*8.1. Calibration Versus Non-Calibration*

Constraining cosmological parameters using GRBs is plagued by several conceptual and practical issues.

First, all GRB correlations by definition are built assuming an *a priori* background cosmology (see Sections 2.5 and 6.1 for details) and, consequently, introducing a circularity problem [166]. Second, the majority of GRB correlations holds for LGRBs (see Section 6.1 for details), whose observational rate falls off rapidly at low-*z* and, in some cases, such a nearby LGRB seems to be intrinsically different from the *cosmological* ones [37,167]. Third, unlike SNe Ia, which are calibrated with a selected sub-sample at a very low redshift<sup>23</sup> by anchoring them to primary distance indicators as Cepheids, the shortage of low-*z* GRBs prevents anchoring them to primary distance indicators.

Focusing on the above circularity problem, this is essentially an epistemological issue due to the lack of very low-*z* GRBs and arising from the need of a background cosmology to compute the above-defined *E*iso, *Eγ*, and *L*iso entering GRB correlations [166]. For example, calibrating GRBs through the standard ΛCDM model, the estimate of cosmological parameters of any dark energy framework inevitably returns an overall agreement with the concordance model. Debates toward its use in cosmology seem to indicate that this effect could be minor, albeit it plagues cosmological constraints obtained from GRBs.

Possible ways out known in the literature involve calibration techniques based on the use of SNe Ia distance moduli, cosmographic series, cosmic chronometers, etc. All of these procedures represent plausible solutions, but at the same time introduce possible issues that we are going to describe below.

The above calibration procedures have to compare with an alternative method which completely by-passes the calibration procedure [10]. This uncalibrated procedure consists of a simultaneous fit of correlation parameters together with the cosmological model parameters. This uncalibrated procedure consists of a simultaneous fit of correlation and cosmological model parameters. This procedure and the related issues are also discussed in the following.
