Pricklier still is to discern something that does not send out any detectable information at all: a black hole.<\/p>\n
What is a black hole, and how do we know it is out there if we cannot detect it?<\/p>\n
A Star Is\u00a0Born<\/strong><\/h3>\nThe idea of a black hole originated in the 18\u1d57\u02b0 century, when John Michell and Pierre-Simon Laplace coined the conjectured phenomenon \u2018dark star<\/a>\u2019.<\/p>\nIn the latter half of the 20\u1d57\u02b0 century, divided global politics led independently to different names<\/a> for the same object: physicists to the east of the Iron Curtain referred to \u2018frozen stars,\u2019 while those to the west spoke of \u2018collapsed stars\u2019.<\/p>\nNotwithstanding the political schism, it was John Wheeler\u200a\u2014\u200aon the suggestion<\/a> of an audience member attending one of his lectures\u200a\u2014\u200awho finally managed to popularize the term \u2018black hole\u2019 among academia in 1968, even though Robert Dicke<\/a> and Ann Ewing<\/a> used the phrase already in 1960 and 1964, respectively.<\/p>\nRisen from the Equations<\/strong><\/h4>\nMathematically, the concept of a black hole arises from Albert Einstein\u2019s equations within his theory of general relativity, in spite of Einstein\u2019s reluctance<\/a> to acknowledge its existence.<\/p>\nGeneral relativity maintains<\/a> that gravity is the result of the bending of spacetime\u200a\u2014\u200awhich in turn is the consequence of the presence of large matter or energy distributions\u200a\u2014\u200aand that matter moves along a curved path in spacetime.<\/p>\nIt was Karl Schwarzschild who in 1916 worked out a solution to Einstein\u2019s formulas and found that within a certain distance from a non-rotating, uncharged, spherically symmetric star\u2019s inner centre (the Schwarzschild radius<\/a>) spacetime starts to curve gradually stronger towards its centre, at which point the curvature becomes apparently infinite\u200a\u2014\u200atechnically called a singularity<\/a>.<\/p>\n\n![\"Spacetime](\"data:image\/svg+xml;base64,PHN2ZyB3aWR0aD0iMSIgaGVpZ2h0PSIxIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg==\" "\\"Spacetime")
Fig. 1. Spacetime configuration of a black hole. (Source: Unnikrishnan Menon<\/a> and Martin Silvertant<\/a>).<\/p><\/div><\/figure>\nA 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<\/a> would turn into a black hole if we could shrink her down to an object with a radius of not more than 2.95km\u200a\u2014\u200athe Sun\u2019s Schwarzschild radius. In the case of the Earth, that would come down to 0.89cm.<\/p>\nOther general relativistic solutions include Reissner-Nordstr\u00f6m<\/a> black holes (charged, spherically symmetric, and non-rotating), Kerr<\/a> black holes (uncharged, axially symmetric, and rotating), and Kerr-Newman<\/a> black holes (charged, axially symmetric, and rotating).<\/p>\nSpacetime Consequences<\/strong><\/h4>\nWhat can we except to happen when a star is being shrunk to its Schwarzschild radius?<\/p>\n
First of all, while the star is collapsing in on itself\u200a\u2014\u200aas first calculated<\/a> by J. Robert Oppenheimer and Hartland Snyder in 1939\u200a\u2014\u200amatter density increases<\/a> 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.<\/p>\nBecause 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\u200a\u2014\u200aa situation referred to as gravitational redshift<\/a>.<\/p>\n\nFig. 2. A schematic view of gravitational redshift: an electromagnetic wave (light) is climbing out of a gravitationally dense region, whereby the frequency reduces and the wavelength\u200a\u2014\u200athe distance from peak to peak\u200a\u2014\u200aincreases. (Source: Shadab\u00a0Alam<\/a>).<\/p><\/div><\/figure>\nGiven that clocks rely on<\/a> the frequency of an atom\u2019s spectral lines to keep track of time\u200a\u2014\u200aspectral lines are the so-called electromagnetic fingerprints<\/a> of atoms and molecules\u200a\u2014\u200ait follows then that time will equally be slowing down.<\/p>\nWhat 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\u2019s surface, i.e., the event horizon<\/a>.<\/p>\nPut 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.<\/p>\n
To sum up, due to the black hole\u2019s immense gravitational field and the singularity at its centre, both matter and radiation\u200a\u2014\u200aand this includes light\u200a\u2014\u200acannot 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.<\/p>\n
Manufacturing Manual<\/strong><\/h3>\nIt 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.<\/p>\n
Prior to a star collapsing under gravitational<\/em> pressure, the star as a whole is kept in balance by a countering internal<\/em> pressure, caused by the heat liberated from fusing nuclei of light elements into heavier ones\u200a\u2014\u200aa process known as stellar nucleosynthesis<\/a>.<\/p>\nBut once the fusion grinds to a halt, gravitational pressure takes over, and the collapse sets in.<\/p>\n
With an initial mass of not more than 8 solar masses<\/a> (M\u209b), 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\u200a\u2014\u200adesignated as Pauli\u2019s exclusion principle<\/a>.<\/p>\nThat outward pressure exerted by the electrons (Fermi degeneracy pressure<\/a>) will be capable of stopping the gravity-induced collapse in its tracks.<\/p>\nAfter shedding off an expanding shell of gas, i.e., the planetary nebula<\/a>, the resultant stellar remnant receives the name of white dwarf<\/a> and is only able to subsist in a stable manner if its mass does not exceed 1.4 M\u209b\u200a\u2014\u200athis threshold is the Chandrasekhar limit<\/a>. More than 97%<\/a> of all stars are destined to become (or are already) white dwarfs.<\/p>\n\nFig. 3. A succinct overview of the different paths of stellar evolution. (Source: Britannica<\/a>).<\/p><\/div><\/figure>\nIf the stellar core mass oversteps the Chandrasekhar limit\u200a\u2014\u200adue to either the initial star possessing a mass above 8 M\u209b or the white dwarf accumulating additional mass during its lifetime\u200a\u2014\u200athe 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<\/a>).<\/p>\nFor stars confined to an initial mass of up to 20 or 30 M\u209b<\/a>, the even more compact stellar core residue is now called a neutron star<\/a>. It remains in this state due to neutron degeneracy pressure\u200a\u2014\u200asimilar to electron degeneracy pressure in white dwarfs\u200a\u2014\u200aunder the condition that its mass does not surpass the range of 2 to 3 M\u209b, i.e., the Oppenheimer-Volkoff limit<\/a>.<\/p>\nBoth an initial stellar mass beyond 20 to 30 M\u209b and the transgression of the Oppenheimer-Volkoff limit get the ball rolling for a next and final phase in stellar collapse: a black hole.<\/p>\n
Many Masses<\/strong><\/h3>\nBefore 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.<\/p>\n
