Chandra X-ray Observatory

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https://en.wikipedia.org/wiki/Chandra_X-ray_Observatory

 

Chandra X-ray Observatory

Illustration of Chandra

Instruments

NASA Great Observatories ← Compton Spitzer → Large Strategic Science Missions Astrophysics Division ← Compton Webb →

The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the Space Shuttle Columbia during STS-93 by NASA on July 23, 1999. Chandra was sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled by the high angular resolution of its mirrors. Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes; therefore space-based telescopes are required to make these observations. Chandra is an Earth satellite in a 64-hour orbit, and its mission is ongoing as of 2024.

Chandra is one of the Great Observatories, along with the Hubble Space Telescope, Compton Gamma Ray Observatory (1991–2000), and the Spitzer Space Telescope (2003–2020). The telescope is named after the Nobel Prize-winning Indian astrophysicist Subrahmanyan Chandrasekhar. Its mission is similar to that of ESA's XMM-Newton spacecraft, also launched in 1999 but the two telescopes have different design foci, as Chandra has a much higher angular resolution and XMM-Newton higher spectroscopy throughput.

In response to a decrease in NASA funding in 2024 by the US Congress, Chandra is threatened with an early cancellation despite having more than a decade of operation left. The cancellation has been referred to as a potential "extinction-level" event for X-ray astronomy in the US. A group of astronomers have put together a public outreach project to try to get enough American citizens to persuade the US Congress to provide enough funding to avoid early termination of the observatory.

History

In 1976, the Chandra X-ray Observatory (called AXAF at the time) was proposed to NASA by Riccardo Giacconi and Harvey Tananbaum. Preliminary work began the following year at Marshall Space Flight Center (MSFC) and the Smithsonian Astrophysical Observatory (SAO), where the telescope is now operated for NASA at the Chandra X-ray Center in the Center for Astrophysics | Harvard & Smithsonian. In the meantime, in 1978, NASA launched the first imaging X-ray telescope, Einstein (HEAO-2), into orbit. Work continued on the AXAF project throughout the 1980s and 1990s. In 1992, to reduce costs, the spacecraft was redesigned. Four of the twelve planned mirrors were eliminated, as were two of the six scientific instruments. AXAF's planned orbit was changed to an elliptical one, reaching one third of the way to the Moon's at its farthest point. This eliminated the possibility of improvement or repair by the Space Shuttle but put the observatory above the Earth's radiation belts for most of its orbit. AXAF was assembled and tested by TRW (now Northrop Grumman Aerospace Systems) in Redondo Beach, California.

Space Shuttle Columbia, STS-93 launches in 1999

AXAF was renamed Chandra as part of a contest held by NASA in 1998, which drew more than 6,000 submissions worldwide. The contest winners, Jatila van der Veen and Tyrel Johnson (then a high school teacher and high school student, respectively), suggested the name in honor of Nobel Prize–winning Indian-American astrophysicist Subrahmanyan Chandrasekhar. He is known for his work in determining the maximum mass of white dwarf stars, leading to greater understanding of high energy astronomical phenomena such as neutron stars and black holes. Fittingly, the name Chandra means "moon" in Sanskrit.

Originally scheduled to be launched in December 1998, the spacecraft was delayed several months, eventually being launched on July 23, 1999, at 04:31 UTC by Space Shuttle Columbia during STS-93. Chandra was deployed by Cady Coleman from Columbia at 11:47 UTC. The Inertial Upper Stage's first stage motor ignited at 12:48 UTC, and after burning for 125 seconds and separating, the second stage ignited at 12:51 UTC and burned for 117 seconds. At 22,753 kilograms (50,162 lb), it was the heaviest payload ever launched by the shuttle, a consequence of the two-stage Inertial Upper Stage booster rocket system needed to transport the spacecraft to its high orbit.

Chandra has been returning data since the month after it launched. It is operated by the SAO at the Chandra X-ray Center in Cambridge, Massachusetts, with assistance from MIT and Northrop Grumman Space Technology. The ACIS CCDs suffered particle damage during early radiation belt passages. To prevent further damage, the instrument is now removed from the telescope's focal plane during passages.

Although Chandra was initially given an expected lifetime of 5 years, on September 4, 2001, NASA extended its lifetime to 10 years "based on the observatory's outstanding results." Physically Chandra could last much longer. A 2004 study performed at the Chandra X-ray Center indicated that the observatory could last at least 15 years. It is active as of 2024 and has an upcoming schedule of observations published by the Chandra X-ray Center.

