H0 Tension: New Physics or Systematics?

Dear Colleagues,

The tension in estimation of the Hubble constant from different astronomical/cosmological observations assuming the standard LCDM model of the Universe has remained a big issue in the current cosmology. Local measurements of the Hubble constant using cepheids and type Ia supernovae seem to be at more than 4\sigma tension with estimation of the Hubble constant assuming the concordance LCDM model fitting the Planck cosmic microwave background data. Inclusion of the other cosmological observations such as Baryon acoustic oscillation (BAO) and weak lensing can only make the inconsistencies more complicated, leaking the tensions to the estimation of other key cosmological parameters, such as matter density.

What are the sources of these prominent tensions? Is there really a necessity to go beyond the standard model of cosmology? If so, in which part? Is it about evolving dark energy or a form of the primordial spectrum or do we need more species of neutrinos? What would then be the theoretical implications? If it is about systematics, where should we look for such systematics, and what are the possibilities? How can other and/or future observations help to shed light on this problem?

In this Special Issue, we would like to discuss all these possibilities, and we invite our colleagues to submit their manuscripts related to this topic. Some key subjects are listed as below.

1. Significance of the tension using different data;
2. Reliability of the Hubble constant measurement from local Cepheids and supernova observations;
3. Reliability of the estimated cosmological parameters using CMB and other cosmological data (BAO, weak lensing, cluster counts, etc.), assuming the standard LCDM model;
4. Alternative approaches for model-independent estimation of the Hubble constant (strong lens systems, gravitational wave sirens, tip of the red giant branch);
5. Evolving or early dark energy models to resolve tensions;
6. Non power-law form of the primordial spectrum to resolve the tensions;
7. Effective number of neutrinos and the tensions;
8. Dark matter interactions with other sectors to resolve the tensions;
9. Theoretical implications of going beyond the standard model of cosmology (single field inflation, string theory and swampland conjecture, multi species of dark matter, etc.).

Dr. Arman Shafieloo
Guest Editor

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