Unravel the enigmatic nature of black holes, celestial entities that have intrigued scientists and stargazers alike. Venture into the depths of these cosmic phenomena, exploring their formation, characteristics, and the profound implications they hold for our understanding of the universe.
Event horizon
A boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole.
- 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 if such an event occurred.
Quasi-periodic oscillations
X-ray emissions from accretion disks sometimes flicker at certain frequencies.
Quiescence and advection-dominated accretion flow
The faintness of the accretion disc of an X-ray binary during quiescence is suspected to be caused by the flow of mass entering a mode called an ADVVF, where 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.
Gravitational collapse
When an object’s internal pressure is insufficient to resist the object’s own gravity.
- For stars this usually occurs when 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.
Microlensing (proposed)
Another way that the black hole nature of an object may be tested in the future is through observation of effects caused by a strong gravitational field in their vicinity.
- One such effect is gravitational lensing, in which the deformation of spacetime around a massive object causes light rays to be deflected much as light passing through an optic lens.
- However, it has never been directly observed for a black hole.
The proper motions of stars near the center of our own Milky Way provide strong observational evidence that these stars are orbiting a supermassive black hole.
Since 1995, astronomers have tracked the motions of 90 stars orbiting an invisible object coincident with the radio source Sagittarius A*.
- By fitting their motions to Keplerian orbits, the astronomers were able to infer, in 1998, that a 2.6 million M ☉ object must be contained in a volume with a radius of 0.02 light-years to cause the motion of those stars.
High-energy Collisions
In principle, black holes could be formed in high-energy collisions that achieve sufficient density. However, 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 (mP=√ħ c/G ≈ 1.2×1019 GeV/c2, where quantum effects are expected to invalidate the predictions of general relativity).
Hawking radiation
In 1974, Stephen Hawking predicted that black holes emit small amounts of thermal radiation at a temperature ℏ c3/(8 π G M kB); this effect has become known as Hawking radiation.
- If Hawking’s theory of black hole radiation is correct, then black holes are expected to shrink and evaporate over time as they lose mass by the emission of photons and other particles.
The firewall paradox
According to quantum field theory in curved spacetime, a single emission of Hawking radiation involves two mutually entangled particles.
- Assume a black hole formed a finite time in the past and will fully evaporate away in some future time in which it will only emit a finite amount of information encoded within its Hawking radiation
- An outgoing particle emitted at time must be entangled with all the Hawking radiation the black hole has previously emitted
- This creates a paradox: a principle called “monogamy of entanglement” requires that, like any quantum system, the outgoing particle cannot be fully entangled with both the infalling particle and, independently, with past Hawking radiation
Ergosphere
Outside of the event horizon, 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.
- The ergosphere of a black hole is a volume whose inner boundary is the black hole’s oblate spheroid event horizon and a pumpkin-shaped outer boundary, which coincides with the poles but noticeably wider around the equator.
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.
- This process of accretion is one of the most efficient energy-producing processes known; up to 40% of the rest mass of the accreted material can be emitted as radiation.
Detection of gravitational waves from merging black holes
On 14 September 2015, the LIGO gravitational wave observatory made the first-ever successful direct observation of GW waves produced by the merger of two black holes: one with about 36 solar masses and the other around 29 solar masses
- This observation provides the most concrete evidence for the existence of black holes to date
- The ringdown is the most direct way of observing a black hole
- From this signal, it is possible to extract the frequency and damping time of the dominant mode of the ringdown, and from these, the mass and angular momentum of the final object, which match independent predictions from numerical simulations.
Innermost stable circular orbit (ISCO)
In general relativity, any infinitesimal perturbations to a circular orbit will lead to inspiral into the black hole.
General Relativity
In 1915, Albert Einstein developed his theory of general relativity
- Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass
- 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
- Arthur Eddington showed that the singularity disappeared after a change of coordinates, although it took until 1933 for Georges Lemaître to realize it was a non-physical coordinate singularity
- 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”
- 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, leading to the term “frozen stars”
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 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
- Solutions of Einstein’s equations that violate this inequality exist, but they do not possess an event horizon
- 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 is a universal feature of compact astrophysical objects
- Black holes are commonly classified according to their mass, independent of angular momentum, J
Growth
Once a black hole has formed, it can continue to grow by absorbing additional matter. This is the primary process through which supermassive black holes seem to have grown.
- A similar process has been suggested for the formation of intermediate-mass black holes found in globular clusters.
Properties and structure
The no-hair conjecture postulates that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, 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.
