Model-independent Constraints Research Articles

Model-independent cosmographic constraints from DESI 2024

Context. We explore model-independent constraints on the Universe kinematics up to the snap and jerk hierarchical terms, considering the latest baryon acoustic oscillation (BAO) release provided by the DESI collaboration. Aims. We intend to place novel and more stringent constraints on the cosmographic series, incorporating three combinations of data catalogs: the first made by BAO and observational cosmic chronometers, the second made by BAO and type Ia supernovae, and the last including all the cited data sets. Methods. Considering the latest BAO data provided by the DESI collaboration and tackling the rd parameter to span within the range [144,152] Mpc, with a fixed step of δrd = 2 Mpc, we employed Monte Carlo Markov chain analyses based on the Metropolis algorithm to fix novel bounds on the cosmographic series, fixing the deceleration, q0 , the jerk, j0 , and the snap, s0, parameters, up to the 2σ level. A comparison between the results of the Planck satellite with those obtained by the DESI collaboration is also reported. Results. Our findings showcase a significant departure in terms of j0 even at the 1σ confidence level, albeit compatible with the ACDM paradigm in regard to q0 and s0 at the 2σ level. Analogously, the h0 tension appears alleviated in the second hierarchy when including snap. Conclusions. Our method excludes models that significantly depart from the standard cosmological model. Particularly, direct comparisons with the ACDM and wCDM models and the Chevallier-Polarski-Linder parameterisation are explored, which definitively favour the wCDM scenario over other approaches, contradicting the findings of the original DESI collaboration.

Different Aspects of Entropic Cosmology

We provide a short review of the recent developments in entropic cosmology based on two thermodynamic laws of the apparent horizon, namely the first and the second laws of thermodynamics. The first law essentially provides the change in entropy of the apparent horizon during the cosmic evolution of the universe; in particular, it is expressed by TdS=−d(ρV)+WdV (where W is the work density and other quantities have their usual meanings). In this way, the first law actually links various theories of gravity with the entropy of the apparent horizon. This leads to a natural question—“What is the form of the horizon entropy corresponding to a general modified theory of gravity?”. The second law of horizon thermodynamics states that the change in total entropy (the sum of horizon entropy + matter fields’ entropy) with respect to cosmic time must be positive, where the matter fields behave like an open system characterised by a non-zero chemical potential. The second law of horizon thermodynamics importantly provides model-independent constraints on entropic parameters. Finally, we discuss the standpoint of entropic cosmology on inflation (or bounce), reheating and primordial gravitational waves from the perspective of a generalised entropy function.

Testing the cosmological Poisson equation in a model-independent way

We show how one can test the cosmological Poisson equation by requiring only the validity of three main assumptions: the energy-momentum conservation equations of matter, the equivalence principle, and the cosmological principle. We first point out that one can only measure the combination M≡Ωm(0)μ, where μ quantifies the deviation of the Poisson equation from the standard one and Ωm(0) is the fraction of matter density at present. Then we employ a recent model-independent forecast for the growth rate f(z) and the expansion rate E(z) to obtain constraints on M for a survey that approximates a combination of the Dark Energy Spectroscopic Instrument (DESI) and Euclid. We conclude that a constant M can be measured with a relative error σM=4.5%, while if M is arbitrarily varying in redshift, it can be measured only to within 13.4% (1 σ c.l.) at redshift z=0.9, and 15-22% up to z=1.5. We also project our constraints on some parametrizations of M proposed in literature, while still maintaining model-independence for the background expansion, the power spectrum shape, and the non-linear corrections. Generally speaking, as expected, we find much weaker model-independent constraints than found so far for such models. This means that the cosmological Poisson equation remains quite open to various alternative gravity and dark energy models.

