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The darkest secret of gravity


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Black hole mass scale

From the point of view of classical theories, black holes can have any mass. A process just has to succeed in compressing a given mass - a star, a chair, the unloved boss - below its Schwarzschild radius: Then a black hole results. Obviously there are processes in space that can naturally create a black hole. For example, the end of a massive star is usually a stellar black hole. But that's not all. Over time, astronomers found many candidate objects in the sky for which black holes of a significantly different mass range could also be considered. In the meantime, very specific mass ranges have been found in astrophysics to which a special class of black holes can be assigned. The distinguishing criteria are Dimensions and evolution:

Primordial black holes

Primordial black holes or Mini holes are very small. They only have masses of around 1018 g or 10-15 Solar masses. In tangible units, this corresponds roughly to the mass of an earthly mountain. The associated radius of the event horizon is only about 10-12 m and thus comes into the subatomic area. If we assume that this mass were evenly distributed over a sphere of this radius, the compactness would be extremely high: It would have an average density of about 1048 g / cm3!
The existence of such mini-holes is highly speculative and their mechanism of formation is completely unclear. It could have collapsed a particularly intense form of gravitational waves, the super-critical Brill waves, leaving these mini-holes. It is also considered that such mini-holes were primordially present, i.e. that they arose in an early phase of the universe, for example from spontaneous symmetry breaking of a scalar field, similar to the Higgs mechanism.

Stephen Hawking showed in 1974 that such 'dwarfs' underneath the holes would quickly be annihilated by quantum effects. Because the emission of Hawking radiation costs the hole mass until at some point the small mass is used up and has disappeared. The end of the annihilation phase is - as is assumed - with a brief outbreak (burst) of high-energy, electromagnetic waves. Unfortunately, astronomers have made observations so far no hints found on the primordial black holes. The Gamma Ray Bursts (GRBs) are associated with completely different scenarios, namely star explosions (so-called hypernovae) and the merging of compact binary stars.
The negative result regarding the annihilation radiation of primordial black holes is surprising, because it is to be expected that signatures can be found in the distribution of the large-scale, cosmic background radiation. If there were primordial black holes in an early phase of the universe, they could have grown through accretion and thus possibly have been the seeds for galaxies. However, these hypotheses are quite speculative.

Stellar black holes

Stellar black holes are formed in the course of star evolution. Einstein was certainly impressed that Schwarzschild had found a solution so soon after the publication of the field equations of general relativity: the Schwarzschild solution. However, Einstein was reluctant to believe that these point masses could exist in nature.
However, the growing knowledge in stellar physics caused a change. The astrophysicists found that stars at the end of their development become unstable and collapse as a result of the increasing gravitational pressure. This gravitational collapse produces very compact stars: white dwarfs and neutron stars are just one variant. Even neutron matter cannot withstand a high residual mass of the dying star. Then - according to widespread doctrine - a singularity must arise. Since the precursor stars rotate, it is very likely that the resulting hole will also rotate. The corresponding appropriate spacetime is therefore the Kerr solution and less the Schwarzschild solution. It was not until the late 1960s that these singularities were called black holes.
The stellar black holes arise from stars. So it is not surprising that they weigh only a few to around a hundred solar masses. One solar mass corresponds to 1,989 × 1030 kg. Like all black holes, these can also grow by picking up matter from their vicinity. This process, which is generally important for astrophysics, is called accretion. Black holes that come too close, for example because they formed together in a multiple star system, can also merge with one another. In these so-called merging scenarios, even larger black holes can arise - which may even go beyond the stellar scale. We will see shortly that these heavier candidates for black holes exist as well.

Good candidates for stellar black holes include the following astronomical sources:

When stars explode

As anticipated, stellar black holes arise from star explosions, from the so-called hypernovae (HN). The energy output of these catastrophic events is enormous and is 1053erg. This explosion energy is 'hurled' into the interstellar space and drives violent shock waves that collide with the environment. This leads to violent radiation processes that mainly emit synchrotron radiation. The result of such an explosion is shown in the colorful picture on the right, a false color composite from observation data from the space telescopes Chandra (X-ray: parts of blue and green), Hubble (optical: yellow) and Spitzer (infrared: red color parts), which shows the supernova remnant Cassiopeia A ( Credit: NASA / CXC / SAO, NASA / STScI, NASA / JPL-Caltech 2005). The surrounding material glows in all possible wavelength ranges of the electromagnetic spectrum due to the excitation of the shock of the star explosion that occurred around 300 years ago.
These processes are of great importance for the complexity of matter, because when the shock wave escapes, those elements are created that are heavier than iron. Astronomers call the processes associated with the explosion wave r-processes and p-processes.

