The violent mergers of black holes launch ripples in spacetime called gravitational wavesand the new technique measures the changes in these signals that occur as they experience the expansion of the universe firsthand. Astronomers have known since the late 1990s that the universe is expanding at an accelerating rate, and they call the speed of this expansion Hubble constant. But when scientists calculate the Hubble constant based on observations of the universe and current theories, they come up with very different values. So scientists hope to use cosmic collisions between tight black hole binaries as the team term “spectral sirens” to provide an alternative measurement technique for the Hubble constant. Ultimately, settling this pressing cosmological concern could reveal in greater detail how the universe has evolved and how it looked in its early years. In particular, a better understanding of the evolution of the universe could help cosmologists solve some key puzzles about dark energy. Dark energy makes up about 68% of the matter and energy content of the universe, and scientists want to determine when this mysterious force began to dominate matter and why this change occurred. At the heart of the spectral siren method are gravitational waves—ripples in the very fabric of space and time—launched by powerful cosmic events such as the collision and merger of large compact objects such as neutron stars and black holes. On Earth, incredibly sensitive laser interferometers like the Laser Interferometer Gravitational Wave Observatory (LIGO)the Italian Virgo observatory and the Japanese Kamioka Gravitational Wave Detector (KAGRA), can measure these faint gravitational wave signals. Since the first detection of gravitational waves in September 2015, LIGO and partner instruments have collected data from about 100 distant mergers. Each detection gives scientists a hint about the size of the black holes involved in the merger. For example, this first detection of gravitational waves came from the collision of two black holes that each contained about 30 times the mass of the sun. The new spectral siren method suggests that gravitational wave signals may also encode other information. Specifically, as these ripples in spacetime travel vast distances and long timescales to reach Earth, the properties of their signals are altered by the expansion of the universe. “For example, if you took a black hole and put it earlier in the universe, the signal would change and look like a bigger black hole than it actually is,” said study co-author and University of Chicago astrophysicist, Daniel Holz. one statement. In order to unlock information about the universe’s expansion rate encoded in gravitational wave data, scientists will need to learn how the signal has changed since it was blasted into space. Holz and his colleague believe that a recently discovered population of local black holes could be used as a tool to assess these changes. “So we measure the masses of nearby black holes and understand their characteristics, and then we look further away and see how much these further ones seem to have shifted,” said Jose María Ezquiaga, co-author and an astrophysicist also at the University of Chicago, reported in the announcement. “And that gives you a measure of the expansion of the universe.” Because gravitational waves, like light, take time to travel from their source Earth, detecting these ripples from more distant black hole mergers allows scientists to look back in time. And the study’s authors say that as LIGO and other probes become even more powerful and collect gravitational wave signals from more distant events, researchers may one day be able to observe collisions that occurred 10 billion years ago—about 3.8 billion years after Big explosion. This is also when researchers believe dark energy began to dominate other forms of matter and energy. “It’s around that time that we jumped from dark matter being the dominant force in the universe to dark energy being the dominant one, and we’re very interested in studying this critical transition,” Ezquiaga said. Ezquiaga and Holz say the spectral siren method for measuring the Hubble constant could have advantages over other techniques, such as measuring the change in frequency of light from distant supernova, or exploding stars. (These approaches depend on an understanding of the physics of stars and galaxies, and thus of complex physics and astrophysics.) This new technique, however, depends on more than just Einstein’s established model of gravity—the theory of general relativity — and uses local black holes as a built-in calibration tool. This calibration will improve as more gravitational wave data are collected from colliding black holes. “We need preferably thousands of these signals, which we should have in a few years, and even more in the next decade or two,” Holz concluded. “At that point, it would be an incredibly powerful method for learning about the universe.” The twin’s research is discussed in an article published on August 3 in the Physical Review Letters. Follow us on Twitter @Spacedotcom and up Facebook.