Proper motions in the sub-kiloparsec jet of 3C 78: novel constraints on the physical nature of relativistic jets

ABSTRACT Jets from active galactic nuclei are thought to play a role in the evolution of their host and local environments, but a detailed prescription is limited by the understanding of the jets themselves. Proper motion studies of compact bright components in radio jets can be used to produce model-independent constraints on their Lorentz factor, necessary to understand the quantity of energy deposited in the intergalactic medium. We present our initial work on the jet of radio–galaxy 3C 78, as part of Catalogue of proper motions in active galactic nuclei using Very Large Array Studies (CAgNVAS), with a goal of constraining nature of jet plasma on larger (>100 parsec) scales. In 3C 78, we find three prominent knots (A, B, and C), where knot B undergoes sub-luminal longitudinal motion (∼0.6c at ∼ 200 pc), while knot C undergoes extreme (apparent) backward motion and eventual forward motion (∼−2.6c, 0.5c, at ∼ 300 pc). Assuming knots are shocks, we infer the bulk speeds from the pattern motion of Knots B and C. We model the spectral energy distribution of the large-scale jet and observe that a physically motivated two-zone model can explain most of the observed emission. We also find that the jet profile remains approximately conical from parsec to kiloparsec scales. Using the parsec-scale speed from very long baseline interferometry studies (∼0.1c) and the derived bulk speeds, we find that the jet undergoes bulk acceleration between the parsec and the kiloparsec scales providing the first direct evidence of jet acceleration in a conical and matter-dominated jet.

Schwinger poles of the three-gluon vertex: symmetry and dynamics

The implementation of the Schwinger mechanism endows gluons with a nonperturbative mass through the formation of special massless poles in the fundamental QCD vertices; due to their longitudinal character, these poles do not cause divergences in on-shell amplitudes, but induce detectable effects in the Green’s functions of the theory. Particularly important in this theoretical setup is the three-gluon vertex, whose pole content extends beyond the minimal structure required for the generation of a gluon mass. In the present work we analyze these additional pole patterns by means of two distinct, but ultimately equivalent, methods: the Slavnov–Taylor identity satisfied by the three-gluon vertex, and the nonlinear Schwinger–Dyson equation that governs the dynamical evolution of this vertex. Our analysis reveals that the Slavnov–Taylor identity imposes strict model-independent constraints on the associated residues, preventing them from vanishing. Approximate versions of these constraints are subsequently recovered from the Schwinger–Dyson equation, once the elements responsible for the activation of the Schwinger mechanism have been duly incorporated. The excellent coincidence between the two approaches exposes a profound connection between symmetry and dynamics, and serves as a nontrivial self-consistency test of this particular mass generating scenario.

Nonparametric analysis of the Hubble diagram with neural networks

The recent extension of the Hubble diagram of supernovae and quasars to redshifts much higher than 1 prompted a revived interest in nonparametric approaches to test cosmological models and to measure the expansion rate of the Universe. In particular, it is of great interest to infer model-independent constraints on the possible evolution of the dark energy component. Here we present a new method, based on neural network regression, to analyze the Hubble diagram in a completely nonparametric, model-independent fashion. We first validated the method through simulated samples with the same redshift distribution as the real ones, and we discuss the limitations related to the “inversion problem” for the distance-redshift relation. We then applied this new technique to the analysis of the Hubble diagram of supernovae and quasars. We confirm that the data up to z ∼ 1 − 1.5 are in agreement with a flat Λ cold dark matter model with ΩM ∼ 0.3, while ∼5-sigma deviations emerge at higher redshifts. A flat Λ cold dark matter model would still be compatible with the data with ΩM > 0.4. Allowing for a generic evolution of the dark energy component, we find solutions that suggest an increasing value of ΩM with redshift, as predicted by interacting dark sector models.