Stellar-Mass Black\u00a0Holes<\/strong><\/h4>\nStellar black holes are the ones formed by the procedure as described in the preceding section \u2018Manufacturing Manual\u2019 and have a mass between 3<\/a> and 100 M\u209b. According to Kelly Holley-Bockelmann, there are about one hundred million<\/a> of them in our galaxy.<\/p>\nBut 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<\/a> matter of a companion star as well as by the merger of two<\/a> neutrons stars<\/a>, two<\/a> black holes<\/a>, or, with still some lingering uncertainty, a neutron star<\/a> and a<\/a> black hole<\/a>.<\/p>\nIntermediate-Mass Black\u00a0Holes<\/strong><\/h4>\nThe 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<\/a> between 100 M\u209b and 1 million M\u209b, and are considered by and large as the potential progenitors or seeds<\/a> giving rise to supermassive black holes (see the next subsection).<\/p>\nSome candidates for this type include M82 X-1 (428 M\u209b<\/a>) in the galaxy M82, one in the globular cluster NGC104 of 2,300 M\u209b<\/a>, HLX-1 in the galaxy ESO 243\u201349 with 10,000 M\u209b<\/a>, NGC 2276\u20133c (in the galaxy NGC 2276) with an estimated mass between 4,300 and 85,000 M\u209b<\/a>, and one in the gas cloud CO-0.40\u20130.22 of our own Milky Way galaxy with 100,000 M\u209b<\/a>.<\/p>\n\nFig. 4. This composite image shows the nearby galaxy Messier 82. The intermediate-mass black hole M82 X-1 is the brightest object in the inset. (Source: Earthsky<\/a>).<\/p><\/div><\/figure>\nIt 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<\/a>), the direct collapse of gas clouds<\/a>, the accumulated collision<\/a> of objects in densely populated star clusters, being the result of inflated primordial black holes<\/a> at the time of the Big Bang, or the remnant of the death of the first generation<\/a> of massive stars\u200a\u2014\u200athese are stars with an approximate mass range of 100 to 250 M\u209b<\/a> that sprung to life supposedly after the first few hundred millions of years<\/a> following the Big Bang.<\/p>\nSupermassive Black\u00a0Holes<\/strong><\/h4>\nWith masses in the order of several million and even billion solar masses, supermassive black holes sit at the centre<\/a> of most galaxies throughout the Universe, including the Milky Way galaxy\u200a\u2014\u200aour supermassive black hole Sagittarius A* is projected to have a mass of 4 million M\u209b<\/a>.<\/p>\nHow these enormous astrophysical objects came to be is still an open area of research. One explanation points to mass segregation<\/a> 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<\/a> of these heavy objects.<\/p>\nThis mechanism might shed light on the hypothesis that the presence of intermediate-mass black hole seeds<\/a> in the early Universe leads up to the existence of quasars that came about within the first billion years after the Big Bang\u200a\u2014\u200aa 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).<\/p>\nSee, for instance, the quasar ULAS J1342+0928<\/a> with a 780 million M\u209b<\/a> supermassive black hole or the quasar SDSS J1148+5251<\/a> nurturing a supermassive black hole of 3 billion M\u209b<\/a> that originated 690 million and 870 million years after the Big Bang, respectively.<\/p>\n\nFig. 5. An overview of some explanations about how intermediate-mass black hole seeds might have contributed to the existence of supermassive black holes within the first billion years after the Big Bang. (Source: UniverseReview<\/a>).<\/p><\/div><\/figure>\nOther descriptions of how these black holes end up being so massive suggest growth by black hole accretion<\/a> 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<\/a> of a galaxy\u200a\u2014\u200athe inner spherical part of a galaxy\u200a\u2014\u200awhich then collapses into a black hole and develops over time through accretion into a supermassive black hole; or, the direct collapse<\/a> of dense gas clouds combined with an exceptional accretion rate (super-Eddington accretion).<\/p>\nUltra-Massive Black\u00a0Holes<\/strong><\/h4>\nSome black holes that inhabit the centre of brightest cluster galaxies\u200a\u2014\u200athis is the brightest galaxy within a galaxy cluster, which is a large group of galaxies held together by gravitation\u200a\u2014\u200aexceed the mass of 10 billion M\u209b. They go by the name of ultra-massive black holes<\/a>.<\/p>\nExamples are the black hole of 17 billion M\u209b<\/a> within the NGC 1277 galaxy of the Perseus cluster, a 21 billion M\u209b<\/a> black hole at the NGC 4889 galaxy within the Coma cluster, and a black hole with a range of 25 to 100 billion M\u209b<\/a> dwelling in the centre of the PKS 0745-BCG galaxy in the PKS 0745 cluster.<\/p>\nThis category of black holes is usually observed as quasars<\/a> 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.<\/p>\nPrimordial Black\u00a0Holes<\/strong><\/h4>\nRemaining hidden to the present day, primordial black holes are thought to have formed at the radiation-dominated era (roughly the first<\/a> 50,000 years of existence of our Universe) as a result of the gravitational collapse<\/a> of inhomogeneities<\/a> in the initial density during the inflationary period, i.e., a fraction<\/a> of a second after the Universe\u2019s birth.<\/p>\nSome of the alternative formation channels<\/a> 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.<\/p>\nContingent upon the precise moment of creation, the primordial black hole\u2019s mass could vary<\/a> 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 \u2018A Loophole in Spacetime\u2019), we would detect only those with an initial mass<\/a> equal to or higher than 10\u00b9\u2075 g.<\/p>\n\nFig. 6. The evolution of the scale factor a\u200a\u2014\u200awhich reflects the relative expansion of the Universe\u200a\u2014\u200aas a function of time in logarithmic scales. (Source: Paper Pierre-Henri Chavanis<\/a>).<\/p><\/div><\/figure>\nNotwithstanding the prediction of evaporation, the confirmation of the presences of primordial black holes below<\/em> that threshold would boost the prospect of string theory<\/a>, because additional spatial dimensions\u200a\u2014\u200awhich are anticipated by the theory\u200a\u2014\u200amight possibly restrict<\/a> the rate of evaporation given that these extra dimensions would impact the way gravity behaves on the smallest of scales.<\/p>\nNot only that, primordial<\/a> black holes<\/a> are a massive compact halo object (MACHO) candidate<\/a> for the existence of dark matter in the outskirts of galaxies (halos).<\/p>\nBesides 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<\/a> (27%) and dark energy (68%). These yet unobserved entities would account for the rotation rate of galaxies, among other astrophysical phenomena, and the Universe\u2019s increasing rate of expansion, respectively.<\/p>\nCommon Characteristics<\/strong><\/h3>\nAcross 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.<\/p>\n
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.<\/p>\n
Let us now focus on these characteristics in greater detail.<\/p>\n
