Spyros Sypsas | King's College London (original) (raw)

Papers by Spyros Sypsas

In this thesis, consisting of two main parts, we study observational signatures of cosmic (super)... more In this thesis, consisting of two main parts, we study observational signatures of cosmic (super)strings in the context of D-brane inflation and properties of scalar perturbations on generic homogeneous inflating backgrounds. In the first part we study the production, nature and decay processes of cosmic superstrings in two widely used effective models of D-brane inflation, namely the D3/D7D3/D7D3/D7 and D3/barD3D3/\bar{D}3D3/barD3 models. Specifically, we show that the strings produced in D3/D7D3/D7D3/D7 are of local axionic type and we place constraints on the tension while arguing that the supersymmetry breaking mechanism of the model needs to be altered according to supergravity constraints on constant Fayet-Iliopoulos terms. Moreover, we study radiative processes of cosmic superstrings on warped backgrounds. We argue that placing the string formation in a natural context such as D3/barD3D3/\bar{D}3D3/barD3 inflation, restricts the forms of possible radiation from these objects. Motivated by these string models, which inevitably result in the presence of heavy moduli fields during inflation, in the second part, using the Effective Field Theory (EFT) of inflation, we construct operators that capture the effects of massive scalars on the low energy dynamics of inflaton perturbations. We compute the energy scales that define the validity window of the EFT such as the scale where ultra violet (UV) degrees of freedom become operational and the scale where the EFT becomes strongly coupled. We show that the low energy operators related to heavy fields induce a dispersion relation for the light modes admitting two regimes: a linear and a non linear/dispersive one. Assuming that these modes cross the Hubble scale within the dispersive regime, we compute observables related to two- and three-point correlators and show how they are directly connected with the scale of UV physics.

Journal of Cosmology and Astroparticle Physics, 2015

We study next-to-leading corrections to the effective action of the curvature perturbation obtain... more We study next-to-leading corrections to the effective action of the curvature perturbation obtained by integrating out the coupled heavy isocurvature perturbation. These corrections result from including higher order derivative operators, weighted by the mass scale of the heavy physics, in the effective theory expansion. We find that the correction terms are suppressed by the ratio of the Hubble parameter to the heavy mass scale. The corresponding corrections to the power spectrum of the curvature perturbation are presented for a simple illustrative example.

Journal of Cosmology and Astroparticle Physics, 2014

In this article we discuss the role of current and future CMB measurements to pin down the model ... more In this article we discuss the role of current and future CMB measurements to pin down the model of inflation responsible for the generation of primordial curvature perturbations. By considering a parameterization of the effective field theory of inflation with a modified dispersion relation arising from heavy fields, we derive the dependence of cosmological observables on the scale of heavy physics Λ UV . Specifically, we show how the f NL non-linearity parameters are related to the phase velocity of curvature perturbations at horizon exit, which is parameterized by Λ UV . Bicep2 and Planck findings are shown to be consistent with a value Λ UV ∼ Λ GUT . However, we find a degeneracy in the parameter space of inflationary models that can only be resolved with a detailed knowledge of the shape of the non-Gaussian bispectrum.

We generalize previous results on N=1, (3+1)-dimensional superconformal block quiver gauge theori... more We generalize previous results on N=1, (3+1)-dimensional superconformal block quiver gauge theories. It is known that the necessary conditions for a theory to be superconformal, i.e. that the beta and gamma functions vanish in addition to anomaly cancellation, translate to a Diophantine equation in terms of the quiver data. We re-derive results for low block numbers revealing an new intriguing algebraic structure underlying a class of possible superconformal fixed points of such theories. After explicitly computing the five block case Diophantine equation, we use this structure to reorganize the result in a form that can be applied to arbitrary block numbers. We argue that these theories can be thought of as vectors in the root system of the corresponding quiver and superconformality conditions are shown to associate them to certain subsets of imaginary roots. These methods also allow for an interpretation of Seiberg duality as the action of the affine Weyl group on the root lattice.

Journal of High Energy Physics, Jan 1, 2010

We examine the consequences of recent developments on Fayet-Iliopoulos (FI) terms for D-term infl... more 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) FI .

Arxiv preprint arXiv:1105.1784, Jan 1, 2011

We analyze theoretical constraints on the consistency of brane inflation leading to cosmic supers... more We analyze theoretical constraints on the consistency of brane inflation leading to cosmic superstring formation and on the subsequent radiation of these cosmic superstrings. We first investigate the implications of recently elucidated supergravity constraints on models of brane inflation. We show that both D3/D7 and D3/D3 brane inflation are subject to non-trivial constraints. Both models can be shown to satisfy those constraints, but for the case of D3/D7 there may be important consequences for the inflationary mechanism. We then analyze the strong theoretical constraints on the types of allowed radiation by cosmic superstrings, should they be formed; such constraints do not apply to their field theoretical analogues. We argue that in a warped background where one might expect axionic radiation to be enhanced relative to gravitational radiation, neither F-strings nor D-strings can emit axionic radiation, while FD-strings cannot give rise to Neveu-Schwarz-Neveu-Schwarz particle emission, and their Ramond-Ramond particle emission is not welldefined.

