A black hole is a region of
space-time where gravity is so strong that nothing, including light
or other electromagnetic waves, has enough energy to escape it. The
theory of general relativity predicts that a sufficiently compact
mass can deform space-time to form a black hole. The boundary of no
escape is called the event horizon. Although it has a great effect on
the fate and circumstances of an object crossing it, it has no
locally detectable features according to general relativity. In many
ways, a black hole acts like an ideal black body, as it reflects no
light. Moreover, quantum field theory in curved space-time predicts
that event horizons emit Hawking radiation, with the same spectrum as
a black body of a temperature inversely proportional to its mass.
This temperature is of the order of billionths of a kelvin for
stellar black holes, making it essentially impossible to observe
directly.
Objects whose gravitational fields are
too strong for light to escape were first considered in the 18th
century by John Michell and Pierre-Simon Laplace. In 1916, Karl
Schwarzschild found the first modern solution of general relativity
that would characterize a black hole. David Finkelstein, in 1958,
first published the interpretation of "black hole"
as a region of space from which nothing can escape. Black holes were
long considered a mathematical curiosity; it was not until the 1960s
that theoretical work showed they were a generic prediction of
general relativity. The discovery of neutron stars by Jocelyn Bell
Burnell in 1967 sparked interest in gravitationally collapsed compact
objects as a possible astrophysical reality. The first black hole
known was Cygnus X-1, identified by several researchers independently
in 1971.
Black holes of stellar mass form when
massive stars collapse at the end of their life cycle. After a black
hole has formed, it can grow by absorbing mass from its surroundings.
Supermassive black holes of millions of solar masses (M☉) may form
by absorbing other stars and merging with other black holes. There is
consensus that supermassive black holes exist in the centres of most
galaxies.
The presence of a black hole can be
inferred through its interaction with other matter and with
electromagnetic radiation such as visible light. Any matter that
falls onto a black hole can form an external accretion disk heated by
friction, forming quasars, some of the brightest objects in the
universe. Stars passing too close to a supermassive black hole can be
shredded into streamers that shine very brightly before being
"swallowed." If other stars are orbiting a black
hole, their orbits can determine the black hole's mass and location.
Such observations can be used to exclude possible alternatives such
as neutron stars. In this way, astronomers have identified numerous
stellar black hole candidates in binary systems and established that
the radio source known as Sagittarius A*, at the core of the Milky
Way galaxy, contains a supermassive black hole of about 4.3 million
solar masses.
On 11 February 2016, the LIGO
Scientific Collaboration and the Virgo collaboration announced the
first direct detection of gravitational waves, representing the first
observation of a black hole merger. On 10 April 2019, the first
direct image of a black hole and its vicinity was published,
following observations made by the Event Horizon Telescope (EHT) in
2017 of the supermassive black hole in Messier 87's galactic centre.
As of 2021, the nearest known body thought to be a black hole is
around 1,500 light-years (460 parsecs) away (see list of nearest
black holes). Though only a couple dozen black holes have been found
so far in the Milky Way, there are thought to be hundreds of
millions, most of which are solitary and do not cause emission of
radiation. Therefore, they would only be detectable by gravitational
lensing.
History
The idea of a body so big that even
light could not escape was briefly proposed by English astronomical
pioneer and clergyman John Michell in a letter published in November
1784. Michell's simplistic calculations assumed such a body might
have the same density as the Sun, and concluded that one would form
when a star's diameter exceeds the Sun's by a factor of 500, and its
surface escape velocity exceeds the usual speed of light. Michell
referred to these bodies as dark stars. He correctly noted that such
supermassive but non-radiating bodies might be detectable through
their gravitational effects on nearby visible bodies. Scholars of the
time were initially excited by the proposal that giant but invisible
'dark stars' might be hiding in plain view, but enthusiasm dampened
when the wavelike nature of light became apparent in the early
nineteenth century, as if light were a wave rather than a particle,
it was unclear what, if any, influence gravity would have on escaping
light waves.