No Hair<\/strong><\/h4>\nFirst off, the no-hair theorem<\/a> states that the only measurable properties of isolated black holes are mass, electric charge, and spin; knowing all the other material properties of matter (\u2018hair\u2019) that has fallen into a black hole is out of our reach, once it has crossed the event horizon.<\/p>\nHaving said that, it is assumed that astrophysical black holes have zero charge<\/a>, because any charged black hole would be quickly neutralized by attracting and engulfing matter with an opposite charge.<\/p>\nIn addition, as all stars<\/a> and colliding systems in the Universe possess some angular momentum\u200a\u2014\u200aspin\u200a\u2014\u200aso will every black hole, since it is originally, among other scenarios, created out of stellar collapse or generated within regions of extremely high density<\/a>, such as places rife with collisions of compact objects or gas.<\/p>\nThe 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 \u2018Risen from the Equations\u2019) the Kerr black hole is the most plausible general description<\/a> of real-world black holes.<\/p>\nWith regard to mass, the black hole\u2019s size<\/a> solely depends on the amount of mass it has absorbed. It is then obvious from the previous section \u2018Many Masses\u2019 that an ultra-massive black hole will be much bigger than a stellar black hole.<\/p>\nA Loophole in Spacetime<\/strong><\/h4>\nIn the upper subsection \u2018Spacetime Consequences\u2019, we mentioned that nothing escapes from within a black hole\u2019s event horizon. Except, Stephen Hawking demonstrated<\/a> that some radiation (called Hawking radiation) does slip through the black hole\u2019s allegedly impermeable barrier and leaks back out into the Universe, albeit too weak to be detectable by our current technology.<\/p>\nThis leads to the consequence that black holes evaporate over time. The larger they are, the longer<\/a> it takes them to evaporate.<\/p>\n\nFig. 7. (1) Shells of matter collapse; (2) A horizon forms, and Hawking radiation (in the form of particles of zero or low mass, such as photons or neutrinos or gravitons) emerges from the horizon; (3) the Hawking radiation carries off energy, causing the black hole\u2019s size and mass to shrink; (4) eventually the black hole evaporates completely, leaving only the Hawking radiation behind. (Source: Matt Strassler<\/a>).<\/p><\/div><\/figure>\nBut this triggers a problem: As Hawking radiation does purportedly not<\/a> 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?<\/p>\nSeveral ideas<\/a> have been formulated to solve this black hole information paradox<\/a>, but research now hints that information does in fact get out<\/a> of a black hole after all, providing a tentative way out of this conundrum.<\/p>\nWhirling Meals<\/strong><\/h4>\nBlack holes born out of at least a binary star system are able to acquire an accretion disk<\/a>. What is more, when they nibble away matter or gas from that disk, large jets of radiation<\/a> soar\u200a\u2014\u200ausually in the X-ray spectrum\u200a\u2014\u200awhich 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%<\/a> of the accreted matter, at a minimum, disappears into the black hole.<\/p>\nThese 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<\/a>, 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<\/a> debate<\/a>.<\/p>\n\nFig. 8. An artist\u2019s conception of radiation jets produced by an accreting black hole. (Source: ScienceBlogs<\/a>).<\/p><\/div><\/figure>\nSome of the most energetic phenomena<\/a> across the Universe are ideal birthplaces of accretion disks. They include active galactic nuclei (e.g., quasars), X-ray binaries<\/a> (a system whereby either a neutron star or a black hole accretes from a normal star, releasing X-rays during the process), tidal disruption<\/a> events<\/a> (when a supermassive black hole rips apart a star that came too close\u200a\u2014\u200aoccurring every ten thousand years or so\u200a\u2014\u200aand devours roughly half of that star, a short-lived radiation discharge flares up), or gamma-ray bursts<\/a> (a multi-second, very intense flash of gamma rays that is believed to stem from an explosion following the merger of neutron stars).<\/p>\nConcerning supermassive black holes, recent experimental data by Meg Urry et al.<\/a> 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.<\/p>\nGalactic Connections<\/strong><\/h4>\nScientists have unveiled several connections between supermassive black holes and the galaxy in which they dwell.<\/p>\n
For one, some researchers have found a positive correlation<\/a> between the mass of a supermassive black hole and various properties of its host galaxy, in particular galaxy mass<\/a>, bulge mass<\/a>, bulge luminosity<\/a>, and stellar velocity dispersion<\/a> (which gives the average radial<\/a> velocity of stars around a central massive object). Moreover, the supermassive black hole\u2019s mass measures typically<\/a> 0.1% of that of the bulge of a galaxy.<\/p>\nThese 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<\/a> of active galactic nuclei as a proxy for black hole accretion.<\/p>\n\nFig. 9. A linear correlation is shown between the galactic bulge mass (horizontal axis) and the mass of its supermassive black hole (vertical axis), in logarithmic scales. (Source: Paper Jenny Greene et\u00a0al.<\/a>).<\/p><\/div><\/figure>\nEven though many researchers indeed evaluate such reinforcing links between black hole growth and galaxy stellar formation, several<\/a> other<\/a> research<\/a> studies<\/a> report an inverse correspondence between these two variables.<\/p>\nOne of their main arguments is that an increased rate of accretion results in a higher amount of radiation<\/a> ejected into the host galaxy which in turn prevents the surrounding gas to cool and condense into molecular clouds (which eventually collapse into stars).<\/p>\nThe 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<\/a>, halo masses<\/a>, and bulge masses<\/a>.<\/p>\nFuture research will help to further clarify the nature of the relationship between galaxy formation and the mass of its central black hole.<\/p>\n
Elusive Stars<\/strong><\/h3>\nAt 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?<\/p>\n
