It goes without saying that it is hard to observe something that we cannot perceive with our own eyes. Luckily, science and technology have come a long way in helping us to detect the unseeable.
Think of optical and atomic force microscopes showing how the Shewanella oneidensis bacterium extracts oxygen from toxic metals, mirrors and optical cavities that single out a rubidium atom, or spectrographs and telescopes revealing the presence of, among other chemical elements, neon in the hot gas cloud Omega Nebula, 52 million billion kilometres away from Earth.
Pricklier still is to discern something that does not send out any detectable information at all: a black hole.
What is a black hole, and how do we know it is out there if we cannot detect it?
A Star Is Born
The idea of a black hole originated in the 18ᵗʰ century, when John Michell and Pierre-Simon Laplace coined the conjectured phenomenon ‘dark star’.
In the latter half of the 20ᵗʰ century, divided global politics led independently to different names for the same object: physicists to the east of the Iron Curtain referred to ‘frozen stars,’ while those to the west spoke of ‘collapsed stars’.
Notwithstanding the political schism, it was John Wheeler — on the suggestion of an audience member attending one of his lectures — who finally managed to popularize the term ‘black hole’ among academia in 1968, even though Robert Dicke and Ann Ewing used the phrase already in 1960 and 1964, respectively.
Risen from the Equations
Mathematically, the concept of a black hole arises from Albert Einstein’s equations within his theory of general relativity, in spite of Einstein’s reluctance to acknowledge its existence.
General relativity maintains that gravity is the result of the bending of spacetime — which in turn is the consequence of the presence of large matter or energy distributions — and that matter moves along a curved path in spacetime.
It was Karl Schwarzschild who in 1916 worked out a solution to Einstein’s formulas and found that within a certain distance from a non-rotating, uncharged, spherically symmetric star’s inner centre (the Schwarzschild radius) spacetime starts to curve gradually stronger towards its centre, at which point the curvature becomes apparently infinite — technically called a singularity.
A black hole is then defined as an object with a radius equal to or less than the Schwarzschild radius of its original star. For instance, with a radius of 695,700km, the Sun would turn into a black hole if we could shrink her down to an object with a radius of not more than 2.95km — the Sun’s Schwarzschild radius. In the case of the Earth, that would come down to 0.89cm.
Other general relativistic solutions include Reissner-Nordström black holes (charged, spherically symmetric, and non-rotating), Kerr black holes (uncharged, axially symmetric, and rotating), and Kerr-Newman black holes (charged, axially symmetric, and rotating).
What can we except to happen when a star is being shrunk to its Schwarzschild radius?
First of all, while the star is collapsing in on itself — as first calculated by J. Robert Oppenheimer and Hartland Snyder in 1939 — matter density increases which, according to general relativity, is responsible for an ever-growing curvature of spacetime and, thus, a stronger gravitational field in the proximity of the shrinking star.
Because of that intensifying gravity, if you are stationary and watching this event unfolding from a distance, the frequency of the light (which is the number of oscillations per second of the electromagnetic radiation wave, i.e., light) that reaches you from the collapsing star will diminish continually — a situation referred to as gravitational redshift.
Given that clocks rely on the frequency of an atom’s spectral lines to keep track of time — spectral lines are the so-called electromagnetic fingerprints of atoms and molecules — it follows then that time will equally be slowing down.
What this means is that you will claim that the formation of the black hole took an infinite amount of time which basically translates in you not knowing what is going on beyond the black hole’s surface, i.e., the event horizon.
Put differently, to you the event horizon will appear to be in a frozen state, therefore blocking off any communication of information from the region inside the Schwarzschild radius.
To sum up, due to the black hole’s immense gravitational field and the singularity at its centre, both matter and radiation — and this includes light — cannot escape from the event horizon, as it would take them an infinite amount of time to crawl out of the black hole. That is, a black hole appears effectively black to anyone who is observing it from the outside.
It seems furthermore that not all stars are eligible to convert into a black hole; the final outcome of gravitational collapse much depends on the initial mass of the star.
Prior to a star collapsing under gravitational pressure, the star as a whole is kept in balance by a countering internal pressure, caused by the heat liberated from fusing nuclei of light elements into heavier ones — a process known as stellar nucleosynthesis.
