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.

Climb the Cosmic Ladder 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 and you get the speed of the galaxy in kilometers per second. So the Hubble constant in today’s universe, H0, which is also called the local expansion rate, can be determined by measuring the distances to and the speeds of galaxies. The most direct measurement of H0 uses a technique called the cosmic distance ladder, pioneered by Hubble (2). In a multistep process, astronomers use what they call standard candles—particular types of stars and supernovae whose intrinsic brightness, or luminosity, is revealed by the way their observed brightness changes with time. These standard candles allow astronomers to bootstrap their way out of the Milky Way. For example, the first step on the ladder involves finding the distance to a standard candle in our galaxy by using its parallax, which is the difference in the position of the star when viewed from two different locations on Earth. The distance and the observed brightness let astronomers calculate the intrinsic brightness or luminosity of the standard candle and relate it to some other observable property that does not depend on its distance from us. The next step then is to find the same standard candle in a galaxy outside the Milky Way and use it to get the distance to the galaxy. Once astronomers step on the last rung of the ladder, they can determine the luminosity of a supernova located in a distant galaxy. From that, astronomers can calculate its distance and that of its host galaxy. “Once we know the luminosity, then we could, from any supernova, measure the expansion rate of the universe,” explains Daniel Scolnic, an astrophysicist at KICP and member of SH0ES, a project that climbs the distance ladder using the Hubble Space Telescope and ground-based telescopes in Chile and Hawaii. To measure the expansion rate, or H0, astronomers need one more piece of data: the amount by which the light from the supernova is shifted toward longer wavelengths. This redshift is a measure of the speed at which the supernova and its host galaxy are receding from us (analogous to how an ambulance’s siren falls in pitch as the ambulance speeds away). The SH0ES team put all this together and found H0 to be about 73.24 kilometers per second per megaparsec (3).

The Other Shoe Drops But another way of measuring H0 is at odds with SH0ES. This technique uses the cosmic microwave background (CMB). About 380,000 years after the Big Bang, the universe cooled down enough for the free electrons and protons to combine and form the first atoms of neutral hydrogen. Photons, which until then had been constantly interacting with free electrons and thus unable to travel far, were unshackled. Now stretched to microwave wavelengths, they form the CMB. “This piece of physics is very well understood,” says Silvia Galli of the Institute of Astrophysics in Paris. “It works extremely well to describe the CMB.” The European Space Agency’s Planck satellite, launched in 2009, measured the CMB for about 4.5 years. The satellite looked for tiny deviations in the temperature of the CMB photons from place to place in the sky. The deviations have their roots in the physics of the early universe, and their angular scale in the sky can be used to infer the local Hubble constant (4). Galli and her Planck colleagues calculate H0 to be about 67.8. It has proved impossible to reconcile this with the SH0ES value of 73, says Galli. Of course, both teams may have overlooked some systematic errors in their measurements, and this discrepancy or tension in the data may go away with further refinements. Nonetheless, the fact that the tension hasn’t evaporated despite two generations of experimentation is troubling, say researchers, because it could undermine some of our assumptions about the universe. “We have seen a lot of tensions come and go,” says cosmologist Miguel Zumalacárregui of the University of California, Berkeley. “This is the most serious that I have seen.” The Planck result depends on the standard model of cosmology, which says that the universe is 68.3% dark energy (the energy of the vacuum of space–time), 26.8% dark matter (the unseen matter whose gravitational influence can be detected in the motions of galaxies and galaxy clusters), and 4.9% normal matter. Although these parameters are tightly constrained by the CMB data, the standard model also assumes that the amount of dark matter hasn’t changed. What if it has? “This tension is staying and both sides are digging in. There are no signs at all of anyone moving.” —Daniel Scolnic Torsten Bringmann of the University of Oslo in Norway and his colleagues showed earlier this year that if a tiny percentage of the universe’s dark matter has decayed into undetectable radiation, that would bump up the estimates of H0 from the Planck data, bringing it in line with that of SH0ES (5). Favored dark matter candidates do not decay, so this would require some new piece of fundamental physics. For example, to accommodate decaying dark matter, physicists would have to introduce a new fundamental force that allows dark matter particles to decay. Another solution could be to add a particle. Among that 4.9% normal matter are three known types of subatomic particles called neutrinos. If an as-yet-undetected type of neutrino exists, this would increase the Planck value of H0. However, all existing empirical data from cosmology and particle physics are consistent with three types (6). The most intriguing change to the standard model would involve dark energy, a concept astronomers introduced in the late 1990s to explain the accelerating expansion of the universe. In the standard model, the amount of dark energy per unit volume of the cosmos doesn’t change with time, but in other models it is dynamic. One class of theories attributes acceleration to the changing nature of gravity as the universe evolves, thanks to a hypothesized field called the Galileon (7). These theories boost the value of the CMB-estimated H0 to bring it closer to distance-ladder measurements.