Understanding large scale CMB anomalies with the generalized nonminimal derivative coupling during inflation

We study the observational implications of a class of inflationary models wherein the inflaton is coupled to the Einstein tensor through a generalized nonminimal derivative coupling (GNMDC). In particular, we explore whether these models can generate suitable features in the primordial spectrum of curvature perturbations as a possible explanation for the large-scale anomalies associated with the angular power spectrum of CMB temperature anisotropies. We derive model-independent constraints on the GNMDC function for such a scenario, considering both the scalar and tensor perturbations. We modify cosmomc to accommodate our GNMDC framework and investigate different classes of inflationary models using a fully consistent numerical approach. We find that the hilltop-quartic model with a specific choice of the GNMDC function provides a considerable improvement over the best-fit reference $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model with a nearly scale-invariant power spectrum. While the large-scale structure observations should be able to provide independent constraints, future CMB experiments, such as CMB-S4 and CMB-Bharat, are expected to constrain further the parameter space of such beyond canonical single-field inflationary models.

Combining gravitational lensing and gravitational redshift to measure the anisotropic stress with future galaxy surveys

Galaxy surveys provide one of the best ways to constrain the theory of gravity at cosmological scales. They can be used to constrain the two gravitational potentials encoding time, $\Psi$, and spatial, $\Phi$, distortions, which are exactly equal at late time within general relativity. Hence, any small variation leading to a nonzero anisotropic stress, i.e. a difference between these potentials, would be an indication for modified gravity. Current analyses usually consider gravitational lensing and redshift-space distortions to constrain the anisotropic stress, but these rely on certain assumptions like the validity of the weak equivalence principle, and a specific time evolution of the functions encoding deviations from general relativity. In this work, we propose a reparametrization of the gravitational lensing observable, together with the use of the relativistic dipole of the correlation function of galaxies to directly measure the anisotropic stress with a minimum amount of assumptions. We consider the future Legacy Survey of Space and Time of the Vera C. Rubin Observatory and the future Square Kilometer Array, and show that combining gravitational lensing and gravitational redshift with the proposed approach we will achieve model-independent constraints on the anisotropic stress at the level of $\sim 20\,\%$.

Dark matter fluid constraints from galaxy rotation curves

Galaxy rotation curves are considered to be convincing evidence for dark matter or some dynamically equivalent alternative mechanism. Starting only from the rotation curve data, we present a model independent approach of testing a general hypothesis that dark matter has the properties of a barotropic fluid. It is shown how the speed of sound squared can be expressed in terms of rotation curve data and their radial derivatives and how model independent constraints can be obtained from the requirements that it is confined between 0 and c^2. Using the Milky Way rotation curve data available in the literature, we obtain the constraints on the barotropic fluid speed of sound and illustrate the potential of this approach. Technical challenges, limitations and possible future extensions and improvements of the proposed approach are discussed.

Current data are consistent with flat spatial hypersurfaces in the ΛCDM cosmological model but favor more lensing than the model predicts

We study the performance of three pairs of tilted $\Lambda$CDM cosmological models, two pairs allowing for non-flat spatial hypersurfaces with CMB temperature and polarization power spectrum data (P18) from Planck, P18 lensing (lensing), and non-CMB data (non-CMB). For the six models, we measure cosmological parameters and study whether or not pairs of the data sets are mutually consistent in these models. Half of these models allow the lensing consistency parameter $A_L$ to be an additional free parameter, while the other three have $A_L = 1$. The tilted spatially-flat models assume the usual primordial spatial inhomogeneity power spectrum. The tilted non-flat models assume either the primordial power spectrum used in the Planck group analyses [Planck $P(q)$] or a recently computed power spectrum [new $P(q)$]. In the tilted non-flat models with $A_L=1$ we find differences between P18 data and non-CMB data cosmological parameter constraints, which are large enough to rule out the Planck $P(q)$ model at 3$\sigma$ but not the new $P(q)$ model. While both P18 data and non-CMB data separately favor a closed geometry when P18+non-CMB data are jointly analyzed the evidence in favor of non-flat hypersurfaces subsides. Differences between P18 data and non-CMB data cosmological constraints subside when $A_L$ is allowed to vary. From the most restrictive P18+lensing+non-CMB data combination we get almost model-independent constraints and find that the $A_L>1$ option is preferred over the $\Omega_k<0$ one, with the $A_L$ parameter, for all models, being larger than unity by $\sim 2.5\sigma$. According to the deviance information criterion, in the P18+lensing+non-CMB analysis, the varying $A_L$ option is on the verge of being strongly favored over the $A_L=1$ one, which could indicate a problem for the standard tilted flat $\Lambda$CDM model (Abridged abstract).

