Black hole science enters its golden age

For centuries, black holes were merely theoretically speculative ideas.

This tiny sliver of the GOODS-N deep field, imaged with many observatories including Hubble, Spitzer, Chandra, XMM-Newton, Herschel, the VLT, and more, contains a seemingly unremarkable red dot. That object, a quasar-galaxy hybrid from just 730 million years after the Big Bang, may be key to unlocking the mystery of galaxy-black hole evolution. Once speculative, the evidence for the physical existence and ubiquity of black holes is now overwhelming.

The concept first arose in 1783, when John Michell proposed them.

The Sun’s light is due to nuclear fusion, which primarily converts hydrogen into helium. When we measure the rotation rate of the Sun, we find that it’s one of the slowest rotators in the entire Solar System, taking from 25-to-33 days to make one 360-degree rotation, dependent on latitude. Emitting a near-constant 3.8 × 10^26 W of power, the Sun is the brightest thing most of us will ever see. Although many other sources are intrinsically brighter, they’re much farther away.

If you maintained the Sun’s density but increased its mass, light couldn’t escape above ~500 solar masses.

An illustration of heavily curved spacetime for a point mass, which corresponds to the physical scenario of being located outside the event horizon of a black hole. As you get closer and closer to the mass’s location in spacetime, space becomes more severely curved, eventually leading to a location from within which even light cannot escape: the event horizon. The radius of that location is set by the mass, charge, and angular momentum of the black hole, the speed of light, and the laws of General Relativity alone. Quite remarkably, if you replace “r/R” with the inverse of it, “R/r”, you can map the interior of a black hole onto the exterior and vice versa, transforming your solution for a black hole into one for a white hole.

Although none were observed, the idea resurged with Karl Schwarzschild’s 1916 solution within Einstein’s General Relativity.

In a Universe that isn’t expanding, you can fill it with stationary matter in any configuration you like, but it will always collapse down to a black hole. Such a Universe is unstable in the context of Einstein’s gravity, and must be expanding to be stable, or we must accept its inevitable fate.

With enough mass in a given spatial volume, collapse to a black hole becomes unavoidable.

From outside a black hole, all the infalling matter will emit light and is always visible, while nothing from behind the event horizon can get out. The event horizon of a rotating black hole ought to be only dependent on its mass and spin, but we have not yet figured out how (or whether) the spinning black hole has a coupling to the exterior Universe.

In 1963, Roy Kerr enhanced Schwarzschild’s solution to incorporate rotation.

In the vicinity of a black hole, space flows like either a moving walkway or a waterfall, depending on how you want to visualize it. Unlike in the non-rotating case, the event horizon splits into two, while the central singularity gets stretched out into a one-dimensional ring. Nobody knows what occurs at the central singularity, but its presence and existence cannot be avoided with our current understanding of physics.

Contemporaneously, suggestive “black hole” evidence appeared with the discovery of the first quasars.

The radio feature of the galaxy Alcyoneus include a central, active black hole, collimated jets, and two giant radio lobes at either end. The Milky Way is shown at the bottom for scale, as well as “10x the Milky Way” for perspective.

These extragalactic QUAsi-StellAr Radio Sources (QUASARs) were ultra-distant, but shone brilliantly in radio light and beyond.

This illustration of a radio-loud quasar that is embedded within a star-forming galaxy gives a close-up look of how giant radio galaxies are expected to emerge. At the center of an active galaxy with a supermassive black hole, jets are emitted that slam into the larger galactic halo, energizing the gas and plasma and causing radio emissions in the form of jets close by the black hole, and then plumes and/or lobes farther away. Both supermassive and stellar-mass black holes have overwhelming evidence supporting their existence, but supermassive black holes may heat matter to the highest temperatures of all, accelerating particles to even beyond the GZK cutoff set by particle physics.

Then Cygnus X-1, an X-ray emitting black hole candidate, was found within the Milky Way.

Discovered in 1964 as an X-ray emitting source consistent with a stellar object orbiting a black hole, Cygnus X-1 represents the first black hole candidate known within the Milky Way. Cygnus X-1 is located near large active regions of star formation in the Milky Way: precisely the location expected to find an X-ray emitting black hole binary.

