arXiv subjects

Giada Bargiacchi

Publications and source records attributed to Giada Bargiacchi.

18 recordsLinked to original sources

arXiv2026

GUEST: Gravitational Universe Exploration with Satellite Tracking. A passive satellite laser-ranging mission for the dark gravitational Universe

GUEST is a space mission concept whose central objective is the detection of gravitational waves (GWs) in the microhertz band -- a physics-rich frequency window that no other present or planned detector can reach at a significant level. The concept is simple: two dense, passive spheres, covered with cube-corner retroreflectors, deployed in {highly eccentric} Earth orbits ($e \gtrsim 0.7$, period $P \gtrsim 33$ h), tracked continuously by the global network of satellite laser-ranging stations over a minimum observation time of 10 years, with an expected total duration of 30 years. The orbits themselves act as resonant detectors of the oscillating gravitational perturbations, with the microhertz sensitivity emerging from the selected orbital parameters. From the same data stream, GUEST delivers a programme of fundamental and applied science that cuts across particle physics, gravitational-wave astronomy, cosmology, astrophysics, and geodesy: the first coherent search for GWs from supermassive black-hole binaries in the $\mu$Hz band, the exploration of primordial GW backgrounds in the unexplored energy-scale gap between pulsar-timing arrays and LISA, a dedicated probe of ultra-light dark matter in a parameter region untouched by any other experiment, a new way to search for ultra-light bosons, order-of-magnitude-improved tests of new gravitational interactions at astronomical ranges, and a step change in the absolute determination of $GM_\oplus$ that underpins the Global Geodetic Observing System and future navigation and Earth-observation missions. This white paper presents the motivation, scientific reach, and mission concept of GUEST.

astro-ph.CO↗

arXiv2026

NovaMoon: A Strategic Lunar Reference Station for Positioning, Timing, and Largely Enhanced Science in the Earth-Moon System

The renewed interest in lunar exploration and the development of future lunar communication and navigation services highlight the need for a precise, stable, and interoperable geodetic and timing infrastructure on the Moon. NovaMoon, proposed as a scientific and navigation payload for ESA's Argonaut lander, is designed as a lunar-based local differential, geodetic, and timing station supporting both operational needs in the Moon's south polar region and a broad range of scientific investigations. The payload integrates a lunar laser retroreflector, a Very Long Baseline Interferometry transmitter, a receiver for navigation signals compatible with LunaNet standards, high-stability atomic clocks, and direct-to-Earth radio links -- making it the first lunar station to co-locate multiple ranging, tracking, and timing techniques. NovaMoon will enable sub-metre to decimetre positioning, provide local differential corrections for lunar users, and ensure an accurate and stable realisation of position and time. Preliminary simulation studies show that this multi-technique dataset improves the lunar reference frame, orientation and ephemerides, and estimates of interior parameters like tidal response and core properties. NovaMoon will also provide the first long-duration physical realisation of a lunar time reference. Beyond its primary goals, it supports improved cartography, precise surface geolocation, and higher-resolution topography, contributing to safer landings and operations. It also enables new tests of fundamental physics, including constraints on relativity and possible deviations from classical gravity.

astro-ph.EP↗

arXiv2025

Cosmological and lunar laser ranging constraints on evolving dark energy in a nonminimally coupled curvature-matter gravity model

We analyze a cosmological solution to the field equations of a modified gravity model where curvature and matter are nonminimally coupled. The current Universe's accelerated expansion is driven by a cosmological constant while the impact of the nonminimal coupling on the expansion history is recast as an effective equation of state for evolving dark energy. The model is analyzed under a tracking solution that follows the minimum of the effective potential for a scalar field that captures the modified theory's effects. We determine the conditions for the existence of this minimum and for the validity of the tracking solution. Cosmological constraints on the parameters of the model are obtained by resorting to recent outcomes of data from the DESI collaboration in combination with the Pantheon+ and Dark Energy Survey supernovae compilations, which give compatible results that point to the presence of a dynamical behavior for dark energy. The gravity model violates the equivalence principle since it gives rise to a fifth force that implies the Earth and Moon fall differently towards the Sun. The cosmological constraints are intersected with limits resulting from a test of the equivalence principle in the Earth-Moon system based on lunar laser ranging data. We find that a variety of model parameters are consistent with both of these constraints, all while producing a dynamical evolution of dark energy with similarities to that found in recent DESI results.