But once the fusion grinds to a halt, gravitational pressure takes over, and the collapse sets in.
With an initial mass of not more than 8 solar masses (Mₛ), a star continues to implode until the electrons refuse to be squeezed together any further, based on the fact that two fermions (the fundamental matter particles, e.g., neutrinos, quarks, electrons, etc.) cannot be in the same quantum state — designated as Pauli’s exclusion principle.
That outward pressure exerted by the electrons (Fermi degeneracy pressure) will be capable of stopping the gravity-induced collapse in its tracks.
After shedding off an expanding shell of gas, i.e., the planetary nebula, the resultant stellar remnant receives the name of white dwarf and is only able to subsist in a stable manner if its mass does not exceed 1.4 Mₛ — this threshold is the Chandrasekhar limit. More than 97% of all stars are destined to become (or are already) white dwarfs.
If the stellar core mass oversteps the Chandrasekhar limit — due to either the initial star possessing a mass above 8 Mₛ or the white dwarf accumulating additional mass during its lifetime — the electrons will cave to the gravitational pressure and unite with protons to produce neutrons and neutrinos, allowing a further collapse of the star. Only this time, it is accompanied by an impressive explosion (supernova).
For stars confined to an initial mass of up to 20 or 30 Mₛ, the even more compact stellar core residue is now called a neutron star. It remains in this state due to neutron degeneracy pressure — similar to electron degeneracy pressure in white dwarfs — under the condition that its mass does not surpass the range of 2 to 3 Mₛ, i.e., the Oppenheimer-Volkoff limit.
Both an initial stellar mass beyond 20 to 30 Mₛ and the transgression of the Oppenheimer-Volkoff limit get the ball rolling for a next and final phase in stellar collapse: a black hole.
Before exploring some more general properties of black holes, it is useful to bring our attention to the various types of black holes distinguished by physicists: stellar-mass black holes, intermediate-mass black holes, supermassive black holes, ultra-massive black holes, and primordial black holes.
Stellar-Mass Black Holes
Stellar black holes are the ones formed by the procedure as described in the preceding section ‘Manufacturing Manual’ and have a mass between 3 and 100 Mₛ. According to Kelly Holley-Bockelmann, there are about one hundred million of them in our galaxy.
But black holes within that mass range can also be created by a neutron star that is going past the Oppenheimer-Volkoff limit by gobbling up matter of a companion star as well as by the merger of two neutrons stars, two black holes, or, with still some lingering uncertainty, a neutron star and a black hole.
Intermediate-Mass Black Holes
The existence of a next class of black holes is somewhat still ambiguous. Intermediate-mass black holes would come in a wide variety of masses, oscillating between 100 Mₛ and 1 million Mₛ, and are considered by and large as the potential progenitors or seeds giving rise to supermassive black holes (see the next subsection).
Some candidates for this type include M82 X-1 (428 Mₛ) in the galaxy M82, one in the globular cluster NGC104 of 2,300 Mₛ, HLX-1 in the galaxy ESO 243–49 with 10,000 Mₛ, NGC 2276–3c (in the galaxy NGC 2276) with an estimated mass between 4,300 and 85,000 Mₛ, and one in the gas cloud CO-0.40–0.22 of our own Milky Way galaxy with 100,000 Mₛ.
It is theorized that this type of black hole sees the light of day through a broad array of possible formation mechanisms, such as the union of two black holes (experimentally observed in 2019), the direct collapse of gas clouds, the accumulated collision of objects in densely populated star clusters, being the result of inflated primordial black holes at the time of the Big Bang, or the remnant of the death of the first generation of massive stars — these are stars with an approximate mass range of 100 to 250 Mₛ that sprung to life supposedly after the first few hundred millions of years following the Big Bang.
Supermassive Black Holes
With masses in the order of several million and even billion solar masses, supermassive black holes sit at the centre of most galaxies throughout the Universe, including the Milky Way galaxy — our supermassive black hole Sagittarius A* is projected to have a mass of 4 million Mₛ.
How these enormous astrophysical objects came to be is still an open area of research. One explanation points to mass segregation whereby heavy objects in a galaxy, e.g., massive stars, neutron stars, and stellar and intermediate-mass black holes, move towards the galactic centre whereas lighter objects drift away from it. The central black hole then gradually grows by multiple mergers of these heavy objects.