Meanwhile, Roger Penrose demonstrated, astrophysically, how black holes could pragmatically form in our Universe.

When matter collapses, it can inevitably form a black hole. Roger Penrose was the first to work out the physics of spacetime, applicable to all observers at all points in space and at all instants in time, that governs a system such as this. His conception has been the gold standard in General Relativity ever since.

John Wheeler gave the name “black holes” in 1968.

The second-largest black hole as seen from Earth, the one at the center of the galaxy M87, is shown in three views here. At the top is optical from Hubble, at the lower-left is radio from NRAO, and at the lower-right is X-ray from Chandra. These differing views have different resolutions dependent on the optical sensitivity, wavelength of light used, and size of the telescope mirrors used to observe them. These are all examples of radiation emitted from the regions around black holes, demonstrating that black holes aren’t so black, after all.

Once speculative, the modern case for them is overwhelming.

This view of the cocoon surrounding the Milky Way’s galactic center is only ~10 light-years across, but contains and is possibly powered by our central, supermassive black hole that weighs in at ~4 million times the mass of our Sun.

X-ray emissions appear from accelerating, infalling, accreted matter.

On September 14, 2013, astronomers caught the largest X-ray flare ever detected from the supermassive black hole at the center of the Milky Way, known as Sagittarius A*. In X-rays, no event horizon is visible at these resolutions; the “light” is purely disk-like. However, we can be certain that only matter remaining outside the event horizon generates light; matter passing within it gets added to the black hole’s mass, inevitably infalling into the black hole’s central singularity. Many types of transients are now known to exist across many different wavelengths of light.

Individual stars orbit these massive, non-luminous objects.

This 20-year time-lapse of stars near the center of our galaxy comes from the ESO, published in 2018. Note how the resolution and sensitivity of the features sharpen and improve toward the end, all orbiting our galaxy’s (invisible) central supermassive black hole. Practically every large galaxy, even at early times, is thought to house a supermassive black hole, but only the one at the center of the Milky Way is close enough to see the motions of individual stars around it, and to thereby accurately determine the black hole’s mass.

Gravitational waves arise from both inspirals

The most up-to-date plot, as of November, 2021, of all the black holes and neutron stars observed both electromagnetically and through gravitational waves. While these include objects ranging from a little over 1 solar mass, for the lightest neutron stars, up to objects a little over 100 solar masses, for post-merger black holes, gravitational wave astronomy is presently only sensitive to a very narrow set of objects. The closest black holes have all been found as X-ray binaries, until the November 2022 discovery of Gaia BH1.

and mergers.

When two neutron stars collide, if their total mass is great enough, they won’t just result in a kilonova explosion and the ubiquitous creation of heavy elements, but will lead to the formation of a novel black hole from the post-merger remnant. Gravitational waves and gamma-rays from the merger appear to travel at indistinguishable speeds: the speed of all massless particles.

And photon emissions now reveal their horizons,

Size comparison of the two black holes imaged by the Event Horizon Telescope (EHT) Collaboration: M87*, at the heart of the galaxy Messier 87, and Sagittarius A* (Sgr A*), at the center of the Milky Way. Although Messier 87’s black hole is easier to image because of the slow time variation, the one around the center of the Milky Way is the largest as viewed from Earth.

including polarizations,

Polarized view of the black hole in M87. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. Note how much swirlier this image appears than the original, which was more blob-like. It’s fully expected that all supermassive black holes will exhibit polarization signatures imprinted upon their radiation, a calculation that requires the interplay of General Relativity with electromagnetism to predict. Additionally, outside of the event horizon, a small amount of radiation is constantly emitted owing to the curvature of space itself: Hawking radiation, which will eventually be responsible for this black hole’s decay.

directly. Welcome to the golden age for black holes.

The time-averaged data from multiple different points in time that show a series of snapshots in the evolution of the radiation coming from Sagittarius A*. The “average” image structure belies the rapid time-evolution of the radiation around this object.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.