External links
Black holes on In Our Time at the BBC
- Stanford Encyclopedia of Philosophy – “Singularities and Black Holes” by Erik Curiel and Peter Bokulich
- Interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
- ESA’s Black Hole Visualization
- Frequently Asked Questions on Black holes
- “Schwarzschild Geometry” at the Hubble site
- 16-year-long study tracks stars orbiting Milky Way black hole
Observational evidence
Predicted appearance of non-rotating black hole with toroidal ring of ionized matter
- By their very nature, black holes do not directly emit any electromagnetic radiation other than the hypothetical Hawking radiation, so astrophysicists searching for black holes must generally rely on indirect observations.
- The Event Horizon Telescope (EHT) is an attempt to directly observe the immediate environment of the event horizon of Sagittarius A*, the black hole at the center of the Milky Way.
Gravitational 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.
- 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. After attaining a certain ideal velocity, it is best[clarification needed] to free fall the rest of the way.[84] When they reach the singularities, they are crushed to infinite density and their mass is added to the total of the black hole.
Primordial black holes and the Big Bang
Gravitational collapse requires great density. In the current epoch, these high densities are only found in stars.
- In the early universe, densities were much greater, possibly allowing for the creation of black holes. However, high density alone is not enough to allow black hole formation since there must be initial density perturbations that could then grow under their own gravity.
X-ray binaries
These are binary star systems that emit X-rays from accreting matter from another star.
- The presence of an ordinary star in such a system provides an opportunity for studying the central object and to determine if it might be a black hole (i.e. neutron star).
Notes Jump up ^ The value of cJ/GM2 can exceed 1 for objects other than black holes. The largest value known for a neutron star is ≤ 0.4, and commonly used equations of state would limit that value to < 0.7.[63]
The (outer) event horizon radius scales as follows: Jump up, Eddington-Finkelstein coordinates, Schwarzschild coordinates, Kruskal-Szekeres coordinates, etc.
- Jump up is true only for 4-dimensional spacetimes. In higher dimensions more complicated horizon topologies like a black ring are possible.
Etymology
In the early 20th century, physicists used the term “gravitationally collapsed object”.
- The term “black hole” was used in print by Life magazine and Science News magazine in 1963, and by science journalist Ann Ewing in her article “‘Black Holes in Space’ in 1964.
- It is likely that the term was first used by physicist Robert H. Dicke, who compared the phenomenon to the Black Hole of Calcutta.
Galactic nuclei
Theoretical and observational studies have shown that the activity in these active galactic nuclei (AGN) may be explained by the presence of supermassive black holes, which can be millions of times more massive than stellar ones.
- Models of AGN consist of a central black hole that may be millions or billions of times larger than the Sun, a disk of gas and dust called an accretion disk, and jets perpendicular to the disk called jets, and an X-ray flare from Sagittarius A* in the center of the Milky Way.
Information loss paradox
Because a black hole has only a few internal parameters, most of the information about the matter that went into forming the black hole is lost.
- In quantum mechanics, loss of information corresponds to the violation of vital property called unitarity, which has to do with the conservation of probability. It has been argued that loss of unitarity would also imply violation of conservation of energy.
History
The idea of a body so massive that even light could not escape was briefly proposed by John Michell in a letter published in November 1784
- Such supermassive but non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies
- Scholars were initially excited by the proposal that giant but invisible stars might be hiding in plain view, but enthusiasm dampened when the wavelike nature of light became apparent in the early nineteenth century
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing-not even particles and electromagnetic radiation such as light-can escape from inside it.
The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. The black hole can continue to grow by absorbing mass from its surroundings.
Photon sphere
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.
- 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 hole, or on an inward spiral where it would eventually cross the event horizon.
Entropy and thermodynamics
In 1971, 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 merge
- This result, now known as the second law of black hole mechanics, is remarkably similar to the Second law of thermodynamics.
- Quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature, which will carry away energy from the black hole causing it to shrink
- The radiation, however, also carries away entropy, and it can be proven that the sum of the entropy of the matter surrounding the hole and one quarter of the area of horizon as measured in Planck units is always increasing
- Although general relativity can be used to perform a semi-classical calculation of entropy, it is theoretically unsatisfying for black holes because there is no satisfactory theory of quantum gravity
Formation and evolution
It was long questioned whether black holes could actually exist in nature or whether they were merely pathological solutions to Einstein’s equations.
- 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.
Alternatives
The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter.
- New exotic phases of matter could push up this bound.
- For example, a phase of free quarks at high density could allow for the formation of dense quark stars.
- Supermassive black holes are much less dense than standard black holes, but have a higher average density.
Golden Age
David Finkelstein in 1958 extended Oppenheimer’s results to include the point of view of infalling observers.
- In 1963 Roy Kerr found the exact solution for a rotating black hole and in 1974 Stephen Hawking proved that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole
