations, we evaluate the scattering of open strings off D-particles in brane world scenarios. The D-particles are viewed as D3 branes wrapped up around three cycles, and their embedding in brane worlds constitutes a case of intersecting branes, among which strings are stretched, representing various types of excitations of the Standard Model (SM) particles in the low-energy limit. Our analysis, reveals interesting and novel selection rules for the resulting causal time delays, proportional to the energy of the incident matter state, from the processes of splitting, capture and re-emission of the latter by the D-particles. In particular, we show that there are relatively large time delays only for excitations that belong to the Cartan subalgebra of the SM gauge group, which notably includes photons. We discuss the possible relevance of these results to the models of space time foam predicting a non-trivial vacuum refractive index for photons and the associated cosmic γ-ray phenomenology. In particular, we demonstrate how low-string-scale models can be falsified already in this context, by current astrophysical observations of cosmic photons, and how such observations serve as a way to discriminate low- from high-string scale models.

hep-th↗

arXiv2009

Decoherence and CPT Violation in a Stringy Model of Space-Time Foam

I discuss a model inspired from the string/brane framework, in which our Universe is represented as a three brane, propagating in a bulk space time punctured by D0-brane (D-particle) defects. As the D3-brane world moves in the bulk, the D-particles cross it, and from an effective observer on D3 the situation looks like a ``space-time foam'' with the defects ``flashing'' on and off (``D-particle foam''). The open strings, with their ends attached on the brane, which represent matter in this scenario, can interact with the D-particles on the D3-brane universe in a topologically non-trivial manner, involving splitting and capture of the strings by the D0-brane defects. Such processes are described by logarithmic conformal field theories on the world-sheet. Physically, they result in effective decoherence of the string matter on the D3 brane, and as a result, of CPT Violation, but of a type that implies an ill-defined nature of the effective CPT operator. Due to electric charge conservation, only electrically neutral (string) matter can exhibit such interactions with the D-particle foam. This may have unique, experimentally detectable, consequences for electrically-neutral entangled quantum matter states on the brane world, in particular the modification of the pertinent EPR Correlation of neutral mesons in a meson factory.

hep-th↗

arXiv2009

CPT Violation and Decoherence in Quantum Gravity

In this brief review I discuss ways and tests of CPT-Violation in the context of quantum gravity theories with space-time foam vacua, which entail quantum decoherence of matter propagating in such backgrounds. I cover a wide variety of sensitive probes, ranging from cosmic neutrinos to meson factories. I pay particular emphasis on associating the latter with specific, probably unique ("smoking-gun"), effects of this type of CPT Violation, related to a modification of Einstein-Podolsky-Rosen (EPR) correlations in the entangled states of the relevant neutral mesons. I also present some semi-microscopic estimates of these latter effects, in the context of a specific string-inspired model of space-time foam ("D-particle foam").

hep-ph↗

arXiv2009

Non-extensive statistics in stringy space-time foam models and entangled meson states

The possibility of generation of non-extensive statistics, in the sense of Tsallis, due to space-time foam is discussed within the context of a particular kind of foam in string/brane-theory, the D-particle foam model . The latter involves point-like brane defects (D-particles), which provide the topologically non-trivial foamy structures of space-time. A stochastic Langevin equation for the velocity recoil of D-particles can be derived from the pinched approximation for a sum over genera in the calculation of the partition function of a bosonic string in the presence of heavy D-particles. The string coupling in standard perturbation theory is related to the exponential of the expectation of the dilaton. Inclusion of fluctuations of the dilaton itself and uncertainties in the string background will then necessitate fluctuations in $g_{s}$. The fluctuation in the string coupling in the sum over genera typically leads to a generic structure of the Langevin equation where the coefficient of the noise term, dependent on the string coupling $g_{s}$, fluctuates. The positivity of $g_{s}$ leads naturally to a stochastic modelling of its distribution with a $χ$- distribution. This then rigorously implies a Tsallis type non-extensive or, more generally, a superstatistics distribution for the recoil velocity of D-particles. As a concrete and physically interesting application, we provide a rigorous estimate of an $ω$-like effect, pertinent to CPT violating modifications of the Einstein-Podolsky-Rosen correlators in entangled states of neutral Kaons. In the case of D-particle foam fluctuations, which respect the Lorentz symmetry of the vacuum on average, we find that the $ω$-effect may be within the range of sensitivity of future meson factories.