This mechanism might shed light on the hypothesis that the presence of intermediate-mass black hole seeds in the early Universe leads up to the existence of quasars that came about within the first billion years after the Big Bang — a quasar is a very luminous dense region at the centre of a galaxy in which a supermassive black hole resides that is enclosed by a gaseous accretion disk (see below for more information on accretion disks).
See, for instance, the quasar ULAS J1342+0928 with a 780 million Mₛ supermassive black hole or the quasar SDSS J1148+5251 nurturing a supermassive black hole of 3 billion Mₛ that originated 690 million and 870 million years after the Big Bang, respectively.
Other descriptions of how these black holes end up being so massive suggest growth by black hole accretion whereby a black hole swallows up large amounts of matter or gas from a cloud of stellar debris swirling around the black hole (the accretion disk); the build-up of mass in the bulge of a galaxy — the inner spherical part of a galaxy — which then collapses into a black hole and develops over time through accretion into a supermassive black hole; or, the direct collapse of dense gas clouds combined with an exceptional accretion rate (super-Eddington accretion).
Ultra-Massive Black Holes
Some black holes that inhabit the centre of brightest cluster galaxies — this is the brightest galaxy within a galaxy cluster, which is a large group of galaxies held together by gravitation — exceed the mass of 10 billion Mₛ. They go by the name of ultra-massive black holes.
Examples are the black hole of 17 billion Mₛ within the NGC 1277 galaxy of the Perseus cluster, a 21 billion Mₛ black hole at the NGC 4889 galaxy within the Coma cluster, and a black hole with a range of 25 to 100 billion Mₛ dwelling in the centre of the PKS 0745-BCG galaxy in the PKS 0745 cluster.
This category of black holes is usually observed as quasars dating back to a time between 1.4 and 3.3 billion years following the Big Bang. The hypothesized paths of development of these massive objects echo to some extent the ones delineated in the previous subsection.
Primordial Black Holes
Remaining hidden to the present day, primordial black holes are thought to have formed at the radiation-dominated era (roughly the first 50,000 years of existence of our Universe) as a result of the gravitational collapse of inhomogeneities in the initial density during the inflationary period, i.e., a fraction of a second after the Universe’s birth.
Some of the alternative formation channels of primordial black holes in the early Universe include gravitational instability of supermassive particles, the collapse of cosmic strings, first order phase transitions, and a double inflation scenario.
Contingent upon the precise moment of creation, the primordial black hole’s mass could vary from one one hundred thousandth of a gram (g) to one hundred thousand solar masses. Nonetheless, as black holes are predicted to evaporate over time (see the below subsection ‘A Loophole in Spacetime’), we would detect only those with an initial mass equal to or higher than 10¹⁵ g.
Notwithstanding the prediction of evaporation, the confirmation of the presences of primordial black holes below that threshold would boost the prospect of string theory, because additional spatial dimensions — which are anticipated by the theory — might possibly restrict the rate of evaporation given that these extra dimensions would impact the way gravity behaves on the smallest of scales.
Besides the ordinary (baryonic) matter from which living organisms, planets, and stars are made, physicists estimate that the total amount of matter in the Universe must equally include dark matter (27%) and dark energy (68%). These yet unobserved entities would account for the rotation rate of galaxies, among other astrophysical phenomena, and the Universe’s increasing rate of expansion, respectively.
Across all the types of black holes discussed in the preceding section, we can identify shared features, such as having no hair and the fact that they evaporate over time.
At the same time, various subtypes of commonalities also exist, namely the notion that spinning black holes sustain an accretion disk and that there are multiple relationships between a supermassive black hole and its host galaxy.
Let us now focus on these characteristics in greater detail.
First off, the no-hair theorem states that the only measurable properties of isolated black holes are mass, electric charge, and spin; knowing all the other material properties of matter (‘hair’) that has fallen into a black hole is out of our reach, once it has crossed the event horizon.
Having said that, it is assumed that astrophysical black holes have zero charge, because any charged black hole would be quickly neutralized by attracting and engulfing matter with an opposite charge.
