The Hubble constant is one of the most critical numbers in cosmology because it tells us how fast the universe is expanding. There are several methods for measuring this percentage. However, determining the accuracy of this number is necessary to better understand fundamental questions such as the age, history, and composition of the universe. The new study by two University of Chicago astrophysicists offers a way to do this calculation: by using pairs of colliding black holes and thus understanding the evolution of the universe, what it’s made of and where it’s going. According to the scientists, the new technique dubbed “spectral siren” could provide insights into the otherwise elusive “teenage” years of the universe. Occasionally, two black holes collide. This event is so powerful that it creates a space-time ripple that travels throughout the universe. These ripples are also known as gravitational waves. The US Gravitational-Wave Observatory (LIGO) and Italy’s Virgo Observatory can capture these ripples here on Earth. 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. UChicago astrophysicist Daniel Holz, one of the paper’s two authors, said: “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 is. is.” Determining a way to estimate how this signal changed could help scientists estimate the expansion rate of the universe. However, the problem is calibration: How do they know how much it changed from the original? In this new study, the scientists suggest that they can use the new knowledge about the entire population of black holes as a calibration tool. For example, current evidence suggests that most of the detected black holes have 5 to 40 times the mass of our sun. First author Jose María Ezquiaga, a NASA Einstein postdoctoral fellow and fellow at the Kavli Institute for Cosmological Physics, who works with Holz at UChicago, said: “So we measure the masses of nearby black holes and understand their characteristics, and then we look further afield. and we see how far these seem to have shifted further. And that gives you a measure of the expansion of the universe.” 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. The authors noted, “The other advantage of this method is that gaps in our scientific knowledge create fewer uncertainties. The method can be calibrated using the entire population of black holes, immediately detecting and correcting for errors. 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. That means the measurements can be thrown off quite a bit if there’s something we don’t know yet.” “Instead, this new black hole method is based almost purely on Einstein’s theory of gravity, which is well-studied and has withstood all the ways scientists have tried to test it so far.” Holz said, “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. At that point, it would be an incredibly powerful method for learning about the universe.” Journal Reference:
