On the trail of invisible black holes: gravitational impact and effects on nearby stars

What is a black hole?

Black holes are astrophysical objects whose gravitational field is so intense that no light can escape, making them invisible. These objects 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 insurmountable limit for any particle or radiation.

Indirect observation through gravitational interactions

A black hole can be detected by its gravitational influence on its surroundings. In astrophysics, this translates into the study of the movement of stars or gas around a region where no light source is visible.

• Star velocity curves: By measuring the orbital velocities of stars near a compact region, one can deduce the mass of the central object. An excess of invisible mass is a strong indication of a black hole, such as at the center of the Milky Way with Sagittarius A*.
• Gravitational lensing: The deflection of light rays by the intense gravitational field can also reveal an isolated black hole via a gravitational lensing effect.

X-ray radiation from accretion disks

Black holes in the accretion phase attract surrounding matter, which forms a hot disk rotating at high speed. Internal friction raises the temperature of the disk to several million 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, rotation speed (spin), and structure of the environment near the black hole.

Radio observation and direct imaging

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), providing sufficient angular resolution to "see" the shadow of the black hole surrounded by the accretion disk.

Detection of gravitational waves

The coalescence of binary black holes produces gravitational waves detected by ground-based interferometers (LIGO, Virgo, KAGRA). These waves are deformations of spacetime, measured using ultra-sensitive laser interferometers capable of detecting length variations on the order of 10^-19 m.

The analysis of the waveforms allows extracting the masses, spins, and distances of the observed systems, providing a new observation channel to study the extreme physics of black holes.

Table: Summary of black hole observation methods

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

Have black holes always existed?

Black holes are not solely the result of stellar evolution: some may have existed since the first seconds following the Big Bang. These are known as primordial black holes. Unlike stellar black holes, these hypothetical objects would not have originated from the collapse of stars but from extreme density fluctuations in the primordial universe, amplified by rapid expansion during cosmic inflation.

According to physical models, certain regions of space could have locally exceeded a critical density, causing an immediate gravitational collapse. If these primordial black holes really 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 notably help explain a fraction of dark matter, although no direct detection has yet confirmed their existence.

In this sense, black holes are not only objects born from stars but could be witnesses to the extreme conditions of the primordial universe. Their study would thus allow testing fundamental theories of physics, such as inflation, quantum gravity, or unification models.

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).