Measuring the Spin of Black Holes: A Journey to the Edge of the Universe
The quest to understand the mysteries of black holes has captivated astronomers and physicists for decades. While these celestial entities are often depicted as insatiable monsters that devour everything, including light, recent research has revealed a fascinating aspect of their nature: black holes spin. And they spin with incredible velocity.
The speed at which a black hole spins is crucial in comprehending its impact on its surroundings, from the accretion disc to the entire galaxy. A groundbreaking paper by Tegan Thomas of the University of Virginia and her team offers both good and bad news in this regard. The bad news is that, for now, we are unable to determine the exact speed at which black holes spin. However, the good news is that a potential solution is on the horizon.
Two competing theories have been proposed to explain the maximum spin velocity of a black hole. The first, developed by Kip Thorne in the 1970s, suggests that the maximum speed is 99.8% of the speed of light, with photons emitted from the accretion disc acting as a braking force. The second theory, introduced by Charles Gammie in 2004, proposes a maximum speed of 93.75% of the speed of light, constrained by highly magnetized jets that slow down the black hole's rotation.
For years, scientists have debated which of these theories is correct. The advent of advanced telescopes, such as the Event Horizon Telescope (EHT), has provided valuable data on black hole properties. However, the EHT's resolution of 20 microarcseconds is insufficient to distinguish between the two spin theories.
To address this challenge, Thomas and her team employed a sophisticated 3D General Relativistic Magnetohydrodynamics (GRMHD) simulation. They modeled Sgr A*, a black hole at the center of our galaxy, under both theoretical maximum spin scenarios. By using ray-tracing software, they generated synthetic radio images that could be compared with EHT observations.
Surprisingly, the results showed that the EHT could not differentiate between the two spin models. The accretion rate and relativistic jets created by the black hole were nearly identical in both scenarios. Moreover, the light curves, linear polarization, and circular polarizations of the signals overlapped significantly at the EHT's resolution.
The solution to this conundrum may lie in the black hole's photon ring, a thin and brilliant circle of light formed by light rays that have been trapped by the black hole's gravity, made multiple rotations, and then escaped towards Earth. However, current Earth-based sensors lack the sensitivity required to observe this phenomenon, which is on the order of 5 microarcseconds.
The Black Hole Explorer (BHEX) mission, currently in the planning stages as a NASA Small Explorer mission, offers a promising solution. BHEX aims to place a radio telescope in Earth's orbit, working in conjunction with the EHT's components like the Green Bank Telescope (GBT) and the Atacama Large Millimeter/submillimeter Array (ALMA). By extending the EHT into space, BHEX will create an interferometer capable of directly observing the photon ring of Sgr A*.
Once launched, BHEX will play a pivotal role in determining the precise shape of the photon ring for Sgr A*. While our local black hole may not be spinning at its theoretical maximum speed, BHEX will guide us in identifying the necessary observations to achieve that goal. The decades-long debate about black hole spin may soon reach a conclusive answer, shedding new light on these enigmatic celestial entities.
As we continue to explore the universe, the quest to measure black hole spin becomes a testament to our relentless pursuit of knowledge and our desire to unravel the secrets of the cosmos.
