arXiv subjects

Mairi Sakellariadou

Publications and source records attributed to Mairi Sakellariadou.

At least 127 records · Page 7Linked to original sources

arXiv2014

Unweaving the Fabric of the Universe: The Interplay between Mathematics and Physics

Our conventional understanding of space-time, as well as our notion of geometry, break down once we attempt to describe the very early stages of the evolution of our universe. The extreme physical conditions near the Big Bang necessitate an intimate interplay between physics and mathematics. The main challenge is the construction of a theory of quantum gravity, the long-sought unification of Einstein's general relativity with quantum mechanics. There are several attempts to formulate such a theory; they can be tested against experimental and observational results coming from high energy physics and astrophysics, leading to a remarkable interplay between gravity, particle physics and cosmology.

gr-qc↗

arXiv2014

Doubling of the Algebra and Neutrino Mixing within Noncommutative Spectral Geometry

We study physical implications of the doubling of the algebra, an essential element in the construction of the noncommutative spectral geometry model, proposed by Connes and his collaborators as offering a geometric explanation for the standard model of strong and electroweak interactions. Linking the algebra doubling to the deformed Hopf algebra, we build Bogogliubov transformations and show the emergence of neutrino mixing.

hep-th↗

arXiv2014

Spectral Regularisation: Induced Gravity and the Onset of Inflation

Using spectral regularisation, we compute the Weyl anomaly and express the anomaly generating functional of the quantum effective action through a collective scalar degree of freedom of all quantum vacuum fluctuations. Such a formulation allows us to describe induced gravity on an equal footing with the anomaly-induced effective action, in a self-consistent way. We then show that requiring stability of the cosmological constant under loop quantum corrections, Sakharov's induced gravity and Starobinsky's anomaly-induced inflation are either both present or both absent, depending on the particle content of the theory.

hep-th↗

arXiv2013

Constraints on NonCommutative Spectral Action from Gravity Probe B and Torsion Balance Experiments

Noncommutative spectral geometry offers a purely geometric explanation for the standard model of strong and electroweak interactions, including a geometric explanation for the origin of the Higgs field. Within this framework, the gravitational, the electroweak and the strong forces are all described as purely gravitational forces on a unified noncommutative space-time. In this study, we infer a constraint on one of the three free parameters of the model, namely the one characterising the coupling constants at unification, by linearising the field equations in the limit of weak gravitational fields generated by a rotating gravitational source, and by making use of recent experimental data. In particular, using data obtained by Gravity Probe B, we set a lower bound on the Weyl term appearing in the noncommutative spectral action, namely $β \gtrsim 10^{-6}$m$^{-1}$. This constraint becomes stronger once we use results from torsion balance experiments, leading to $β \gtrsim 10^{4}$m$^{-1}$. The latter is much stronger than any constraint imposed so far to curvature squared terms.

gr-qc↗

arXiv2013

Inflation and cosmic (super)strings: implications of their intimate relation revisited

We briefly discuss constraints on supersymmetric hybrid inflation models and examine the consistency of brane inflation models. We then address the implications for inflationary scenarios resulting from the strong constraints on the cosmic (super)string tension imposed from the most recent cosmic microwave background temperature anisotropies data.

hep-th↗

arXiv2013

Is F-term hybrid inflation natural within minimal supersymmetric SO(10)?

We examine whether F-term supersymmetric hybrid inflation can be embedded within the minimal SO(10) model, in a natural way. We show that none of the singlets of the Standard Model symmetries in the minimal set of SO(10) representations can satisfy the conditions which are necessary for a scalar field to play the role of the inflaton. As a consequence, one has to introduce an extra scalar field, which however may spoil the naturalness of inflation within the context of SO(10). Nevertheless, if we add an extra scalar field, we are then able to construct a model that can accommodate flat directions while it preserves the stability of the inflationary valley.

hep-ph↗

arXiv2013

Effective field theory of weakly coupled inflationary models

The application of Effective Field Theory (EFT) methods to inflation has taken a central role in our current understanding of the very early universe. The EFT perspective has been particularly useful in analyzing the self-interactions determining the evolution of co-moving curvature perturbations (Goldstone boson modes) and their influence on low-energy observables. However, the standard EFT formalism, to lowest order in spacetime differential operators, does not provide the most general parametrization of a theory that remains weakly coupled throughout the entire low-energy regime. Here we study the EFT formulation by including spacetime differential operators implying a scale dependence of the Goldstone boson self-interactions and its dispersion relation. These operators are shown to arise naturally from the low-energy interaction of the Goldstone boson with heavy fields that have been integrated out. We find that the EFT then stays weakly coupled all the way up to the cutoff scale at which ultraviolet degrees of freedom become operative. This opens up a regime of new physics where the dispersion relation is dominated by a quadratic dependence on the momentum ω~ p^2. In addition, provided that modes crossed the horizon within this energy range, the prediction of inflationary observables - including non-Gaussian signatures - are significantly affected by the new scales characterizing it.

