Black hole
From Wikipedia, the free
encyclopedia
Simulated view of a black hole in front of the Large
Magellanic Cloud. The ratio between the black hole
Schwarzschild radius and the observer distance to it is 1:9. Of note
is the gravitational lensing effect known as an Einstein
ring, which produces a set of two fairly bright and
large but highly distorted images of the Cloud as compared to its
actual angular size.
History
The idea of a body so massive that even light could not escape was
put forward by geologist
John
Michell in a letter written to Henry
Cavendish in 1783 to the Royal
Society:
If the semi-diameter of a sphere of the same density as the Sun were
to exceed that of the Sun in the proportion of 500 to 1, a body
falling from an infinite height towards it would have acquired at its
surface greater velocity than that of light, and consequently
supposing light to be attracted by the same force in proportion to
its vis inertiae, with other bodies, all light emitted from such a
body would be made to return towards it by its own proper gravity.
In 1796, mathematician Pierre-Simon
Laplace promoted the same idea in the first and second
editions of his book Exposition du système du Monde (it was
removed from later editions).[3][4]
Such "dark
stars" were largely ignored in the nineteenth
century, since light was then thought to be a massless wave and
therefore not influenced by gravity. Unlike the modern black hole
concept, the object behind the horizon of a dark star is assumed to
be stable against collapse.
General relativity
In 1915, Albert
Einstein developed his general theory of relativity,
having earlier shown that gravity does in fact influence light's
motion. A few months later, Karl
Schwarzschild gave the solution
for the gravitational field of a point mass and a spherical mass,[5]
showing that a black hole could theoretically exist. The
Schwarzschild
radius is now known to be the radius of the event
horizon of a non-rotating black hole, but this was not
well understood at that time, for example Schwarzschild himself
thought it was not physical. Johannes Droste, a student of Hendrik
Lorentz, independently gave the same solution for the
point mass a few months after Schwarzschild and wrote more
extensively about its properties. In 1930, astrophysicist
Subrahmanyan
Chandrasekhar calculated, using general relativity,
that a non-rotating body of electron-degenerate
matter above 1.44 solar masses (the Chandrasekhar
limit) would collapse. His arguments were opposed by
Arthur
Eddington, who believed that something would
inevitably stop the collapse. Eddington was partly correct: a white
dwarf slightly more massive than the Chandrasekhar
limit will collapse into a neutron
star, which is itself stable because of the Pauli
exclusion principle. But in 1939, Robert
Oppenheimer and others predicted that stars above
approximately three solar masses (the Tolman-Oppenheimer-Volkoff
limit) would collapse into black holes for the reasons
presented by Chandrasekhar.[6]
Oppenheimer and his co-authors used Schwarzschild's
system of coordinates (the only coordinates available
in 1939), which produced mathematical
singularities at the Schwarzschild
radius, in other words some of the terms in the
equations became infinite
at the Schwarzschild radius. This was interpreted as indicating that
the Schwarzschild radius 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,"[7]
because an outside observer would see the surface of the star frozen
in time at the instant where its collapse takes it inside the
Schwarzschild radius. This is a known property of modern black holes,
but it must be emphasized that the light from the surface of the
frozen star becomes redshifted very fast, turning the black hole
black very quickly. Many physicists could not accept the idea of time
standing still at the Schwarzschild radius, and there was little
interest in the subject for over 20 years.
Golden age
In 1958, David
Finkelstein introduced the concept of the event
horizon by presenting Eddington-Finkelstein
coordinates, which enabled him to show that "The
Schwarzschild surface r = 2 m is not a singularity, but that it
acts as a perfect unidirectional membrane: causal influences can
cross it in only one direction".[8]
This did not strictly contradict Oppenheimer's results, but extended
them to include the point of view of infalling observers. All
theories up to this point, including Finkelstein's, covered only
non-rotating black holes. In 1963, Roy
Kerr found the exact solution for a rotating black
hole. The rotating singularity of this solution was a ring, and not a
point. A short while later, Roger
Penrose was able to prove that singularities occur
inside any black hole. In 1967, astronomers discovered
pulsars,[9][10]
and within a few years could show that the known pulsars were rapidly
rotating neutron
stars. Until that time, neutron stars were also
regarded as just theoretical curiosities. So the discovery of pulsars
awakened interest in all types of ultra-dense objects that might be
formed by gravitational collapse.
