The 'Local Distance Network' Path Toward A More Accurate Hubble Constant

 

                   Artist's conception of the 'ladder' technique to obtain H₀

For scientific journal papers, few titles can compete in power and heft with:

The Local Distance Network: A community consensus report on the measurement of the Hubble constant at ∼1% precision

Which was published in the April 26 issue of Astronomy & Astrophysics (Vol. 708). In the paper 38 authors contributed to a detailed investigation exploring the so-called ‘Hubble tension’ and the degree to which two competing domains for the rate of expansion of the universe can be reconciled.  Different techniques for measuring what is called the Hubble Constant — the rate at which the universe is expanding — keep coming up with different estimates. Lead author of the University of Chicago paper, Wendy Freedman, said the Hubble Tension may be no big deal, that the scientific community may merely need to improve the way it estimates the distances between galaxies and thus the speed they recede from one another. If that happens, the Hubble Tension could be resolved without the requirement of revolutionary theories.

The Hubble tension refers to the two main ways cosmologists measure the Hubble constant (H₀), which represents the universe’s expansion rate.  (The ‘tension’ arises because the two methods don’t agree)

The method that uses the cosmic microwave background (CMB), or the leftover radiation from the Big Bang, has the constant at 41 or 42 miles (67 or 68 kilometers) per second per megaparsec (a unit of distance about 3.3 million light-years). The other approach that uses local observations of galaxies and supernova puts it at 45 miles (73 kilometers per second per megaparsec).

From their paper, we see the end result:

"The local H₀ is robustly determined, with first-rank indicators internally consistent within their uncertainties. The baseline result is H₀ = 73.50 ± 0.81 km s⁻¹ Mpc⁻¹, 7.1σ from the early Universe plus ΛCDM result 67.24 ± 0.35 km s⁻¹ Mpc⁻¹ and 5.0σ from BBN+BAO within a flat ΛCDM DESI DR2 (68.51 ± 0.58 km s⁻¹ Mpc⁻¹)."

The key component for achieving the result is the 'local distance network' - with the graphic (and codes) below from the paper:

Basically, dozens of distance indicators were assembled from dozens of teams of researchers to arrive at a coordinated product and conclusion. In a way the process could be analogized to building a 'ladder' (see top graphic from WSJ) to achieve the final result. In words of the authors:

The various subsets of the astronomical community working on aspects related to the Hubble tension have interacted at different junctures and have a general understanding of each other’s methodologies. However, accurately and reliably combining results while considering all interdependencies requires a hands-on, collaborative approach and a careful and thorough treatment rooted in transparency, engagement, and scientific discourse.

This was the underlying motivation and raison d’être for the workshop¹ “What’s under the H0od?” held at the International Space Sciences Institute (ISSI) in Bern, Switzerland, in March 2025. The goal of this workshop was to arrive at a consensus set of "baseline" and "variant" datasets that should be included, to define statistically rigorous analysis procedures that account for datasetnt covariance, and to begin developing the open access tools required to measure H₀ within a networked formalism. The Local Distance Network (Fig. 1) extends the distance ladder concept “horizontally” by linking multiple overlapping calibration paths.

In other words, this 'laddering' of different methods to reach an accurate value for  H₀ was one of the largest, most demanding cooperative astronomical ventures ever undertaken to finally try to nail down the uncertainties in the Hubble constant.  The remaining bugbear?  The end result is about 10 percent faster than what the standard model of cosmology says it should be.

This means there is probably something missing from the standard model, or a force we don't fully understand.  This according to study co-author Stefano Casertano, quoted in the Wall Street Journal. Two likely culprits here are dark matter and dark energy.

Readers can access the full paper here:

https://doi.org/10.1051/0004-6361/202557993

See Also:

• Dark Energy - Evidence for a New Law of Physics?
• And:
• All Experts Redux: The Cosmological Basis Of Dark Matter, Dark Energy

      And:

       Gravitational Lensing Exposes Further Discrepancy in Hubble Constant - Past 5 Standard Deviations