Efficiently exploring multidimensional parameter spaces beyond the Standard Model

We propose a method to ease the challenges of exploring multidimensional parameter spaces in beyond-the-Standard Model theories. We evaluate the model likelihood for any choice of parameters by sampling the theory parameters intelligently and building a kernel density estimator. By reducing the number of expensive Monte Carlo simulations, this method provides an efficient way to test complex theories. We illustrate our technique to set new limits on a short-lived heavy neutrino $N$, proposed as an explanation of anomalies in neutrino experiments. Using a search for lepton pairs in the T2K near detector, we find exclusion limits on the model parameters in a vast region of parameter space, fully exploiting the advantages of our new method. With a single Monte Carlo simulation, we obtain the differential event rate for arbitrary choices of model parameters, allowing us to cast limits on any slice of the model parameter space. We conclude that $N$ particles with lifetimes greater than $c{\ensuremath{\tau}}^{0}\ensuremath{\gtrsim}3\text{ }\text{ }\mathrm{cm}$ are excluded by T2K data. We also derive model-independent constraints in terms of the total rate, lifetime, and $N$ mass and provide an approximated analytical formula. This method can be applied in other branches of physics to explore the landscape of theory parameters efficiently.

Cosmological model-independent constraints on the baryon fraction in the IGM from fast radio bursts and supernovae data

Fast Radio Bursts (FRBs) are millisecond-duration radio transients with an observed dispersion measure (DM) greater than the expected Milky Way contribution, which suggests that such events are of extragalactic origin. Although some models have been proposed to explain the physics of the pulse, the mechanism behind the FRBs emission is still unknown. From FRBs data with known host galaxies, the redshift is directly measured and can be combined with estimates of the DM to constrain the cosmological parameters, such as the baryon number density and the Hubble constant. However, the poor knowledge of the fraction of baryonic mass in the intergalactic medium (f_{IGM}) and its degeneracy with the cosmological parameters impose limits on the cosmological application of FRBs. In this work we present a cosmological model-independent method to determine the evolution of f_{IGM} combining the latest FRBs observations with localized host galaxy and current supernovae data. We consider constant and time-dependent f_{IGM} parameterizations and show, through a Bayesian model selection analysis, that a conclusive answer about the time-evolution of f_{IGM} depend strongly on the DM fluctuations due to the spatial variation in cosmic electron density (delta ). In particular, our analysis show that the evidence varies from strong (in favor of a growing evolution of f_{IGM} with redshift) to inconclusive, as larger values of delta are considered.

Neutron star mass estimates from gamma-ray eclipses in spider millisecond pulsar binaries

Reliable neutron star mass measurements are key to determining the equation of state of cold nuclear matter, but such measurements are rare. Black widows and redbacks are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Spectroscopy of the optically bright companions can determine their radial velocities, providing inclination-dependent pulsar mass estimates. Although inclinations can be inferred from subtle features in optical light curves, such estimates may be systematically biased due to incomplete heating models and poorly understood variability. Using data from the Fermi Large Area Telescope, we have searched for gamma-ray eclipses from 49 spider systems, discovering significant eclipses in 7 systems, including the prototypical black widow PSR B1957+20. Gamma-ray eclipses require direct occultation of the pulsar by the companion, and so the detection, or significant exclusion, of a gamma-ray eclipse strictly limits the binary inclination angle, providing new robust, model-independent pulsar mass constraints. For PSR B1957+20, the eclipse implies a much lighter pulsar (1.81 ± 0.07 solar masses) than inferred from optical light curve modelling.