The Third Way What’s needed is an entirely new way to measure H0—and that is where GW170817 comes in. “This tension is staying and both sides are digging in. There are no signs at all of anyone moving,” says Scolnic. “Everyone is extremely excited about what this gravitational wave measurement will be able to say.” Importantly, it will be an alternate, independent measurement of the local value of the Hubble constant, Scolnic notes. GW170817 was the outcome of two neutron stars, each a dense remnant of a past supernova explosion, that had been orbiting each other closely. They spiraled inward, radiating ripples in the fabric of space–time known as gravitational waves that were picked up on Earth by the Laser Interferometer Gravitational-Wave Observatory (LIGO). The neutron stars eventually collided in a massive explosion, generating a final burst of gravitational waves and gamma rays. Holz and colleagues found the distance to GW170817 by tracking the rate of change of the frequency of the gravitational waves and their amplitude. The changing frequency as the neutron stars orbit each other faster and faster gives astronomers an indication of the mass of the binary star system and, therefore, the absolute amplitude of the waves. Comparing the absolute amplitude with the observed amplitude gives the distance. Because gravitational waves are more like waves of sound than light, Holz calls such events standard sirens. Measuring H0 with a standard siren involves physics completely different from what is used by either Planck or SH0ES—it’s simple general relativity with no assumptions. “It’s very clean in that sense,” says Holz. That means it can potentially be an arbiter between the CMB and supernova approaches. The data from GW170817 give a distance to it of about 40 megaparsecs. By combining that with the redshift of its host galaxy, the LIGO team ends up with a value for H0 of about 70 kilometers per second per megaparsec. That is right in the middle of the Planck and SH0ES values and with error bars large enough to accommodate either (8). So this lone siren does not settle the dispute. But it has narrowed the exotic physics options. In most Galileon models and in some other theories of dark energy gravitational waves don’t travel at the speed of light. But telescopes saw the light from the collision arrive at almost exactly the same time as LIGO saw the gravitational waves. Based on this observation, Zumalacárregui and his colleagues, as well as other teams, ruled out such models of dark energy (9). Soon the method should be able to do much better. Holz says that once LIGO comes back online in January 2019 after an upgrade, it should detect many more standard sirens. The previous run of LIGO was sensitive to neutron–star mergers out to a distance of about 80 megaparsecs. The upgraded LIGO will see out to 120 megaparsecs, encompassing more than three times the volume. Finding more standard sirens will help Holz and others calculate H0 with increasing precision. That could support Planck, in which case the SH0ES team is probably doing something wrong, or support SH0ES, in which case Planck may have to reexamine its error bars, or—“by far the most exciting” option, according to Scolnic—the discrepancy is real, and suspicions fall squarely on the assumptions in the standard model. The consequences are hard to overstate. Such a scenario would suggest that “there is physics beyond the standard model,” says Scolnic. “Everything is on the table.”