In addition, as all stars and colliding systems in the Universe possess some angular momentum — spin — so will every black hole, since it is originally, among other scenarios, created out of stellar collapse or generated within regions of extremely high density, such as places rife with collisions of compact objects or gas.
The two aforementioned observations about charge and spin bring us to the conclusion that from the four theoretical solutions to general relativity (see the above subsection ‘Risen from the Equations’) the Kerr black hole is the most plausible general description of real-world black holes.
With regard to mass, the black hole’s size solely depends on the amount of mass it has absorbed. It is then obvious from the previous section ‘Many Masses’ that an ultra-massive black hole will be much bigger than a stellar black hole.
A Loophole in Spacetime
In the upper subsection ‘Spacetime Consequences’, we mentioned that nothing escapes from within a black hole’s event horizon. Except, Stephen Hawking demonstrated that some radiation (called Hawking radiation) does slip through the black hole’s allegedly impermeable barrier and leaks back out into the Universe, albeit too weak to be detectable by our current technology.
This leads to the consequence that black holes evaporate over time. The larger they are, the longer it takes them to evaporate.
But this triggers a problem: As Hawking radiation does purportedly not contain any information about what lives behind the event horizon, then what happens to that stored information inside the black hole once it has evaporated away completely?
Several ideas have been formulated to solve this black hole information paradox, but research now hints that information does in fact get out of a black hole after all, providing a tentative way out of this conundrum.
Black holes born out of at least a binary star system are able to acquire an accretion disk. What is more, when they nibble away matter or gas from that disk, large jets of radiation soar — usually in the X-ray spectrum — which oust up to 42% of that disk debris in the form of heat energy (compare that to a matter-energy conversion rate of 0.7% inside the Sun) whilst 58% of the accreted matter, at a minimum, disappears into the black hole.
These radiative streams emerge because the rotating material around the black hole is being heated up through friction and compressed extensively when approaching the event horizon. But whether the black hole spin itself (see, for instance, the Blandford-Znajek process and the Penrose mechanism) or the accretion disk dynamics are responsible for this powerful outward energy propulsion is still up for debate.
Some of the most energetic phenomena across the Universe are ideal birthplaces of accretion disks. They include active galactic nuclei (e.g., quasars), X-ray binaries (a system whereby either a neutron star or a black hole accretes from a normal star, releasing X-rays during the process), tidal disruption events (when a supermassive black hole rips apart a star that came too close — occurring every ten thousand years or so — and devours roughly half of that star, a short-lived radiation discharge flares up), or gamma-ray bursts (a multi-second, very intense flash of gamma rays that is believed to stem from an explosion following the merger of neutron stars).
Concerning supermassive black holes, recent experimental data by Meg Urry et al. indicate that they generally display a relatively high spin, suggesting that these black holes obtain their enormous size predominately by accreting gas instead of merging with black holes.
Scientists have unveiled several connections between supermassive black holes and the galaxy in which they dwell.
For one, some researchers have found a positive correlation between the mass of a supermassive black hole and various properties of its host galaxy, in particular galaxy mass, bulge mass, bulge luminosity, and stellar velocity dispersion (which gives the average radial velocity of stars around a central massive object). Moreover, the supermassive black hole’s mass measures typically 0.1% of that of the bulge of a galaxy.
These relations insinuate that galaxy formation and black hole growth go hand in hand. This is additionally corroborated by other studies that use X-ray luminosity of active galactic nuclei as a proxy for black hole accretion.
Even though many researchers indeed evaluate such reinforcing links between black hole growth and galaxy stellar formation, several other research studies report an inverse correspondence between these two variables.
One of their main arguments is that an increased rate of accretion results in a higher amount of radiation ejected into the host galaxy which in turn prevents the surrounding gas to cool and condense into molecular clouds (which eventually collapse into stars).
The context of the above discussion becomes even more subtle when considering that a lower rate of star formation has equally been associated with, among other galactic properties, higher stellar masses, halo masses, and bulge masses.
Future research will help to further clarify the nature of the relationship between galaxy formation and the mass of its central black hole.
At this point, we have attained a more comprehensive picture of the nature and properties of black holes. But there is one question that remains unanswered: How do we know black holes are out there, since not only can we not see them, but they also do not give off any detectable electromagnetic radiation?