gr-qc↗

arXiv2024

High-redshift Cosmology by Gamma-Ray Bursts: an overview

Several correlations among Gamma-Ray Bursts (GRBs) quantities, both in the prompt and afterglow emissions, have been established during the last decades, thus enabling the standardization of GRBs as cosmological probes. Since GRBs are observed up to redshift $z \sim 9$, they represent a valuable tool to fill in the gap of information on the Universe evolution between the farthest type Ia supernovae and the Cosmic Microwave Background Radiation and to shed new light on the current challenging cosmological tensions. Without claiming for completeness, here we describe the state of the art of GRB correlations, their theoretical interpretations, and their cosmological applications both as standalone probes and in combination with other probes. In this framework, we pinpoint the importance of correcting the correlations for selection biases and redshift evolution to derive intrinsic relations, the assets of combining probes at different scales, and the need for the employment of the appropriate cosmological likelihood to precisely constrain cosmological parameters. Furthermore, we emphasize the benefits of the cosmographic approach to avoid any cosmological assumptions and the valuable applications of machine learning techniques to reconstruct GRB light curves and predict unknown GRB redshifts. Finally, we stress the relevance of all these factors, along with future observations, to definitely boost the power of GRBs in cosmology.

astro-ph.CO↗

arXiv2024

Quasars as standard candles VI: spectroscopic validation of the cosmological sample

A sample of quasars has been recently assembled to investigate the non-linear relation between their monochromatic luminosities at 2500{\AA}, and 2 keV and to exploit quasars as a new class of standardized candles. The use of this technique for cosmological purposes relies on the non-evolution with redshift of the UV-optical spectral properties of quasars, as well as on the absence of possible contaminants such as dust extinction and host-galaxy contribution. We address these possible issues by analysing the spectral properties of our cosmological quasar sample. We produced composite spectra in different bins of redshift and accretion parameters (black hole mass, bolometric luminosity), to investigate any possible evolution of the spectral properties of the continuum of the composites with these parameters. We found a remarkable similarity amongst the various stacked spectra. The overall shape of the continuum does not show any statistically significant trend with the accretion parameters nor with the redshift. The composite spectrum of our quasar sample is consistent with negligible levels of both intrinsic reddening (with a colour excess E(B-V)< 0.01) and host-galaxy emission (less than 10%) in the optical. We tested whether unaccounted dust extinction could explain the discrepancy between our cosmographic fit of the Hubble-Lemaitre diagram and the concordance {\Lambda}CDM model. The average colour excess required to solve the tension should increase with redshift up to unphysically high values (E(B-V)=0.1 at z>3) that would imply that the intrinsic emission of quasars is much bluer and more luminous than ever reported in observed spectra. The similarity of quasar spectra across the parameter space excludes a significant evolution of the average continuum properties with any of the explored parameters, confirming the reliability of our sample for cosmological applications.

astro-ph.GA↗

arXiv2024

The Scavenger Hunt for Quasar Samples to Be Used as Cosmological Tools

Although the $Λ$ Cold Dark Matter model is the most accredited cosmological model, information at high redshifts ($z$) between type Ia supernovae ($z=2.26$) and the Cosmic Microwave Background ($z=1100$) is crucial to validate this model further. To this end, we have discovered a sample of 1132 quasars up to $z=7.54$ exhibiting a reduced intrinsic dispersion of the relation between ultraviolet and X-ray fluxes, $δ_\mathrm{F}=0.22$ vs. $δ_\mathrm{F}=0.29$ ($24\%$ less), than the original sample. This gold sample, once we correct the luminosities for selection biases and redshift evolution, enables us to determine the matter density parameter $Ω_M$ with a precision of 0.09. Unprecedentedly, this quasar sample is the only one that, as a standalone cosmological probe, yields such tight constraints on $Ω_M$ while being drawn from the same parent population of the initial sample.

