Illustration of a black hole tearing the fabric of spacetime and the stars around it, with gravitational lensing effects. At the center, the dark singularity is surrounded by a blazing accretion disk of golden orange, distorted by extreme gravity according to the laws of general relativity.
Image source: astronoo.com
This article presents the methods for detecting black holes, astrophysical objects whose intense gravitational field makes them invisible. Four approaches are detailed: the study of gravitational interactions (stellar velocity curves, gravitational lensing), the detection of X-rays emitted by accretion disks heated to millions of kelvins (Chandra, XMM-Newton), VLBI radio imaging (Event Horizon Telescope, first image of M87's shadow in 2019), and the detection of gravitational waves from binary black hole mergers (LIGO, Virgo). The article distinguishes between stellar black holes, resulting from the collapse of massive stars, and primordial black holes, formed in the first seconds after the Big Bang and potential candidates for dark matter.
Black holes, although invisible, are detected by their gravitational effects on their surroundings and by the signals they generate. Four main methods are used: 1) gravitational influence (measuring the orbital speeds of stars around a compact region, such as Sagittarius A* at the center of the Milky Way; gravitational lensing effect); 2) X-ray radiation emitted by accretion disks (matter heated to millions of degrees as it falls into the black hole); 3) VLBI radio imaging (Event Horizon Telescope, which produced the first image of M87's shadow in 2019); 4) gravitational waves (spacetime distortions detected by LIGO and Virgo during binary black hole mergers). Black holes can be stellar (resulting from the collapse of massive stars) or primordial (formed in the first seconds after the Big Bang, hypothetical candidates for dark matter).
Black holes are astrophysical objects with a gravitational field so intense that no light can escape, making them invisible. They are formed by the gravitational collapse of the core of massive stars (supernova). This reality, anticipated as early as the 18th century by John Michell (1724 - 1793), is now rigorously described by general relativity, which defines the event horizon as the point of no return for any particle or radiation.
A black hole can be detected by its gravitational influence on its surroundings. In astrophysics, this is observed through the motion of stars or gas around a region where no light source is visible.
Black holes in the accretion phase attract surrounding matter, which forms a hot disk rotating at high speed. Internal friction heats the disk to millions of kelvins, causing the emission of X-rays detectable by specialized satellites (e.g., Chandra, XMM-Newton).
The spectrum and variability of X-ray radiation provide information about the mass, spin (rotation), and structure of the environment near the black hole.
The historic image of the shadow of the supermassive black hole in M87 by the Event Horizon Telescope (EHT) in 2019 was a major breakthrough. This global network of radio telescopes operates using very long baseline interferometry (VLBI), achieving an angular resolution sufficient to "see" the black hole's shadow surrounded by the accretion disk.
The coalescence of binary black holes produces gravitational waves detected by ground-based interferometers (LIGO, Virgo, KAGRA). These waves are distortions in spacetime, measured using ultra-sensitive laser interferometers capable of detecting length variations on the order of 10-19 m.
Analyzing the waveforms allows the extraction of the masses, spins, and distances of the observed systems, providing a new observation channel to study the extreme physics of black holes.
| Method | Physical Principle | Type of Detected Signal | Example Instruments |
|---|---|---|---|
| Gravitational Influence | Effect on the orbital motion of stars and gas | Velocity curves, gravitational lenses | Optical Observatories: VLT, Keck |
| X-ray Radiation from Accretion Disks | Heat due to friction and ionization in the disk | X-rays | Satellites: Chandra, XMM-Newton |
| VLBI Radio Imaging | High angular resolution interferometry | Direct image of the black hole's shadow | Event Horizon Telescope (EHT) |
| Gravitational Waves | Temporal distortions of spacetime during mergers | Gravitational signals in audio frequency | LIGO, Virgo, KAGRA |
Sources:
• Misner, C.W., Thorne, K.S., Wheeler, J.A., Gravitation, 1973.
• Abbott B. et al., Observation of Gravitational Waves from a Binary Black Hole Merger, Phys. Rev. Lett. 116, 2016.
