How fast is the universe expanding? Clashing measurements may point to new physics

Abstract

The siren sounded on August 17, 2017. Astronomers picked up a burst of gravitational waves from the collision of two neutron stars (1), an event that Daniel Holz had been dreaming about for more than a decade. “You write these papers, and it sounds like fantasy. The equations say this might happen, but it’s completely different when nature cooperates and lets you make the measurement,” says Holz, an astrophysicist at the University of Chicago’s Kavli Institute for Cosmological Physics (KICP). “It’s just too good. It’s ridiculous. It’s embarrassing. But there it is.” One way of measuring the Hubble constant (H0) uses the CMB, shown here as observed by the European Space Agency’s Planck satellite, which launched in 2009 and measured the CMB for about 4.5 years. But this technique yields a different value for the constant than does an approach that calculates H0 by measuring the distances to and the speeds of galaxies. Image © ESA and the Planck Collaboration. The neutron star merger, named GW170817, gave Holz and his colleagues an entirely new way to measure how fast the universe is expanding. This method could settle a simmering dispute between the two established ways of measuring expansion, and it could mean rethinking the makeup of our universe—perhaps requiring new types of a subatomic particle or unexpected forms of dark matter or dark energy. Astronomer Edwin Hubble discovered in the 1920s that our universe is expanding. The expansion is causing all galaxies to speed away from us, and the rate at which a galaxy is receding is equal to its distance times a number called the Hubble constant. The distance to the galaxy is calculated in megaparsecs, where 1 megaparsec is about 3.26 million light years, and the constant has units of kilometers per second per megaparsec. Multiply the two …