astro-ph.CO↗

arXiv2024

Reduced uncertainties up to 43\% on the Hubble constant and the matter density with the SNe Ia with a new statistical analysis

Type Ia Supernovae (SNe Ia) are considered the most reliable \textit{standard candles} and they have played an invaluable role in cosmology since the discovery of the Universe's accelerated expansion. During the last decades, the SNe Ia samples have been improved in number, redshift coverage, calibration methodology, and systematics treatment. These efforts led to the most recent \textit{``Pantheon"} (2018) and \textit{``Pantheon +"} (2022) releases, which enable to constrain cosmological parameters more precisely than previous samples. In this era of precision cosmology, the community strives to find new ways to reduce uncertainties on cosmological parameters. To this end, we start our investigation even from the likelihood assumption of Gaussianity, implicitly used in this domain. Indeed, the usual practise involves constraining parameters through a Gaussian distance moduli likelihood. This method relies on the implicit assumption that the difference between the distance moduli measured and the ones expected from the cosmological model is Gaussianly distributed. In this work, we test this hypothesis for both the \textit{Pantheon} and \textit{Pantheon +} releases. We find that in both cases this requirement is not fulfilled and the actual underlying distributions are a logistic and a Student's t distribution for the \textit{Pantheon} and \textit{Pantheon +} data, respectively. When we apply these new likelihoods fitting a flat $Λ$CDM model, we significantly reduce the uncertainties on $Ω_M$ and $H_0$ of $\sim 40 \%$. This boosts the SNe Ia power in constraining cosmological parameters, thus representing a huge step forward to shed light on the current debated tensions in cosmology.

astro-ph.CO↗

arXiv2023

Quasars as Standard Candles V. Evaluation of a $\leq$ 0.06 dex intrinsic dispersion in the LX-LUV relation

A characteristic feature of quasars is the observed non-linear relationship between their monochromatic luminosities at rest-frame 2500 {\AA} and 2 keV. This relationship is evident across all redshifts and luminosities and, due to its non-linearity, can be implemented to estimate quasar distances and construct a Hubble Diagram for quasars. Historically, a significant challenge in the cosmological application of this relation has been its high observed dispersion. Recent studies have demonstrated that this dispersion can be reduced by excluding biased objects from the sample. Nevertheless, the dispersion remains considerable ($\delta \sim 0.20$ dex), especially when compared to the Phillips relation for supernovae Ia. Given the absence of a comprehensive physical model for the relation, it remains unclear how much of the remaining dispersion is intrinsically tied to the relation itself and how much can be attributed to observational factors not addressed by the sample selection and by the choice of X-ray and UV indicators. Potential contributing factors include (i) the scatter produced by using X-ray photometric results instead of spectroscopic ones, (ii) the intrinsic variability of quasars, and (iii) the inclination of the accretion disc relative to our line of sight. In this study, we thoroughly examine these three factors and quantify their individual contributions to the observed dispersion. Based on our findings, we argue that the intrinsic dispersion of the X-ray/UV luminosity relation is likely below 0.06 dex. We also discuss why high-redshift subsamples can show a significantly lower dispersion than the average one.

astro-ph.CO↗

arXiv2023

Quasars as Standard Candles IV. Analysis of the X-ray and UV indicators of the disc-corona relation