Gravitational Motion<\/strong><\/h4>\nOne 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\u200a\u2014\u200awith the support of the equations of general relativity<\/a>, or even Kepler\u2019s laws<\/a>\u200a\u2014\u200areveal the mass<\/a> of that central object, from which it is possible to infer<\/a> the presence of a black hole.<\/p>\nNonetheless, 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<\/a> 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.<\/p>\n<\/figure>\n\n\nFig. 10. The individual orbit of 8 stars around the supermassive massive black hole Sagittarius A* within the centre of the Milky Way. (Source:\u00a0UCLA<\/a>).<\/p><\/div>\n<\/div>\n<\/div>\nRegarding our own galaxy, Andrea Ghez and Reinhard Genzel have recently been awarded<\/a> the Nobel Prize in Physics\u200a\u2014\u200ashared with Roger Penrose\u200a\u2014\u200afor meticulously mapping numerous stellar orbits in the Milky Way\u2019s innermost central region, providing strong evidence for the presence of a supermassive black hole named Sagittarius A*.<\/p>\nApart from stellar motion, researchers have also turned to other gravitational signposts, such as gas cloud<\/a> movements<\/a>, to lay bare certain black hole characteristics.<\/p>\nVelocity Profile<\/strong><\/h4>\nA next approach engages with the concept of velocity: Astronomical spectroscopy<\/a>, which studies the electromagnetic radiation emitted by astronomical objects, uncovers information on the velocity dispersion<\/a> of stars from which a black hole mass<\/a> can be derived.<\/p>\nFor example, leaning on near-infrared spectroscopy, the work by Laura Ferrarese et al.<\/a> 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.<\/p>\nAnother example resorts to the combination of spectroscopy within the visible spectrum and photometry<\/a> (which analyses the received electromagnetic energy\u200a\u2014\u200aflux\u200a\u2014\u200aor brightness from celestial objects) to calculate radial velocities and ultimately determine the presence<\/a> of a black hole within the binary star system XTEJ1859+226.<\/p>\nIntense Energies<\/strong><\/h4>\nA 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.<\/p>\n
As a case in point, through the combined use of the Swift\u2019s Burst Alert Telescope<\/a> and the Chandra X-ray Observatory<\/a>, scientists were able to figure out what the mass<\/a> 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<\/em> J1644+57<\/a>).<\/p>\nBesides 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<\/a> and the GALEX<\/a> telescopes enabled the detection<\/a> of a tidal disruption event in the inactive<\/a> galaxy PS1\u201310jh, leading to the identification of a 2.8 million M\u209b supermassive black hole.<\/p>\nCosmic Lenses<\/strong><\/h4>\nAnother modus operandi that guides us towards the whereabouts of black holes is called gravitational lensing<\/a> and falls back on a prediction of general relativity, namely that strong gravitational regions of spacetime bend<\/a> light.<\/p>\nGravitational 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<\/a> source image\u200a\u2014\u200aor even multiple source images\u200a\u2014\u200atogether with, under certain circumstances, a ring of light around that massive object (Einstein ring<\/a>).<\/p>\n\nFig. 11. The light from the single quasar PG 1115+080 is split and distorted in this infrared image. PG 1115+080 is at a distance of about 8 billion light years in the constellation Leo, and it is viewed through an elliptical galaxy lens at a distance of 3 billion light years. It shows the four images of the quasar (the two on the left are nearly merging) surrounding the galaxy that causes the light to be lensed. The quasar is a variable light source and the light in each of the four images travels a different path to reach the Earth. (Source: Spacetelescope<\/a>).<\/p><\/div><\/figure>\nThis method implies that the effect of gravitational lensing should, at least in theory, be visible and detectable<\/a>. Within the context of supermassive black holes, several sources<\/a> 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<\/a> in the galactic halo, it might help to disentangle the enigma of dark matter.<\/p>\nIn one example, the gravitational lensing was caused by the presence of an entire galaxy: researchers identified<\/a> the location of a supermassive black hole within the active galactic nucleus PKS 1830\u2013211 only because a galaxy in between the observing telescopes and PKS 1830\u2013211 acted as a gravitational magnifying lens, producing an Einstein ring and multiple source images in the process.<\/p>\nWhat is more, one of the distinct manifestations of the magnifying<\/a> effect of gravitational lensing is the broadening<\/a> 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<\/a> shifts and gravitational redshift<\/a> which become especially prominent on a strongly rotating accretion disk.<\/p>\nFor instance, both gravitational lensing<\/a> and strong light-bending<\/a> explain the registered Fe spectral variability\u200a\u2014\u200aeven though not<\/a> everyone agrees\u200a\u2014\u200awithin the innermost region of the accretion disk twirling around the supermassive black hole of galaxy MCG-6\u201330\u201315.<\/p>\nAccreting Snapshots<\/strong><\/h4>\nA 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<\/a> be endowed with an accretion disk, making the detection of a black hole a sticky<\/a> endeavour.<\/p>\nOne example highlights how the inner accretion disk of a black hole within the active galactic nucleus IRAS 13224\u20133809 can be made accessible<\/a> to surveillance by studying the rapid variability in flux of the X-ray spectral band, which is gathered by the XMM-Newton<\/a> and nuSTAR<\/a> telescopes.<\/p>\nBut perhaps the most solid evidence for the existence of a black hole to date comes from the production of a direct<\/a> image<\/a> of the shadow and the accretion disk of a 6.5 billion M\u209b<\/a> supermassive black hole within the active galaxy M87, taken by the Event Horizon Telescope.<\/p>\n\nFig. 12. Direct image of the shadow of the supermassive black hole dwelling within the galaxy M87. The event horizon is indicated by the white circle. (Source: Paper George Smoot et\u00a0al.<\/a>).<\/p><\/div><\/figure>\nSpacetime Ripples<\/strong><\/h4>\nAnother, relatively young monitoring method makes use of relativistic physics: The recording of gravitational waves (GW) by ground-based observatories\u200a\u2014\u200athink of the Laser Interferometer Gravitational-Wave Observatory (LIGO<\/a>) and Virgo<\/a> in particular\u200a\u2014\u200aprovides data<\/a> (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.<\/p>\nFor instance, both the GW190425<\/a> and GW190426_152155<\/a> events might qualify<\/a> as a black hole-neutron star merger and produced<\/a> a 3.4 M\u209b and 7.2 M\u209b stellar-mass black hole, respectively. Also, the black hole merging event GW190521<\/a> gave birth to a 150 M\u209b intermediate-mass black hole, while the neutron star merger GW170817<\/a> brought about, arguably<\/a>, a 2.7 M\u209b<\/a> stellar-mass black hole. As a final example, the black hole merger GW190517_055101<\/a> resulted in a 61.9 M\u209b<\/a> stellar-mass black hole, exhibiting the largest effective aligned spin.<\/p>\nIt seems furthermore to be the case that stellar black holes discovered through gravitational waves are on average of higher<\/a> mass than the ones picked up by electromagnetic radiation-based telescopes.<\/p>\n\n