One way to go about detecting a black hole relies on the orbital movement of stars around such massive object. Based on radio and infrared telescope observations, the knowledge of full stellar orbits can — with the support of the equations of general relativity, or even Kepler’s laws — reveal the mass of that central object, from which it is possible to infer the presence of a black hole.
Nonetheless, in order to apply this method with respect to supermassive black holes at the centre of galaxies, the catch is that we need to be able to distinguish individual orbits of stars around a black hole which is not always feasible due to insufficient resolution of our instruments (distances are vast in our Universe) or having the line of sight blocked by interstellar gas, dust, or intense light sources.
Regarding our own galaxy, Andrea Ghez and Reinhard Genzel have recently been awarded the Nobel Prize in Physics — shared with Roger Penrose — for meticulously mapping numerous stellar orbits in the Milky Way’s innermost central region, providing strong evidence for the presence of a supermassive black hole named Sagittarius A*.
A next approach engages with the concept of velocity: Astronomical spectroscopy, which studies the electromagnetic radiation emitted by astronomical objects, uncovers information on the velocity dispersion of stars from which a black hole mass can be derived.
For example, leaning on near-infrared spectroscopy, the work by Laura Ferrarese et al. established a velocity profile within the nucleus of galaxy M33 which is not consistent with the presence of a supermassive black hole, despite being a region with an extremely high mass density.
Another example resorts to the combination of spectroscopy within the visible spectrum and photometry (which analyses the received electromagnetic energy — flux — or brightness from celestial objects) to calculate radial velocities and ultimately determine the presence of a black hole within the binary star system XTEJ1859+226.
A third technique involves the scrutiny of X-ray emissions that originate from jets propagated by quasars, gamma-ray bursts, or X-ray binaries, among other high-energy events.
As a case in point, through the combined use of the Swift’s Burst Alert Telescope and the Chandra X-ray Observatory, scientists were able to figure out what the mass of the central black hole is by looking at the duration of the measured flares of the gamma-ray burst GRB 110328A (which was later on reclassified as a tidal disruption event named Swift J1644+57).
Besides X-rays, other wavelengths in the electromagnetic spectrum prove valuable, too. In one instance, through the inspection of ultraviolet and optic light, the joined effort of the Pan-STARRS1 and the GALEX telescopes enabled the detection of a tidal disruption event in the inactive galaxy PS1–10jh, leading to the identification of a 2.8 million Mₛ supermassive black hole.
Another modus operandi that guides us towards the whereabouts of black holes is called gravitational lensing and falls back on a prediction of general relativity, namely that strong gravitational regions of spacetime bend light.
Gravitational lensing refers to the situation whereby a large mass, e.g., a black hole or a whole galaxy, that is situated in between an observer and a distant light source bends the light transmitted by this distant source, generating as a result a distorted, magnified source image — or even multiple source images — together with, under certain circumstances, a ring of light around that massive object (Einstein ring).
This method implies that the effect of gravitational lensing should, at least in theory, be visible and detectable. Within the context of supermassive black holes, several sources of gravitational lensing have been proposed, including closely orbiting objects, such as stars or low-mass X-ray binaries, and the appearance of hot spots (flares) on the accretion disk. Even more intriguingly, if this lensing technique would be conducive to the pinpointing of primordial black holes in the galactic halo, it might help to disentangle the enigma of dark matter.
In one example, the gravitational lensing was caused by the presence of an entire galaxy: researchers identified the location of a supermassive black hole within the active galactic nucleus PKS 1830–211 only because a galaxy in between the observing telescopes and PKS 1830–211 acted as a gravitational magnifying lens, producing an Einstein ring and multiple source images in the process.
What is more, one of the distinct manifestations of the magnifying effect of gravitational lensing is the broadening of iron (Fe) spectral lines, which reside within the black hole accretion disk and have been excited by scattered X-ray photons. This scaling effect is further enhanced by Doppler shifts and gravitational redshift which become especially prominent on a strongly rotating accretion disk.
For instance, both gravitational lensing and strong light-bending explain the registered Fe spectral variability — even though not everyone agrees — within the innermost region of the accretion disk twirling around the supermassive black hole of galaxy MCG-6–30–15.
