Key Takeaways:
- Astronomers detect colossal black hole duo hurtling towards collision, emitting powerful gravitational waves.
- Supermassive black hole pairs produce universe’s loudest gravitational waves, surpassing those from smaller black hole mergers.
- Discovery aids understanding of galaxy mergers and fate of supermassive black hole pairs.
- Detection techniques involve pulsar timing arrays due to gravitational waves’ frequency.
- First detection of gravitational wave background from supermassive black hole pairs expected within five years, offering insights into cosmic phenomena.
These black holes each possess a mass exceeding 800 million times that of our sun. As they gradually approach one another in a spiral of doom, they will initiate the propagation of gravitational waves throughout the fabric of space-time. These cosmic ripples will merge with the yet-to-be-identified background noise of gravitational waves originating from other supermassive black holes.
Even prior to their inevitable collision, the gravitational waves emitted by this pair of supermassive black holes will surpass those previously detected from the mergers of considerably smaller black holes and neutron stars.
Chiara Mingarelli, a co-discoverer and an associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics in New York City, remarks, “Supermassive black hole binaries produce the most powerful gravitational waves in the universe.” Gravitational waves from pairs of supermassive black holes “are a million times more intense than those observed by LIGO.”
The study, led by Andy Goulding, an associate research scholar at Princeton University, along with collaborators from Princeton and the U.S. Naval Research Laboratory in Washington, D.C., announces the discovery in The Astrophysical Journal Letters.
These two supermassive black holes hold particular interest due to their location approximately 2.5 billion light-years away from Earth. Considering that observing distant objects in astronomy is akin to peering into the past, these black holes belong to a universe that existed 2.5 billion years prior to our own. Coincidentally, this aligns with the estimated time frame during which the black holes will commence emitting potent gravitational waves.
CREDIT: A.D. Goulding et al./Astrophysical Journal Letters
In the present universe, these black holes are already emitting gravitational waves, but given the limitations of light speed, these waves won’t reach us for billions of years. Nevertheless, their discovery remains significant as it aids scientists in estimating the number of nearby supermassive black holes currently emitting gravitational waves, which could be detected at present.
Detection of the gravitational wave background holds the potential to resolve some of the most pressing questions in astronomy, such as the frequency of galaxy mergers and whether pairs of supermassive black holes eventually merge or remain in a perpetual dance around each other.
Jenny Greene, a study co-author and professor of astrophysical sciences at Princeton, comments, “It’s a significant challenge for astronomy that we are uncertain whether supermassive black holes merge. Observationally, this is a longstanding enigma that requires resolution.”
Supermassive black holes contain masses equivalent to millions or even billions of suns. Virtually all galaxies, including the Milky Way, host at least one of these behemoths at their core. When galaxies merge, their supermassive black holes converge and commence orbiting one another. Over time, this orbit shrinks as gas and stars interact between the black holes, siphoning off energy.
However, once the supermassive black holes draw near enough, the energy exchange dwindles. Some theoretical studies propose that black holes reach a standstill when approximately 1 parsec (about 3.2 light-years) apart, a phenomenon known as the final parsec problem. In such a scenario, only rare groupings of three or more supermassive black holes lead to mergers.
Identifying stalled pairs of black holes poses a challenge as they are indistinguishable when they are too close together long before they produce discernible gravitational waves. Furthermore, they only emit strong gravitational waves when they overcome the final parsec hurdle and draw closer together. (Observing them as they were 2.5 billion years ago, the newfound supermassive black holes appear to be approximately 430 parsecs apart.)
If the final parsec problem is non-existent, astronomers anticipate that the universe reverberates with the gravitational waves of supermassive black hole pairs. “This background noise is reminiscent of a chaotic chorus of crickets chirping in the night,” states Goulding. “While individual crickets may not be distinguishable, the volume of the noise aids in estimating their abundance.” (However, when two supermassive black holes eventually collide and merge, they emit a resounding chirp that eclipses all others. Nevertheless, such an event is transient and exceedingly rare, making near-term detection unlikely.)
The gravitational waves produced by pairs of supermassive black holes lie beyond the frequencies currently observable by experiments like LIGO and Virgo. Instead, gravitational wave researchers rely on arrays of specialized stars called pulsars, which serve as cosmic metronomes. These rapidly spinning stars emit radio waves in a consistent rhythm. If a passing gravitational wave stretches or compresses the space between Earth and the pulsar, it slightly disrupts this rhythm.
Detecting the gravitational wave background using pulsar timing arrays necessitates patience and monitoring of numerous stars. A single pulsar’s rhythm might experience disruptions of only a few hundred nanoseconds over a decade. The magnitude of timing disruptions increases with the intensity of the background noise, hastening the likelihood of initial detection.
Goulding, Greene, and other observational astronomers on the team identified the two black holes using the Hubble Space Telescope. While supermassive black holes are not directly observable through optical telescopes, they are surrounded by bright clusters of stars and warm gas attracted by their immense gravitational pull. During the epoch in which these black holes were observed, the galaxy housing them was among the most luminous in the universe. Furthermore, the galaxy’s core emitted two exceptionally large plumes of gas. Upon directing the Hubble Space Telescope towards the galaxy to investigate the origins of these striking gas formations, researchers uncovered not one, but two massive black holes.
Subsequently, the observational team collaborated with gravitational wave physicists Mingarelli and Princeton graduate student Kris Pardo to contextualize their findings within the realm of the gravitational wave background. This discovery serves as a reference point for estimating the prevalence of supermassive black hole pairs within detectable range of Earth. Previous estimates relied on computational models of galaxy merger frequencies rather than actual observations of supermassive black hole pairs.
Based on their findings, Pardo and Mingarelli predict that under optimistic circumstances, there are approximately 112 nearby supermassive black holes emitting gravitational waves. Consequently, the first detection of the gravitational wave background from supermassive black holes should occur within the next five years. Failure to make such a detection would suggest that the final parsec problem may be insurmountable. The team is presently surveying other galaxies resembling the one harboring the newly discovered supermassive black hole pair. Discovering additional pairs will refine their predictions further.
