A secular ruler An important ongoing scientific debate is how fast the universe is expanding — a number called the Hubble constant. The different methods available so far give slightly different answers, and scientists are eager to find alternative ways to measure this percentage. Testing the accuracy of this number is especially important because it affects our understanding of fundamental questions such as the age, history, and composition of the universe. The new study offers a way to do this calculation, using special detectors that detect the cosmic echoes of black hole collisions. Occasionally, two black holes will hit each other — an event so powerful that it literally creates a ripple in spacetime that travels throughout the universe. Here on Earth, the US Gravitational-Wave Observatory (LIGO) and Italy’s Virgo Observatory can pick up these ripples, called gravitational waves. In recent years, LIGO and Virgo have collected measurements from nearly 100 pairs of colliding black holes. The signal from each collision contains information about how massive the black holes were. But the signal travels through space, and during that time the universe has expanded, which changes the properties of the signal. “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 really is,” explained UChicago astrophysicist Daniel Holz, one of the two authors on paper. If scientists can find a way to measure how this signal has changed, they can calculate the expansion rate of the universe. The problem is calibration: How do they know how much it changed from the original? In their new paper, Holz and first author Jose María Ezquiaga suggest that they can use our new knowledge of the entire population of black holes as a calibration tool. For example, current evidence suggests that most of the black holes that have been detected are 5 to 40 times the mass of our sun. “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 appear to have shifted,” said Ezquiaga, a postdoctoral fellow at NASA’s Einstein and Kavli Institute for Cosmological Physics. works with Holz at UChicago. “And that gives you a measure of the expansion of the universe.” The authors call it the “spectral siren” method, a new approach to the “standard siren” method that Holz and colleagues pioneered. (The name is a reference to the “standard candle” methods also used in astronomy.) Scientists are excited because in the future, as LIGO’s capabilities expand, the method may provide a unique window into the universe’s “teenage” years — about 10 billion years ago — that are difficult to study with other methods. Researchers can use the cosmic microwave background to see the earliest moments of the universe, and they can look around at galaxies near our own to study the universe’s more recent history. But the intermediate period is more difficult to achieve and is an area of particular scientific interest. “It’s around that time that we switched 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. The other advantage of this method, the authors said, is that there are fewer uncertainties created by gaps in our scientific knowledge. “By using the entire population of black holes, the method can be calibrated, detecting and correcting errors immediately,” Holz said. The other methods used to calculate the Hubble constant are based on our current understanding of the physics of stars and galaxies, which includes very complex physics and astrophysics. This means that the measurements can be thrown off quite a bit if there is something we don’t know yet. Instead, this new black hole method is based almost entirely on Einstein’s theory of gravity, which is well-studied and has withstood all the ways scientists have tried to test it so far. The more measurements they have of all the black holes, the more accurate this calibration will be. “We need preferably thousands of these signals, which we should have in a few years, and even more in the next decade or two,” Holtz said. “At that point it would be an incredibly powerful method for learning about the universe.” Story source: Material provided by the University of Chicago. Originally written by Louise Lerner. Note: Content can be edited for style and length.