Physicist John
Wheeler is widely credited with coining the term black
hole in his 1967 public lecture Our Universe: the Known and
Unknown, as an alternative to the more cumbersome
"gravitationally completely collapsed star." However,
Wheeler insisted that someone else at the conference had coined the
term and he had merely adopted it as useful shorthand. The term was
also cited in a 1964 letter by Anne Ewing to the AAAS:
According to Einstein’s general theory of relativity, as mass is
added to a degenerate star a sudden collapse will take place and the
intense gravitational field of the star will close in on itself. Such
a star then forms a "black hole" in the universe.
—Ann Ewing , letter to AAAS[11]
Properties and structure
The No
hair theorem states that, once it achieves a stable
condition after formation, a black hole has only three independent
physical properties: mass, charge, and angular momentum.[12]
Any two black holes that share the same values for these properties,
or parameters, are classically indistinguishable.
These properties are special because they are visible from outside
the 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, the ADM
mass, far away from the black hole.[13]
Likewise, the angular momentum can be measured from far away using
frame
dragging by the gravitomagnetic field.
When a black hole swallows any form of matter, its horizon oscillates
like a stretchy membrane with friction, a dissipative
system, until it reaches a simple final state (see
membrane
paradigm).[14]
Similary, any information about the charge distribution of the matter
is lost as the field is evenly distributed alone the event horizon as
if the black hole was acting like a conducting sphere with a definite
resistivity. This is different from other field theories like
electromagnetism, which does not have any friction or resistivity at
the microscopic level, because they are time reversible. Because the
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 the
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 the total
baryon
number, lepton
number, and all the other nearly conserved
pseudo-charges of particle physics. This behavior is so puzzling,
that it has been called the black
hole information loss paradox.[15][16][17]
Classification
By physical properties
The simplest black hole has mass but neither charge nor angular
momentum. These black holes are often referred to as Schwarzschild
black holes after the physicist Karl
Schwarzschild who discovered this solution
in 1915.[5]
It was the first non-trivial exact
solution to the Einstein
field equations to be discovered, and according to
Birkhoff's
theorem, the only vacuum
solution that is spherically
symmetric.[18]
This means that there is no observable difference 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
only correct near the black hole horizon; far away, the external
gravitational field is identical to that of any other body of the
same mass.[19]
More general black hole solutions were discovered later in the 20th
century. The Reissner-Nordström
metric describes a black hole with electric charge,
while the Kerr
metric yields a rotating black hole. The more
generally known stationary
black hole solution, the Kerr-Newman
metric, describes 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. In natural
units , the total charge and the
total angular momentum are expected to satisfy
for a black hole of mass M.
Black holes saturating this inequality are called extremal.
Solutions of Einstein's equations violating the inequality do exist,
but do not have a horizon. These solutions have naked
singularities and are deemed unphysical, as the
cosmic
censorship hypothesis rules out such singularities due
to the generic gravitational collapse of realistic
matter.[20]
This is supported by numerical simulations.[21]
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 common feature of compact
objects, and the black-hole candidate binary X-ray source GRS
1915+105[22]
appears to have an angular momentum near the maximum allowed value.
By mass
|
Class
|
Mass
|
Size
|
|
~105–109
MSun
|
~0.001–10
AU
|
|
|
~103
MSun
|
~103
km = REarth
|
|
|
~10 MSun
|
~30 km
|
|
|
up to
~MMoon
|
up to
~0.1 mm
|
where is the Schwarzschild radius and is
the mass
of the Sun. A black hole's size and mass are thus
simply related independent
of rotation. According to this criterion, black holes
are classed as:
- Supermassive – contain hundreds of thousands to billions of solar masses, and are thought to exist in the center of most galaxies,[23][24] including the Milky Way.[25] They are thought to be responsible for active galactic nuclei, and presumably form either from the coalescence of smaller black holes, or by the accretion of stars and gas onto them. The largest known supermassive black hole is located in OJ 287 weighing in at 18 billion solar masses.[26]
- Intermediate – contain thousands of solar masses. They have been proposed as a possible power source for ultraluminous X-ray sources.[27] There is no known mechanism for them to form directly, so they likely form via collisions of lower mass black holes, either in the dense stellar cores of globular clusters or galaxies.[citation needed] Such creation events should produce intense bursts of gravitational waves, which may be observed soon. The boundary between super- and intermediate-mass black holes is a matter of convention. Their lower mass limit, the maximum mass for direct formation of a single black hole from collapse of a massive star, is poorly known at present, but is thought to be somewhere well below 200 solar masses.