In the latter half of the 20\u1d57\u02b0 century, divided global politics led independently to different names<\/a> for the same object: physicists to the east of the Iron Curtain referred to \u2018frozen stars,\u2019 while those to the west spoke of \u2018collapsed stars\u2019.<\/p>\n Notwithstanding the political schism, it was John Wheeler\u200a\u2014\u200aon the suggestion<\/a> of an audience member attending one of his lectures\u200a\u2014\u200awho finally managed to popularize the term \u2018black hole\u2019 among academia in 1968, even though Robert Dicke<\/a> and Ann Ewing<\/a> used the phrase already in 1960 and 1964, respectively.<\/p>\n Mathematically, the concept of a black hole arises from Albert Einstein\u2019s equations within his theory of general relativity, in spite of Einstein\u2019s reluctance<\/a> to acknowledge its existence.<\/p>\n General relativity maintains<\/a> that gravity is the result of the bending of spacetime\u200a\u2014\u200awhich in turn is the consequence of the presence of large matter or energy distributions\u200a\u2014\u200aand that matter moves along a curved path in spacetime.<\/p>\n It was Karl Schwarzschild who in 1916 worked out a solution to Einstein\u2019s formulas and found that within a certain distance from a non-rotating, uncharged, spherically symmetric star\u2019s inner centre (the Schwarzschild radius<\/a>) spacetime starts to curve gradually stronger towards its centre, at which point the curvature becomes apparently infinite\u200a\u2014\u200atechnically called a singularity<\/a>.<\/p>\n Fig. 1. Spacetime configuration of a black hole. (Source: Unnikrishnan Menon<\/a> and Martin Silvertant<\/a>).<\/p><\/div><\/figure>\n 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<\/a> would turn into a black hole if we could shrink her down to an object with a radius of not more than 2.95km\u200a\u2014\u200athe Sun\u2019s Schwarzschild radius. In the case of the Earth, that would come down to 0.89cm.<\/p>\n Other general relativistic solutions include Reissner-Nordstr\u00f6m<\/a> black holes (charged, spherically symmetric, and non-rotating), Kerr<\/a> black holes (uncharged, axially symmetric, and rotating), and Kerr-Newman<\/a> black holes (charged, axially symmetric, and rotating).<\/p>\n What can we except to happen when a star is being shrunk to its Schwarzschild radius?<\/p>\n First of all, while the star is collapsing in on itself\u200a\u2014\u200aas first calculated<\/a> by J. Robert Oppenheimer and Hartland Snyder in 1939\u200a\u2014\u200amatter density increases<\/a> 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.<\/p>\n 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\u200a\u2014\u200aa situation referred to as gravitational redshift<\/a>.<\/p>\n Fig. 2. A schematic view of gravitational redshift: an electromagnetic wave (light) is climbing out of a gravitationally dense region, whereby the frequency reduces and the wavelength\u200a\u2014\u200athe distance from peak to peak\u200a\u2014\u200aincreases. (Source: Shadab\u00a0Alam<\/a>).<\/p><\/div><\/figure>\n Given that clocks rely on<\/a> the frequency of an atom\u2019s spectral lines to keep track of time\u200a\u2014\u200aspectral lines are the so-called electromagnetic fingerprints<\/a> of atoms and molecules\u200a\u2014\u200ait follows then that time will equally be slowing down.<\/p>\n 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\u2019s surface, i.e., the event horizon<\/a>.<\/p>\n 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.<\/p>\n To sum up, due to the black hole\u2019s immense gravitational field and the singularity at its centre, both matter and radiation\u200a\u2014\u200aand this includes light\u200a\u2014\u200acannot 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.<\/p>\n 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.<\/p>\n Prior to a star collapsing under gravitational<\/em> pressure, the star as a whole is kept in balance by a countering internal<\/em> pressure, caused by the heat liberated from fusing nuclei of light elements into heavier ones\u200a\u2014\u200aa process known as stellar nucleosynthesis<\/a>.<\/p>\n But once the fusion grinds to a halt, gravitational pressure takes over, and the collapse sets in.<\/p>\n With an initial mass of not more than 8 solar masses<\/a> (M\u209b), 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\u200a\u2014\u200adesignated as Pauli\u2019s exclusion principle<\/a>.<\/p>\n That outward pressure exerted by the electrons (Fermi degeneracy pressure<\/a>) will be capable of stopping the gravity-induced collapse in its tracks.<\/p>\n After shedding off an expanding shell of gas, i.e., the planetary nebula<\/a>, the resultant stellar remnant receives the name of white dwarf<\/a> and is only able to subsist in a stable manner if its mass does not exceed 1.4 M\u209b\u200a\u2014\u200athis threshold is the Chandrasekhar limit<\/a>. More than 97%<\/a> of all stars are destined to become (or are already) white dwarfs.<\/p>\n Fig. 3. A succinct overview of the different paths of stellar evolution. (Source: Britannica<\/a>).<\/p><\/div><\/figure>\n If the stellar core mass oversteps the Chandrasekhar limit\u200a\u2014\u200adue to either the initial star possessing a mass above 8 M\u209b or the white dwarf accumulating additional mass during its lifetime\u200a\u2014\u200athe 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<\/a>).<\/p>\n For stars confined to an initial mass of up to 20 or 30 M\u209b<\/a>, the even more compact stellar core residue is now called a neutron star<\/a>. It remains in this state due to neutron degeneracy pressure\u200a\u2014\u200asimilar to electron degeneracy pressure in white dwarfs\u200a\u2014\u200aunder the condition that its mass does not surpass the range of 2 to 3 M\u209b, i.e., the Oppenheimer-Volkoff limit<\/a>.<\/p>\n Both an initial stellar mass beyond 20 to 30 M\u209b and the transgression of the Oppenheimer-Volkoff limit get the ball rolling for a next and final phase in stellar collapse: a black hole.<\/p>\n 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.<\/p>\n Stellar black holes are the ones formed by the procedure as described in the preceding section \u2018Manufacturing Manual\u2019 and have a mass between 3<\/a> and 100 M\u209b. According to Kelly Holley-Bockelmann, there are about one hundred million<\/a> of them in our galaxy.<\/p>\n 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<\/a> matter of a companion star as well as by the merger of two<\/a> neutrons stars<\/a>, two<\/a> black holes<\/a>, or, with still some lingering uncertainty, a neutron star<\/a> and a<\/a> black hole<\/a>.<\/p>\n 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<\/a> between 100 M\u209b and 1 million M\u209b, and are considered by and large as the potential progenitors or seeds<\/a> giving rise to supermassive black holes (see the next subsection).<\/p>\n Some candidates for this type include M82 X-1 (428 M\u209b<\/a>) in the galaxy M82, one in the globular cluster NGC104 of 2,300 M\u209b<\/a>, HLX-1 in the galaxy ESO 243\u201349 with 10,000 M\u209b<\/a>, NGC 2276\u20133c (in the galaxy NGC 2276) with an estimated mass between 4,300 and 85,000 M\u209b<\/a>, and one in the gas cloud CO-0.40\u20130.22 of our own Milky Way galaxy with 100,000 M\u209b<\/a>.<\/p>\n Fig. 4. This composite image shows the nearby galaxy Messier 82. The intermediate-mass black hole M82 X-1 is the brightest object in the inset. (Source: Earthsky<\/a>).