Above all, its mass is decisive for the fate of a star. If at the end of the stellar development a mass that exceeds about three solar masses collapses, the gravitational collapse on a stellar black hole cannot be prevented. As a consequence, the material collapses to a point.

The collapse can also take place in double star systems (binary systems), which are more common in the universe than single stars (such as the sun). It is possible that a binary star has already become a white dwarf in the manner described. This conversion is comparatively mild and hardly affects the companion. If its companion star is massive and spatially relatively close, it can happen that matter can overflow from it onto the compact component. If one imagines the two overlapping gravitational fields of the stars, then it is clear that there must be at least one point in this system where the entire gravitational force disappears: In this so-called inner Lagrange point, the gravitational force of one component is equal that of the other component. A detailed invoice (restricted three-body problem) shows that there are a total of five such Lagrange points in a binary star system. They are entered as L1 to L5 in the following representation of the effective gravitational potential of both masses (detailed description in the lexicon under Roche volumes):

If the companion star is so extensive, usually a giant star, that its surface extends to a Lagrange point or beyond, then matter can overflow. Astrophysicists then say that the companion star exceeds its Roche volume. The companion star rotates, i.e. its matter has angular momentum. For this reason, the material does not fall in directly radially, but forms a flattened, disk-shaped structure: the standard accretion disk. There are processes in this accretion disk (dissipation due to turbulent viscosity or magnetic rotational instability, MRI), which transport the angular momentum to the outside. Therefore, the material can eventually get to the compact component.
If the compact component is now a white dwarf, the material falls from the accretion disk onto its surface and accumulates. The white dwarf is gradually gaining mass, but white dwarfs cannot become as heavy as they want. If so much mass has been accreted that it reaches the Chandrasekhar limit of about 1.46 solar masses, it tears the white dwarf apart in a powerful hydrogen explosion, a type Ia supernova. It can be assumed that the companion star is thrown out of the binary system during this explosion because it receives a 'kick' from the expiring shock wave. However, this hypothesis and other details of supernova explosions have yet to be confirmed in computer simulations. Simulating a single supernova from a pre-neutron star is currently causing enough problems.

The brightest in the universe!

The hypernova is in a particularly violent form of a star explosion. Here it is to be expected that from the collapse of a very massive star, e.g. of the Wolf-Rayet type with a few tens of solar masses, always a stellar black hole is formed. The astronomers assume that long-term gamma ray bursts (GRBs) are associated with hypernovae. With these bursts of brightness in the most energetic range of electromagnetic radiation, the gravitational collapse drives an ultra-relativistic jet made of stellar matter, which makes its way through the collapsing star. In the context of the anisotropic fireball model, it is assumed that this jet finally leaves the area of ​​the collapsar and passes through the interstellar medium (then alsoCircum burst medium called) propagated. The characteristic is created when the shock wave gradually slows down Afterglow of the GRBs in low-energy areas of the spectrum GRB afterglow) by synchrotron cooling. GRBs are with luminosities of 1051...54 erg / s the most luminous thing astronomers know - they even trump the active galaxy nuclei (AGN), of which the most luminous quasars are around 1047 have erg / s! (We will come to the AGN in the course of the chapter)

The figure on the left shows an observation photo of the gamma ray outbreak GRB030227, taken with the gamma satellite Integral of the European Space Agency (Credit: Beckmann et al., SPI-Team, Integral / ESA 2003). The radiation shown here has energies between 20 and 200 keV and is in the range of hard X-ray and gamma radiation. To the left of the GRB is a continuous source, the Crab Nebula. Crab nebulae) to see. When comparing the intensity you can see that the GRB even outshines the crab pulsar for a short time. There a rapidly rotating neutron star stimulates its surroundings to glow. The crab pulsar is such a prominent and powerful source in gamma astronomy that astronomers put luminosities and fluxes from cosmic gamma sources in its units.