- Stellar-mass – have masses ranging from a lower limit of about 1.4–3 solar masses (1.4 is the Chandrasekhar limit and 3 is the Tolman-Oppenheimer-Volkoff limit for the maximum mass of neutron stars) up to perhaps 15–20 solar masses. They are created by the collapse of individual stars, or by the coalescence (inevitable, due to gravitational radiation) of binary neutron stars. Stars may form with initial masses up to about 100 solar masses, or in the distant past, possibly even higher, but these shed most of their outer massive layers during earlier phases of their evolution, either blown away in stellar winds during the red giant, AGB, and Wolf-Rayet stages, or expelled in supernova explosions for stars that turn into neutron stars or black holes. Being known mostly by theoretical models for late-stage stellar evolution, the upper limit for the mass of stellar-mass black holes is somewhat uncertain at present. The cores of still lighter stars form white dwarfs.
- Micro – (also mini black holes) have masses much less than that of a star. At these sizes, quantum mechanics is expected to take effect. There is no known mechanism for them to form via normal processes of stellar evolution, but certain inflationary scenarios predict their production during the early stages of the evolution of the universe.[citation needed] According to some theories of quantum gravity they may also be produced in the highly energetic reaction produced by cosmic rays hitting the atmosphere or even in particle accelerators such as the Large Hadron Collider.[citation needed] The theory of Hawking radiation predicts that such black holes will evaporate in bright flashes of gamma radiation. NASA's Fermi Gamma-ray Space Telescope satellite (formerly GLAST) launched in 2008 is searching for such flashes.[28]
Event horizon
The defining feature of a black hole is the appearance of an
event horizon—a boundary in spacetime
through which matter and light can only pass inward towards the mass
of the black hole. Nothing, including light, can escape from inside
the event horizon. The event horizon is referred to as such because
if an event occurs within the boundary, light from that event cannot
reach an outside observer, making it impossible to determine if such
an event occurred.[29]
As predicted by general relativity, the presence of a large mass
deforms spacetime in such a way that the paths particles take 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.[30]
To a distant observer, clocks near a black hole appear to tick more
slowly than those further away from the black hole.[31]
Due to this effect, known as gravitational
time dilation, an object falling into a black hole
appears to slow down as it approaches the event horizon, taking an
infinite time to reach it.[32]
At the same time, all processes on this object slow down causing
emitted light to appear redder and dimmer, an effect known as
gravitational
redshift.[33]
Eventually, at a point just before it reaches the event horizon, the
falling object becomes so dim that it can no longer be seen.
On the other hand, an observer falling into a black hole does not
notice any of these effects as he crosses the event horizon.
According to his own clock, he crosses the event horizon after a
finite time, although he is unable to determine exactly when he
crosses it, as it is impossible to determine the location of the
event horizon from local observations.[34]
For a non rotating (static) black hole, the Schwarzschild
radius delimits a spherical event horizon. The
Schwarzschild radius of an object is proportional to the mass.[35]
Rotating black holes have distorted, nonspherical event horizons.
Since the event horizon is not a material surface but rather merely a
mathematically defined demarcation boundary, nothing prevents matter
or radiation from entering a black hole, only from exiting one. The
description of black holes given by general relativity is known to be
an approximation, and it is expected that quantum
gravity effects become significant near the vicinity
of the event horizon.[36]
This allows observations of matter in the vicinity of a black hole's
event horizon to be used to indirectly study general
relativity and proposed extensions to it.
Singularity
At the center of a black hole as described by general relativity lies
a gravitational
singularity, a region where the spacetime curvature
becomes infinite.[37]
For a non-rotating black hole this region takes the shape of a single
point and for a rotating
black hole it is smeared out to form a ring
shapelying in the plane of rotation.[38]
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.[39]
The singular region can thus be thought of as having infinite
density.
