arXiv subjects

Andrei Lazanu

Publications and source records attributed to Andrei Lazanu.

At least 19 recordsLinked to original sources

arXiv2025

Accelerated cosmological expansion from pseudo-Hermiticity

We show that a well-studied pseudo-Hermitian field theory composed of two complex scalar fields can generate accelerated cosmological expansion through a novel mechanism. The dynamics is unique to the pseudo-Hermitian field theory, and it arises in the regime of broken antilinear symmetry, wherein a growth instability from the resulting complex eigenspectrum competes with the Hubble damping. The azimuthal components of the complex scalar fields asymptote to a constant rate of rolling at late times, reminiscent of motion around the infinite staircase of M.C. Escher's lithograph "Ascending and Descending". The resulting centripetal acceleration drives the radial components of the field away from the minimum of the potential, and the system generates a self-sustaining and constant Hubble rate at late times, even when tuning the minimum of the potential such that the classical vacuum energy is vanishing. This result evidences the potential to generate novel and physically relevant dynamics that are unique to pseudo-Hermitian field theories, and that their regimes of broken antilinear symmetry can be physically relevant in dynamical spacetimes.

hep-th↗

arXiv2025

Power Spectrum Emulators from Neural Networks and Tree-Based Methods

We use two subsets of 2000 and 1000 Quijote simulations to build two power spectrum emulators, allowing for fast computations of the non-linear matter power spectrum. The first emulator is built in terms of seven cosmological parameters: the matter and baryon fraction of the energy density of the Universe $\Omega_m$ and $\Omega_b$, the reduced Hubble constant $h$, the scalar spectral index $n_s$, the amplitude of matter density fluctuations $\sigma_8$, the total neutrino mass $M_{\nu}$ and the dark energy equation of state parameter $w$, on scales $k \in [0.015,1.8]\,h/ \rm{Mpc^{-1}}$. The power spectra can be directly determined at redshifts 0, 0.5, 1, 2 and 3, while for intermediate redshifts these can be interpolated. The second emulator is based on five cosmological parameters, $\Omega_m$, $h$, $n_s$, $\sigma_8$ and the amplitude of equilateral non-Gaussianity $f_{\rm NL}^{\rm eq}$, at redshifts 0, 0.503, 0.733, 0.997 for $k \in [0.015,1.8]\,h/ \rm{Mpc^{-1}}$. The emulators are built on machine learning techniques. In both cases we have investigated both neural networks and tree-based methods and we have shown that the best accuracy is obtained for a neural network with two hidden layers. Both emulators achieve a root-mean-squared relative error of less then 5\% for all the redshifts considered on the scales discussed.

astro-ph.CO↗

arXiv2025

Primordial gravitational waves in DHOST inflation

We consider DHOST inflationary models with a shift symmetry leading to a de Sitter space-time at the background cosmological level. Deviations from scale invariance of the scalar and tensor perturbations follow from the breaking of the shift symmetry by quadratic and quartic operators. These models show a strong violation of the consistency relation of single-field inflationary models with a very flat spectrum of tensor perturbations. This opens up the prospects of future detection of primordial gravitational waves by mHz experiments.

astro-ph.CO↗

arXiv2024

Recasting scalar-tensor theories of gravity for colliders

Diagrammatic approaches to perturbation theory transformed the practicability of calculations in particle physics. In the case of extended theories of gravity, however, obtaining the relevant diagrammatic rules is non-trivial: we must expand in metric perturbations and around (local) minima of the scalar field potentials, make multiple field redefinitions, and diagonalize kinetic and mass mixings. In this note, we will motivate these theories, introduce the package FeynMG -- a Mathematica extension of FeynRules that automates the process described above -- and highlight an application to a model with unique collider phenomenology.

hep-ph↗

arXiv2024

Scalar radiation zeros at the LHC

We consider a class of singlet scalar extensions of the Standard Model of particle physics in which the scalar couples only to off-shell states. As a result, low-order tree-level processes involving the singlet scalar vanish, providing a unique phenomenology that may allow to evade existing constraints on new singlet scalar fields. We describe search strategies for such states at the Large Hadron Collider and identify the parameter space that can be explored in the future.