The super star Eta Carinae (η Car) in the constellation ship (Carina) is a celebrity Galactic Hypernova Candidate. η Car has about 100 solar masses, is 7500 light years away and - typical for massive stars - blows off a violent particle wind in the form of a bipolar discharge. It is an interesting question whether this star could pose a threat to humanity should it one day light up as a Gamma Ray Burst. An estimate (made by me) for the deposited equivalent dose of the hypotheticalEta Carinae-GRBs with known parameters and plausible assumptions (burst lasting 100s) results in a value of about 1 Sv (Sievert), based on one day. This corresponds to almost 300 times the usual annual exposure (3 mSv) of a person! Brief full-body irradiation of over 7 Sv leads to death after a few days. With a really long GRB of around 1000 seconds, which is not uncommon from an astronomical point of view, the danger could not be dismissed out of hand! However, this danger would “only” exist for the life of the earth hemisphere that is facing the GRB at the moment of the burst. In addition, many estimates are included in this calculation: the source is point-shaped, which makes whole-body irradiation difficult; The orientation of the GRB jet is of particular relevance. With a bit of luck, it won't point in the direction of earth ...

Unfortunately, a numerical treatment has not yet come to the point where the collapse to neutron stars or black holes in the simulation would have succeeded. However, attempts are being made. It is important to adequately implement radiation and neutrino physics as well as Einstein's theory. When simulating the gravitational collapse to black holes, one needs a criterion that locally fixes the formation of a horizon. This research is also just beginning. The observation of nature, however, shows that the collapse to compact objects occurs (e.g. SN type II in the Crab Nebula or SN 1987a).
Stellar black holes are the classic representatives. For a long time it was believed that this narrow mass domain was characteristic of black holes. Finally, in the 1960s, astrophysicists discovered the Standard Model for Active Galactic Nuclei (AGN), called the AGN paradigm, according to which the activity of particularly bright galaxy nuclei is explained by accretion to supermassive black holes. We come to that at the end of this chapter.

Moderate black holes

Let us first stay on the mass scale, which we want to gradually expand. Medium-weight black holes are the newest type on the mass scale. They have larger masses of 102 until 106 Solar masses. Recently there has been strong evidence that this intermediate type must exist in the center of globular clusters. The very topical question is whether the mysterious, ultra-luminous X-ray sources (ultra-luminous X-ray sources, ULXs) can be explained physically with massive black holes.

The candidate objects in the center of globular clusters are called

Globular clusters are ancient galactic components and are located in a spheroidal edge region of a galaxy, the galactic halo. About a hundred thousand stars form a spherical cluster, the star density of which increases sharply towards the center. Optically they look like the example on the right, the globular cluster M80, taken with the Hubble Space Telescope (Credit: Ferraro et al. 1999, AURA / STScI / NASA). M80 is one of about 150 globular clusters in the Milky Way and 28,000 light years away. Globular clusters are the oldest objects in a galaxy and reach the age of the universe. Due to their old age, globular clusters hardly contain any gas that could be accreted. Therefore, should they exist, their black holes must starve.
The massive black holes could have emerged from lighter stellar black holes, because frequent merger events in the dense globular star clusters. merging events) happened. They would then have gained their current high mass through accretion. The characteristic rotation curves of stars around the cluster center (the velocity dispersion curves) can be explained very elegantly with massive black holes. It would also be possible that whole stars are accreted. Then astronomers would have the chance to indirectly deduce the existence of a black hole in the associated high-energy bursts of radiation (X-ray bursts).
The problem with this nice idea: There are alternative explanations for most globular clusters, the without massive black hole. It is conceivable that a large collection of massive or compact stars (boson stars, fermion balls) sits in the center of the globular cluster, which would also explain the measured velocity dispersion. Clear evidence must therefore be awaited. In the case of G1, a globular cluster in our neighboring galaxy Andromeda, indications of a massive black hole of around 17,000 solar masses have in any case increased (Gebhardt et al. 2005).

In the smallest representatives of galaxies, the Dwarf galaxies candidates for black holes have now also been found.The method is also based on the measurement of the stellar velocity dispersion. These objects are called i.a.