(Nearly) Model-independent Constraints on the Neutral Hydrogen Fraction in the Intergalactic Medium at z ∼ 5–7 Using Dark Pixel Fractions in Lyα and Lyβ Forests

Cosmic reionization was the last major phase transition of hydrogen from neutral to highly ionized in the intergalactic medium (IGM). Current observations show that the IGM is significantly neutral at z > 7 and largely ionized by z ∼ 5.5. However, most methods to measure the IGM neutral fraction are highly model dependent and are limited to when the volume-averaged neutral fraction of the IGM is either relatively low () or close to unity (). In particular, the neutral fraction evolution of the IGM at the critical redshift range of z = 6–7 is poorly constrained. We present new constraints on at z ∼ 5.1–6.8 by analyzing deep optical spectra of 53 quasars at 5.73 < z < 7.09. We derive model-independent upper limits on the neutral hydrogen fraction based on the fraction of “dark” pixels identified in the Lyα and Lyβ forests, without any assumptions on the IGM model or the intrinsic shape of the quasar continuum. They are the first model-independent constraints on the IGM neutral hydrogen fraction at z ∼ 6.2–6.8 using quasar absorption measurements. Our results give upper limits of (1σ), (1σ), and (1σ). The dark pixel fractions at z > 6.1 are consistent with the redshift evolution of the neutral fraction of the IGM derived from Planck 2018.

The charged Higgs from the Bottom-Up: probing flavor at the LHC

We systematically study model-independent constraints on the three generic charged Higgs couplings to b-quarks and up-type quarks. While existing LHC searches have focussed on the tb coupling, we emphasize that the LHC plays a crucial role in probing also ub and cb couplings, since constraints from flavor physics are weak. In particular we propose various new searches that can significantly extend the present reach on the parameter space by: i) looking for light charged Higgses that decay into ub-quarks, ii) probing charged Higgs couplings to light and top quarks using multi-b-jet signatures, iii) looking for single b-quarks in low-mass dijet searches, iv) searching for charge asymmetries induced by charged Higgs production via ub couplings.

Fisher matrix for the one-loop galaxy power spectrum: measuring expansion and growth rates without assuming a cosmological model

We introduce a methodology to extend the Fisher matrix forecasts to mildly non-linear scales without the need of selecting a cosmological model. We make use of standard non-linear perturbation theory for biased tracers complemented by counterterms, and assume that the cosmological distances can be measured accurately with standard candles. Instead of choosing a specific model, we parametrize the linear power spectrum and the growth rate in several k and z bins. We show that one can then obtain model-independent constraints of the expansion rate E(z) = E(z)/H 0 and the growth rate f(k,z), besides the bias functions. We apply the technique to both Euclid and DESI public specifications in the range 0.6 ≤ z ≤ 1.8 and show that the gain in precision when going from k max = 0.1 to 0.2 h/Mpc is around two- to threefold, while it reaches four- to ninefold when extending to k max = 0.3 h/Mpc. In absolute terms, with k max = 0.2 h/Mpc, one can reach high precision on E(z) at each z-shell: 8–10% for DESI with Δz = 0.1, 5–6% for Euclid with Δz = 0.2–0.3. This improves to 1–2% if the growth rate f is taken to be k-independent. The growth rate itself has in general much weaker constraints, unless assumed to be k-independent, in which case the gain is similar to the one for E(z) and uncertainties around 5–15% can be reached at each z-bin. We also discuss how neglecting the non-linear corrections can have a large effect on the constraints even for k max = 0.1 h/Mpc, unless one has independent strong prior information on the non-linear parameters.