In this thesis, consisting of two main parts, we study observational signatures of cosmic (super)... more In this thesis, consisting of two main parts, we study observational signatures of cosmic (super)strings in the context of D-brane inflation and properties of scalar perturbations on generic homogeneous inflating backgrounds. In the first part we study the production, nature and decay processes of cosmic superstrings in two widely used effective models of D-brane inflation, namely the D3/D7D3/D7D3/D7 and D3/barD3D3/\bar{D}3D3/barD3 models. Specifically, we show that the strings produced in D3/D7D3/D7D3/D7 are of local axionic type and we place constraints on the tension while arguing that the supersymmetry breaking mechanism of the model needs to be altered according to supergravity constraints on constant Fayet-Iliopoulos terms. Moreover, we study radiative processes of cosmic superstrings on warped backgrounds. We argue that placing the string formation in a natural context such as D3/barD3D3/\bar{D}3D3/barD3 inflation, restricts the forms of possible radiation from these objects. Motivated by these string models, which inevitably result in the presence of heavy moduli fields during inflation, in the second part, using the Effective Field Theory (EFT) of inflation, we construct operators that capture the effects of massive scalars on the low energy dynamics of inflaton perturbations. We compute the energy scales that define the validity window of the EFT such as the scale where ultra violet (UV) degrees of freedom become operational and the scale where the EFT becomes strongly coupled. We show that the low energy operators related to heavy fields induce a dispersion relation for the light modes admitting two regimes: a linear and a non linear/dispersive one. Assuming that these modes cross the Hubble scale within the dispersive regime, we compute observables related to two- and three-point correlators and show how they are directly connected with the scale of UV physics.

Journal of Cosmology and Astroparticle Physics, 2015

We study next-to-leading corrections to the effective action of the curvature perturbation obtain... more We study next-to-leading corrections to the effective action of the curvature perturbation obtained by integrating out the coupled heavy isocurvature perturbation. These corrections result from including higher order derivative operators, weighted by the mass scale of the heavy physics, in the effective theory expansion. We find that the correction terms are suppressed by the ratio of the Hubble parameter to the heavy mass scale. The corresponding corrections to the power spectrum of the curvature perturbation are presented for a simple illustrative example.

Journal of Cosmology and Astroparticle Physics, 2014

In this article we discuss the role of current and future CMB measurements to pin down the model ... more In this article we discuss the role of current and future CMB measurements to pin down the model of inflation responsible for the generation of primordial curvature perturbations. By considering a parameterization of the effective field theory of inflation with a modified dispersion relation arising from heavy fields, we derive the dependence of cosmological observables on the scale of heavy physics Λ UV . Specifically, we show how the f NL non-linearity parameters are related to the phase velocity of curvature perturbations at horizon exit, which is parameterized by Λ UV . Bicep2 and Planck findings are shown to be consistent with a value Λ UV ∼ Λ GUT . However, we find a degeneracy in the parameter space of inflationary models that can only be resolved with a detailed knowledge of the shape of the non-Gaussian bispectrum.

We generalize previous results on N=1, (3+1)-dimensional superconformal block quiver gauge theori... more We generalize previous results on N=1, (3+1)-dimensional superconformal block quiver gauge theories. It is known that the necessary conditions for a theory to be superconformal, i.e. that the beta and gamma functions vanish in addition to anomaly cancellation, translate to a Diophantine equation in terms of the quiver data. We re-derive results for low block numbers revealing an new intriguing algebraic structure underlying a class of possible superconformal fixed points of such theories. After explicitly computing the five block case Diophantine equation, we use this structure to reorganize the result in a form that can be applied to arbitrary block numbers. We argue that these theories can be thought of as vectors in the root system of the corresponding quiver and superconformality conditions are shown to associate them to certain subsets of imaginary roots. These methods also allow for an interpretation of Seiberg duality as the action of the affine Weyl group on the root lattice.

Journal of High Energy Physics, Jan 1, 2010

We examine the consequences of recent developments on Fayet-Iliopoulos (FI) terms for D-term infl... more 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) FI .

Arxiv preprint arXiv:1105.1784, Jan 1, 2011

We analyze theoretical constraints on the consistency of brane inflation leading to cosmic supers... more We analyze theoretical constraints on the consistency of brane inflation leading to cosmic superstring formation and on the subsequent radiation of these cosmic superstrings. We first investigate the implications of recently elucidated supergravity constraints on models of brane inflation. We show that both D3/D7 and D3/D3 brane inflation are subject to non-trivial constraints. Both models can be shown to satisfy those constraints, but for the case of D3/D7 there may be important consequences for the inflationary mechanism. We then analyze the strong theoretical constraints on the types of allowed radiation by cosmic superstrings, should they be formed; such constraints do not apply to their field theoretical analogues. We argue that in a warped background where one might expect axionic radiation to be enhanced relative to gravitational radiation, neither F-strings nor D-strings can emit axionic radiation, while FD-strings cannot give rise to Neveu-Schwarz-Neveu-Schwarz particle emission, and their Ramond-Ramond particle emission is not welldefined.