For centuries, black holes were just theoretically speculative ideas.
This tiny fragment of the GOODS-N deep field, imaged by many observatories including Hubble, Spitzer, Chandra, XMM-Newton, Herschel, the VLT and many others, contains a seemingly unremarkable red dot. This object, a quasar-galaxy hybrid dating to just 730 million years after the Big Bang, could hold the key to unraveling 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 appeared in 1783, when John Michell proposed them.
This image of the Sun, taken on April 20, 2015, shows a number of features common to all stars: magnetic loops, prominences, plasma filaments, and regions of higher and lower temperatures. The Sun is less dense than the Earth, but much larger and more massive, and has a much greater escape velocity from its surface than the Earth. If the Sun maintained the same density but had 500 times its current mass, with the corresponding increase in volume, it would itself collapse into a black hole, as first shown in 1783 by John Michell, even in Newtonian gravity.
If you maintained the Sun’s density but increased its mass, light could not escape above about 500 solar masses.
Inside a black hole, the curvature of spacetime is so great that light cannot escape, nor particles, under any circumstances. A singularity, based on our current laws of physics, must be inevitable, although the nature of this singularity is not well understood in the context of general relativity alone.
Although none were observed, the idea resurfaced with Karl Schwarzschild’s 1916 solution to Einstein’s General Relativity.
If you start with a fixed, bound configuration of mass, and there are no non-gravitational forces or effects present (or they are all negligible compared to gravity), that mass will collapse always inevitably into a black hole. This is one of the main reasons why a static, non-expanding universe is incompatible with Einstein’s relativity.
With sufficient mass in a given spatial volume, black hole collapse becomes inevitable.
From outside a black hole, all incoming matter will emit light and will always be visible, while nothing behind the event horizon will be able to come out. But if you were the one who fell into a black hole, your energy could in theory reappear as part of a hot Big Bang in a newborn Universe; the link between black holes and the birth of new universes is still speculative, but is dismissed at our peril.
In 1963 Roy Kerr improved Schwarzschild’s solution to incorporate rotation.
Even for a complicated entity like a spinning massive black hole (a Kerr black hole), once you cross the (outer) event horizon, whatever type of matter or radiation you are made of, you will fall toward the central singularity and add mass to the black hole. What happens at the central singularity is however not well described by current physics, because its behavior is pathological.
At the same time, evidence suggestive of a “black hole” emerged with the discovery of the first quasars.
The radio feature of the Alcyoneus galaxy includes a central active black hole, collimated jets, and two giant radio lobes at either end. The Milky Way is displayed at the bottom for scale, along with “10x the Milky Way” for perspective.
These extragalactic QUAsi-StellAr (QUASAR) radio sources were ultra-distant, but shone brightly in the radio light and beyond.
This illustration of a radio-strong quasar embedded in a star-forming galaxy provides insight into 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 gas and plasma and causing radio emissions in the form of jets near the black hole, then plumes and/or more distant lobes. Supermassive and stellar-mass black holes have overwhelming evidence supporting their existence.
Then Cygnus X-1, a candidate X-ray emitting black hole, was found in the Milky Way.
Discovered in 1964 as an X-ray emitting source compatible with a stellar object orbiting a black hole, Cygnus X-1 represents the first known candidate black hole in the Milky Way. Cygnus X-1 is located near large active star forming regions in the Milky Way: precisely the predicted location to find an X-ray emitting black hole binary.
Meanwhile, Roger Penrose demonstrated, astrophysically, how pragmatically black holes could form in our Universe.
When matter collapses, it can inevitably form a black hole. Penrose was the first to develop the physics of space-time, applicable to all observers at all points in space and all instants of time, which governs such a system. Its design has been the gold standard of general relativity ever since.
John Wheeler coined the name “black holes” in 1968.
This three-panel view features the central region of the galaxy Messier 87, home to the largest black hole (about 6.5 billion solar masses) known to be about 100 million light-years away. Optical jet (top), radio lobes (bottom left) and ultra-hot X-ray emitting signatures (bottom right) all indicate the presence of an ultramassive black hole, recently confirmed by direct measurements of the Event Horizon Telescope.
Once speculative, the modern case for them is overwhelming.
This view of the cocoon surrounding the galactic center of the Milky Way is only about 10 light-years across, but contains and is possibly powered by our central supermassive black hole which weighs about 4 million times the mass of our Sun. .
X-ray emissions come from the acceleration, fall and accretion of matter.
On September 14, 2013, astronomers captured the largest X-ray flare ever detected in the supermassive black hole at the center of the Milky Way, known as Sagittarius A*. Emission from the black hole in many wavelengths of light has hinted at its properties, but there is no substitute for direct observation of its event horizon.
Individual stars orbit these massive, non-luminous objects.
This 20-year time lapse of stars near the center of our galaxy is from ESO, published in 2018. Note how the resolution and sensitivity of features sharpens and improves towards the end, all orbiting around of the central (invisible) supermassive black of our galaxy. hole. Virtually every large galaxy, even in early times, is thought to harbor a supermassive black hole, but only the one at the center of the Milky Way is close enough to see the movements of individual stars around it.
Gravitational waves come from the two inspirations
The most up-to-date plot, as of November 2021, of all black holes and neutron stars observed both electromagnetically and by gravitational waves. As you can clearly see, there is no longer a “gap” between 2 and 5 solar masses; on the contrary, this population exists and is probably composed of black holes formed from the merger of neutron stars, according to the event of August 17, 2017.
and mergers.
When two neutron stars collide, if their total mass is large enough, they will not only result in a kilonova explosion and the ubiquitous creation of heavy elements, but will lead to the formation of a new black hole from the post-merger residue. Gravitational waves and gamma rays from fusion appear to travel at indistinguishable speeds: the speed of all massless particles.
And the 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 Messier 87 galaxy, and Sagittarius A* (Sgr A*), at the center of the Milky Way. Although Messier 87’s black hole is easier to image due to slow weather variation, the one around the center of the Milky Way is the largest seen from Earth.
including polarizations,
Polarized view of the black hole in M87. The lines mark the orientation of the polarization, which is related to the magnetic field around the shadow of the black hole. Notice how much more swirly this image appears than the original, which looked more like a drop. All supermassive black holes are expected to exhibit polarization signatures imprinted on their radiation.
directly. Welcome to the golden age of black holes.
Time-averaged data from several different time points that show a series of snapshots of the evolution of radiation from Sagittarius A*. The “average” image structure contradicts the rapid temporal evolution of the radiation around this object.
Mostly Mute Monday tells an astronomical story in pictures, visuals and no more than 200 words. Talk less; smile more.