With around 100,000 solar masses, these candidates for black holes lie precisely in the transition area between massive and supermassive black holes. Again, the evidence is weak. Overall, it can be provisionally balanced: The hypothesis of black holes of medium mass in globular clusters and dwarf galaxies is plausible, but the models are still very uncertain and a reliable verification of a closed mass gap remains to be seen. The existence of massive black holes in ULXs is still a very vague thesis because astronomers still don't know enough about the physics of these sources. In part, some ULXs can also be explained well with accreting stellar black holes.
From a theoretical point of view, it has long been a mystery why these masses should not also have black holes. The observations must conclusively support this hypothesis. The almost closed Mass hierarchy is certainly - from a theoretical point of view - extraordinarily attractive!

Super massive black holes

Super-massive black holes are usually centers of galaxies. In a unifying model, almost all astrophysicists today agree that every galaxy in the center a supermassive black hole supermassive black hole, SMBH) (there may be a few exceptions, e.g. where the galaxy core was torn away in a collision event.).
The masses of supermassive (also called superheavy) black holes can be found on the scale from millions to billions of solar masses! The maximum masses are derived from old elliptical galaxies, such as M87, which hardly contain any gas that could be accreted. These giant ellipses are based on a common evolutionary model from the merging of spiral galaxies (merging spirals) emerged and have gone through a violent development phase. The galactic collision processes can currently be read from numerous irregular galaxies. Such mixing processes promote the formation of new stars because the interstellar medium (ISM) of the galaxies involved is violently mixed. Astronomers recognize this by the blue coloration of such star formation regions. starburst regions). Young, massive stars of the spectral type O or B shine blue and blue-white.
The Schwarzschild radius of supermassive black holes is on the order of billions of kilometers or a few astronomical units. If one of these giants were to sit in the center of our solar system, its Schwarzschild radius would extend roughly to the orbit of the dwarf planet Pluto! These largest representatives of black holes are indeed gigantic, but in turn tiny relative to their host galaxy in which they are embedded.

The AGN standard model

A central supermassive black hole in particular is the vital ingredient of Active Galactic Nuclei (AGN) (see this web article for details). Because the hole feeds the enormous brightness of the AGN through accretion of plasma. That is the core message of the AGN paradigm. According to this standard model for active galaxies, the following structures are found: The cold, molecular material of the large-scale dust torus falls from the pc-scale (several tens to hundreds of light-years) into the heart of the AGN. The flow of matter develops a special shape, the so-called standard disc, which is a thin disc of matter. It gets hotter and hotter towards the center and emits heat radiation very efficiently. For this reason the material is cooled well and collects in the flat structure. In the inner area of ​​the pane, the material is finally extremely heated and therefore ionized. On a small scale (a few light years), the hot accretion flow inflates and is drawn into the black hole. The now very hot and thinned out accretion flow now takes on a new appearance: As a so-called ADAF, it is now gaining spatial thickness and can hardly be cooled by radiation.

The glow of the AGN

The radiation processes involved on this long journey are very diverse: thermal radiation is omnipresent and can radiate in the X-ray range in the hot plasma a few light years away from the black hole. Cold ambient radiation (from the cosmic background or the cold accretion disk) can also be Comptonized, i.e. low-energy photons become high-energy photons through inverse Compton scattering in hot gas. Charged particles that move against the background of existing magnetic fields generate synchrotron radiation and cyclotron radiation. This can also be Comptonized (Synchrotron Self Compton, SSC). In addition, there is bremsstrahlung, i.e. electromagnetic emission from slowed-down, charged particles. To put it in a nutshell: An active galactic nucleus is nothing more than an accreting, supermassive black hole, which makes its surroundings shine in all possible facets. In this context, astrophysicists like to speak of the black hole as an AGN engine. AGN engine). The brightest AGNs are the most luminous quasars, the (bolometric) luminosities of about 1047 reach erg / s. The observation photo at the top left shows the optical appearance of the luminous quasar 3C 273, taken with the Hubble space telescope (Credit: Martel et al. 2003, STScI / NASA). For comparison: our sun creates a luminosity of 1033 erg / s. Less bright AGNs such as the Seyfert galaxies come to about a hundredth of the quasar luminosity, i.e. about 1045 erg / s.

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Overview


© Andreas Müller, August 2007