A fifth approach to indirectly see a black hole focuses on exposing its surrounding accretion disk. It is nevertheless instructive to remain mindful of the fact that neutron stars can also be endowed with an accretion disk, making the detection of a black hole a sticky endeavour.
One example highlights how the inner accretion disk of a black hole within the active galactic nucleus IRAS 13224–3809 can be made accessible to surveillance by studying the rapid variability in flux of the X-ray spectral band, which is gathered by the XMM-Newton and nuSTAR telescopes.
But perhaps the most solid evidence for the existence of a black hole to date comes from the production of a direct image of the shadow and the accretion disk of a 6.5 billion Mₛ supermassive black hole within the active galaxy M87, taken by the Event Horizon Telescope.
Another, relatively young monitoring method makes use of relativistic physics: The recording of gravitational waves (GW) by ground-based observatories — think of the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo in particular — provides data (e.g., distance, mass, luminosity, and spin) about stellar-mass and even intermediate-mass black holes that were born out of binary black hole and neutron star systems.
For instance, both the GW190425 and GW190426_152155 events might qualify as a black hole-neutron star merger and produced a 3.4 Mₛ and 7.2 Mₛ stellar-mass black hole, respectively. Also, the black hole merging event GW190521 gave birth to a 150 Mₛ intermediate-mass black hole, while the neutron star merger GW170817 brought about, arguably, a 2.7 Mₛ stellar-mass black hole. As a final example, the black hole merger GW190517_055101 resulted in a 61.9 Mₛ stellar-mass black hole, exhibiting the largest effective aligned spin.
It seems furthermore to be the case that stellar black holes discovered through gravitational waves are on average of higher mass than the ones picked up by electromagnetic radiation-based telescopes.
And when it comes to other types of black holes and merging events, such as primordial black holes and supermassive black hole mergers, the odds of observing them will improve considerably when the Laser Interferometer Space Antenna (LISA) is up and running in 2034 (which is, unlike LIGO and Virgo, not ground-based, but consists of three spacecrafts orbiting the Sun in an equilateral triangle formation).
A final strategy that we will discuss in this section to track down black holes brings us to particle physics.
At the particle accelerator the Large Hadron Collider (LHC) at CERN, mostly protons and heavy ions — ions are particles that possess an electrical charge — are smashed into each other at extremely high energies. By analysing the aftermath of collisions, physicists look for answers about, inter alia, the early Universe, the nature of dark matter and dark energy, and the existence of supersymmetric particles, i.e., a class of hypothetical particles that would, if confirmed, upgrade the Standard Model into a more complete model describing our physical Universe.
Some scientists anticipate that microscopic black holes will pop up at the LHC, since the collision of particles with high enough energies might turn gravity into an increasingly relevant factor — remember that, as per general relativity, the higher the mass or energy (which are equivalent), the stronger the gravitational field, which at some point will eventually facilitate the presence of a black hole.
Not only that, spotting these black holes at the LHC would strengthen the robustness of string theory, as it is built on the premise of additional spatial dimensions (see also the above subsection ‘Primordial Black Holes’).
What is more, and still under the assumption of a reality that comprises extra dimensions, high-energy cosmic neutrinos entering Earth may equally foster the conception of microscopic black holes. Upon their creation, they would immediately evaporate and leave signatures traceable by neutrino telescopes beneath the Earth’s surface, e.g., the IceCube or Super-Kamiokande observatory.
For the time being, neither microscopic black holes nor extra dimensions of spacetime have been experimentally detected.
A Cosmic Wonder of Spacetime
Scientists have come a long way in developing tools and techniques to make the unobservable black hole observable.
Whether it has been conceived from the remnants of a dying star, from density inhomogeneities within the first moments of the big Bang, or from collapsing gas clouds, whether it is microscopic or ultra-massive, or whether it accretes or not, the very existence of a black hole will continue to tantalize our imagination about the nature and possible limits of our Universe for many more years to come.
If the recent technological breakthroughs, including the detection of gravitational waves and the first direct image of a black hole’s shadow, are just a glimpse of what the Universe has in store for us, then science might indeed hold the key to open doors to worlds that only our imagination could have embraced.