In July 2008, the International X-ray Observatory, a joint project between ESA, NASA and JAXA, was proposed as the next major X-ray observatory but was later canceled. ESA later resurrected a downsized version of the project as the Advanced Telescope for High Energy Astrophysics (ATHENA), with a proposed launch in 2028.

On October 10, 2018, Chandra entered safe mode operations, due to a gyroscope glitch. NASA reported that all science instruments were safe. Within days, the 3-second error in data from one gyro was understood, and plans were made to return Chandra to full service. The gyroscope that experienced the glitch was placed in reserve and is otherwise healthy.

In March 2024, Congress decided to reduce funding for NASA and its missions. This may lead to the premature end of this mission. In June 2024, Senators urged NASA to reconsider the cuts to Chandra.

Example discoveries

Crew of STS-93 with a scale model

The data gathered by Chandra has greatly advanced the field of X-ray astronomy. Here are some examples of discoveries supported by observations from Chandra:

• The first light image, of supernova remnant Cassiopeia A, gave astronomers their first glimpse of the compact object at the center of the remnant, probably a neutron star or black hole.
• In the Crab Nebula, another supernova remnant, Chandra showed a never-before-seen ring around the central pulsar and jets that had only been partially seen by earlier telescopes.
• The first X-ray emission was seen from the supermassive black hole, Sagittarius A*, at the center of the Milky Way.
• Chandra found much more cool gas than expected spiraling into the center of the Andromeda Galaxy.
• Pressure fronts were observed in detail for the first time in Abell 2142, where clusters of galaxies are merging.
• The earliest images in X-rays of the shock wave of a supernova were taken of SN 1987A.
• Chandra showed for the first time the shadow of a small galaxy as it is being cannibalized by a larger one, in an image of Perseus A.
• Anew type of black hole was discovered in galaxy M82, mid-mass objects purported to be the missing link between stellar-sized black holes and super massive black holes.
• X-ray emission lines were associated for the first time with a gamma-ray burst, Beethoven Burst GRB 991216.
• High school students, using Chandra data, discovered a neutron star in supernova remnant IC 443.
• Observations by Chandra and BeppoSAX suggest that gamma-ray bursts occur in star-forming regions.
• Chandra data suggested that RX J1856.5-3754 and 3C58, previously thought to be pulsars, might be even denser objects: quark stars. These results are still debated.
• Sound waves from violent activity around a super massive black hole were observed in the Perseus Cluster (2003).

• 

  CXO image of the brown dwarf TWA 5B

  TWA 5B, a brown dwarf, was seen orbiting a binary system of Sun-like stars.
• Nearly all stars on the main sequence are X-ray emitters.
• The X-ray shadow of Titan was seen when it transited the Crab Nebula.
• X-ray emissions from materials falling from a protoplanetary disc into a star.
• Hubble constant measured to be 76.9 km/s/Mpc using Sunyaev-Zel'dovich effect.
• 2006 Chandra found strong evidence that dark matter exists by observing super cluster collision.
• 2006 X-ray emitting loops, rings and filaments discovered around a super massive black hole within Messier 87 imply the presence of pressure waves, shock waves and sound waves. The evolution of Messier 87 may have been dramatically affected.
• Observations of the Bullet cluster put limits on the cross-section of the self-interaction of dark matter.
• "The Hand of God" photograph of PSR B1509-58.
• Jupiter's x-rays coming from poles, not auroral ring.
• A large halo of hot gas was found surrounding the Milky Way.
• Extremely dense and luminous dwarf galaxy M60-UCD1 observed.
• On January 5, 2015, NASA reported that CXO observed an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*, the supermassive black hole in the center of the Milky Way galaxy. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers.
• In September 2016, it was announced that Chandra had detected X-ray emissions from Pluto, the first detection of X-rays from a Kuiper belt object. Chandra had made the observations in 2014 and 2015, supporting the New Horizons spacecraft for its July 2015 encounter.
• In September 2020, Chandra reportedly may have made an observation of an exoplanet in the Whirlpool Galaxy, which would be the first planet discovered beyond the Milky Way.
• In April 2021, NASA announced findings from the observatory in a tweet saying "Uranus gives off X-rays, astronomers find". The discovery would have "intriguing implications for understanding Uranus" if it is confirmed that the X-rays originate from the planet and are not emitted by the Sun.

