Shown above: values of the expansion rate of the universe (Hubble constant H0) from direct and indirect measurements by different missions, with the grey and pink bands showing the 68% confidence-level values from SH0ES and Planck, respectively.
Problem
“Local measurements of the current expansion rate of the universe, for example based on supernovae as standard candles, which do not rely heavily on assumptions about cosmological models, give values that cluster around 73 km s–1 Mpc–1. Then there is another, indirect route to measuring what we believe is the same quantity but only within a model, the lambda-cold-dark-matter (ΛCDM) model, which is looking at the baby universe via the cosmic microwave background (CMB). When we look at the CMB, we don’t measure recession velocities, but we interpret a parameter within the model as the expansion rate of the universe. The ΛCDM model is extremely successful, but the value of the Hubble constant using this method comes out at around 67 km s–1 Mpc–1, and the discrepancy with local measurements is now 4σ or more.” – Licia Verde [2]
CDM stand for “cold dark matter”. CMB stands for “cosmic background radiation”. Λ is the cosmological constant. For a historical perspective on the development of the ΛCDM model I recommend Joel Primack’s excellent presentation.
Possible theoretical solutions [1]
“The ΛCDM cosmological model, a simple and elegant framework, has been found to provide a very good fit to almost all of the observational probes available until present. Despite its great success, the model is based on the assumption of three basic ingredients (CDM, a cosmological constant, and inflation) whose underlying physics are largely unknown.”
“The significant discrepancy in the Hubble constant measurements by early and local observations has raised a giant question mark over the ΛCDM scenario.”
“The estimated value of H0 from early time data by the Planck satellite within the ΛCDM paradigm is significantly differing (at 4.2σ) from the measured values of H0 in model independent approaches, e.g. using the latest local distance ladders by SH0ES collaboration.”
References
Suresh Emre
Renaissance Universal 