Modern physics discredits Michell's
notion of a light ray shooting directly from the surface of a
supermassive star, being slowed down by the star's gravity, stopping,
and then free-falling back to the star's surface.
General relativity
In 1915, Albert Einstein developed his
theory of general relativity, having earlier shown that gravity does
influence light's motion. Only a few months later, Karl Schwarzschild
found a solution to the Einstein field equations that describes the
gravitational field of a point mass and a spherical mass. A few
months after Schwarzschild, Johannes Droste, a student of Hendrik
Lorentz, independently gave the same solution for the point mass and
wrote more extensively about its properties. This solution had a
peculiar behaviour at what is now called the Schwarzschild radius,
where it became singular, meaning that some of the terms in the
Einstein equations became infinite. The nature of this surface was
not quite understood at the time. In 1924, Arthur Eddington showed
that the singularity disappeared after a change of coordinates (see
Eddington–Finkelstein coordinates), although it took until 1933 for
Georges Lemaître to realize that this meant the singularity at the
Schwarzschild radius was a non-physical coordinate singularity.
Arthur Eddington did however comment on the possibility of a star
with mass compressed to the Schwarzschild radius in a 1926 book,
noting that Einstein's theory allows us to rule out overly large
densities for visible stars like Betelgeuse because "a star
of 250 million km radius could not possibly have so high a density as
the Sun. Firstly, the force of gravitation would be so great that
light would be unable to escape from it, the rays falling back to the
star like a stone to the earth. Secondly, the red shift of the
spectral lines would be so great that the spectrum would be shifted
out of existence. Thirdly, the mass would produce so much curvature
of the space-time metric that space would close up around the star,
leaving us outside (i.e., nowhere)."
In 1931, Subrahmanyan Chandrasekhar
calculated, using special relativity, that a non-rotating body of
electron-degenerate matter above a certain limiting mass (now called
the Chandrasekhar limit at 1.4 M☉) has no stable solutions. His
arguments were opposed by many of his contemporaries like Eddington
and Lev Landau, who argued that some yet unknown mechanism would stop
the collapse. They were partly correct: a white dwarf slightly more
massive than the Chandrasekhar limit will collapse into a neutron
star, which is itself stable. But in 1939, Robert Oppenheimer and
others predicted that neutron stars above another limit (the
Tolman–Oppenheimer–Volkoff limit) would collapse further for the
reasons presented by Chandrasekhar, and concluded that no law of
physics was likely to intervene and stop at least some stars from
collapsing to black holes. Their original calculations, based on the
Pauli exclusion principle, gave it as 0.7 M☉; subsequent
consideration of neutron-neutron repulsion mediated by the strong
force raised the estimate to approximately 1.5 M☉ to 3.0 M☉.
Observations of the neutron star merger GW170817, which is thought to
have generated a black hole shortly afterward, have refined the TOV
limit estimate to ~2.17 M☉.
Oppenheimer and his co-authors
interpreted the singularity at the boundary of the Schwarzschild
radius as indicating that this was the boundary of a bubble in which
time stopped. This is a valid point of view for external observers,
but not for infalling observers. Because of this property, the
collapsed stars were called "frozen stars", because
an outside observer would see the surface of the star frozen in time
at the instant where its collapse takes it to the Schwarzschild
radius.
Golden age
In 1958, David Finkelstein identified
the Schwarzschild surface as an event horizon, "a perfect
unidirectional membrane: causal influences can cross it in only one
direction". This did not strictly contradict Oppenheimer's
results, but extended them to include the point of view of infalling
observers. Finkelstein's solution extended the Schwarzschild solution
for the future of observers falling into a black hole. A complete
extension had already been found by Martin Kruskal, who was urged to
publish it.
These results came at the beginning of
the golden age of general relativity, which was marked by general
relativity and black holes becoming mainstream subjects of research.