Technical description

Assembly of the telescope

The main mirror of AXAF (Chandra)

HRC flight unit of Chandra

Unlike optical telescopes which possess simple aluminized parabolic surfaces (mirrors), X-ray telescopes generally use a Wolter telescope consisting of nested cylindrical paraboloid and hyperboloid surfaces coated with iridium or gold. X-ray photons would be absorbed by normal mirror surfaces, so mirrors with a low grazing angle are necessary to reflect them. Chandra uses four pairs of nested mirrors, together with their support structure, called the High Resolution Mirror Assembly (HRMA); the mirror substrate is 2 cm-thick glass, with the reflecting surface a 33 nm iridium coating, and the diameters are 65 cm, 87 cm, 99 cm and 123 cm. The thick substrate and particularly careful polishing allowed a very precise optical surface, which is responsible for Chandra's unmatched resolution: between 80% and 95% of the incoming X-ray energy is focused into a one-arcsecond circle. However, the thickness of the substrate limits the proportion of the aperture which is filled, leading to the low collecting area compared to XMM-Newton.

Chandra's highly elliptical orbit allows it to observe continuously for up to 55 hours of its 65-hour orbital period. At its furthest orbital point from Earth, Chandra is one of the most distant Earth-orbiting satellites. This orbit takes it beyond the geostationary satellites and beyond the outer Van Allen belt.

With an angular resolution of 0.5 arcsecond (2.4 μrad), Chandra possesses a resolution over 1000 times better than that of the first orbiting X-ray telescope.

CXO uses mechanical gyroscopes, which are sensors that help determine what direction the telescope is pointed. Other navigation and orientation systems on board CXO include an aspect camera, Earth and Sun sensors, and reaction wheels. It also has two sets of thrusters, one for movement and another for offloading momentum.

Instruments

The Science Instrument Module (SIM) holds the two focal plane instruments, the Advanced CCD Imaging Spectrometer (ACIS) and the High Resolution Camera (HRC), moving whichever is called for into position during an observation.

ACIS consists of 10 CCD chips and provides images as well as spectral information of the object observed. It operates in the photon energy range of 0.2–10 keV. The HRC has two micro-channel plate components and images over the range of 0.1–10 keV. It also has a time resolution of 16 microseconds. Both of these instruments can be used on their own or in conjunction with one of the observatory's two transmission gratings.

The transmission gratings, which swing into the optical path behind the mirrors, provide Chandra with high resolution spectroscopy. The High Energy Transmission Grating Spectrometer (HETGS) works over 0.4–10 keV and has a spectral resolution of 60–1000. The Low Energy Transmission Grating Spectrometer (LETGS) has a range of 0.09–3 keV and a resolution of 40–2000.

Summary:

• High Resolution Camera (HRC)
• Advanced CCD Imaging Spectrometer (ACIS)
• High Energy Transmission Grating Spectrometer (HETGS)
• Low Energy Transmission Grating Spectrometer (LETGS)

Direct collapse black hole

From Wikipedia, the free encyclopedia

Artist's impression for the formation of a massive black hole seed via the direct black hole channel.

Direct collapse black holes (DCBHs) are high-mass black hole seeds that form from the direct collapse of a large amount of material. They putatively formed within the redshift range z=15–30, when the Universe was about 100–250 million years old. Unlike seeds formed from the first population of stars (also known as Population III stars), direct collapse black hole seeds are formed by a direct, general relativistic instability. They are very massive, with a typical mass at formation of ~10⁵ M_☉. This category of black hole seeds was originally proposed theoretically to alleviate the challenge in building supermassive black holes already at redshift z~7, as numerous observations to date have confirmed.

Formation

Direct collapse black holes (DCBHs) are massive black hole seeds theorized to have formed in the high-redshift Universe and with typical masses at formation of ~10⁵ M_☉, but spanning between 10⁴ M_☉ and 10⁶ M_☉. The environmental physical conditions to form a DCBH (as opposed to a cluster of stars) are the following:

1. Metal-free gas (gas containing only hydrogen and helium).
2. Atomic-cooling gas.
3. Sufficiently large flux of Lyman–Werner photons, in order to destroy hydrogen molecules, which are very efficient gas coolants.

The previous conditions are necessary to avoid gas cooling and, hence, fragmentation of the primordial gas cloud. Unable to fragment and form stars, the gas cloud undergoes a gravitational collapse of the entire structure, reaching extremely high matter density at its core, on the order of ~10⁷ g/cm³. At this density, the object undergoes a general relativistic instability, which leads to the formation of a black hole of a typical mass ~10⁵ M_☉, and up to 1 million M_☉. The occurrence of the general relativistic instability, as well as the absence of the intermediate stellar phase, led to the denomination of direct collapse black hole. In other words, these objects collapse directly from the primordial gas cloud, not from a stellar progenitor as prescribed in standard black hole models.