• Event Horizon Telescope Collaboration, First M87 Event Horizon Telescope Results, Astrophys. J. Lett. 875, 2019.
• NASA Chandra X-ray Observatory, https://chandra.harvard.edu
Black holes are not only the result of stellar evolution: some may have existed since the first seconds after the Big Bang. These are known as primordial black holes. Unlike stellar black holes, these hypothetical objects would not result from the collapse of stars but from extreme density fluctuations in the early universe, amplified by rapid expansion during cosmic inflation.
According to physical models, certain regions of space may have locally exceeded a critical density, causing immediate gravitational collapse. If these primordial black holes truly existed (or still exist), they could have a wide range of masses, from less than an asteroid to several thousand solar masses. Their presence could help explain a fraction of dark matter, although no direct detection has yet confirmed their existence.
In this sense, black holes are not just objects born from stars but could be witnesses to the extreme conditions of the early universe. Their study would thus allow testing fundamental theories of physics, such as inflation, quantum gravity, or unification models.
| Characteristic | Primordial Black Holes | Stellar Black Holes |
|---|---|---|
| Origin | Density fluctuations in the early universe, post-Big Bang | Gravitational collapse of the core of massive stars after a supernova |
| Formation Period | Within the first second after the Big Bang | Hundreds of millions of years after the Big Bang (after the formation of massive stars) |
| Mass Range | From ~10-5 g (Planck mass) to several thousand solar masses | From a few to tens of solar masses |
| Observation | Hypothetical to date, no direct detection | Confirmed by X-ray radiation, gravitational waves, stellar dynamics |
| Potential Cosmological Role | Possible dark matter candidates; testing physics beyond the Standard Model | Common products of stellar evolution in galaxies |
References:
• Carr B.J., Hawking S.W., Black holes in the early Universe, MNRAS, 168, 399–416 (1974).
• Carr B.J., Kühnel F., Primordial Black Holes as Dark Matter Candidates, Annual Review of Nuclear and Particle Science, 70, 355–394 (2020).
• Sasaki M. et al., Primordial Black Holes—Perspectives in Gravitational Wave Astronomy, Classical and Quantum Gravity, 35(6), 063001 (2018).
• Zel’dovich Y.B., Novikov I.D., Relativistic Astrophysics Vol. 1, University of Chicago Press (1971).
• Abbott B. et al. (LIGO Scientific Collaboration and Virgo Collaboration), GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo During the Second Part of the Third Observing Run, Phys. Rev. X 11, 021053 (2021).
Stellar black holes are formed by the gravitational collapse of the core of massive stars (more than 20 solar masses) after a supernova explosion. Their typical mass ranges from a few to several tens of solar masses, and they form hundreds of millions of years after the Big Bang. Primordial black holes are hypothetical objects that may have formed in the first seconds after the Big Bang, from extreme density fluctuations amplified by cosmic inflation. They could have a wide range of masses (from 10-5 g to several thousand solar masses) and are excellent candidates for explaining part of dark matter, although no direct detection has yet been confirmed.
When a black hole is in the accretion phase, it attracts surrounding matter (gas, dust) that forms a spiral accretion disk. Internal friction and tidal forces heat this disk to temperatures of several million kelvins, causing the emission of intense X-ray radiation. This radiation is not emitted by the black hole itself but by the matter before it crosses the event horizon. Specialized satellites such as Chandra (NASA) and XMM-Newton (ESA) detect these X-rays. Analyzing the spectrum and variability of this radiation allows the deduction of the black hole's mass, spin (rotation), and other properties.
The Event Horizon Telescope (EHT) is not a single telescope but a global network of radio telescopes (spread across several continents) that operates using very long baseline interferometry (VLBI). This technique combines signals from multiple distant telescopes to achieve an exceptional angular resolution (equivalent to that of a telescope the size of the Earth). In 2019, the EHT produced the first direct image of the shadow of the supermassive black hole at the center of the M87 galaxy, located 55 million light-years away. The image shows a dark region (the black hole's shadow) surrounded by a bright ring, formed by matter from the accretion disk whose light is bent by the black hole's intense gravity.
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