This process was helped by the discovery of pulsars by Jocelyn Bell
Burnell in 1967, which, by 1969, were shown to be rapidly rotating
neutron stars. Until that time, neutron stars, like black holes, were
regarded as just theoretical curiosities; but the discovery of
pulsars showed their physical relevance and spurred a further
interest in all types of compact objects that might be formed by
gravitational collapse.[citation needed]
In this period more general black hole
solutions were found. In 1963, Roy Kerr found the exact solution for
a rotating black hole. Two years later, Ezra Newman found the
axisymmetric solution for a black hole that is both rotating and
electrically charged. Through the work of Werner Israel, Brandon
Carter, and David Robinson the no-hair theorem emerged, stating that
a stationary black hole solution is completely described by the three
parameters of the Kerr–Newman metric: mass, angular momentum, and
electric charge.
At first, it was suspected that the
strange features of the black hole solutions were pathological
artifacts from the symmetry conditions imposed, and that the
singularities would not appear in generic situations. This view was
held in particular by Vladimir Belinsky, Isaak Khalatnikov, and
Evgeny Lifshitz, who tried to prove that no singularities appear in
generic solutions. However, in the late 1960s Roger Penrose and
Stephen Hawking used global techniques to prove that singularities
appear generically. For this work, Penrose received half of the 2020
Nobel Prize in Physics, Hawking having died in 2018. Based on
observations in Greenwich and Toronto in the early 1970s, Cygnus X-1,
a galactic X-ray source discovered in 1964, became the first
astronomical object commonly accepted to be a black hole.
Work by James Bardeen, Jacob
Bekenstein, Carter, and Hawking in the early 1970s led to the
formulation of black hole thermodynamics. These laws describe the
behaviour of a black hole in close analogy to the laws of
thermodynamics by relating mass to energy, area to entropy, and
surface gravity to temperature. The analogy was completed when
Hawking, in 1974, showed that quantum field theory implies that black
holes should radiate like a black body with a temperature
proportional to the surface gravity of the black hole, predicting the
effect now known as Hawking radiation.
Etymology
John Michell used the term "dark
star" in a November 1783 letter to Henry Cavendish, and in
the early 20th century, physicists used the term "gravitationally
collapsed object". Science writer Marcia Bartusiak traces
the term "black hole" to physicist Robert H. Dicke,
who in the early 1960s reportedly compared the phenomenon to the
Black Hole of Calcutta, notorious as a prison where people entered
but never left alive.
The term "black hole"
was used in print by Life and Science News magazines in 1963, and by
science journalist Ann Ewing in her article "'Black Holes' in
Space", dated 18 January 1964, which was a report on a
meeting of the American Association for the Advancement of Science
held in Cleveland, Ohio.
In December 1967, a student reportedly
suggested the phrase "black hole" at a lecture by
John Wheeler; Wheeler adopted the term for its brevity and
"advertising value", and it quickly caught on,
leading some to credit Wheeler with coining the phrase.
Properties and structure
The no-hair theorem postulates that,
once it achieves a stable condition after formation, a black hole has
only three independent physical properties: mass, electric charge,
and angular momentum; the black hole is otherwise featureless. If the
conjecture is true, any two black holes that share the same values
for these properties, or parameters, are indistinguishable from one
another. The degree to which the conjecture is true for real black
holes under the laws of modern physics is currently an unsolved
problem.
These properties are special because
they are visible from outside a black hole. For example, a charged
black hole repels other like charges just like any other charged
object. Similarly, the total mass inside a sphere containing a black
hole can be found by using the gravitational analog of Gauss's law
(through the ADM mass), far away from the black hole. Likewise, the
angular momentum (or spin) can be measured from far away using frame
dragging by the gravitomagnetic field, through for example the
Lense–Thirring effect.
