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Chicago: Black holes are often thought of as cosmic voids where information disappears, but scientists may have discovered a way to use their final moments to unveil the universe’s history. In a recent study published in Physical Review Letters, University of Chicago astrophysicists proposed a method for measuring the universe's expansion rate by analyzing the collisions of black hole pairs.
This technique, called the “spectral siren,” could shed light on the mysterious “teenage” years of the universe.
A New Cosmic Ruler
One of the most debated questions in cosmology is the universe's rate of expansion, known as the Hubble constant. Current methods produce slightly varying results, and scientists are eager to find alternative approaches to refine this measurement. The new study suggests using gravitational waves ripples in space-time caused by black hole collisions detected by observatories like LIGO and Virgo.
These collisions generate signals containing details about the black holes' masses. However, as the signals traverse the expanding universe, their properties change. For example, a black hole observed from an earlier point in cosmic history would appear more massive due to the effects of expansion. By comparing the masses of nearby black holes with those observed further away, scientists can estimate the universe’s expansion rate.
The Spectral Siren Approach
This “spectral siren” method builds on the ‘standard siren’ approach, refining it with insights from the broader black hole population. Current evidence shows most black holes fall within a mass range of five to forty times that of the sun. Researchers believe this new technique can self-calibrate, minimizing errors and uncertainties associated with traditional methods.
Unlocking the Universe’s Teenage Years
The method holds promise for studying a critical period in cosmic history around 10 billion years ago when dark energy began to dominate over dark matter. While the cosmic microwave background reveals details about the universe's infancy, and nearby galaxies provide insights into recent history, this “in-between” era remains challenging to explore.
As gravitational wave detection capabilities improve, the spectral siren approach could offer a clearer view of this pivotal phase. Unlike other methods that rely on complex astrophysical models, this approach primarily depends on Einstein’s theory of gravity, a well-tested framework.
A Promising Future
The researchers anticipate needing thousands of black hole collision signals to achieve highly precise measurements. As gravitational wave observatories continue to advance, this groundbreaking method may revolutionize our understanding of the universe’s past, present, and future.