Using teleseismic backprojection and InSAR to obtain segmentation information for large earthquakes: a case study of the 2016Mw 6.6 Muji earthquake

SUMMARYA good understanding of earthquake rupture segmentation is important to characterize fault geometries at depth for follow-up tectonic, stress-field or other analyses. We propose a data-driven strategy and develop pre-optimization methods to support finite fault inversions with independent prior estimates on earthquake source parameters. The first method we develop is a time-domain, multi-array and novel multiphase backprojection (BP) of teleseismic data. This method infers the spatio-temporal evolution of the rupture process, including a potential occurrence of rupture segmentation. Secondly, we apply image analysis methods on InSAR surface displacement maps to infer rupture characteristics (e.g. strike and length) and the number of potential segments. Both methods can provide model-independent constraints on fault location, dimension, orientation and rupture timing, applicable to form priors of model parameters before detailed modelling. We demonstrate and test our methods based on synthetic tests and an application to the 25.11.2016 Muji Mw 6.6 earthquake. Our results indicate segmentation and bilateral rupturing for the 2016 Muji earthquake. The results of the BP of the Muji Mw 6.6 earthquake using high-frequency filtered teleseismic waveforms in particular shows the capability to illuminate the rupture history with the potential to resolve the start and stop phases of individual fault segments.

Absolute ν Mass Measurement with the DUNE Experiment.

Time of flight delay in the supernova neutrino signal offers a unique tool to set model-independent constraints on the absolute neutrino mass. The presence of a sharp time structure during a first emission phase, the so-called neutronization burst in the electron neutrino flavor time distribution, makes this channel a very powerful one. Large liquid argon underground detectors will provide precision measurements of the time dependence of the electron neutrino fluxes. We derive here a new ν mass sensitivity attainable at the future DUNE far detector from a future supernova collapse in our galactic neighborhood, finding a sub-eV reach under favorable scenarios. These values are competitive with those expected for laboratory direct neutrino mass searches.

Factorizing hidden particle production rates

A method is proposed to streamline the computation of hidden particle production rates by factorizing them into i) a model-independent SM contribution, and ii) a observable-independent hidden sector contribution. The SM contribution can be computed once for each observable and re-used for a wide array of hidden sector models, while the hidden sector contribution can be computed once for each model, and re-used for a wide array of observables. The SM contribution also facilitates extracting model independent constraints on hidden particle production. The method is compatible with effective field theory (EFT) and simplified model approaches. It is illustrated by factorizing the rate of charged kaon decays into a charged lepton and a number of hidden particles, and a single form factor Fℓ is found to parametrize the impact of general hidden sectors. We derive model-independent constraints for the form factor Fe that governs decays into positrons and hidden particles.

Nearly model-independent constraints on dense matter equation of state in a Bayesian approach

We apply a Bayesian approach to construct a large number of minimally constrained equations of state (EOSs) and study their correlations with a few selected properties of a neutron star (NS). Our set of minimal constraints includes a few basic properties of saturated nuclear matter and low-density pure neutron matter EOS which is obtained from a precise next-to-next-to-next-to-leading-order (${\mathrm{N}}^{3}\mathrm{LO}$) calculation in chiral effective field theory. The tidal deformability and radius of a NS with mass $1--2\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ are found to be strongly correlated with the pressure of $\ensuremath{\beta}$-equilibrated matter at densities higher than the saturation density (${\ensuremath{\rho}}_{0}=0.16\text{ }\text{ }{\mathrm{fm}}^{\ensuremath{-}3}$) in a nearly model-independent manner. These correlations are employed to parametrize the pressure for $\ensuremath{\beta}$-equilibrated matter, around $2{\ensuremath{\rho}}_{0}$, as a function of neutron star mass and the corresponding tidal deformability. The maximum mass of the neutron star is also found to be strongly correlated with the pressure of $\ensuremath{\beta}$-equilibrated matter at densities $\ensuremath{\sim}4.5{\ensuremath{\rho}}_{0}$.