hep-th↗

arXiv2013

Noncommutative Spectral Geometry: A Short Review

We review the noncommutative spectral geometry, a gravitational model that combines noncommutative geometry with the spectral action principle, in an attempt to unify General Relativity and the Standard Model of electroweak and strong interactions. Despite the phenomenological successes of the model, the discrepancy between the predicted Higgs mass and the current experimental data indicate that one may have to go beyond the simple model considered at first. We review the current status of the phenomenological consequences and their implications. Since this model lives by construction at high energy scales, namely at the Grand Unified Theories scale, it provides a natural framework to investigate early universe cosmology. We briefly review some of its cosmological consequences.

hep-th↗

arXiv2013

Noncommutative spectral geometry and the deformed Hopf algebra structure of quantum field theory

We report the results obtained in the study of Alain Connes noncommutative spectral geometry construction focusing on its essential ingredient of the algebra doubling. We show that such a two-sheeted structure is related with the gauge structure of the theory, its dissipative character and carries in itself the seeds of quantization. From the algebraic point of view, the algebra doubling process has the same structure of the deformed Hops algebra structure which characterizes quantum field theory.

hep-th↗

arXiv2012

Confronting MOND and TeVeS with strong gravitational lensing over galactic scales: an extended survey

The validity of MOND and TeVeS models of modified gravity has been recently tested by using lensing techniques, with the conclusion that a non-trivial component in the form of dark matter is needed in order to match the observations. In this work those analyses are extended by comparing lensing to stellar masses for a sample of nine strong gravitational lenses that probe galactic scales. The sample is extracted from a recent work that presents the mass profile out to a few effective radii, therefore reaching into regions that are dominated by dark matter in the standard (general relativity) scenario. A range of interpolating functions are explored to test the validity of MOND/TeVeS in these systems. Out of the nine systems, there are five robust candidates with a significant excess (higher that 50%) of lensing mass with respect to stellar mass, irrespective of the stellar initial mass function. One of these lenses (Q0957) is located at the centre of a galactic cluster. This system might be accommodated in MOND/TeVeS via the addition of a hot component, like a 2 eV neutrino, that contribute over cluster scales. However, the other four robust candidates (LBQS1009, HE1104, B1600, HE2149) are located in field/group regions, so that a cold component (CDM) would be required even within the MOND/TeVeS framework. Our results therefore do not support recent claims that these alternative scenarios to CDM can survive astrophysical data.

astro-ph.CO↗

arXiv2012

Stringy Models of Modified Gravity: Space-time defects and Structure Formation

Starting from microscopic models of space-time foam, based on brane universes propagating in bulk space-times populated by D0-brane defects ("D-particles"), we arrive at effective actions used by a low-energy observer on the brane world to describe his/her observations of the Universe. These actions include, apart from the metric tensor field, also scalar (dilaton) and vector fields, the latter describing the interactions of low-energy matter on the brane world with the recoiling point-like space-time defect (D-particle). The vector field is proportional to the recoil velocity of the D-particle and as such it satisfies a certain constraint. The vector breaks locally Lorentz invariance, which however is assumed to be conserved on average in a space-time foam situation, involving the interaction of matter with populations of D-particle defects. In this paper we demonstrate that, already at the end of the radiation era, the (constrained) vector field associated with the recoil of the defects provides the seeds for a growing mode in the evolution of the Universe. Such a growing mode survives during the matter dominated era, provided the variance of the D-particle recoil velocities on the brane is larger than a critical value.