When an object falls into a black hole,
any information about the shape of the object or distribution of
charge on it is evenly distributed along the horizon of the black
hole, and is lost to outside observers. The behavior of the horizon
in this situation is a dissipative system that is closely analogous
to that of a conductive stretchy membrane with friction and
electrical resistance—the membrane paradigm. This is different from
other field theories such as electromagnetism, which do not have any
friction or resistivity at the microscopic level, because they are
time-reversible. Because a black hole eventually achieves a stable
state with only three parameters, there is no way to avoid losing
information about the initial conditions: the gravitational and
electric fields of a black hole give very little information about
what went in. The information that is lost includes every quantity
that cannot be measured far away from the black hole horizon,
including approximately conserved quantum numbers such as the total
baryon number and lepton number. This behavior is so puzzling that it
has been called the black hole information loss paradox.
Physical properties
The simplest static black holes have
mass but neither electric charge nor angular momentum. These black
holes are often referred to as Schwarzschild black holes after Karl
Schwarzschild who discovered this solution in 1916. According to
Birkhoff's theorem, it is the only vacuum solution that is
spherically symmetric. This means there is no observable difference
at a distance between the gravitational field of such a black hole
and that of any other spherical object of the same mass. The popular
notion of a black hole "sucking in everything" in
its surroundings is therefore correct only near a black hole's
horizon; far away, the external gravitational field is identical to
that of any other body of the same mass.
Solutions describing more general black
holes also exist. Non-rotating charged black holes are described by
the Reissner–Nordström metric, while the Kerr metric describes a
non-charged rotating black hole. The most general stationary black
hole solution known is the Kerr–Newman metric, which describes a
black hole with both charge and angular momentum.
While the mass of a black hole can take
any positive value, the charge and angular momentum are constrained
by the mass. The total electric charge Q and the total angular
momentum J are expected to satisfy the inequality
Q 2 4 π ϵ 0 + c 2 J 2 G M 2 ≤
G M 2 {\displaystyle {\frac {Q^{2}}{4\pi \epsilon _{0}}}+{\frac
{c^{2}J^{2}}{GM^{2}}}\leq GM^{2}}
for a black hole of mass M. Black holes
with the minimum possible mass satisfying this inequality are called
extremal. Solutions of Einstein's equations that violate this
inequality exist, but they do not possess an event horizon. These
solutions have so-called naked singularities that can be observed
from the outside, and hence are deemed unphysical. The cosmic
censorship hypothesis rules out the formation of such singularities,
when they are created through the gravitational collapse of realistic
matter. This is supported by numerical simulations.
Due to the relatively large strength of
the electromagnetic force, black holes forming from the collapse of
stars are expected to retain the nearly neutral charge of the star.
Rotation, however, is expected to be a universal feature of compact
astrophysical objects. The black-hole candidate binary X-ray source
GRS 1915+105 appears to have an angular momentum near the maximum
allowed value. That uncharged limit is
J ≤ G M 2 c , {\displaystyle
J\leq {\frac {GM^{2}}{c}},}
allowing definition of a dimensionless
spin parameter such that
0 ≤ c J G M 2 ≤ 1.
{\displaystyle 0\leq {\frac {cJ}{GM^{2}}}\
Black holes are commonly classified
according to their mass, independent of angular momentum, J. The size
of a black hole, as determined by the radius of the event horizon, or
Schwarzschild radius, is proportional to the mass, M, through
r s = 2 G M c 2 ≈ 2.95 M M ⊙
k m , {\displaystyle r_{\mathrm {s} }={\frac {2GM}{c^{2}}}\approx
2.95\,{\frac {M}{M_{\odot }}}~\mathrm {km,} }
where rs is the Schwarzschild radius
and M☉ is the mass of the Sun. For a black hole with nonzero spin
and/or electric charge, the radius is smaller, until an extremal
black hole could have an event horizon close to
r + = G M c 2 . {\displaystyle
r_{\mathrm {+} }={\frac {GM}{c^{2}}}.}
Event horizon
The defining feature of a black hole is
the appearance of an event horizon—a boundary in space-time through
which matter and light can pass only inward towards the mass of the
black hole. Nothing, not even light, can escape from inside the event
horizon. The event horizon is referred to as such because if an event
occurs within the boundary, information from that event cannot reach
an outside observer, making it impossible to determine whether such
an event occurred.