hep-th↗

arXiv2009

High-Energy gamma-ray Astronomy and String Theory

There have been observations, first from the MAGIC Telescope (July 2005) and quite recently (September 2008) from the FERMI Satellite Telescope, on non-simultaneous arrival of high-energy photons from distant celestial sources. In each case, the highest energy photons were delayed, as compared to their lower-energy counterparts. Although the astrophysics at the source of these energetic photons is still not understood, and such non simultaneous arrival might be due to non simultaneous emission as a result of conventional physics effects, nevertheless, rather surprisingly, the observed time delays can also fit excellently some scenarios in quantum gravity, predicting Lorentz violating space-time "foam" backgrounds with a non-trivial subluminal vacuum refractive index suppressed linearly by a quantum gravity scale of the order of the reduced Planck mass. In this pedagogical talk, I discuss the MAGIC and FERMI findings in this context and I argue on a theoretical model of space-time foam in string/brane theory that can accommodate the findings of those experiments in agreement with all other stringent tests of Lorentz invariance. However, I stress the current ambiguities/uncertainties on the source mechanisms, which need to be resolved first before definite conclusions are reached regarding quantum gravity foam scenarios.

astro-ph.HE↗

arXiv2009

The MAGIC of SSC and how it affects LHC

We discuss the phenomenology of Supercritical String Cosmology (SSC) in the context of Dark Matter constraints on supersymmetric particle physics models at LHC. We also link our results with recent findings of the MAGIC, H.E.S.S. and Fermi Telescopes on delayed arrival of highly energetic photons from the distant Galaxies and GRBs. The link is provided by a concrete model of space-time foam in (supercritical) string theory, involving space-time defects and their interaction with matter in a brane world scenario.

hep-ph↗

arXiv2009

Can TeVeS avoid Dark Matter on galactic scales?

A fully relativistic analysis of gravitational lensing in TeVeS is presented. By estimating the lensing masses for a set of six lenses from the CASTLES database, and then comparing them to the stellar mass, the deficit between the two is obtained and analysed. Considering a parametrised range for the TeVeS function $mu(y)$, which controls the strength of the modification to gravity, it is found that on galactic scales TeVeS requires additional dark matter with the commonly used $mu(y)$. A soft dependence of the results on the cosmological framework and the TeVeS free parameters is discussed. For one particular form of $mu(y)$, TeVeS is found to require very little dark matter. This choice is however ruled out by rotation curve data. The inability to simultaneously fit lensing and rotation curves for a single form of $mu(y)$ is a challenge to a "no dark matter" TeVeS proposal.

astro-ph.GA↗

arXiv2008

Tachyon-Dilaton-induced Inflation as an alpha'-resummed String Background

Within the framework of a novel functional method on the world-sheet of the string, we discuss simple but re-summed (in the Regge slope) inflationary scenarios in the context of closed Bosonic strings, living in four target-space dimensions, in the presence of non-trivial tachyon, dilaton and graviton cosmological backgrounds. The inflationary solutions are argued to guarantee the vanishing of the corresponding Weyl anomaly coefficients in a given world-sheet renormalization scheme, thereby ensuring conformal invariance of the corresponding sigma-model to all orders in the Regge slope. The key property is the requirement of "homogeneity" of the corresponding Weyl anomaly coefficients. Inflation entails appropriate relations between the dilaton and tachyon field configurations, whose form can lead to either a de Sitter vacuum, incompatible though (due to the cosmic horizons) with the perturbative string scattering amplitudes, or to cosmic space-times involving brief inflationary periods, interpolating smoothly between power-law and/or Minkowski Universes. The latter situation is characterized by well-defined scattering amplitudes, and is thus compatible with a perturbative string framework. It is this scenario that we consider a self-consistent ground state in our framework, which is based on local field redefinitions of background fields.

hep-th↗

arXiv2008

Quantum Gravity, CPT symmetry and Entangled States

There may unique ("smoking-gun") signatures of the breakdown of CPT symmetry, induced in some models of Quantum Gravity entailing decoherence for quantum matter. Such effects can be observed in entangled states of neutral mesons via modifications of the respective Einstein-Podolsky-Rosen (EPR) correlators ("omega"-effect). In the talk I discuss experimental signatures and bounds of the omega-effect in Phi- and B-factories, and argue that the effect might be falsifiable at the next generation facilities.

hep-ph↗