An observer falling into a schwarzschild black hole (i.e.
non-rotating and no charges) cannot avoid the singularity. In fact,
any attempt to do so will only shorten the time taken to get
there.[40]
When he reaches the singularity he is crushed to infinite density and
his mass is added to the total of the black hole. Before that happens
he will have been torn apart by the growing tidal
forces in a process sometimes referred to as
spaghettification
or the noodle effect.[41]
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 worm hole.[42]
It also appears to be possible to follow closed
timelike curves around the Kerr singularity, which
lead to problems with causality
like the grandfather
paradox. [43]
It is expected that none of these peculiar effects would survive in a
proper quantum mechanical treatment of rotating and charged black
holes.[44]
The appearance of singularities in general relativity is commonly
perceived as signaling the breakdown of the theory.[45]
This breakdown, however, is expected; it occurs in a situation where
quantum
mechanical 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. It is generally expected that a theory
of quantum
gravity will feature black holes without
singularities.[46][47]
Photon sphere
The photon sphere is a spherical boundary of zero thickness such that
photons moving along tangents
to the sphere will be trapped in a circular orbit. For non-rotating
black holes, the photon sphere has a radius 1.5 times the
Schwarzschild
radius. The orbits are dynamically
unstable, hence any small perturbation (such as a
particle of infalling matter) will grow over time, either setting it
on an outward trajectory escaping the black hole or on an inward
spiral eventually crossing the event horizon.
While light can still escape from inside the photon sphere, any light
that crosses the photon sphere on an inbound trajectory will be
captured by the black hole. Hence any light reaching an outside
observer from inside the photon sphere must have been emitted by
objects inside the photon sphere but still outside of the event
horizon.
Other compact
objects, such as neutron
stars, can also have photon spheres.[48]
This follows from the fact that the gravitational field of an object
does not depend on its actual size, hence any object that is smaller
than 1.5 times the Schwarzschild radius corresponding to its mass
will indeed have a photon sphere.
Ergosphere
The ergosphere is an oblate spheroid region outside of the event
horizon, where objects cannot remain stationary.
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
becomes 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.[49]
The ergosphere of a black hole is bounded by, the (outer) event
horizon on the inside and an oblate
spheroid, which coincides with the event horizon at the poles and is
noticeably wider around the equator. The outer boundary is sometimes
called the ergosurface.
Objects and radiation can escape normally from the ergosphere. In
fact through the Penrose
process objects can emerge from the ergosphere with
more energy than they entered. This energy is taken from the
rotational energy of the black hole causing it to slow down.[50]
Formation and evolution
Considering the exotic nature of black holes, it may be natural to
question if such bizarre objects could actually exist in nature or to
suggest that they are merely pathological solutions to Einstein's
equations. Einstein himself wrongly thought that black holes would
not form, because he held that the angular momentum of collapsing
particles would stabilize their motion at some radius.[51]
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,[52]
and by the end of the 1960s, they had persuaded the majority of
researchers in the field that there is no obstacle to forming an
event horizon.
Once an event horizon forms, Roger
Penrose proved that a singularity will form somewhere
inside it. Shortly afterwards, Stephen
Hawking showed that many cosmological solutions
describing the big
bang have singularities, in the absence of scalar
fields or other exotic matter (see Penrose-Hawking
singularity theorems). 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.[53]
The primary formation process for black holes is expected to be the
gravitational
collapse of heavy objects such as stars, but there are
also more exotic processes that can lead to the production of black
holes.
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, or because a star which would have
been stable receives a lot of extra matter in a way which 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 ideal
gas law explains the connection between pressure,
temperature, and volume).
The collapse may be stopped by the degeneracy
pressure of the star's constituents, condensing the
matter in an exotic denser
state. The result is one of the various types of
compact
star. Which type of compact star is formed depends on
the mass of the remnant - the matter left over after changes
triggered by the collapse (such as supernova
or pulsations leading to a planetary
nebula) have blown away the outer layers. Note that
this can be substantially less than the original star - remnants
exceeding 5 solar masses are produced by stars which were over 20
solar masses before the collapse.
If the mass of the remnant exceeds ~3-4 solar masses (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. After this no known mechanism
(except possibly quark degeneracy pressure, see quark
star) is powerful enough to stop the collapse and the
object will inevitably collapse to a black hole.