A computer simulation reported in July 2022 showed that a halo at the rare convergence of strong, cold accretion flows can create massive black holes seeds without the need for ultraviolet backgrounds, supersonic streaming motions or even atomic cooling. Cold flows produced turbulence in the halo, which suppressed star formation. In the simulation, no stars formed in the halo until it had grown to 40 million solar masses at a redshift of 25.7 when the halo's gravity was finally able to overcome the turbulence; the halo then collapsed and formed two supermassive stars that died as DCBHs of 31,000 and 40,000 M_☉.

Demography

Direct collapse black holes are generally thought to be extremely rare objects in the high-redshift Universe, because the three fundamental conditions for their formation (see above in section Formation) are challenging to be met all together in the same gas cloud. Current cosmological simulations suggest that DCBHs could be as rare as only about 1 per cubic gigaparsec at redshift 15. The prediction on their number density is highly dependent on the minimum flux of Lyman–Werner photons required for their formation and can be as large as ~10⁷ DCBHs per cubic gigaparsec in the most optimistic scenarios.

Detection

In 2016, a team led by Harvard University astrophysicist Fabio Pacucci identified the first two candidate direct collapse black holes, using data from the Hubble Space Telescope and the Chandra X-ray Observatory. The two candidates, both at redshift ${\displaystyle z>6}$, were found in the CANDELS GOODS-S field and matched the spectral properties predicted for this type of astrophysical sources. In particular, these sources are predicted to have a significant excess of infrared radiation, when compared to other categories of sources at high redshift. Additional observations, in particular with the James Webb Space Telescope, will be crucial to investigate the properties of these sources and confirm their nature.

Difference from primordial and stellar collapse black holes

A primordial black hole is the result of the direct collapse of energy, ionized matter, or both, during the inflationary or radiation-dominated eras, while a direct collapse black hole is the result of the collapse of unusually dense and large regions of gas. Note that a black hole formed by the collapse of a Population III star is not considered "direct" collapse.

Photon sphere

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Photon_sphere

An animation of how light rays can be gravitationally bent to form a photon sphere

A photon sphere or photon circle arises in a neighbourhood of the event horizon of a black hole where gravity is so strong that emitted photons will not just bend around the black hole but also return to the point where they were emitted from and consequently display boomerang-like properties. As the source emitting photons falls into the gravitational field towards the event horizon the shape of the trajectory of each boomerang photon changes, tending to a more circular form. At a critical value of the radial distance from the singularity the trajectory of a boomerang photon will take the form of a non-stable circular orbit, thus forming a photon circle and hence in aggregation a photon sphere. The circular photon orbit is said to be the last photon orbit. The radius of the photon sphere, which is also the lower bound for any stable orbit, is, for a Schwarzschild black hole,

${\displaystyle r=\frac{3GM}{c^2}=\frac{3r_{\text{s}}}{2},}$

where G is the gravitational constant, M is the mass of the black hole, c is the speed of light in vacuum, and r_s is the Schwarzschild radius (the radius of the event horizon); see below for a derivation of this result.

This equation entails that photon spheres can only exist in the space surrounding an extremely compact object (a black hole or possibly an "ultracompact" neutron star).

The photon sphere is located farther from the center of a black hole than the event horizon. Within a photon sphere, it is possible to imagine a photon that is emitted (or reflected) from the back of one's head and, following an orbit of the black hole, is then intercepted by the person's eye, allowing one to see the back of the head, see e.g. For non-rotating black holes, the photon sphere is a sphere of radius 3/2 r_s. There are no stable free-fall orbits that exist within or cross the photon sphere. Any free-fall orbit that crosses it from the outside spirals into the black hole. Any orbit that crosses it from the inside escapes to infinity or falls back in and spirals into the black hole. No unaccelerated orbit with a semi-major axis less than this distance is possible, but within the photon sphere, a constant acceleration will allow a spacecraft or probe to hover above the event horizon.

Another property of the photon sphere is centrifugal force (note: not centripetal) reversal. Outside the photon sphere, the faster one orbits, the greater the outward force one feels. Centrifugal force falls to zero at the photon sphere, including non-freefall orbits at any speed, i.e. an object weighs the same no matter how fast it orbits, and becomes negative inside it. Inside the photon sphere, faster orbiting leads to greater weight or inward force. This has serious ramifications for the fluid dynamics of inward fluid flow.

A rotating black hole has two photon spheres. As a black hole rotates, it drags space with it. The photon sphere that is closer to the black hole is moving in the same direction as the rotation, whereas the photon sphere further away is moving against it. The greater the angular velocity of the rotation of a black hole, the greater the distance between the two photon spheres. Since the black hole has an axis of rotation, this only holds true if approaching the black hole in the direction of the equator. In a polar orbit, there is only one photon sphere. This is because when approaching at this angle, the possibility of traveling with or against the rotation does not exist. The rotation will instead cause the orbit to precess.

Further information: Photon surface

Derivation for a Schwarzschild black hole

Since a Schwarzschild black hole has spherical symmetry, all possible axes for a circular photon orbit are equivalent, and all circular orbits have the same radius.

This derivation involves using the Schwarzschild metric, given by

${\displaystyle d{s}^2=\left(1-\frac{r_{\text{s}}}{r}\right)c^{2}d{t}^2-{\left(1-\frac{r_{\text{s}}}{r}\right)}^{-1}d{r}^2-r^2({\sin }^2\theta d{\varphi }^2+d{\theta }^2).}$

For a photon traveling at a constant radius r (i.e. in the φ-coordinate direction), ${\displaystyle dr=0}$. Since it is a photon, ${\displaystyle ds=0}$ (a "light-like interval"). We can always rotate the coordinate system such that ${\displaystyle \theta }$ is constant, ${\displaystyle d\theta =0}$ (e.g., ${\displaystyle \theta =\pi /2}$).

Setting ds, dr and dθ to zero, we have

${\displaystyle \left(1-\frac{r_{\text{s}}}{r}\right)c^{2}d{t}^2=r^2{\sin }^2\theta d{\varphi }^2.}$

Re-arranging gives

${\displaystyle \frac{d\varphi }{dt}=\frac{c}{r\sin \theta }\sqrt{1-\frac{r_{\text{s}}}{r}}.}$

To proceed, we need the relation ${\displaystyle \frac{d\varphi }{dt}}$. To find it, we use the radial geodesic equation

${\displaystyle \frac{d^{2}r}{d{\tau }^2}+{\mathrm{\Gamma }}_{\mu \nu }^r{u}^{\mu }{u}^{\nu }=0.}$

Non vanishing ${\displaystyle \mathrm{\Gamma }}$-connection coefficients are

${\displaystyle {\mathrm{\Gamma }}_{tt}^r=\frac{c^{2}B{B}^{\prime }}{2},{\mathrm{\Gamma }}_{rr}^r=-\frac{B^{-1}{B}^{\prime }}{2},{\mathrm{\Gamma }}_{\theta \theta }^r=-rB,{\mathrm{\Gamma }}_{\varphi \varphi }^r=-Br{\sin }^2\theta ,}$

where ${\displaystyle B^{\prime }=\frac{dB}{dr},B=1-\frac{r_{\text{s}}}{r}}$.

We treat photon radial geodesics with constant r and ${\displaystyle \theta }$, therefore

${\displaystyle \frac{dr}{d\tau }=\frac{d^{2}r}{d{\tau }^2}=\frac{d\theta }{d\tau }=0.}$

Substituting it all into the radial geodesic equation (the geodesic equation with the radial coordinate as the dependent variable), we obtain

${\displaystyle {\left(\frac{d\varphi }{dt}\right)}^2=\frac{c^2{r}_{\text{s}}}{2r^3{\sin }^2\theta }.}$

Comparing it with what was obtained previously, we have

${\displaystyle c\sqrt{\frac{r_{\text{s}}}{2r}}=c\sqrt{1-\frac{r_{\text{s}}}{r}},}$

where we have inserted ${\displaystyle \theta =\pi /2}$ radians (imagine that the central mass, about which the photon is orbiting, is located at the centre of the coordinate axes. Then, as the photon is travelling along the ${\displaystyle \varphi }$-coordinate line, for the mass to be located directly in the centre of the photon's orbit, we must have ${\displaystyle \theta =\pi /2}$ radians).

Hence, rearranging this final expression gives

${\displaystyle r=\frac{3}{2}{r}_{\text{s}},}$

which is the result we set out to prove.

Views from the side (l) and from above a pole (r). A rotating black hole has 9 radii between which light can orbit on a constant r coordinate. In this animation, all photon orbits for a = M are shown.