As predicted by general relativity, the
presence of a mass deforms space-time in such a way that the paths
taken by particles bend towards the mass. At the event horizon of a
black hole, this deformation becomes so strong that there are no
paths that lead away from the black hole.
To a distant observer, clocks near a
black hole would appear to tick more slowly than those farther away
from the black hole. Due to this effect, known as gravitational time
dilation, an object falling into a black hole appears to slow as it
approaches the event horizon, taking an infinite time to reach it. At
the same time, all processes on this object slow down, from the
viewpoint of a fixed outside observer, causing any light emitted by
the object to appear redder and dimmer, an effect known as
gravitational red-shift. Eventually, the falling object fades away
until it can no longer be seen. Typically this process happens very
rapidly with an object disappearing from view within less than a
second.
On the other hand, indestructible
observers falling into a black hole do not notice any of these
effects as they cross the event horizon. According to their own
clocks, which appear to them to tick normally, they cross the event
horizon after a finite time without noting any singular behaviour; in
classical general relativity, it is impossible to determine the
location of the event horizon from local observations, due to
Einstein's equivalence principle.
The topology of the event horizon of a
black hole at equilibrium is always spherical. For non-rotating
(static) black holes the geometry of the event horizon is precisely
spherical, while for rotating black holes the event horizon is
oblate.
Singularity
At the center of a black hole, as
described by general relativity, may lie a gravitational singularity,
a region where the space-time curvature becomes infinite. For a
non-rotating black hole, this region takes the shape of a single
point; for a rotating black hole it is smeared out to form a ring
singularity that lies in the plane of rotation. In both cases, the
singular region has zero volume. It can also be shown that the
singular region contains all the mass of the black hole solution. The
singular region can thus be thought of as having infinite density.
Observers falling into a Schwarzschild
black hole (i.e., non-rotating and not charged) cannot avoid being
carried into the singularity once they cross the event horizon. They
can prolong the experience by accelerating away to slow their
descent, but only up to a limit. When they reach the singularity,
they are crushed to infinite density and their mass is added to the
total of the black hole. Before that happens, they will have been
torn apart by the growing tidal forces in a process sometimes
referred to as spaghettification or the "noodle effect".
In the case of a charged
(Reissner–Nordström) or rotating (Kerr) black hole, it is possible
to avoid the singularity. Extending these solutions as far as
possible reveals the hypothetical possibility of exiting the black
hole into a different spacetime with the black hole acting as a
wormhole. The possibility of traveling to another universe is,
however, only theoretical since any perturbation would destroy this
possibility. It also appears to be possible to follow closed
time-like curves (returning to one's own past) around the Kerr
singularity, which leads to problems with causality like the
grandfather paradox. It is expected that none of these peculiar
effects would survive in a proper quantum treatment of rotating and
charged black holes.
The appearance of singularities in
general relativity is commonly perceived as signaling the breakdown
of the theory. This breakdown, however, is expected; it occurs in a
situation where quantum effects should describe these actions, due to
the extremely high density and therefore particle interactions. To
date, it has not been possible to combine quantum and gravitational
effects into a single theory, although there exist attempts to
formulate such a theory of quantum gravity. It is generally expected
that such a theory will not feature any singularities.
Photon sphere
The photon sphere is a spherical
boundary of zero thickness in which photons that move on tangents to
that sphere would be trapped in a circular orbit about the black
hole. For non-rotating black holes, the photon sphere has a radius
1.5 times the Schwarzschild radius. Their orbits would be dynamically
unstable, hence any small perturbation, such as a particle of
infalling matter, would cause an instability that would grow over
time, either setting the photon on an outward trajectory causing it
to escape the black hole, or on an inward spiral where it would
eventually cross the event horizon.
While light can still escape from the
photon sphere, any light that crosses the photon sphere on an inbound
trajectory will be captured by the black hole. Hence any light that
reaches an outside observer from the photon sphere must have been
emitted by objects between the photon sphere and the event horizon.