<\/p><\/div><\/figure>\n 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<\/a>), the direct collapse of gas clouds<\/a>, the accumulated collision<\/a> of objects in densely populated star clusters, being the result of inflated primordial black holes<\/a> at the time of the Big Bang, or the remnant of the death of the first generation<\/a> of massive stars\u200a\u2014\u200athese are stars with an approximate mass range of 100 to 250 M\u209b<\/a> that sprung to life supposedly after the first few hundred millions of years<\/a> following the Big Bang.<\/p>\n With masses in the order of several million and even billion solar masses, supermassive black holes sit at the centre<\/a> of most galaxies throughout the Universe, including the Milky Way galaxy\u200a\u2014\u200aour supermassive black hole Sagittarius A* is projected to have a mass of 4 million M\u209b<\/a>.<\/p>\n How these enormous astrophysical objects came to be is still an open area of research. One explanation points to mass segregation<\/a> 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<\/a> of these heavy objects.<\/p>\n This mechanism might shed light on the hypothesis that the presence of intermediate-mass black hole seeds<\/a> in the early Universe leads up to the existence of quasars that came about within the first billion years after the Big Bang\u200a\u2014\u200aa 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).<\/p>\n See, for instance, the quasar ULAS J1342+0928<\/a> with a 780 million M\u209b<\/a> supermassive black hole or the quasar SDSS J1148+5251<\/a> nurturing a supermassive black hole of 3 billion M\u209b<\/a> that originated 690 million and 870 million years after the Big Bang, respectively.<\/p>\n Fig. 5. An overview of some explanations about how intermediate-mass black hole seeds might have contributed to the existence of supermassive black holes within the first billion years after the Big Bang. (Source: UniverseReview<\/a>).<\/p><\/div><\/figure>\n Other descriptions of how these black holes end up being so massive suggest growth by black hole accretion<\/a> 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<\/a> of a galaxy\u200a\u2014\u200athe inner spherical part of a galaxy\u200a\u2014\u200awhich then collapses into a black hole and develops over time through accretion into a supermassive black hole; or, the direct collapse<\/a> of dense gas clouds combined with an exceptional accretion rate (super-Eddington accretion).<\/p>\n Some black holes that inhabit the centre of brightest cluster galaxies\u200a\u2014\u200athis is the brightest galaxy within a galaxy cluster, which is a large group of galaxies held together by gravitation\u200a\u2014\u200aexceed the mass of 10 billion M\u209b. They go by the name of ultra-massive black holes<\/a>.<\/p>\n Examples are the black hole of 17 billion M\u209b<\/a> within the NGC 1277 galaxy of the Perseus cluster, a 21 billion M\u209b<\/a> black hole at the NGC 4889 galaxy within the Coma cluster, and a black hole with a range of 25 to 100 billion M\u209b<\/a> dwelling in the centre of the PKS 0745-BCG galaxy in the PKS 0745 cluster.<\/p>\n This category of black holes is usually observed as quasars<\/a> 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.<\/p>\n Remaining hidden to the present day, primordial black holes are thought to have formed at the radiation-dominated era (roughly the first<\/a> 50,000 years of existence of our Universe) as a result of the gravitational collapse<\/a> of inhomogeneities<\/a> in the initial density during the inflationary period, i.e., a fraction<\/a> of a second after the Universe\u2019s birth.<\/p>\n Some of the alternative formation channels<\/a> 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.<\/p>\n Contingent upon the precise moment of creation, the primordial black hole\u2019s mass could vary<\/a> 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 \u2018A Loophole in Spacetime\u2019), we would detect only those with an initial mass<\/a> equal to or higher than 10\u00b9\u2075 g.<\/p>\n Fig. 6. The evolution of the scale factor a\u200a\u2014\u200awhich reflects the relative expansion of the Universe\u200a\u2014\u200aas a function of time in logarithmic scales. (Source: Paper Pierre-Henri Chavanis<\/a>).<\/p><\/div><\/figure>\n Notwithstanding the prediction of evaporation, the confirmation of the presences of primordial black holes below<\/em> that threshold would boost the prospect of string theory<\/a>, because additional spatial dimensions\u200a\u2014\u200awhich are anticipated by the theory\u200a\u2014\u200amight possibly restrict<\/a> the rate of evaporation given that these extra dimensions would impact the way gravity behaves on the smallest of scales.<\/p>\n Not only that, primordial<\/a> black holes<\/a> are a massive compact halo object (MACHO) candidate<\/a> for the existence of dark matter in the outskirts of galaxies (halos).<\/p>\n 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<\/a> (27%) and dark energy (68%). These yet unobserved entities would account for the rotation rate of galaxies, among other astrophysical phenomena, and the Universe\u2019s increasing rate of expansion, respectively.<\/p>\n 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.<\/p>\n 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.<\/p>\n Let us now focus on these characteristics in greater detail.<\/p>\n First off, the no-hair theorem<\/a> states that the only measurable properties of isolated black holes are mass, electric charge, and spin; knowing all the other material properties of matter (\u2018hair\u2019) that has fallen into a black hole is out of our reach, once it has crossed the event horizon.<\/p>\n Having said that, it is assumed that astrophysical black holes have zero charge<\/a>, because any charged black hole would be quickly neutralized by attracting and engulfing matter with an opposite charge.<\/p>\n In addition, as all stars<\/a> and colliding systems in the Universe possess some angular momentum\u200a\u2014\u200aspin\u200a\u2014\u200aso will every black hole, since it is originally, among other scenarios, created out of stellar collapse or generated within regions of extremely high density<\/a>, such as places rife with collisions of compact objects or gas.<\/p>\n 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 \u2018Risen from the Equations\u2019) the Kerr black hole is the most plausible general description<\/a> of real-world black holes.<\/p>\n With regard to mass, the black hole\u2019s size<\/a> solely depends on the amount of mass it has absorbed. It is then obvious from the previous section \u2018Many Masses\u2019 that an ultra-massive black hole will be much bigger than a stellar black hole.<\/p>\n In the upper subsection \u2018Spacetime Consequences\u2019, we mentioned that nothing escapes from within a black hole\u2019s event horizon. Except, Stephen Hawking demonstrated<\/a> that some radiation (called Hawking radiation) does slip through the black hole\u2019s allegedly impermeable barrier and leaks back out into the Universe, albeit too weak to be detectable by our current technology.