In contrast to a Schwarzschild black hole, a Kerr (spinning) black hole does not have spherical symmetry, but only an axis of symmetry, which has profound consequences for the photon orbits, see e.g. Cramer for details and simulations of photon orbits and photon circles. There are two circular photon orbits in the equatorial plane (prograde and retrograde), with different Boyer–Lindquist radii:

${\displaystyle r_{\pm }^{\circ }=r_{\text{s}}\left[1+\cos \left(\frac{2}{3}\arccos \frac{\pm |a|}{M}\right)\right],}$

where ${\displaystyle a=J/M}$ is the angular momentum per unit mass of the black hole. There exist other constant-radius orbits, but they have more complicated paths which oscillate in latitude about the equator.

Event horizon

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Event_horizon

 

General relativity
${\displaystyle G_{\mu \nu }+\mathrm{\Lambda }{g}_{\mu \nu }=\kappa T_{\mu \nu }}$

In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer. Wolfgang Rindler coined the term in the 1950s.

In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive compact objects that even light cannot escape. At that time, the Newtonian theory of gravitation and the so-called corpuscular theory of light were dominant. In these theories, if the escape velocity of the gravitational influence of a massive object exceeds the speed of light, then light originating inside or from it can escape temporarily but will return. In 1958, David Finkelstein used general relativity to introduce a stricter definition of a local black hole event horizon as a boundary beyond which events of any kind cannot affect an outside observer, leading to information and firewall paradoxes, encouraging the re-examination of the concept of local event horizons and the notion of black holes. Several theories were subsequently developed, some with and some without event horizons. One of the leading developers of theories to describe black holes, Stephen Hawking, suggested that an apparent horizon should be used instead of an event horizon, saying, "Gravitational collapse produces apparent horizons but no event horizons." He eventually concluded that "the absence of event horizons means that there are no black holes – in the sense of regimes from which light can't escape to infinity."

Any object approaching the horizon from the observer's side appears to slow down, never quite crossing the horizon. Due to gravitational redshift, its image reddens over time as the object moves closer to the horizon.

In an expanding universe, the speed of expansion reaches — and even exceeds — the speed of light, preventing signals from traveling to some regions. A cosmic event horizon is a real event horizon because it affects all kinds of signals, including gravitational waves, which travel at the speed of light.

More specific horizon types include the related but distinct absolute and apparent horizons found around a black hole. Other distinct types include:

• The Cauchy and Killing horizons.
• The photon spheres and ergospheres of the Kerr solution.
• Particle and cosmological horizons relevant to cosmology.
• Isolated and dynamical horizons, which are important in current black hole research.

Cosmic event horizon

Main article: Cosmological horizon

The reachable Universe as a function of time and distance, in context of the expanding Universe.

In cosmology, the event horizon of the observable universe is the largest comoving distance from which light emitted now can ever reach the observer in the future. This differs from the concept of the particle horizon, which represents the largest comoving distance from which light emitted in the past could reach the observer at a given time. For events that occur beyond that distance, light has not had enough time to reach our location, even if it was emitted at the time the universe began. The evolution of the particle horizon with time depends on the nature of the expansion of the universe. If the expansion has certain characteristics, parts of the universe will never be observable, no matter how long the observer waits for the light from those regions to arrive. The boundary beyond which events cannot ever be observed is an event horizon, and it represents the maximum extent of the particle horizon.

The criterion for determining whether a particle horizon for the universe exists is as follows. Define a comoving distance d_p as

${\displaystyle d_p={\int }_0^{t_0}\frac{c}{a(t)}dt.}$

In this equation, a is the scale factor, c is the speed of light, and t₀ is the age of the Universe. If d_p → ∞ (i.e., points arbitrarily as far away as can be observed), then no event horizon exists. If d_p ≠ ∞, a horizon is present.

Examples of cosmological models without an event horizon are universes dominated by matter or by radiation. An example of a cosmological model with an event horizon is a universe dominated by the cosmological constant (a de Sitter universe).

A calculation of the speeds of the cosmological event and particle horizons was given in a paper on the FLRW cosmological model, approximating the Universe as composed of non-interacting constituents, each one being a perfect fluid.

Apparent horizon of an accelerated particle

See also: Hyperbolic motion (relativity)

Spacetime diagram showing a uniformly accelerated particle, P, and an event E that is outside the particle's apparent horizon. The event's forward light cone never intersects the particle's world line.

