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The longstanding search for the cosmological model that best describes the Universe has been made more intriguing since the recent discovery of the Hubble constant, H 0, tension observed between the value of H 0 from the Cosmic Microwave Background and from type Ia supernovae (SNe Ia). Hence, the commonly trusted flat Λ cold dark matter (ΛCDM) model is under investigation. In this scenario, cosmography is a very powerful technique to investigate the evolution of the Universe without any cosmological assumption, thus revealing tensions between observational data and predictions from cosmological models in a completely model-independent way. We here employ a robust cosmographic technique based on an orthogonal logarithmic polynomial expansion of the luminosity distance to fit quasars (QSOs) alone and QSOs combined with Gamma-Ray Bursts (GRBs), SNe Ia, and Baryon Acoustic Oscillations. To apply QSOs and GRBs as probes we use, respectively, the Risaliti–Lusso relation between ultraviolet and X-ray luminosities and the 'Dainotti GRB 3D relation' among the rest-frame end time of the X-ray plateau emission, its corresponding luminosity, and the peak prompt luminosity. We also correct QSOs and GRBs for selection biases and redshift evolution and we employ both the traditional Gaussian likelihood and the newly discovered best-fitting likelihoods for each probe investigated. This comprehensive analysis reveals a strong tension (⁠|$\gt 4 \, \sigma$|⁠) between our data sets and the flat ΛCDM model proving the power of both the cosmographic approach and high-redshift sources, such as QSOs and GRBs, which can probe the Universe at early epochs. [ABSTRACT FROM AUTHOR]

The recent |$\sim 4 \, \sigma$| Hubble constant, H 0, tension is observed between the value of H 0 from the cosmic microwave background (CMB) and Type Ia supernovae (SNe Ia). It is a decade since this tension is excruciating the modern astrophysical community. To shed light on this problem is key to consider probes at intermediate redshifts between SNe Ia and CMB and reduce the uncertainty on H 0. Toward these goals, we fill the redshift gap by employing gamma-ray bursts (GRBs) and quasars (QSOs), reaching z  = 9.4 and z  = 7.6, respectively, combined with baryonic acoustic oscillations (BAO), and SNe Ia. To this end, we employ the 'Dainotti GRB 3D relation' among the rest-frame end time of the X-ray plateau emission, its corresponding luminosity, and the peak prompt luminosity, and the 'Risaliti–Lusso' QSO relation between ultraviolet and X-ray luminosities. We inquire the commonly adopted Gaussianity assumption on GRBs, QSOs, and BAO. With the joint sample, we fit the flat Λ Cold Dark Matter model with both the Gaussian and the newly discovered likelihoods. We also investigate the impact of the calibration assumed for Pantheon and Pantheon  + SNe Ia on this analysis. Remarkably, we show that only GRBs fulfil the Gaussianity assumption. We achieve small uncertainties on the matter-density parameter Ω M and H 0. We find H 0 values compatible within 2σ with the one from the Tip of the Red Giant Branch. Finally, we show that the cosmological results are heavily biased against the arbitrary calibration choice for SNe Ia. [ABSTRACT FROM AUTHOR]

In the past few years, we built a Hubble diagram of quasars up to redshift z∼7$$ z\sim 7 $$, based on the nonlinear relation between quasars' x‐ray and UV luminosities. Such a Hubble diagram shows a >4σ$$ >4\sigma $$ deviation from the standard flat Λ$$ \Lambda $$CDM model at z>1.5$$ z>1.5 $$. 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$$ z>2 $$ will confirm the deviation from the concordance model found with the Hubble diagram of quasars. [ABSTRACT FROM AUTHOR]

We present a new catalogue of ∼2400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM–Newton. The sample can be used to investigate the non-linear relation between the ultraviolet (UV) and X-ray luminosity of quasars as well as to build a Hubble diagram up to a redshift of z ∼ 7.5. We selected sources that are neither reddened by dust in the optical and UV nor obscured by gas in the X-rays, and whose X-ray fluxes are free from flux-limit-related biases. After checking for any possible systematics, we confirm, in agreement with our previous works, that the X-ray to UV relation provides distance estimates matching those from supernovae up to z ∼ 1.5, and its slope shows no redshift evolution up to z ∼ 5. We provide a full description of the methodology for testing cosmological models, further supporting a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat cold dark matter model up to z ∼ 1.5–2, but strong deviations emerge at higher redshifts. Since we have minimised all non-negligible systematic effects and proven the stability of the LX − LUV relation at high redshifts, we conclude that an evolution of the expansion rate of the Universe should be considered as a possible explanation for the observed deviation, rather than some systematic (redshift-dependent) effect associated with high-redshift quasars. [ABSTRACT FROM AUTHOR]