hep-ph↗

arXiv2024

Recent Developments in Degenerate Higher Order Scalar Tensor Theories

Degenerate Higher Order Scalar Tensor (DHOST) theories are the most general scalar-tensor theories whose Lagrangian depends on the metric tensor and a single scalar field and its derivatives up to second order. They propagate only one scalar degree of freedom, without being plagued by Ostrogradsky instabilities. This is achieved through certain degeneracies of the functions forming their Lagrangian. They generalise the Horndeski and beyond-Horndeski theories. Originally proposed to describe the late-time acceleration of the expansion of the universe, generalising the cosmological constant, they can also be used to build models of the early universe, to describe inflation or alternatives to standard inflation. In the late universe, they modify the standard Vainstein screening mechanism from Horndeski theories (which can have observable consequences) and are suited to build black hole models, featuring non-stealth Kerr black hole solutions. In this work their phenomenology is reviewed, looking at their basic properties, their parameterisations and classifications, focusing on solutions in the early and the late universe and at cosmological and astrophysical constraints.

astro-ph.CO↗

arXiv2024

A DHOST model of inflation: CMB constraints from the power spectrum and the bispectrum

We build an inflationary model based on Degenerate Higher Order Scalar Tensor (DHOST) theories in a de Sitter background. We determine the scale-dependent power spectrum of curvature perturbations in these theories and we show that such a model can be compatible with the latest Planck measurements on the Cosmic Microwave Background (CMB). We calculate the bispectrum of curvature perturbations in DHOST models and we directly constrain them using the Planck results on non-Gaussianities. We show that the bispectrum consists of a contribution depending only on the power spectrum parameters and a linear combination of terms depending on new parameters. The former peaks in the squeezed limit, while the latter in the equilateral limit. We use the publicly available CMB-BEST code to directly compare the model predictions to the CMB bispectrum statistics and to marginalise over the free parameters, explicitly showing that there are viable DHOST inflationary models that satisfy both the power spectrum and bispectrum constraints from Planck.

astro-ph.CO↗

arXiv2023

CMB bispectrum constraints on DHOST inflation

We present the first direct constraints on a Degenerate Higher Order Scalar Tensor (DHOST) inflation model using the Planck 2018 Cosmic Microwave Background (CMB) results on non-Gaussianities. We identify that the bispectrum consists of a fixed contribution following from the power spectrum and a linear combination of terms depending on five free parameters defining the cubic perturbations to the DHOST model. The former peaks in the squeezed limit, while the latter is maximised in the equilateral limit. We directly confront the model predictions to the CMB bispectrum statistics via the public code CMB-BEST and marginalize over the free parameters. We explicitly show that there are viable DHOST inflationary models satisfying both power spectrum and bispectrum constraints from Planck. However, rather surprisingly, the constraints exclude certain models at the $6\sigma$-level even though they pass the conventional fudge factor tests. In this case and despite having a handful of free parameters, the model's large squeezed bispectrum cannot be cancelled out without introducing a large bispectrum in other limits which are strongly constrained by Planck's non-detection of primordial non-Gaussianity. We emphasize that first-order approximations such as fudge factors, albeit commonly used in the literature, may be misleading and provide weaker constraints. A proper analysis of the constraints from Planck requires a more robust approach, such as the one provided by the CMB-BEST code.

astro-ph.CO↗

arXiv2021

Non-Gaussianity in DHOST inflation

DHOST inflation models where deviations from a pure de Sitter background are induced by an axion-like potential can lead to large non-Gaussianities. We investigate the nature of non-Gaussianities in these models and compare to the results given by the Planck experiment. The overlap between the DHOST non-Gaussianities and the equilateral, orthogonal and local templates can be rendered arbitrarily small. On the other hand, this does not preclude DHOST models from showing large non-Gaussianities as exemplified by their reduced bispectrum. As a result, they could be probed by future experiments and also by a more thorough analysis of the existing Planck data.

astro-ph.CO↗

arXiv2021

Extracting cosmological parameters from N-body simulations using machine learning techniques

We make use of snapshots taken from the Quijote suite of simulations, consisting of 2000 simulations where five cosmological parameters have been varied ($Ω_m$, $Ω_b$, $h$, $n_s$ and $σ_8$) in order to investigate the possibility of determining them using machine learning techniques. In particular, we show that convolutional neural networks can be employed to accurately extract $Ω_m$ and $σ_8$ from the N-body simulations, and that these parameters can also be found from the non-linear matter power spectrum obtained from the same suite of simulations using both random forest regressors and deep neural networks. We show that the power spectrum provides competitive results in terms of accuracy compared to using the simulations and that we can also estimate the scalar spectral index $n_s$ from the power spectrum, at a lower precision.