<\/p>\n This leads to the consequence that black holes evaporate over time. The larger they are, the longer<\/a> it takes them to evaporate.<\/p>\n Fig. 7. (1) Shells of matter collapse; (2) A horizon forms, and Hawking radiation (in the form of particles of zero or low mass, such as photons or neutrinos or gravitons) emerges from the horizon; (3) the Hawking radiation carries off energy, causing the black hole\u2019s size and mass to shrink; (4) eventually the black hole evaporates completely, leaving only the Hawking radiation behind. (Source: Matt Strassler<\/a>).<\/p><\/div><\/figure>\n But this triggers a problem: As Hawking radiation does purportedly not<\/a> 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?<\/p>\n Several ideas<\/a> have been formulated to solve this black hole information paradox<\/a>, but research now hints that information does in fact get out<\/a> of a black hole after all, providing a tentative way out of this conundrum.<\/p>\n Black holes born out of at least a binary star system are able to acquire an accretion disk<\/a>. What is more, when they nibble away matter or gas from that disk, large jets of radiation<\/a> soar\u200a\u2014\u200ausually in the X-ray spectrum\u200a\u2014\u200awhich 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%<\/a> of the accreted matter, at a minimum, disappears into the black hole.<\/p>\n 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<\/a>, 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<\/a> debate<\/a>.<\/p>\n Fig. 8. An artist\u2019s conception of radiation jets produced by an accreting black hole. (Source: ScienceBlogs<\/a>).<\/p><\/div><\/figure>\n Some of the most energetic phenomena<\/a> across the Universe are ideal birthplaces of accretion disks. They include active galactic nuclei (e.g., quasars), X-ray binaries<\/a> (a system whereby either a neutron star or a black hole accretes from a normal star, releasing X-rays during the process), tidal disruption<\/a> events<\/a> (when a supermassive black hole rips apart a star that came too close\u200a\u2014\u200aoccurring every ten thousand years or so\u200a\u2014\u200aand devours roughly half of that star, a short-lived radiation discharge flares up), or gamma-ray bursts<\/a> (a multi-second, very intense flash of gamma rays that is believed to stem from an explosion following the merger of neutron stars).<\/p>\n Concerning supermassive black holes, recent experimental data by Meg Urry et al.<\/a> 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.<\/p>\n Scientists have unveiled several connections between supermassive black holes and the galaxy in which they dwell.<\/p>\n For one, some researchers have found a positive correlation<\/a> between the mass of a supermassive black hole and various properties of its host galaxy, in particular galaxy mass<\/a>, bulge mass<\/a>, bulge luminosity<\/a>, and stellar velocity dispersion<\/a> (which gives the average radial<\/a> velocity of stars around a central massive object). Moreover, the supermassive black hole\u2019s mass measures typically<\/a> 0.1% of that of the bulge of a galaxy.<\/p>\n 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<\/a> of active galactic nuclei as a proxy for black hole accretion.<\/p>\n Fig. 9. A linear correlation is shown between the galactic bulge mass (horizontal axis) and the mass of its supermassive black hole (vertical axis), in logarithmic scales. (Source: Paper Jenny Greene et\u00a0al.<\/a>).<\/p><\/div><\/figure>\n Even though many researchers indeed evaluate such reinforcing links between black hole growth and galaxy stellar formation, several<\/a> other<\/a> research<\/a> studies<\/a> report an inverse correspondence between these two variables.<\/p>\n One of their main arguments is that an increased rate of accretion results in a higher amount of radiation<\/a> ejected into the host galaxy which in turn prevents the surrounding gas to cool and condense into molecular clouds (which eventually collapse into stars).<\/p>\n 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<\/a>, halo masses<\/a>, and bulge masses<\/a>.<\/p>\n Future research will help to further clarify the nature of the relationship between galaxy formation and the mass of its central black hole.<\/p>\n 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?<\/p>\n 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\u200a\u2014\u200awith the support of the equations of general relativity<\/a>, or even Kepler\u2019s laws<\/a>\u200a\u2014\u200areveal the mass<\/a> of that central object, from which it is possible to infer<\/a> the presence of a black hole.<\/p>\n 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<\/a> 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.<\/p>\n Fig. 10. The individual orbit of 8 stars around the supermassive massive black hole Sagittarius A* within the centre of the Milky Way. (Source:\u00a0UCLA<\/a>).<\/p><\/div>\n<\/div>\n<\/div>\n Regarding our own galaxy, Andrea Ghez and Reinhard Genzel have recently been awarded<\/a> the Nobel Prize in Physics\u200a\u2014\u200ashared with Roger Penrose\u200a\u2014\u200afor meticulously mapping numerous stellar orbits in the Milky Way\u2019s innermost central region, providing strong evidence for the presence of a supermassive black hole named Sagittarius A*.<\/p>\n Apart from stellar motion, researchers have also turned to other gravitational signposts, such as gas cloud<\/a> movements<\/a>, to lay bare certain black hole characteristics.<\/p>\n A next approach engages with the concept of velocity: Astronomical spectroscopy<\/a>, which studies the electromagnetic radiation emitted by astronomical objects, uncovers information on the velocity dispersion<\/a> of stars from which a black hole mass<\/a> can be derived.<\/p>\n For example, leaning on near-infrared spectroscopy, the work by Laura Ferrarese et al.<\/a> 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.<\/p>\n Another example resorts to the combination of spectroscopy within the visible spectrum and photometry<\/a> (which analyses the received electromagnetic energy\u200a\u2014\u200aflux\u200a\u2014\u200aor brightness from celestial objects) to calculate radial velocities and ultimately determine the presence<\/a> of a black hole within the binary star system XTEJ1859+226.<\/p>\n 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.<\/p>\n As a case in point, through the combined use of the Swift\u2019s Burst Alert Telescope<\/a> and the Chandra X-ray Observatory<\/a>, scientists were able to figure out what the mass<\/a> 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<\/em> J1644+57<\/a>).<\/p>\n 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<\/a> and the GALEX<\/a> telescopes enabled the detection<\/a> of a tidal disruption event in the inactive<\/a> galaxy PS1\u201310jh, leading to the identification of a 2.8 million M\u209b supermassive black hole.