Hubble constant

Hubble constant
Name	Hubble constant
Unit	km·s−1·Mpc−1
Discovered	1920s
Discoverer	Edwin Hubble
Field	Cosmology

Hubble constant The Hubble constant quantifies the present-day rate of cosmic expansion, relating recessional velocity to distance for extragalactic objects. It is central to models used by observatories, space agencies, and research institutes to infer the age, size, and fate of the universe, and it underpins analyses conducted with instruments and collaborations across astronomy and physics.

Introduction

The Hubble constant appears in the Friedmann equations used by theorists at institutions like Institute for Advanced Study, California Institute of Technology, and Max Planck Society. Observational determinations involve facilities such as Hubble Space Telescope, Planck satellite, Very Large Telescope, Keck Observatory, and missions from European Space Agency and National Aeronautics and Space Administration. Measurements influence work by researchers associated with universities like Harvard University, University of Cambridge, and Princeton University, and connect to landmark studies published in journals like Nature (journal), Science (journal), and The Astrophysical Journal.

Measurement Techniques

Direct distance ladder methods employ standard candles and geometric calibrators developed by teams at Carnegie Institution for Science, Space Telescope Science Institute, and Royal Astronomical Society. Key standard candles include Cepheid variables measured by projects led by figures such as Adam Riess and collaborations linked to Supernova Cosmology Project and High-Z Supernova Search Team, and Type Ia supernovae catalogues compiled with support from Sloan Digital Sky Survey. Parallax baselines come from missions like Hipparcos and Gaia (spacecraft), while maser-based geometric distances are provided by arrays including Very Long Baseline Array and programs affiliated with National Radio Astronomy Observatory.

Cosmic microwave background inference relies on anisotropy measurements from instruments such as Planck satellite, Wilkinson Microwave Anisotropy Probe, and ground-based experiments like Atacama Cosmology Telescope and South Pole Telescope. These employ theoretical frameworks from researchers connected with University of Chicago, Columbia University, and University of California, Berkeley, invoking models tested against datasets from collaborations including BICEP/Keck. Baryon acoustic oscillation methods use surveys like Baryon Oscillation Spectroscopic Survey and 2dF Galaxy Redshift Survey executed by teams at Lawrence Berkeley National Laboratory and Australian National University.

Historical Development

The observational program that led to the constant began with work at Mount Wilson Observatory by astronomers such as Edwin Hubble and contemporaries like Milton Humason, building on theoretical predictions from Alexander Friedmann and Georges Lemaître. Subsequent refinements involved measurements by groups at Palomar Observatory, Royal Greenwich Observatory, and institutions represented by Walter Baade and Harlow Shapley. Debates over scale factors and calibration saw contributions from Allan Sandage and later reanalyses by teams at Johns Hopkins University and University of Chicago. The age crisis and tension episodes drew in projects like Hubble Space Telescope Key Project and analyses by the Cosmic Background Explorer team.

Current Tension and Discrepancies

Contemporary disagreements, often framed as a tension between direct and indirect routes, pit local distance ladder values advocated by researchers such as Adam Riess against lower inferences from cosmic microwave background analyses led by Planck Collaboration scientists including George Efstathiou. Independent intermediate methods using gravitational lens time delays from groups like H0LiCOW and megamaser programs at Megamaser Cosmology Project offer complementary values, while alternative probes from surveys by Dark Energy Survey and experiments at European Southern Observatory provide cross-checks. The statistical and systematic debates involve experts affiliated with Stanford University, University of Oxford, and Yale University and intersect with theoretical proposals from researchers at Perimeter Institute and CERN exploring new physics scenarios.

Cosmological Implications

The numerical value inferred for the Hubble constant affects estimates of the universe's age used by scholars at Princeton University and Cambridge University, modifies constraints on dark energy parameters central to work at Fermilab and SLAC National Accelerator Laboratory, and influences models of cosmic inflation developed at institutes like Kavli Institute for Theoretical Physics. It alters derived densities reported by collaborations such as Planck Collaboration and impacts parameter degeneracies studied in seminars at Institute of Physics. Tension-driven hypotheses include additional relativistic species considered in publications from Institute for Cosmic Ray Research and modifications to dark matter behavior explored by teams at MIT and University of California, Santa Cruz.

Future Prospects and Experiments

Planned and current facilities will refine measurements: space missions such as James Webb Space Telescope, follow-on concepts proposed at European Space Agency and NASA, and new surveys like Legacy Survey of Space and Time at Vera C. Rubin Observatory aim to reduce systematics. Next-generation cosmic microwave background projects, including CMB-S4 and successors conceived at Brookhaven National Laboratory and Lawrence Livermore National Laboratory, will tighten indirect inferences. Radio interferometers such as Square Kilometre Array and expanded very long baseline networks coordinated with National Radio Astronomy Observatory promise improved geometric distances, while gravitational wave standard siren detections by collaborations like LIGO Scientific Collaboration and Virgo (detector) could provide independent calibration. Interdisciplinary efforts span institutions including European Research Council-funded consortia and national programs at Japan Aerospace Exploration Agency, ensuring progressive convergence on the expansion rate.

Category:Cosmology