For a Kerr black hole the radius of the photon sphere depends on the
spin parameter and on the details of the photon orbit, which can be
pro-grade (the photon rotates in the same sense of the black hole
spin) or retrograde.
Ergosphere
Rotating black holes are surrounded by
a region of spacetime in which it is impossible to stand still,
called the ergosphere. This is the result of a process known as
frame-dragging; general relativity predicts that any rotating mass
will tend to slightly "drag" along the spacetime
immediately surrounding it. Any object near the rotating mass will
tend to start moving in the direction of rotation. For a rotating
black hole, this effect is so strong near the event horizon that an
object would have to move faster than the speed of light in the
opposite direction to just stand still.
The ergosphere of a black hole is a
volume bounded by the black hole's event horizon and the ergosurface,
which coincides with the event horizon at the poles but is at a much
greater distance around the equator.
Objects and radiation can escape
normally from the ergosphere. Through the Penrose process, objects
can emerge from the ergosphere with more energy than they entered
with. The extra energy is taken from the rotational energy of the
black hole. Thereby the rotation of the black hole slows down. A
variation of the Penrose process in the presence of strong magnetic
fields, the Blandford–Znajek process is considered a likely
mechanism for the enormous luminosity and relativistic jets of
quasars and other active galactic nuclei.
Innermost stable circular orbit
(ISCO)
In Newtonian gravity, test particles
can stably orbit at arbitrary distances from a central object. In
general relativity, however, there exists an innermost stable
circular orbit (often called the ISCO), inside of which, any
infinitesimal perturbations to a circular orbit will lead to inspiral
into the black hole. The location of the ISCO depends on the spin of
the black hole, in the case of a Schwarzschild black hole (spin zero)
is:
r I S C O = 3 r s = 6 G M c 2 ,
{\displaystyle r_{\rm {ISCO}}=3\,r_{s}={\frac {6\,GM}{c^{2}}},}
and decreases with increasing black
hole spin for particles orbiting in the same direction as the spin.
Formation and evolution
Given the bizarre character of black
holes, it was long questioned whether such objects could actually
exist in nature or whether they were merely pathological solutions to
Einstein's equations. Einstein himself wrongly thought black holes
would not form, because he held that the angular momentum of
collapsing particles would stabilize their motion at some radius.
This led the general relativity community to dismiss all results to
the contrary for many years. However, a minority of relativists
continued to contend that black holes were physical objects, and by
the end of the 1960s, they had persuaded the majority of researchers
in the field that there is no obstacle to the formation of an event
horizon.
Simulation of two black holes
colliding
Penrose demonstrated that once an event
horizon forms, general relativity without quantum mechanics requires
that a singularity will form within. Shortly afterwards, Hawking
showed that many cosmological solutions that describe the Big Bang
have singularities without scalar fields or other exotic matter. The
Kerr solution, the no-hair theorem, and the laws of black hole
thermodynamics showed that the physical properties of black holes
were simple and comprehensible, making them respectable subjects for
research. Conventional black holes are formed by gravitational
collapse of heavy objects such as stars, but they can also in theory
be formed by other processes.
Gravitational collapse
Gravitational collapse occurs when an
object's internal pressure is insufficient to resist the object's own
gravity. For stars this usually occurs either because a star has too
little "fuel" left to maintain its temperature
through stellar nucleosynthesis, or because a star that would have
been stable receives extra matter in a way that does not raise its
core temperature. In either case the star's temperature is no longer
high enough to prevent it from collapsing under its own weight. The
collapse may be stopped by the degeneracy pressure of the star's
constituents, allowing the condensation of matter into an exotic
denser state. The result is one of the various types of compact star.
Which type forms depends on the mass of the remnant of the original
star left if the outer layers have been blown away (for example, in a
Type II supernova). The mass of the remnant, the collapsed object
that survives the explosion, can be substantially less than that of
the original star. Remnants exceeding 5 M☉ are produced by stars
that were over 20 M☉ before the collapse.
