Black holes are some of the most mysterious and fascinating objects in the universe. They are regions of space where gravity is so strong that nothing, not even light, can escape. When two black holes collide, they produce ripples in the fabric of space and time, called gravitational waves. These waves carry information about the nature and history of the black holes, as well as the universe itself. But how can we decode this information from the faint and fleeting signals that we detect on Earth?
A new study by researchers at the Heidelberg Institute for Theoretical Studies (HITS) has found a surprising answer to this question. They predict that black hole mergers produce gravitational waves in two universal frequency ranges, depending on the masses of the black holes. This means that we can use the sound of the gravitational waves to infer the properties of the black holes and their origins. The study was published in The Astrophysical Journal Letters.
How Black Holes Have Standard Masses
To understand how black hole mergers produce universal frequency ranges, we need to understand how black holes form in the first place. Black holes are the remnants of massive stars that have exhausted their nuclear fuel and collapsed under their own gravity. However, not all stars end up as black holes. Some stars explode in spectacular supernovae, while others lose their outer layers to stellar winds or companion stars.
The researchers used simulations to show that some black holes have standard masses that result from the stripping of their stellar progenitors’ envelopes. This means that these black holes have lost most of their original mass before collapsing, leaving behind a compact core. For example, a star with an initial mass of 40 solar masses could end up as a black hole with a mass of 5 solar masses, after losing 35 solar masses to stellar winds or a binary companion.
These standard masses affect the masses of the black holes that merge because they determine how often they encounter each other in dense stellar environments. For example, a black hole with a mass of 5 solar masses is more likely to merge with another black hole with a similar mass, than with a black hole with a much larger or smaller mass. Therefore, most black hole mergers involve pairs of black holes with standard masses.
How Black Holes Chirp in Two Universal Tones
The masses of the black holes that merge also affect the frequency evolution of the gravitational waves that they produce. The frequency evolution is characterized by a quantity called the chirp mass, which is a mathematical combination of the two individual black hole masses. The chirp mass determines how fast and how high the frequency of the gravitational waves increases as the black holes spiral towards each other and eventually merge.
The researchers found that there are two universal frequency ranges for the gravitational waves produced by black hole mergers, depending on whether the chirp mass is above or below a certain threshold. The threshold is about 21.2 solar masses, which corresponds to a pair of black holes with standard masses of about 10 solar masses each. If the chirp mass is above this threshold, the frequency range is between 35 and 350 hertz, which is within the sensitivity band of current ground-based detectors such as LIGO and Virgo. If the chirp mass is below this threshold, the frequency range is between 2 and 20 millihertz, which is within the sensitivity band of future space-based detectors such as LISA.
These universal frequency ranges mean that we can use the sound of the gravitational waves to infer the chirp mass of the black hole mergers, and thus their individual masses and origins. For example, if we detect a gravitational wave signal with a frequency range between 35 and 350 hertz, we can conclude that it came from a pair of black holes with standard masses above 10 solar masses each, which likely formed from massive stars that exploded in supernovae. If we detect a gravitational wave signal with a frequency range between 2 and 20 millihertz, we can conclude that it came from a pair of black holes with standard masses below 10 solar masses each, which likely formed from less massive stars that lost their envelopes to stellar winds or binary companions.
How Black Holes Reveal Secrets of the Universe
The finding of universal frequency ranges for gravitational waves from black hole mergers has important implications for understanding how black holes form, how stars explode in supernovae, and how to measure the cosmological expansion of the universe. By using these frequency ranges as clues, we can test different models of stellar evolution and supernova physics, and compare them with observations from other sources such as electromagnetic radiation or neutrinos. We can also use these frequency ranges to estimate how many black hole mergers occur in different regions of space and time, and how they contribute to the growth and evolution of supermassive black holes at the centers of galaxies.
Moreover, we can use these frequency ranges to measure one of the most fundamental parameters of the universe: the Hubble constant, which describes how fast the universe is expanding. The Hubble constant can be inferred from the luminosity distance of the gravitational wave sources, which is the distance that corresponds to their apparent brightness. However, the luminosity distance depends on both the intrinsic brightness and the redshift of the sources, which is the amount by which their frequency is shifted due to the expansion of space. To break this degeneracy, we need an independent measurement of either the intrinsic brightness or the redshift of the sources. The universal frequency ranges provide us with a way to estimate the intrinsic brightness of the gravitational wave sources, based on their chirp masses. By combining this information with the luminosity distance, we can obtain an estimate of the redshift, and thus the Hubble constant.
Of course, this finding is not without challenges and limitations. The researchers acknowledge that their simulations are based on simplified assumptions and idealized scenarios, and that they do not account for all the possible effects that could influence the masses and frequencies of the black hole mergers, such as spin, eccentricity, metallicity, or environmental factors. They also note that their predictions need to be tested and confirmed by more observations and data analysis from current and future gravitational wave detectors. However, they are optimistic that their finding will open new avenues for exploring and understanding the mysteries of black holes, gravitational waves, and the universe.
Conclusion
Black hole mergers are one of the most powerful and spectacular events in the universe. They produce gravitational waves that carry valuable information about the nature and history of the black holes, as well as the universe itself. A new study by researchers at HITS predicts that black hole mergers produce gravitational waves in two universal frequency ranges, depending on the masses of the black holes. This finding could help us to decode this information from the sound of the gravitational waves, and to learn more about how black holes form, how stars explode in supernovae, and how to measure the cosmological expansion of the universe. This finding could also inspire us to listen more carefully to the symphony of space and time, and to appreciate its beauty and diversity.