If a particle is moving at a constant velocity in a non-expanding universe free of gravitational fields, any event that occurs in that Universe will eventually be observable by the particle, because the forward light cones from these events intersect the particle's world line. On the other hand, if the particle is accelerating, in some situations light cones from some events never intersect the particle's world line. Under these conditions, an apparent horizon is present in the particle's (accelerating) reference frame, representing a boundary beyond which events are unobservable.

For example, this occurs with a uniformly accelerated particle. A spacetime diagram of this situation is shown in the figure to the right. As the particle accelerates, it approaches, but never reaches, the speed of light with respect to its original reference frame. On the spacetime diagram, its path is a hyperbola, which asymptotically approaches a 45-degree line (the path of a light ray). An event whose light cone's edge is this asymptote or is farther away than this asymptote can never be observed by the accelerating particle. In the particle's reference frame, there is a boundary behind it from which no signals can escape (an apparent horizon). The distance to this boundary is given by ${\displaystyle c^2/a}$, where a is the constant proper acceleration of the particle.

While approximations of this type of situation can occur in the real world (in particle accelerators, for example), a true event horizon is never present, as this requires the particle to be accelerated indefinitely (requiring arbitrarily large amounts of energy and an arbitrarily large apparatus).

Interacting with a cosmic horizon

In the case of a horizon perceived by a uniformly accelerating observer in empty space, the horizon seems to remain a fixed distance from the observer no matter how its surroundings move. Varying the observer's acceleration may cause the horizon to appear to move over time or may prevent an event horizon from existing, depending on the acceleration function chosen. The observer never touches the horizon and never passes a location where it appeared to be.

In the case of a horizon perceived by an occupant of a de Sitter universe, the horizon always appears to be a fixed distance away for a non-accelerating observer. It is never contacted, even by an accelerating observer.

Event horizon of a black hole

Main article: Black hole

Far away from the black hole, a particle can move in any direction. It is only restricted by the speed of light.

Closer to the black hole spacetime starts to deform. In some convenient coordinate systems, there are more paths going towards the black hole than paths moving away.[Note 1]

Inside the event horizon all future time paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape, no matter the direction the particle is traveling.

One of the best-known examples of an event horizon derives from general relativity's description of a black hole, a celestial object so dense that no nearby matter or radiation can escape its gravitational field. Often, this is described as the boundary within which the black hole's escape velocity is greater than the speed of light. However, a more detailed description is that within this horizon, all lightlike paths (paths that light could take) (and hence all paths in the forward light cones of particles within the horizon) are warped so as to fall farther into the hole. Once a particle is inside the horizon, moving into the hole is as inevitable as moving forward in time – no matter in what direction the particle is travelling – and can be thought of as equivalent to doing so, depending on the spacetime coordinate system used.

The surface at the Schwarzschild radius acts as an event horizon in a non-rotating body that fits inside this radius (although a rotating black hole operates slightly differently). The Schwarzschild radius of an object is proportional to its mass. Theoretically, any amount of matter will become a black hole if compressed into a space that fits within its corresponding Schwarzschild radius. For the mass of the Sun, this radius is approximately 3 kilometers (1.9 miles); for Earth, it is about 9 millimeters (0.35 inches). In practice, however, neither Earth nor the Sun have the necessary mass (and, therefore, the necessary gravitational force) to overcome electron and neutron degeneracy pressure. The minimal mass required for a star to collapse beyond these pressures is the Tolman–Oppenheimer–Volkoff limit, which is approximately three solar masses.

According to the fundamental gravitational collapse models, an event horizon forms before the singularity of a black hole. If all the stars in the Milky Way would gradually aggregate towards the galactic center while keeping their proportionate distances from each other, they will all fall within their joint Schwarzschild radius long before they are forced to collide. Up to the collapse in the far future, observers in a galaxy surrounded by an event horizon would proceed with their lives normally.

Black hole event horizons are widely misunderstood. Common, although erroneous, is the notion that black holes "vacuum up" material in their neighborhood, where in fact they are no more capable of seeking out material to consume than any other gravitational attractor. As with any mass in the universe, matter must come within its gravitational scope for the possibility to exist of capture or consolidation with any other mass. Equally common is the idea that matter can be observed falling into a black hole. This is not possible. Astronomers can detect only accretion disks around black holes, where material moves with such speed that friction creates high-energy radiation that can be detected (similarly, some matter from these accretion disks is forced out along the axis of spin of the black hole, creating visible jets when these streams interact with matter such as interstellar gas or when they happen to be aimed directly at Earth). Furthermore, a distant observer will never actually see something reach the horizon. Instead, while approaching the hole, the object will seem to go ever more slowly, while any light it emits will be further and further redshifted.

