H₀ Tension: Observational Status, Theoretical Models, and the Road Ahead

Dear Colleagues,

The discrepancy in the value of the Hubble parameter H₀ as obtained from Planck Cosmic Microwave Background (CMB)–Baryon Acoustic Oscillation (BAO) data (H₀ = 67.4 ± 0.5 km s⁻¹ Mpc⁻¹) and various independent local measurements (H₀ = 74.03 ± 1.42 km s⁻¹ Mpc⁻¹) has reached a level close to 6σ and constitutes a major issue for modern cosmology, challenging the validity of the standard ΛCDM model. The independent local determinations of H₀ are based on Cepheids/Type Ia Supernovae (SNe) (SH0ES), strongly-lensed quasar time delay (H0LiCOW), and water megamasers (The Megamaser Cosmology Project). These local determinations converge to the higher value of H₀. Replacing the Cepheid-based local distance ladder calibration by a Tip of the Red Giant Branch-calibration for Type Ia SNe (Carnegie–Chicago Hubble Program) has led to an intermediate value of H₀, which is about 1.2σ away from the Planck/CMB value and 1.7σ, 1.3σ, 1.2σ from SH0ES, H0LiCOW, and Megamaser Cosmology Project, respectively.

If the above tension among different cosmological observations is not due to a very strong and unnoticed systematic error in the data, it may indicate the presence of a new degree of freedom that needs to be included in the standard ΛCDM model to make it simultaneously consistent with all the above cosmological observations. A wide range of cosmological models has been constructed, introducing various new degrees of freedom that may aleviate the H₀ tension. These models include mechanisms that modify the scale of the sound horizon at last scattering using early dark energy or other types of early species, interaction between dark energy and matter, screened fifth forces on the cosmic distance ladder, modified gravity, new properties of late dark energy, including new types of dark energy equation of state parameter, etc.

In view of the importance of this problem for cosmology, it would be useful to have a resource that will present, in a concentrated and concise manner, the current status of the tension on the observational level and the main theoretical directions that are currently pursued and are likely to lead to alleviation of the tension. The current Special Issue in Universe, an open access journal with a 2018 Impact Factor of 2.165, aims at providing this resource. We invite short or longer contributions from the leaders of this research subject on both the observational and the theoretical aspects. These contributions may summarize the main recent research results of a group working on this subject and propose specific directions for future research. Alternatively, they may also present original research results or longer reviews related to H₀ tension. In particular, we invite contributions of the following forms:

1. Communication (short review summaries, 8–10 pages) of recent projects and results of leading groups in this field. A discussion of proposed future research directions is also strongly encouraged in the context of these contributions.
2. Original research papers.
3. Longer reviews of the current status of the field on both the observational and the theoretical level.

With the proper quality and quantity of contributions, this Special Issue will constitute a useful resource for the community as it will include, in a concentrated and concise manner, the main results and insights of leading groups in the field. Thus ,we hope that it will inspire new research directions that may lead to the final resolution of this important cosmological puzzle. We also hope that you will consider representing the research of your group in this Special Issue, thus further improving its quality and usefulness.

Prof. Dr. Leandros Perivolaropoulos
Guest Editor

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• Hubble parameter H₀
• Cosmological Tension
• Sound horizon Scale
• Dark energy
• Distance ladder
• Modified gravity
• Standard ΛCDM model
• Interacting dark energy
• SH0ES, H0LiCOW, Megamaser, Planck
• Cepheid calibratio