hep-th↗

arXiv2012

Noncommutative spectral geometry: A guided tour for theoretical physicists

We review a gravitational model based on noncommutative geometry and the spectral action principle. The space-time geometry is described by the tensor product of a four-dimensional Riemanian manifold by a discrete noncommutative space consisting of only two points. With a specific choice of the finite dimensional involutive algebra, the noncommutative spectral action leads to the standard model of electroweak and strong interactions minimally coupled to Einstein and Weyl gravity. We present the main mathematical ingredients of this model and discuss their physical implications. We argue that the doubling of the algebra is intimately related to dissipation and the gauge field structure. We then show how this noncommutative spectral geometry model, a purely classical construction, carries implicit in the doubling of the algebra the seeds of quantization. After a short review on the phenomenological consequences of this geometric model as an approach to unification, we discuss some of its cosmological consequences. In particular, we study deviations of the Friedmann equation, propagation of gravitational waves, and investigate whether any of the scalar fields in this model could play the role of the inflaton.

hep-th↗

arXiv2012

Effects of anisotropic dynamics on cosmic strings

The dynamics of cosmic strings is considered in anisotropic backgrounds. In particular, the behaviour of infinitely long straight cosmic strings and of cosmic string loops is determined. Small perturbations of a straight cosmic string are calculated. The relevance of these results is discussed with respect to the possible observational imprints of an anisotropic phase on the behaviour of a cosmic string network.

hep-th↗

arXiv2012

Highlights of Noncommutative Spectral Geometry

A summary of noncommutative spectral geometry as an approach to unification is presented. The role of the doubling of the algebra, the seeds of quantization and some cosmological implications are briefly discussed.

hep-th↗

arXiv2012

Noncommutative spectral geometry, dissipation and the origin of quantization

We present a physical interpretation of the doubling of the algebra, which is the basic ingredient of the noncommutative spectral geometry, developed by Connes and collaborators as an approach to unification. We discuss its connection to dissipation and to the gauge structure of the theory. We then argue, following 't Hooft's conjecture, that noncommutative spectral geometry classical construction carries implicit in its feature of the doubling of the algebra the seeds of quantization.

math-ph↗

arXiv2011

Noncommutative spectral geometry, algebra doubling and the seeds of quantization

A physical interpretation of the two-sheeted space, the most fundamental ingredient of noncommutative spectral geometry proposed by Connes as an approach to unification, is presented. It is shown that the doubling of the algebra is related to dissipation and to the gauge structure of the theory, the gauge field acting as a reservoir for the matter field. In a regime of completely deterministic dynamics, dissipation appears to play a key role in the quantization of the theory, according to 't Hooft's conjecture. It is thus argued that the noncommutative spectral geometry classical construction carries implicit in its feature of the doubling of the algebra the seeds of quantization.

math-ph↗

arXiv2011

Cosmic strings from pseudo-anomalous Fayet-Iliopoulos U(1) in D3/D7 brane inflation

We examine the consequences of recent developments on Fayet-Iliopoulos (FI) terms for D-term inflationary models. There is currently no known way to couple constant FI terms to supergravity consistently; only field-dependent FI terms are allowed. These are natural in string theory and we argue that the FI term in D3/D7 inflation turns out to be of this type, corresponding to a pseudo-anomalous U(1). T he anomaly is canceled by the Green-Schwarz mechanism in 4 dimensions. Inflation proceeds as usual, except that the scale is set by the GS parameter. Cosmic strings resulting from a pseudo-anomalous U(1) have potentially interesting characteristics. Originally expected to be global, they turn out to be local in the string theory context and can support currents. We outline the nature of these strings, discuss bounds on their formation, and summarize resulting cosmological consequences.

hep-th↗

arXiv2011

Cosmology within Noncommutative Spectral Geometry

Close to the Planck energy scale, the quantum nature of space-time reveals itself and all forces, including gravity, should be unified so that all interactions correspond to just one underlying symmetry. In the absence of a full quantum gravity theory, one may follow an effective approach and consider space-time as the product of a four-dimensional continuum compact Riemanian manifold by a tiny discrete finite noncommutative space. Since all available data are of a spectral nature, one may argue that it is more appropriate to apply the spectral action principle in this almost commutative space. Following this procedure one obtains an elegant geometric explanation for the most successful particle physics model, namely the standard model (and supersymmetric extensions) of electroweak and strong interactions in all its details, as determined by experimental data. Moreover, since this gravitational theory lives by construction at very high energy scales, it offers a perfect framework to address some of the early universe cosmological questions still awaiting for an answer. After introducing some of the main mathematical elements of noncommutative spectral geometry, I will discuss various cosmological and phenomenological consequences of this theory, focusing in particular on constraints imposed on the gravitational sector of the theory.

hep-th↗