Context: A non-linear relation between quasar monochromatic luminosities at 2500A and 2 keV holds at all observed redshifts and luminosities, and it has been used to derive quasar distances and to build a Hubble Diagram of quasars. The choice of the X-ray and UV indicators has so far been somewhat arbitrary, and has typically relied on photometric data. Aims: We want to determine the X-ray and UV proxies that provide the smallest dispersion of the relation, in order to obtain more precise distance estimates, and to confirm the reliability of the X-ray to UV relation as a distance indicator. Methods: We performed a complete UV spectroscopic analysis of a sample of $\sim$1800 quasars with SDSS optical spectra and XMM- Newton X-ray serendipitous observations. In the X-rays, we analysed the spectra of all the sample objects at redshift z $>$1.9, while we relied on photometric measurements at lower redshifts. As done in previous studies, we analysed the relation in small redshift bins, using fluxes instead of luminosities. Results: We show that the monochromatic fluxes at 1 keV and 2500A are, respectively, the best X-ray and UV continuum indicators among those that are typically available. We also find a tight relation between soft X-ray and Mg ii2800A line fluxes, and a marginal dependence of the X-ray to UV relation on the width of the Mg ii line. Conclusions: Our analysis suggests that the physical quantities that are more tightly linked to one another are the soft X-ray flux at $\sim$1 keV and the ionizing UV flux blueward of the Lyman limit. However, the "usual" monochromatic fluxes at 2 keV and 2500A estimated from photometric data provide an almost as-tight X-ray to UV relation, and can be used to derive quasar distances. The Hubble diagram obtained using spectroscopic indicators is fully consistent with the one presented in previous papers, based on photometric data.

astro-ph.CO↗

arXiv2023

Quasars: standard candles up to z=7.5 with the precision of Supernovae Ia

Currently, the $Λ$ Cold Dark Matter model, which relies on the existence of cold dark matter and a cosmological constant $Λ$, best describes the Universe. However, we lack information in the high-redshift ($z$) region between Type Ia Supernovae (SNe Ia) (up to $z=2.26$) and the Cosmic Microwave Background ($z=1100$), an interval crucial to test cosmological models and their possible evolution. We have defined a sample of 983 Quasars up to $z=7.54$ with reduced intrinsic dispersion $δ=0.007$ which determines the matter density parameter $Ω_M$ with the same precision of SNe Ia. Although previous analysis have been used Quasars as cosmological tools (e.g. Risaliti and Lusso 2019), this is the first time that high-redshift sources, in this case Quasars, as standalone cosmological probes yield such tight constraints on $Ω_M$. Our results show the importance of correcting cosmological relationships for selection biases and redshift evolution and how the choice of a golden sample reduces considerably the intrinsic scatter. This proves the reliability of Quasars as standard cosmological candles.

astro-ph.CO↗

arXiv2023

Reducing the uncertainty on the Hubble constant up to 35\% with an improved statistical analysis: different best-fit likelihoods for Supernovae Ia, Baryon Acoustic Oscillations, Quasars, and Gamma-Ray Bursts

Cosmological models and their parameters are widely debated, especially about whether the current discrepancy between the values of the Hubble constant, $H_{0}$, obtained by type Ia supernovae (SNe Ia), and the Planck data from the Cosmic Microwave Background Radiation could be alleviated when alternative cosmological models are considered. Thus, combining high-redshift probes, such as Gamma-Ray Bursts (GRBs) and Quasars (QSOs), together with Baryon Acoustic Oscillations (BAO) and SNe Ia is important to assess the viability of these alternative models and if they can cast further light on the Hubble tension. In this work, for GRBs, we use a 3-dimensional relation between the peak prompt luminosity, the rest-frame time at the end of the X-ray plateau, and its corresponding luminosity in X-rays: the 3D Dainotti fundamental plane relation. Regarding QSOs, we use the Risaliti-Lusso relation among the UV and X-ray luminosities for a sample of 2421 sources. We correct both the QSO and GRB relations by accounting for selection and evolutionary effects with a reliable statistical method. We here use both the traditional Gaussian likelihoods ($\cal L_G$) and the new best-fit likelihoods ($\cal L_N$) to infer cosmological parameters of a non-flat $Λ$CDM and flat $w$CDM models. We obtain for all the parameters reduced uncertainties, up to $35\%$ for $H_{0}$, when applying the new $\cal L_N$ likelihoods in place of the Gaussian ones. Our results remain consistent with a flat $Λ$CDM model, although with a shift of the dark energy parameter $w$ toward $w<-1$ and a curvature density parameter toward $Ω_k<0$.