This gravitational collapse of heavy stars is assumed to be
responsible for the formation of most (if not all) stellar
mass black holes.
Primordial black holes in The Big Bang
Gravitational collapse requires great densities. In the current epoch
of the universe these high densities are only found in stars, but in
the early universe shortly after the big
bang densities were much greater, possibly allowing
for the creation of black holes. The high density alone is not enough
to allow the formation of black holes since a uniform mass
distribution will not allow the mass to bunch up. In order for
primordial
black holes to form in such a dense medium, there must
be initial density perturbations which can then grow under their own
gravity. Different models for the early universe vary widely in their
predictions of the size of these perturbations. Various models
predict the creation of black holes, ranging from a Planck
mass to hundreds of thousands of solar masses.[54]
Primordial black holes could thus account for the creation of any
type of black hole.
High energy collisions
A simulated event in the CMS detector, a collision in which a micro
black hole may be created.
Gravitational collapse is not the only process that could create
black holes. In principle, black holes could also be created in high
energy collisions that create sufficient density. However, to date,
no such events have ever been detected either directly or indirectly
as a deficiency of the mass balance in particle
accelerator experiments.[55]
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 (~1019 GeV/c2
= ~2 × 10−8 kg), where quantum effects are expected to
make the theory of general relativity break down completely.[citation
needed] This would put the
creation of black holes firmly out of reach of any high energy
process occurring on or near the Earth. Certain developments in
quantum gravity however suggest that this bound could be much lower.
Some braneworld
scenarios for example put the Planck mass much lower, maybe even as
low as 1 TeV/c2.[56]
This would make it possible for micro
black holes to be created in the high energy
collisions occurring when cosmic rays hit the Earth's atmosphere, or
possibly in the new Large
Hadron Collider at CERN.
These theories are however very speculative, and the creation of
black holes in these processes is deemed unlikely by many
specialists.[citation
needed]
Growth
Once a black hole has formed, it can continue to grow by absorbing
additional matter. Any black hole will continually absorb
interstellar
dust from its direct surroundings and omnipresent
cosmic
background radiation, but neither of these processes
should significantly affect the mass of a stellar black hole. More
significant contributions can occur when the black hole formed in a
binary
star system. After formation the black hole can then
leech significant amounts of matter from its companion.
Much larger contributions can be obtained when a black hole merges
with other stars or compact objects. The supermassive
black holes suspected in the center of most galaxies
are expected to have formed from the coagulation of many smaller
objects. The process has also been proposed as the origin of some
intermediate-mass
black holes.
As an object approaches the event horizon, the horizon near the
object bulges up and swallows the object. Shortly thereafter the
increase in radius (due to the extra mass) is distributed evenly
around the hole.
Evaporation
In 1974, Stephen
Hawking showed that black holes are not entirely black
but emit small amounts of thermal radiation.[57]
He got this result by applying quantum
field theory in a static black hole background. The
result of his calculations is that a black hole should emit particles
in a perfect black
body spectrum. This effect has become known as Hawking
radiation. Since Hawking's result, many others have
verified the effect through various methods.[58]
If his theory of black hole radiation is correct then black holes are
expected to emit a thermal spectrum of radiation, and thereby lose
mass, because according to the theory of relativity mass is just
highly condensed energy (E = mc2).[57]
Black holes will shrink and evaporate over time. The temperature of
this spectrum (Hawking
temperature) is proportional to the surface
gravity of the black hole, which for a Schwarzschild
black hole is inversely proportional to the mass. Large black holes,
therefore, emit less radiation than small black holes.
A stellar black hole of 5 solar masses has a Hawking temperature of
about 12 nanokelvins. This is far less than the 2.7 K produced by the
cosmic
microwave background. Stellar mass (and larger) black
holes receive more mass from the cosmic microwave background than
they emit through Hawking radiation and will thus grow instead of
shrink. In order to have a Hawking temperature larger than 2.7 K (and
be able to evaporate) a black hole needs to be lighter than the Moon
(and therefore a diameter of less than a tenth of a millimeter).
