Tension with the flat ΛCDM model from a high redshift Hubble Diagram of supernovae, quasars and gamma-ray bursts (1907.07692v4)

Abstract: In the current framework, the standard parametrization of our Universe is the so-called Lambda Cold Dark Matter ({\Lambda}CDM) model. Recently, Risaliti & Lusso (2019) have shown a ~4{\sigma} tension with the {\Lambda}CDM model through a model-independent parametrization of a Hubble Diagram of supernovae Ia (SNe Ia) from the JLA survey and quasars. Model-independent approaches and independent samples over a wide redshift range are key to testing this tension and any possible systematics. Here we present an analysis of a combined Hubble Diagram of SNe Ia, quasars, and gamma-ray bursts (GRBs) to check the agreement of the quasar and GRB cosmological parameters at high redshifts (z>2) and to test the concordance flat {\Lambda}CDM model with improved statistical accuracy. We build a Hubble diagram with SNe Ia from the Pantheon sample (Scolnic et al. 2018), quasars from the Risaliti & Lusso (2019) sample, and GRBs from the Demianski et al. (2017a) sample, where quasars are standardised through the observed non-linear relation between their ultraviolet and X-ray emission and GRBs through the correlation between the spectral peak energy and the isotropic-equivalent radiated energy (the so-called "Amati relation"). We fit the data with cosmographic models consisting of a fourth-order logarithmic polynomial and a fifth-order linear polynomial, and compare the results with the expectations from a flat {\Lambda}CDM model. We confirm the tension between the best fit cosmographic parameters and the {\Lambda}CDM model at ~4{\sigma} with SNe Ia and quasars, at ~2{\sigma} with SNe Ia and GRBs, and at >4{\sigma} with the whole SNe Ia+quasars+GRB data set. The completely independent high-redshift Hubble diagrams of quasars and GRBs are fully consistent with each other, strongly suggesting that the deviation from the standard model is not due to unknown systematic effects but to new physics.

Tension with the Flat $\Lambda$CDM Model from a High-Redshift Hubble Diagram of Supernovae, Quasars, and Gamma-Ray Bursts

The paper conducted by Lusso et al. presents a thorough investigation into the discrepancies between the observed high-redshift Hubble diagram and the predictions made by the standard cosmological model, known as the Lambda Cold Dark Matter ($\Lambda$CDM) model. By analyzing a combined dataset that includes type Ia supernovae (SNe Ia), quasars, and gamma-ray bursts (GRBs), the authors aim to evaluate the validity of the $\Lambda$CDM model, which posits a Universe comprised of dark energy and dark matter.

Data Sources and Methodology

The research utilizes a robust dataset comprising three core astronomical observations:

1. Pantheon Supernovae: This dataset includes 1,048 spectroscopically confirmed SNe Ia, offering a redshift range up to $z = 2.26$. The Pantheon compilation is recognized for its precise photometric calibrations and comprehensive redshift coverage.
2. Quasars: Utilizing a sample of 1,598 quasars, the paper explores their ultraviolet and X-ray emissions to construct a cosmological distance-redshift relation, thereby acting as a cosmological probe due to their high luminosity and redshift range ($0.04 < z < 5.1$).
3. Gamma-Ray Bursts (GRBs): Featuring 162 GRBs, this sample extends the redshift reach to $z = 6.67$. The GRBs are standardized using the Amati relation, which correlates their spectral peak energy with isotropic-equivalent radiated energy.

The authors employ a model-independent technique through cosmographic methods that refrains from assuming specific dark energy or matter components beyond the observable parameters of the Hubble diagram. Two cosmographic approaches are used: a traditional Taylor expansion of the scale factor and a novel fourth-order logarithmic polynomial expansion.

Key Findings

The analysis reveals a significant tension between the observed cosmographic parameters and those expected under the flat $\Lambda$CDM model:

• The cosmographic parameters indicate a discrepancy at a level of $\sim4\sigma$ when using the SNe Ia and quasars. This tension is also present, albeit less pronounced ($\sim2\sigma$), in the combination of SNe Ia and GRBs.
• The aggregate dataset comprising SNe Ia, quasars, and GRBs demonstrates a $>4\sigma$ deviation, suggesting the presence of factors beyond current $\Lambda$CDM assumptions.

Implications and Discussion

The implications of these results are multifaceted and suggest potential avenues for future research in cosmology:

• New Physics: The observed tension suggests possible new physics or modifications to the standard model, such as evolving dark energy or alternative gravitational theories. This deviation, especially evident at redshifts greater than 1, underscores the necessity for a reevaluation of the cosmological constants within the $\Lambda$CDM framework.
• Robustness of Methodology: The concordance between independent measurements from quasars and GRBs at high redshifts further corroborates the findings, reducing the likelihood that systematic uncertainties could explain these anomalies.
• Further Observations: Enhanced data quality from pointed observations of high-redshift quasars and GRBs is essential to refine these findings and to better characterize the nature of dark energy.

Conclusion

The paper by Lusso et al. highlights significant tensions between high-redshift observations and the $\Lambda$CDM model, proposing a compelling case for novel cosmological theories or modifications. The results facilitate an exciting frontier in cosmology—one that calls into question the sufficiency of current models and encourages continued observation and theoretical exploration to uncover the true nature of our Universe's expansion. Future work should incorporate a broader dataset and explore theoretical models that could potentially integrate these new insights.