Scientists plan to use colliding black holes to measure how fast universe is expanding

Next to nothing can escape the strong gravitational pull of a black hole but now scientists are planning to use the collisions of black holes to understand the age of the universe and its evolution.

In a new study published in Physical Review Letters, University of Chicago astrophysicists have developed a method for using pairs of colliding black holes to measure how fast the universe is expanding and with it, how the universe evolved and where it is going. Scientists are particularly interested in using the new “spectral siren” technique to learn more about the “teenage years” of the universe.

Sometimes, two black holes will slam into each other in spectacular collisions. Such powerful events send space-time ripples across the universe. These ripples, called gravitational waves, can be detected by Earth-based observatories like the American Laser Interferometer Gravitational-Wave Observatory and Italian Virgo observatory.

The signal from these collisions contains important information about the size of the black holes. But since this signal travels vast distances across space, the expansion of the universe changes its properties. “For example, if you took a black hole and put it earlier in the universe, the signal would change and it would look like a bigger black hole than it really is,” explained astrophysicist Daniel Holz, one of the two authors on the paper, in a UChicago press statement.

Scientists need to figure out a way to measure how these signals have changed and it could help them calculate the expansion rate of the universe. The problem lies in understanding how much the signal changed from the original.

Current evidence suggests that most of the black holes we have detected are between 5 and 40 times the mass of our sun. Holz and first author Jose María Ezquiaga plan to use this and other newfound knowledge about the population of black holes as a calibration tool.

As the capabilities of LIGO and other interferometer observatories expand, they would be able to observe “fainter” gravitational waves. This gets scientists excited because that data, combined with the silent siren method could potentially offer unique insights into what is referred to as the “teenage years” of the years: a period about 10 billion years ago. This is interesting because this particular period is difficult to study with current methodologies.

Astrophysicists can use cosmic microwave background radiation to look at the earliest moments of the universe. They can also look around at galaxies near our own Milky Way to study the most recent history of the universe. But it is the in-between period that is a hard nut to crack.

“It’s around that time that we switched from dark matter being the predominant force in the universe to dark energy taking over, and we are very interested in studying this critical transition,” said Ezquiaga, in the press statement.

According to Holz, the new method can be an “incredibly powerful method” to learn about the universe if it could be used with data of thousands of such signals.

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