If the mass of the remnant exceeds
about 3–4 M☉ (the Tolman–Oppenheimer–Volkoff limit), either
because the original star was very heavy or because the remnant
collected additional mass through accretion of matter, even the
degeneracy pressure of neutrons is insufficient to stop the collapse.
No known mechanism (except possibly quark degeneracy pressure, see
quark star) is powerful enough to stop the implosion and the object
will inevitably collapse to form a black hole.
The gravitational collapse of heavy
stars is assumed to be responsible for the formation of stellar mass
black holes. Star formation in the early universe may have resulted
in very massive stars, which upon their collapse would have produced
black holes of up to 103 M☉. These black holes could be the seeds
of the supermassive black holes found in the centres of most
galaxies. It has further been suggested that massive black holes with
typical masses of ~105 M☉ could have formed from the direct
collapse of gas clouds in the young universe. These massive objects
have been proposed as the seeds that eventually formed the earliest
quasars observed already at redshift z ∼ 7 {\displaystyle z\sim 7}.
Some candidates for such objects have been found in observations of
the young universe.
While most of the energy released
during gravitational collapse is emitted very quickly, an outside
observer does not actually see the end of this process. Even though
the collapse takes a finite amount of time from the reference frame
of infalling matter, a distant observer would see the infalling
material slow and halt just above the event horizon, due to
gravitational time dilation. Light from the collapsing material takes
longer and longer to reach the observer, with the light emitted just
before the event horizon forms delayed an infinite amount of time.
Thus the external observer never sees the formation of the event
horizon; instead, the collapsing material seems to become dimmer and
increasingly red-shifted, eventually fading away.
Primordial black holes and the Big
Bang
Gravitational collapse requires great
density. In the current epoch of the universe these high densities
are found only in stars, but in the early universe shortly after the
Big Bang densities were much greater, possibly allowing for the
creation of black holes. High density alone is not enough to allow
black hole formation since a uniform mass distribution will not allow
the mass to bunch up. In order for primordial black holes to have
formed in such a dense medium, there must have been initial density
perturbations that could then grow under their own gravity. Different
models for the early universe vary widely in their predictions of the
scale of these fluctuations. Various models predict the creation of
primordial black holes ranging in size from a Planck mass ( m P = ℏ
c / G {\displaystyle m_{P}={\sqrt {\hbar c/G}}} ≈ 1.2×1019 GeV/c2
≈ 2.2×10−8 kg) to hundreds of thousands of solar masses.
Despite the early universe being
extremely dense—far denser than is usually required to form a black
hole—it did not re-collapse into a black hole during the Big Bang.
Models for the gravitational collapse of objects of relatively
constant size, such as stars, do not necessarily apply in the same
way to rapidly expanding space such as the Big Bang.
High-energy collisions
Gravitational collapse is not the only
process that could create black holes. In principle, black holes
could be formed in high-energy collisions that achieve sufficient
density. As of 2002, no such events have been detected, either
directly or indirectly as a deficiency of the mass balance in
particle accelerator experiments. This suggests that there must be a
lower limit for the mass of black holes. Theoretically, this boundary
is expected to lie around the Planck mass, where quantum effects are
expected to invalidate the predictions of general relativity. This
would put the creation of black holes firmly out of reach of any
high-energy process occurring on or near the Earth. However, certain
developments in quantum gravity suggest that the minimum black hole
mass could be much lower: some braneworld scenarios for example put
the boundary as low as 1 TeV/c2. This would make it conceivable for
micro black holes to be created in the high-energy collisions that
occur when cosmic rays hit the Earth's atmosphere, or possibly in the
Large Hadron Collider at CERN. These theories are very speculative,
and the creation of black holes in these processes is deemed unlikely
by many specialists. Even if micro black holes could be formed, it is
expected that they would evaporate in about 10−25 seconds, posing
no threat to the Earth.
.jpg)
