Decades of experimental effort paid off spectacularly on 14 September 2015, when the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) spotted the gravitational waves generated by a pair of coalescing black holes.To get a sense of the effort leading to that breakthrough, consider that the gravitational waves caused the mirrors at the ends of each interferometer’s 4 km arms to oscillate with an amplitude of about 10m, roughly a factor of a thousand smaller than the classical proton radius. The detection was also a triumph for theory. The frequency and amplitude evolution of the measured waves precisely matched general relativity’s predictions for the signal produced by a binary black hole merger, even though the system’s gravity was orders of magnitude stronger than that of any system that had been precisely probed before that detection. As figureshows, gravitational-wave astronomy began not with a bang but with a chirp.

Labeled GW150914, that first reported event was soon joined by other detections of binary black hole mergers. Each of those events appeared to be totally dark to traditional astronomical instruments—the matter and electromagnetic fields near the merging black holes were not sufficient to generate any signal other than gravitational. As had long been promised, gravitational waves have opened a window onto an otherwise invisible sector of the universe.

Although celebrated by the physics and astronomy communities and feted by the broader public, gravitational-wave astronomy did not initially overlap significantly with more traditional astronomy. That changed on 17 August 2017, when a gravitational-wave signal, followed by a burst of gamma rays, triggered one of the most intense observing campaigns in the history of astronomy. LIGO, joined now by the Virgo detector in Pisa, Italy, recorded a minute-long chirp (see figure) encoding the final several thousand orbits in the coalescence of two neutron stars.The stars’ collision, at about ⅓ the speed of light, was an astronomical cataclysm. Just 1.7 s after the end of the gravitational-wave signal, the orbitingandobservatory recorded a short gamma-ray burst.

The LIGO–Virgo alert provided the sky position and, importantly, the distance to the event. Just 11 hours later, optical astronomers identified a violent event in the galaxy NGC 4993, a kilonova explosion that shone 1000 times brighter than a typical nova. More than 70 teams made follow-up electromagnetic observations. The effort represents the first time a source has been detected through both its gravitational and electromagnetic radiation. A significant portion of the world’s professional astronomers are coauthors with the gravitational-wave teams on the summary paper describing those observations.Observations in the x-ray and radio bands continue as we write. From the event, astronomers are learning much about gamma-ray bursts, neutron stars, and their associated physics and astronomy.

Because a gravitational wave encodes the distance to its source, GW170817 provided the astrophysical community with another advance: the first measurement of the local cosmic expansion rate—the Hubble constant—via gravitational waves.That milestone opened up a completely new way to measure the dynamics of the universe: the standard-siren technique.

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The Hubble constant has been the single most important parameter describing cosmology since Edwin Hubble discovered the expansion of the cosmos in 1929. On the largest scales, the universe expands homogeneously and isotropically, so every part of it recedes from every other part. General relativity shows that due to the cosmic expansion, radiation emitted from a distant object is redshifted as it propagates from its source to an observer.

λ emitted from a source that is a distance D away. Observers on Earth will measure the light to have a wavelength of 1 + z λ , where z is the light’s redshift. To leading order in z , the source distance and redshift are proportional: c z = H 0 D , (1) where the Hubble constant H 0 is today’s value of the Hubble parameter H. It has dimensions of inverse time; the reciprocal 1 / H 0 , known as the Hubble time, provides a rough estimate of the age of the universe. Astronomers conventionally express H 0 in units of km s−1 Mpc−1, because the megaparsec (1 Mpc = 3.26 million light-years) is convenient for intergalactic distances. As mentioned above, equation 6 6. See, for example, B. Schutz, A First Course in General Relativity, 2nd ed., Cambridge U. Press (2009), chap. 12. Consider light with a wavelengthemitted from a source that is a distanceaway. Observers on Earth will measure the light to have a wavelength of, whereis the light’s redshift. To leading order in, the source distance and redshift are proportional:where the Hubble constantis today’s value of the Hubble parameter. It has dimensions of inverse time; the reciprocal, known as the Hubble time, provides a rough estimate of the age of the universe. Astronomers conventionally expressin units of km sMpc, because the megaparsec (1 Mpc = 3.26 million light-years) is convenient for intergalactic distances. As mentioned above, equation 1 is a leading-order expression. For far distant objects, it needs to be corrected with higher-order terms that depend on the nature of the matter and energy that fill the universe.