<\/p>\n Another modus operandi that guides us towards the whereabouts of black holes is called gravitational lensing<\/a> and falls back on a prediction of general relativity, namely that strong gravitational regions of spacetime bend<\/a> light.<\/p>\n 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<\/a> source image\u200a\u2014\u200aor even multiple source images\u200a\u2014\u200atogether with, under certain circumstances, a ring of light around that massive object (Einstein ring<\/a>).<\/p>\n Fig. 11. The light from the single quasar PG 1115+080 is split and distorted in this infrared image. PG 1115+080 is at a distance of about 8 billion light years in the constellation Leo, and it is viewed through an elliptical galaxy lens at a distance of 3 billion light years. It shows the four images of the quasar (the two on the left are nearly merging) surrounding the galaxy that causes the light to be lensed. The quasar is a variable light source and the light in each of the four images travels a different path to reach the Earth. (Source: Spacetelescope<\/a>).<\/p><\/div><\/figure>\n This method implies that the effect of gravitational lensing should, at least in theory, be visible and detectable<\/a>. Within the context of supermassive black holes, several sources<\/a> 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<\/a> in the galactic halo, it might help to disentangle the enigma of dark matter.<\/p>\n In one example, the gravitational lensing was caused by the presence of an entire galaxy: researchers identified<\/a> the location of a supermassive black hole within the active galactic nucleus PKS 1830\u2013211 only because a galaxy in between the observing telescopes and PKS 1830\u2013211 acted as a gravitational magnifying lens, producing an Einstein ring and multiple source images in the process.<\/p>\n What is more, one of the distinct manifestations of the magnifying<\/a> effect of gravitational lensing is the broadening<\/a> 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<\/a> shifts and gravitational redshift<\/a> which become especially prominent on a strongly rotating accretion disk.<\/p>\n For instance, both gravitational lensing<\/a> and strong light-bending<\/a> explain the registered Fe spectral variability\u200a\u2014\u200aeven though not<\/a> everyone agrees\u200a\u2014\u200awithin the innermost region of the accretion disk twirling around the supermassive black hole of galaxy MCG-6\u201330\u201315.<\/p>\n 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<\/a> be endowed with an accretion disk, making the detection of a black hole a sticky<\/a> endeavour.<\/p>\n One example highlights how the inner accretion disk of a black hole within the active galactic nucleus IRAS 13224\u20133809 can be made accessible<\/a> to surveillance by studying the rapid variability in flux of the X-ray spectral band, which is gathered by the XMM-Newton<\/a> and nuSTAR<\/a> telescopes.<\/p>\n But perhaps the most solid evidence for the existence of a black hole to date comes from the production of a direct<\/a> image<\/a> of the shadow and the accretion disk of a 6.5 billion M\u209b<\/a> supermassive black hole within the active galaxy M87, taken by the Event Horizon Telescope.<\/p>\n Fig. 12. Direct image of the shadow of the supermassive black hole dwelling within the galaxy M87. The event horizon is indicated by the white circle. (Source: Paper George Smoot et\u00a0al.<\/a>).<\/p><\/div><\/figure>\n Another, relatively young monitoring method makes use of relativistic physics: The recording of gravitational waves (GW) by ground-based observatories\u200a\u2014\u200athink of the Laser Interferometer Gravitational-Wave Observatory (LIGO<\/a>) and Virgo<\/a> in particular\u200a\u2014\u200aprovides data<\/a> (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.<\/p>\n For instance, both the GW190425<\/a> and GW190426_152155<\/a> events might qualify<\/a> as a black hole-neutron star merger and produced<\/a> a 3.4 M\u209b and 7.2 M\u209b stellar-mass black hole, respectively. Also, the black hole merging event GW190521<\/a> gave birth to a 150 M\u209b intermediate-mass black hole, while the neutron star merger GW170817<\/a> brought about, arguably<\/a>, a 2.7 M\u209b<\/a> stellar-mass black hole. As a final example, the black hole merger GW190517_055101<\/a> resulted in a 61.9 M\u209b<\/a> stellar-mass black hole, exhibiting the largest effective aligned spin.<\/p>\n It seems furthermore to be the case that stellar black holes discovered through gravitational waves are on average of higher<\/a> mass than the ones picked up by electromagnetic radiation-based telescopes.<\/p>\nRisen from the Equations<\/strong><\/h4>\n
![\"Spacetime](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1600\/1%2A8xS8jjACDZgDEo1_CNB_Zg.png?resize=629%2C333&ssl=1\" "\\"Spacetime")
Spacetime Consequences<\/strong><\/h4>\n
![\"A](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1200\/1%2AA9P5lI7_q_zOjhZxQJx-EQ.png?resize=629%2C331&ssl=1\" "\\"A")
Manufacturing Manual<\/strong><\/h3>\n
![\"A](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1600\/1%2AifmQFgj7LfQpD8tytuxs3Q.png?resize=629%2C389&ssl=1\" "\\"A")
Many Masses<\/strong><\/h3>\n
Stellar-Mass Black\u00a0Holes<\/strong><\/h4>\n
Intermediate-Mass Black\u00a0Holes<\/strong><\/h4>\n
![\"This](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1200\/1%2A60dJqx8EJtycj75OvpXH4w.png?resize=629%2C478&ssl=1\" "\\"This")
Supermassive Black\u00a0Holes<\/strong><\/h4>\n
![\"How](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1600\/1%2AbcCoNh4QF3_9PzIf2PemdQ.png?resize=629%2C871&ssl=1\" "\\"How")
Ultra-Massive Black\u00a0Holes<\/strong><\/h4>\n
Primordial Black\u00a0Holes<\/strong><\/h4>\n
![\"The](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1200\/1%2AoWSj6LWKN-yONrJ6-UgnBw.png?resize=629%2C458&ssl=1\" "\\"The")
Common Characteristics<\/strong><\/h3>\n
No Hair<\/strong><\/h4>\n
A Loophole in Spacetime<\/strong><\/h4>\n
![\"Given](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1600\/1%2ADnz_tcBGQbHQ7E9MjLFJIw.png?resize=629%2C480&ssl=1\" "\\"Given")
Whirling Meals<\/strong><\/h4>\n
![\"An](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1600\/1%2AZ3Vl_3kKXYLYuWWJQ7a7cw.png?resize=629%2C536&ssl=1\" "\\"An")
Galactic Connections<\/strong><\/h4>\n
![\"A](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1200\/1%2AOR-zL0tMDRdZuKH9QhOQqA.png?resize=629%2C617&ssl=1\" "\\"A")
Elusive Stars<\/strong><\/h3>\n
Gravitational Motion<\/strong><\/h4>\n
![\"The](\"https:\/\/i0.wp.com\/miro.medium.com\/max\/1400\/1%2A4S80KU9x62wf3ojMzTHOdA.png?resize=629%2C628&ssl=1\" "\\"The")
Velocity Profile<\/strong><\/h4>\n
Intense Energies<\/strong><\/h4>\n
Cosmic Lenses<\/strong><\/h4>\n
![\"This](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1200\/1%2ABtPnhEoleNUTBE-UGV3BjQ.png?resize=629%2C591&ssl=1\" "\\"This")
Accreting Snapshots<\/strong><\/h4>\n
![\"Direct](\"https:\/\/i0.wp.com\/cdn-images-1.medium.com\/max\/1600\/1%2AHetNmtXi1rtZgw9tfdfz5w.png?resize=629%2C413&ssl=1\" "\\"Direct")
Spacetime Ripples<\/strong><\/h4>\n