astro-ph.CO↗

arXiv2021

The reach of next-to-leading-order perturbation theory for the matter bispectrum

We provide a comparison between the matter bispectrum derived with different flavours of perturbation theory at next-to-leading order and measurements from an unprecedentedly large suite of $N$-body simulations. We use the $χ^2$ goodness-of-fit test to determine the range of accuracy of the models as a function of the volume covered by subsets of the simulations. We find that models based on the effective-field-theory (EFT) approach have the largest reach, standard perturbation theory has the shortest, and `classical' resummed schemes lie in between. The gain from EFT, however, is less than in previous studies. We show that the estimated range of accuracy of the EFT predictions is heavily influenced by the procedure adopted to fit the amplitude of the counterterms. For the volumes probed by galaxy redshift surveys, our results indicate that it is advantageous to set three counterterms of the EFT bispectrum to zero and measure the fourth from the power spectrum. We also find that large fluctuations in the estimated reach occur between different realisations. We conclude that it is difficult to unequivocally define a range of accuracy for the models containing free parameters. Finally, we approximately account for systematic effects introduced by the $N$-body technique either in terms of a scale- and shape-dependent bias or by boosting the statistical error bars of the measurements (as routinely done in the literature). We find that the latter approach artificially inflates the reach of EFT models due to the presence of tunable parameters.

astro-ph.CO↗

arXiv2021

Scale-dependence in DHOST inflation

We study the inflationary consequences of Degenerate Higher Order Scalar Tensor (DHOST) theories in a de Sitter background. We perturb the de Sitter background by operators breaking either the degeneracy condition, i.e scordatura DHOST, or the shift symmetry in the scalar field. We first consider derivative scordatura and find that in all cases the power spectra of curvature perturbations are scale-invariant. We then investigate small perturbations by an axion-like potential, and show that in this scenario the power spectrum becomes scale-dependent. The modifications to the spectral index and its first two derivatives are compatible with the latest inflationary constraints. Moreover the tensor to scalar ratio and the non-Gaussianities of these models could be within reach of future experiments.

astro-ph.CO↗

arXiv2018

K-mouflage Imprints on Cosmological Observables and Data Constraints

We investigate cosmological constraints on K-mouflage models of modified gravity. We consider two scenarios: one where the background evolution is free to deviate from $Λ$CDM (K-mouflage) and another one which reproduces a $Λ$CDM expansion (K-mimic), implementing both of them into the EFTCAMB code. We discuss the main observational signatures of these models and we compare their cosmological predictions to different datasets, including CMB, CMB lensing, SNIa and different galaxy catalogues. We argue about the possibility of relieving the $H_0$ and weak lensing tensions within these models, finding that K-mouflage scenarios effectively ease the tension on the Hubble Constant. Our final 95\% C.L. bounds on the $ε_{2,0}$ parameter that measures the overall departure from $Λ$CDM (corresponding to $ε_{2,0}=0$) are $-0.04\leq ε_{2,0} <0$ for K-mouflage and $0< ε_{2,0} <0.002$ for K-mimic. In the former case the main constraining power comes from changes in the background expansion history, while in the latter case the model is strongly constrained by measurements of the amplitude of matter perturbations. The sensitivity of these cosmological constraints closely matches that of solar system probes. We show that these constraints could be significantly tightened with future ideal probes like CORE.

astro-ph.CO↗

arXiv2018

Modelling the matter bispectrum towards nonlinear scales - two and three loops in perturbation theories

I compute the matter bispectrum of large-scale structure up to two loops in the Standard Perturbation Theory and up to three loops in the MPTbreeze renormalised perturbation theory, determining the contributing loop diagrams and evaluating them numerically. In the process I remove the leading divergences in the integrands, thus making them infrared-safe. By comparing the results to numerical simulations, I show that in the case of the Standard Perturbation Theory, the bispectrum at two loops is more accurate than at one loop, up to $k_{\textrm{max}} \sim 0.09 \, h/\textrm{Mpc}$ at $z=0$ and $k_{\textrm{max}} \sim 0.11 \, h/\textrm{Mpc}$ at $z=1$. The MPTbreeze can be employed to accurately model the matter bispectrum up to $k_{\textrm{max}} \sim 0.17 \, h/\textrm{Mpc}$ at $z=0$ and $k_{\textrm{max}} \sim 0.24 \, h/\textrm{Mpc}$ at $z=1$ using the results at three loops.