In principle, just one object of known distance and cosmological redshift suffices to determine the Hubble constant. The redshift of many objects can be determined from spectral measurements, but determining astronomical distances is much more challenging.

For nearby objects, distance can be determined using parallax—that is, measuring the apparent angular shift in the position of an astronomical object as Earth orbits the Sun. The technique does not work well for larger distances, as the angular shift due to Earth’s orbital motion becomes too small to measure.

L and observers on Earth measure it to have a flux F . From the inverse square law and assuming the star radiates isotropically, you obtain the luminosity distance D = L 4 π F . (2) For objects beyond our galaxy, an important tool for measuring distances is the standard candle: an astronomical source whose intrinsic luminosity is assumed to be known. Suppose a star has luminosityand observers on Earth measure it to have a flux. From the inverse square law and assuming the star radiates isotropically, you obtain the luminosity distance

Nature does not provide observers with stars whose luminosities are precisely known. However, it does provide stars and other objects whose luminosities can be inferred accurately. Celebrated examples are the Cepheid variables, giant stars whose luminosities vary periodically. By studying a group of such stars in the Small Magellanic Cloud—a dwarf galaxy near the Milky Way—Henrietta Leavitt discovered in 1912 that each star’s oscillation period correlates with its intrinsic luminosity. Some Cepheids are close enough that their distances can be determined using parallax, and thus their luminosity can be calibrated. With the luminosity–period relationship empirically established, Cepheid variable stars can serve as standard candles for measuring distances beyond the limits of parallax.

Physics Today, By putting together multiple methods for measuring distances, astronomers construct what is called the cosmic distance ladder (see the article by Mario Livio and Adam Riess, October 2013, page 41 ). On each rung of the ladder, objects thought to be of standard luminosity are identified and calibrated in terms of measurements contributing to the previous rung.

H 0 , but many depend in one way or another on the distance ladder. One method relies on an important standard candle that can be seen very far away: the type Ia supernova explosion. Supernova observations not only helped determine H 0 , they also implied nonlinear contributions to equation Physics Today, Various sophisticated methods now exist for measuring, but many depend in one way or another on the distance ladder. One method relies on an important standard candle that can be seen very far away: the type Ia supernova explosion. Supernova observations not only helped determine, they also implied nonlinear contributions to equation 1 that showed the expansion of the universe is accelerating. That result led to the awarding of the 2011 Nobel Prize in Physics to Saul Perlmutter, Adam Riess, and Brian Schmidt (see December 2011, page 14 ).

7 et al. , Astrophys. J. 826, 56 (2016). 7. A. G. Riess, 56 (2016). https://doi.org/10.3847/0004-637X/826/1/56 H 0 = 73.24 ± 1.74 km s−1 Mpc−1. An alternative method, based on the Planck satellite’s measurements 8 et al. (Planck collaboration), Astron. Astrophys. 594, A13 (2016). 8. P. A. R. Ade(Planck collaboration),, A13 (2016). https://doi.org/10.1051/0004-6361/201525830 H 0 = 67.74 ± 0.46 km s−1 Mpc−1. The two values are uncomfortably far apart if their uncertainties are to be believed. Given the many rungs on the distance ladder that must be empirically calibrated, it would not be surprising for one or both of the H 0 determinations to be affected by undiscovered systematic errors. Or the universe might be more complicated than the community now thinks. Perhaps it is more inhomogeneous; perhaps it is less isotropic; perhaps an important contribution to its mass–energy budget has been overlooked; or perhaps general relativity does not describe the universe well on the largest scales. The most recent measurement of the expansion using supernovaeyields= 73.24 ± 1.74 km sMpc. An alternative method, based on thesatellite’s measurementsof fluctuations in the cosmic microwave background gives= 67.74 ± 0.46 km sMpc. The two values are uncomfortably far apart if their uncertainties are to be believed. Given the many rungs on the distance ladder that must be empirically calibrated, it would not be surprising for one or both of thedeterminations to be affected by undiscovered systematic errors. Or the universe might be more complicated than the community now thinks. Perhaps it is more inhomogeneous; perhaps it is less isotropic; perhaps an important contribution to its mass–energy budget has been overlooked; or perhaps general relativity does not describe the universe well on the largest scales.