On the other hand if a black hole is very small, the radiation
effects are expected to become very strong. Even a black hole that is
heavy compared to a human would evaporate in an instant. A black hole
the weight of a car (~10−24 m) would only take a
nanosecond to evaporate, during which time it would briefly have a
luminosity more than 200 times that of the sun. Lighter black holes
are expected to evaporate even faster, for example a black hole of
mass 1 TeV/c2 would take less than 10−88
seconds to evaporate completely. Of course, for such a small black
hole quantum
gravitation effects are expected to play an important
role and could even – although current developments in quantum
gravity do not indicate so – hypothetically make such a small
black hole stable.
Observational evidence
By their very nature black holes do not directly emit any signals
other than the hypothetical Hawking radiation. Since the Hawking
radiation for an astrophysical black hole is predicted to be very
weak, this makes it impossible to directly detect astrophysical black
holes from the Earth. A possible exception to the Hawking radiation
being weak is the last stage of the evaporation of light (primordial)
black holes. Searches for such flashes in the past has proven
unsuccessful and provides stringent limits on the possibility of
existence of light primordial black holes.[59]
NASA's Fermi
Gamma-ray Space Telescope launched in 2008 will
continue the search for these flashes.[60]
Astrophysicists searching for black holes thus have to rely on
indirect observations. A black hole's existence can sometimes be
inferred by observing its gravitational interactions with its
surroundings.
Accretion of matter
Due to conservation
of angular momentum gas falling into the gravitational
well created by a massive object will typically form a
disc-like structure around the object. Friction within the disc
causes angular momentum to be transported outward allowing matter to
fall further inward releasing potential energy and increasing the
temperature of the gas.[61]
In the case of compact
objects such as white
dwarfs, neutron
stars and black holes the gas in the inner regions
becomes so hot that it will emit vast amounts of radiation (mainly
X-rays), which may be detected by telescopes. This process of
accretion is one of the most efficient energy producing process
known; up to 40% of the rest mass of the accreted material can be
emitted in radiation.[61]
(In nuclear fusion only about 1% of the rest mass will be emitted as
energy.) In many cases accretion discs are accompanied by
relativistic
jets emitted along the poles, carry away a lot of the
energy. The mechanism for the creation of these jets is currently not
well understood.
As such many of the universe's more energetic phenomena have been
attributed to the accretion of matter on black holes. In particular
Active
Galactic Nuclei and quasars
are thought to be the accretion discs of supermassive black
holes.[citation
needed] Similarly, X-ray binaries
are thought to be binary
star systems in which one of the two stars is a
compact object accreting matter from its companion.[citation
needed] It has also been
suggested that some ultraluminous
X-ray sources may be the accretion disks of
intermediate-mass
black holes.[62]
X-ray binaries
X-ray
binaries are binary
star systems that are luminous in the X-ray
part of the spectrum. These X-ray emissions are generally thought to
be caused by one of the component stars being a compact object
accreting matter from the other (regular) star. The presence of an
ordinary star in such a system provides a unique opportunity for
studying the central object and determining if it might be a black
hole.
The first strong candidate for a black hole, Cygnus
X-1, was discovered in this way by Webster and
Murdin[64]
and Bolton[65]
in 1972.[66][67]
Some doubt however remained due to the uncertainties resultant from
the companion star being much heavier than the candidate black
hole.[63]
Currently, better candidates for black holes are found in a class of
X-ray binaries called soft X-ray transients.[63]
In this class of system the companion star is relatively low mass
allowing for more accurate estimates in the black hole mass.
Moreover, these system are only active in X-ray for a period of
several months once every 10–50 years. During the period of low
X-ray emission (called quiescence) the accretion disc is extemely
faint allowing for detailed observation of the companion star during
this period. One of the best such candidates is V404
Cyg.
Quiescence and advection-dominated accretion flow
The faintness of the accretion disc during quiescence is thought to
be caused by the flow entering a mode called an advection-dominated
accretion flow (ADAF). In this mode, almost all the
energy generated by friction in the disc is swept along with the flow
instead of radiated away. If this model is correct, then it forms
strong qualitative evidence for the presence of an event horizon.