Topologically, the event horizon is defined from the causal structure as the past null cone of future conformal timelike infinity. A black hole event horizon is teleological in nature, meaning that it is determined by future causes. More precisely, one would need to know the entire history of the universe and all the way into the infinite future to determine the presence of an event horizon, which is not possible for quasilocal observers (not even in principle). In other words, there is no experiment and/or measurement that can be performed within a finite-size region of spacetime and within a finite time interval that answers the question of whether or not an event horizon exists. Because of the purely theoretical nature of the event horizon, the traveling object does not necessarily experience strange effects and does, in fact, pass through the calculated boundary in a finite amount of its proper time.

Interacting with black hole horizons

A misconception concerning event horizons, especially black hole event horizons, is that they represent an immutable surface that destroys objects that approach them. In practice, all event horizons appear to be some distance away from any observer, and objects sent towards an event horizon never appear to cross it from the sending observer's point of view (as the horizon-crossing event's light cone never intersects the observer's world line). Attempting to make an object near the horizon remain stationary with respect to an observer requires applying a force whose magnitude increases unboundedly (becoming infinite) the closer it gets.

In the case of the horizon around a black hole, observers stationary with respect to a distant object will all agree on where the horizon is. While this seems to allow an observer lowered towards the hole on a rope (or rod) to contact the horizon, in practice this cannot be done. The proper distance to the horizon is finite, so the length of rope needed would be finite as well, but if the rope were lowered slowly (so that each point on the rope was approximately at rest in Schwarzschild coordinates), the proper acceleration (G-force) experienced by points on the rope closer and closer to the horizon would approach infinity, so the rope would be torn apart. If the rope is lowered quickly (perhaps even in freefall), then indeed the observer at the bottom of the rope can touch and even cross the event horizon. But once this happens it is impossible to pull the bottom of rope back out of the event horizon, since if the rope is pulled taut, the forces along the rope increase without bound as they approach the event horizon and at some point the rope must break. Furthermore, the break must occur not at the event horizon, but at a point where the second observer can observe it.

Assuming that the possible apparent horizon is far inside the event horizon, or there is none, observers crossing a black hole event horizon would not actually see or feel anything special happen at that moment. In terms of visual appearance, observers who fall into the hole perceive the eventual apparent horizon as a black impermeable area enclosing the singularity. Other objects that had entered the horizon area along the same radial path but at an earlier time would appear below the observer as long as they are not entered inside the apparent horizon, and they could exchange messages. Increasing tidal forces are also locally noticeable effects, as a function of the mass of the black hole. In realistic stellar black holes, spaghettification occurs early: tidal forces tear materials apart well before the event horizon. However, in supermassive black holes, which are found in centers of galaxies, spaghettification occurs inside the event horizon. A human astronaut would survive the fall through an event horizon only in a black hole with a mass of approximately 10,000 solar masses or greater.

Beyond general relativity

A cosmic event horizon is commonly accepted as a real event horizon, whereas the description of a local black hole event horizon given by general relativity is found to be incomplete and controversial. When the conditions under which local event horizons occur are modeled using a more comprehensive picture of the way the Universe works, that includes both relativity and quantum mechanics, local event horizons are expected to have properties that are different from those predicted using general relativity alone.

At present, it is expected by the Hawking radiation mechanism that the primary impact of quantum effects is for event horizons to possess a temperature and so emit radiation. For black holes, this manifests as Hawking radiation, and the larger question of how the black hole possesses a temperature is part of the topic of black hole thermodynamics. For accelerating particles, this manifests as the Unruh effect, which causes space around the particle to appear to be filled with matter and radiation.

According to the controversial black hole firewall hypothesis, matter falling into a black hole would be burned to a crisp by a high energy "firewall" at the event horizon.

An alternative is provided by the complementarity principle, according to which, in the chart of the far observer, infalling matter is thermalized at the horizon and reemitted as Hawking radiation, while in the chart of an infalling observer matter continues undisturbed through the inner region and is destroyed at the singularity. This hypothesis does not violate the no-cloning theorem as there is a single copy of the information according to any given observer. Black hole complementarity is actually suggested by the scaling laws of strings approaching the event horizon, suggesting that in the Schwarzschild chart they stretch to cover the horizon and thermalize into a Planck length-thick membrane.

A complete description of local event horizons generated by gravity is expected to, at minimum, require a theory of quantum gravity. One such candidate theory is M-theory. Another such candidate theory is loop quantum gravity.