astro-ph.CO↗

arXiv2023

The most luminous blue quasars at 3.0<z<3.3 -- III. LBT spectra and accretion parameters

We present the analysis of the rest frame ultraviolet and optical spectra of 30 bright blue quasars at $z\sim3$, selected to examine the suitability of AGN as cosmological probes. In our previous works, we found an unexpectedly high fraction ($\approx 25 \%$) of X-ray weak quasars in the sample. The latter sources also display a flatter UV continuum and a broader and fainter CIV profile in the archival UV data with respect to their X-ray normal counterparts. Here we present new observations with the LBT in both the $zJ$ (rest-frame $\simeq$2300-3100 $\rm \mathring{A}$) and the $K_S$ ($\simeq$4750-5350 $\rm \mathring{A}$) bands. We estimated black hole masses ($M_{\rm BH}$) and Eddington ratios ($λ_{\rm Edd}$) from the from the H$β$ and MgII emission lines, finding that our $z\sim3$ quasars are on average highly accreting ($\langle λ_{\rm Edd} \rangle\simeq 1.2$ and $\langle M_{\rm BH} \rangle\simeq 10^{9.7}M_\odot$), with no difference in $λ_{\rm Edd}$ or $M_{\rm BH}$ between X-ray weak and X-ray normal quasars. From the $zJ$ spectra, we derive flux and equivalent width of MgII and FeII, finding that X-ray weak quasars display higher FeII/MgII ratios with respect to typical quasars. FeII/MgII ratios of X-ray normal quasars are instead consistent with other estimates up to $z\simeq6.5$, corroborating the idea of already chemically mature BLRs at early cosmic time. From the $K_S$ spectra, we find that all the X-ray weak quasars present generally weaker [OIII] emission (EW<10 $\rm \mathring{A}$) than the normal ones. The sample as a whole, however, abides by the known X-ray/[OIII] luminosity correlation, hence the different [OIII] properties are likely due to an intrinsically weaker [OIII] emission in X-ray weak objects, associated to the shape of the spectral energy distribution. We interpret these results in the framework of accretion-disc winds.

astro-ph.GA↗

arXiv2023

Quasars as high-redshift standard candles

In the past few years, we built a Hubble diagram of quasars up to redshift z$\sim$7, based on the nonlinear relation between quasars' x-ray and UV luminosities. Such a Hubble diagram shows a >4$σ$ deviation from the standard flat $Λ$CDM model at z>1.5. Given the important consequences of this result, it is fundamental to rule out any systematic effect in the selection of the sample and/or in the flux measurements, and to investigate possible redshift dependences of the relation, that would invalidate the use of quasars as standard candles. Here we review all the observational results supporting our method: the match of the Hubble diagram of quasars with that of supernovae in the common redshift range, the constant slope of the relation at all redshifts, the redshift non-evolution of the spectral properties of our sources both in the x-rays and in the UV. An independent test of our results requires the observation of other standard candles at high redshift. In particular, we expect that future observations of supernovas at z>2 will confirm the deviation from the concordance model found with the Hubble diagram of quasars.

astro-ph.CO↗

arXiv2023

A bias-free cosmological analysis with quasars alleviating $H_0$ tension

Cosmological models and their parameters are widely debated because of theoretical and observational mismatches of the standard cosmological model, especially the current discrepancy between the value of the Hubble constant, $H_{0}$, obtained by Type Ia supernovae (SNe Ia), and the Cosmic Microwave Background Radiation (CMB). Thus, considering high-redshift probes like quasars (QSOs), having intermediate redshifts between SNe Ia and CMB, is a necessary step. In this work, we use SNe Ia and the most updated QSO sample, reaching redshifts up to $z\sim7.5$, applying the Risaliti-Lusso QSO relation based on a non-linear relation between ultraviolet and X-ray luminosities. We consider this relation both in its original form and corrected for selection biases and evolution in redshift through a reliable statistical method also accounting for the circularity problem. We also explore two approaches: with and without calibration on SNe Ia. We then investigate flat and non-flat standard cosmological models and a flat $w$CDM model, with a constant dark energy equation of state parameter $w$. Remarkably, when correcting for the evolution as a function of cosmology, we obtain closed constraints on $Ω_M$ using only non-calibrated QSOs. We find that considering non-calibrated QSOs combined with SNe Ia and accounting for the same correction, our results are compatible with a flat $Λ$CDM model with $Ω_M = 0.3$ and $H_0 = 70 \, \mathrm{km\,s^{-1}\,Mpc^{-1}}$. Intriguingly, the $H_0$ values obtained place halfway between the one from SNe Ia and CMB, paving the way for new insights into the $H_0$ tension.

astro-ph.CO↗

arXiv2022

Revealing Intrinsic Flat $Λ$CDM Biases with Standardizable Candles

Emerging high redshift cosmological probes, in particular quasars (QSOs), show a preference for larger matter densities, $Ω_{m} \approx 1$, within the flat $Λ$CDM framework. Here, using the Risaliti-Lusso relation for standardizable QSOs, we demonstrate that the QSOs recover the \textit{same} Planck-$Λ$CDM Universe as Type Ia supernovae (SN), $Ω_m \approx 0.3$ at lower redshifts $ 0 < z \lesssim 0.7$, before transitioning to an Einstein-de Sitter Universe ($Ω_m =1$) at higher redshifts $z \gtrsim 1$. We illustrate the same trend, namely increasing $Ω_{m}$ and decreasing $H_0$ with redshift, in SN but poor statistics prevent a definitive statement. We explain physically why the trend is expected in the flat $Λ$CDM cosmology, illustrate the intrinsic bias and non-Gaussian tails with mock Pantheon data, and identify a similar trend in BAO below $z=1$. Our results highlight an intrinsic bias in the flat $Λ$CDM Universe, whereby $Ω_m$ increases, $H_0$ decreases and $S_8$ increases with effective redshift, thus providing a new perspective on $Λ$CDM tensions; even in a Planck-$Λ$CDM Universe the current tensions might have been expected.

astro-ph.CO↗

arXiv2022

Optical and X-ray GRB Fundamental Planes as Cosmological Distance Indicators

Gamma-Ray Bursts (GRBs), can be employed as standardized candles, extending the distance ladder beyond Supernovae Type Ia (SNe Ia, $z=2.26$). We standardize GRBs using the 3D fundamental plane relation (the Dainotti relation) among the rest-frame end time of the X-ray plateau emission, its corresponding luminosity, and the peak prompt luminosity. Combining SNe Ia and GRBs, we constrain $Ω_{\text{M}}= 0.299 \pm 0.009$ assuming a flat $Λ$CDM cosmology with and without correcting GRBs for selection biases and redshift evolution. Using a 3D optical Dainotti correlation, we find this sample is as efficacious in the determination of $Ω_{\text{M}}$ as the X-ray sample. We trimmed our GRB samples to achieve tighter planes to simulate additional GRBs. We determined how many GRBs are needed as standalone probes to achieve a comparable precision on $Ω_{\text{M}}$ to the one obtained by SNe Ia only. We reach the same error measurements derived using SNe Ia in 2011 and 2014 with 142 and 284 simulated optical GRBs, respectively, considering the errorbars on the variables halved. These error limits will be reached in 2038 and in 2047, respectively. Using a doubled sample (obtained by future machine learning approaches allowing a lightcurve reconstruction and the estimates of GRB redhifts when z is unknown) compared to the current sample, with errorbars halved we will reach the same precision as SNe Ia in 2011 and 2014, now and in 2026, respectively. If we consider the current SNe precision, this will be reached with 390 optical GRBs by 2054.

astro-ph.CO↗

arXiv2021

The most luminous blue quasars at 3.0<z<3.3. II. CIV/X-ray emission and accretion disc physics

We analyse the properties of the CIV broad emission line in connection with the X-ray emission of 30 bright SDSS quasars at z~3.0-3.3 with pointed XMM-Newton observations, which were selected to test the suitability of AGN as cosmological tools. In our previous work, we found that a large fraction (~25%) of the quasars in this sample are X-ray underluminous by factors of >3-10. As absorbing columns of >10$^{23}$ cm$^{-2}$ can be safely ruled out, their weakness is most likely intrinsic. Here we explore possible correlations between the UV and X-ray features of these sources to investigate the origin of X-ray weakness. We fit their UV SDSS spectra and analyse their CIV properties (e.g., equivalent width, EW; line peak velocity, $\upsilon_{\rm peak}$) as a function of the X-ray photon index and 2-10 keV flux. We confirm the trends of CIV $\upsilon_{\rm peak}$ and EW with UV luminosity at 2500 angstrom for both X-ray weak and X-ray normal quasars, as well as the correlation between X-ray weakness and CIV EW. In contrast to some recent work, we do not observe any clear relation between the 2-10 keV luminosity and $\upsilon_{\rm peak}$. We find a correlation between the hard X-ray flux and the integrated CIV flux for X-ray normal quasars, whilst X-ray weak quasars deviate from the main trend by more than 0.5 dex. We argue that X-ray weakness might be interpreted in a starved X-ray corona picture associated with an ongoing disc-wind phase. If the wind is ejected in the vicinity of the black hole, the extreme-UV radiation that reaches the corona will be depleted, depriving the corona of seeds photons and generating an X-ray weak quasar. Yet, at the largest UV luminosities (>10$^{47}$ erg s$^{-1}$), there will still be an ample reservoir of ionising photons that can explain the excess CIV emission observed in the X-ray weak quasars with respect to normal sources of similar X-ray luminosities.

astro-ph.GA↗

arXiv2021

Cosmography by orthogonalized logarithmic polynomials

Cosmography is a powerful tool to investigate the Universe kinematic and then to reconstruct dynamics in a model-independent way. However, recent new measurements of supernovae Ia and quasars have populated the Hubble diagram up to high redshifts ($z \sim 7.5$) and the application of the traditional cosmographic approach has become less straightforward due to the large redshifts implied. Here we investigate this issue through an expansion of the luminosity distance-redshift relation in terms of "orthogonal" logarithmic polynomials. In particular we point out the advantages of a new procedure of "orthogonalization" and we show that such an expansion provides a very good fit in the whole $z=0÷7.5$ range to both real and mock data obtained assuming various cosmological models. Moreover, despite of the fact that the cosmographic series is tested well beyond its convergence radius, the parameters obtained expanding the luminosity distance - redshift relation for the $Λ$CDM model are broadly consistent with the results from a fit of mock data obtained with the same cosmological model. This provides a method to test the reliability of a cosmographic function to study cosmological models at high redshifts and it demonstrates that the logarithmic polynomial series can be used to test the consistency of the $Λ$CDM model with the current Hubble diagram of quasars and supernovae Ia. We confirm a strong tension (at $>4σ$) between the concordance cosmological model and the Hubble diagram at $z>1.5$. Such a tension is dominated by the contribution of quasars at $z>2$ and starts to be present also in the few supernovae Ia observed at $z>1$.

astro-ph.CO↗