astro-ph.CO↗

arXiv2018

The two and three-loop matter bispectrum in perturbation theories

We evaluate for the first time the dark matter bispectrum of large-scale structure at two loops in the Standard Perturbation Theory and at three loops in the Renormalised Perturbation Theory (MPTbreeze formalism), removing in each case the leading divergences in the integrals in order to make them infrared-safe. We show that the Standard Perturbation Theory at two loops can be employed to model the matter bispectrum further into the quasi-nonlinear regime compared to one loop, up to $k_{\text{max}} \sim 0.1 \, h/{\text{Mpc}}$ at $z = 0$, but without reaching a high level of accuracy. In the case of the MPTbreeze method, we show that its bispectra decay at smaller and smaller scales with increasing loop order, but with smaller improvements. At three loops, this model predicts the bispectrum accurately up to scales $k_{\text{max}} \sim 0.17 \, h/{\text{Mpc}}$ at $z = 0$ and $k_{\text{max}} \sim 0.24 \, h/{\text{Mpc}}$ at $z = 1$.

astro-ph.CO↗

arXiv2018

Constraining Primordial non-Gaussianity with Bispectrum and Power Spectum from Upcoming Optical and Radio Surveys

We forecast constraints on primordial non-Gaussianity (PNG) and bias parameters from measurements of galaxy power spectrum and bispectrum in future radio continuum and optical surveys. In the galaxy bispectrum, we consider a comprehensive list of effects, including the bias expansion for non-Gaussian initial conditions up to second order, redshift space distortions, redshift uncertainties and theoretical errors. These effects are all combined in a single PNG forecast for the first time. Moreover, we improve the bispectrum modelling over previous forecasts, by accounting for trispectrum contributions. All effects have an impact on final predicted bounds, which varies with the type of survey. We find that the bispectrum can lead to improvements up to a factor $\sim 5$ over bounds based on the power spectrum alone, leading to significantly better constraints for local-type PNG, with respect to current limits from \textit{Planck}. Future radio and photometric surveys could obtain a measurement error of $σ(f_{\mathrm{NL}}^{\mathrm{loc}}) \approx 0.2$. In the case of equilateral PNG, galaxy bispectrum can improve upon present bounds only if significant improvements in the redshift determinations of future, large volume, photometric or radio surveys could be achieved. For orthogonal non-Gaussianity, expected constraints are generally comparable to current ones.

astro-ph.CO↗

arXiv2017

Bispectrum Modelling in Large Scale Structure - A Three Shape Model

We study the matter bispectrum of large structure by comparing theoretical models (perturbation theories and halo models) to numerical simulations using shape and amplitude correlators. We show that among the perturbation theories at one loop the effective field theory of large scale structure extends the furthest into the non-linear regime. We analyse the one and two-loop bispectra in the renormalised perturbation theory and we show that there is a significant extension in the range where results are accurate when going to two loops. In the case of the halo model, we show that there are deficiencies in the modelling of the two-halo term at redshifts z>0 that worsen in the past. Based on this observation and on the shapes identified in the halo model, we build a simple `three-shape model' that provides a good fit to N-body simulations on all scales, at both low and high redshifts. We show that this model can be easily extended to local and equilateral primordial non-Gaussianity using the same shapes.

astro-ph.CO↗

arXiv2017

Matter bispectrum of large-scale structure with Gaussian and non-Gaussian initial conditions: Halo models, perturbation theory, and a three-shape model

We study the matter bispectrum of large-scale structure by comparing the predictions of different perturbative and phenomenological models with the full three-dimensional bispectrum from $N$-body simulations estimated using modal methods. We show that among the perturbative approaches, effective field theory succeeds in extending the range of validity furthest on intermediate scales, at the cost of free additional parameters. By studying the halo model, we show that although it is satisfactory in the deeply non-linear regime, it predicts a deficit of power on intermediate scales, worsening at redshifts $z>0$. By comparison with the $N$-body bispectrum on those scales, we show that there is a significant squeezed component underestimated in the halo model. On the basis of these results, we propose a new three-shape model, based on the tree-level, squeezed and constant bispectrum shapes we identified in the halo model; after calibration this fits the simulations on all scales and redshifts of interest. We extend this model further to primordial non-Gaussianity of the local and equilateral types by showing that the same shapes can be used to describe the additional non-Gaussian component in the matter bispectrum. This method provides a HALOFIT-like prototype of the bispectrum that could be used to describe and test parameter dependencies and should be relevant for the bispectrum of weak gravitational lensing and wider applications.

astro-ph.CO↗