Because, if the object at the center of the disc had a solid surface,
it would emit large amounts of radiation is the highly energetic gas
hits the surface, an effect that is observed for neutron stars in a
similar state.[61]
Quasi-periodic oscillations
The X-ray emissions from accretion disks sometimes exhibit a
flickering around certain frequencies. These signals are called
quasi-periodic
oscillations and are thought to be caused by material
moving along the inner edge of the accretion disk (the innermost
stable circular orbit). As such their frequency is linked to the mass
of the compact object. They can thus be used as an alternative way to
determine the mass of potential black holes.[68]
Gamma ray bursts
Intense but one-time gamma
ray bursts (GRBs) may signal the birth of "new"
black holes, because astrophysicists think that GRBs are caused
either by the gravitational
collapse of giant stars[69]
or by collisions between neutron stars,[70]
and both types of event involve sufficient mass and pressure to
produce black holes. But it appears that a collision between a
neutron star and a black hole can also cause a GRB,[71]
so a GRB is not proof that a "new" black hole has been
formed. All known GRBs come from outside our own galaxy, and most
come from billions of light
years away[72]
so the black holes associated with them are actually billions of
years old.
Galactic nuclei
The jet originating from the center of M87
in this image comes from an active
galactic nucleus that may contain a supermassive
black hole. Credit: Hubble
Space Telescope/NASA/ESA.
It is now widely accepted that the center of every or at least nearly
every galaxy contains a supermassive black hole.[73][74]
The close observational correlation between the mass of this hole and
the velocity dispersion of the host galaxy's bulge, known as the
M-sigma
relation, strongly suggests a connection between the
formation of the black hole and the galaxy itself.[73]
For decades, astronomers have used the term "active
galaxy" to describe galaxies with unusual
characteristics, such as unusual spectral
line emission and very strong radio
emission.[75][76]
However, theoretical and observational studies have shown that the
active
galactic nuclei (AGN) in these galaxies may contain
supermassive
black holes.[75][76]
The models of these AGN consist of a central black hole that may be
millions or billions of times more massive than the Sun;
a disk of gas
and dust
called an accretion
disk; and two jets
that are perpendicular to the accretion disk.[76]
Although supermassive black holes are expected to be found in most
AGN, only some galaxies' nuclei have been more carefully studied in
attempts to both identify and measure the actual masses of the
central supermassive black hole candidates. Some of the most notable
galaxies with supermassive black hole candidates include the
Andromeda
Galaxy, M32,
M87,
NGC
3115, NGC
3377, NGC
4258, and the Sombrero
Galaxy.[77]
Currently, the best evidence for a supermassive black hole comes from
the center of our own Milky
way.[78]
For sixteen years astronomers have tracked the positions of stars
orbiting a central massive object in a region called Sagittarius
A*, one of which—as star called S2—
has completed a full orbit in that period. From the orbital data they
were able to infer that there was a spherical mass of 4.3 million
solar
masses contained within a radius of less than 0.002
lightyears.
This is still more than 3000 times the Schwarzschild radius
corresponding to that mass. This is consistent with the central
object being a supermassive black hole.
Gravitational lensing
The deformation of spacetime around a massive object causes light
rays to be deflected much like light passing through an optic lens.
The phenomenon is known as gravitational
lensing, and has been observed for very large massive
objects like galaxies and galaxy clusters. It has however never been
observed for a black hole. Observation of lensing by a black hole
would allow general relativity to be tested in the region close to
the horizon, where up till now it has only been weakly constrained.
As such, it would significantly strengthen the other evidence for
black holes which crucially depends on the extrapolation of general
relativity in this regime. The best candidate for such observations
would be the supermassive black hole at Sgr A* in the center of our
milky way due to its great mass and close proximity.[79]
Open questions
Entropy and Hawking radiation
If ultra-high-energy collisions of particles in a particle
accelerator can create microscopic black holes, it is
expected that all types of particles will be emitted by black hole
evaporation, providing key evidence for any grand
unified theory. Above are the high energy particles
produced in a gold ion collision on the RHIC.
In 1971, Stephen
Hawking showed that the total area of the event
horizons of any collection of classical black holes can never
decrease, even if they collide and swallow each other; that is
merge.[80]
This is remarkably similar to the Second Law of Thermodynamics,
with area playing the role of entropy.
As a classical object with zero temperature it was assumed that black
holes had zero entropy. If this were the case, the second law of
thermodynamics would be violated by entropy-laden matter entering the
black hole, resulting in a decrease of the total entropy of the
universe. Therefore, Jacob
Bekenstein proposed that a black hole should have an
entropy, and that it should be proportional to its horizon area.
Since black holes do not classically emit radiation, the
thermodynamic viewpoint seemed simply an analogy, since zero
temperature implies infinite changes in entropy with any addition of
heat, which implies infinite entropy. However, in 1974, Hawking
applied quantum
field theory to the curved spacetime around the event
horizon and discovered that black holes emit Hawking
radiation, a form of thermal
radiation, allied to the Unruh
effect, which implied they had a positive temperature.
This strengthened the analogy being drawn between black hole dynamics
and thermodynamics: using the first
law of black hole mechanics, it follows that the
entropy of a non-rotating black hole is one quarter of the area of
the horizon. This is a universal result and can be extended to apply
to cosmological horizons such as in de
Sitter space. It was later suggested that black holes
are maximum-entropy objects, meaning that the maximum possible
entropy of a region of space is the entropy of the largest black hole
that can fit into it. This led to the holographic
principle.
The Hawking radiation reflects a characteristic temperature
of the black hole, which can be calculated from its entropy. The more
its temperature falls, the more massive a black hole becomes: the
more energy a black hole absorbs, the colder it gets. A black hole
with roughly the mass
of the planet Mercury would have a temperature in
equilibrium with the cosmic microwave background radiation (about
2.73 K). More massive than this, a black hole will be colder than the
background radiation, and it will gain energy from the background
faster than it gives energy up through Hawking radiation, becoming
even colder still. However, for a less massive black hole the effect
implies that the mass of the black hole will slowly evaporate with
time, with the black hole becoming hotter and hotter as it does so.
Although these effects are negligible for black holes massive enough
to have been formed astronomically, they would rapidly become
significant for hypothetical smaller
black holes, where quantum-mechanical effects
dominate. Indeed, small black holes are predicted to undergo runaway
evaporation and eventually vanish in a burst of radiation.
Although general relativity can be used to perform a semi-classical
calculation of black hole entropy, this situation is theoretically
unsatisfying. In statistical
mechanics, entropy is understood as counting the
number of microscopic configurations of a system which have the same
macroscopic qualities (such as mass,
charge,
pressure,
etc.). But without a satisfactory theory of quantum
gravity, one cannot perform such a computation for
black holes. Some promise has been shown by string
theory, however, which posits that the microscopic
degrees of freedom of the black hole are D-branes.
By counting the states of D-branes with given charges and energy, the
entropy for certain supersymmetric
black holes has been reproduced. Extending the region of validity of
these calculations is an ongoing area of research.
Black hole unitarity
An open question in fundamental physics is the so-called information
loss paradox, or black
hole unitarity paradox. Classically, the laws of
physics are the same run forward or in reverse (T-symmetry).
Liouville's
Theorem dictates conservation of phase space volume,
which can be thought of as 'conservation of information', so there is
some problem even in classical (non-quantum general relativity)
physics. In quantum mechanics, this corresponds to a vital property
called unitarity,
which has to do with the conservation of probability (It can also be
thought of as a conservation of quantum phase space volume as
expressed by the density
matrix).[81]
Fuzzballs
Fuzzballs are theorized by some superstring
theory scientists to be the true quantum
description of black holes. The theory resolves the information
paradox by eliminating the need for a singularity
at the heart of the black hole with infinite spacetime
curvature due to an infinitely intense gravitational field from a
region of zero volume. Modern physics breaks down when such
parameters are infinite and zero.
Samir Mathur of Ohio
State University, with postdoctoral researcher Oleg
Lunin, proposed via two papers in 2002 that black holes are actually
spheres of strings with a definite volume; they are not a
singularity,
which the classic view holds to be a zero-dimensional, zero-volume
point into which a black hole’s entire mass is concentrated.[82]
String
theory holds that the fundamental constituents of
subatomic
particles, including the force
carriers (e.g., quarks
leptons,
photons,
and gluons),
all comprise a one-dimensional string of energy that takes on its
identity by vibrating in different modes and/or frequencies. Quite
unlike the view of a black hole as a singularity, a small fuzzball
can be thought of as an extra-dense neutron
star where its neutrons have decomposed, or “melted,”
liberating the quarks
(strings in string theory) comprising them. Accordingly, fuzzballs
can be regarded as the most extreme form of degenerate
matter.
