Neither Einstein nor the great astrophysicist Arthur Eddington, who provided the first convincing evidence of the validity of Einstein’s general relativity theory and wrote one of the first treatises on Einstein’s theory of gravitation, believed in the existence of what we now call black holes in the 1930s. Yet, even back then, the work of Chandrasekhar and Oppenheimer with their students showed that sufficiently massive stars should not end their lives as white dwarfs or neutron stars, but rather gravitationally collapse into stellar black holes.
Today, black holes are found everywhere, including at the heart of large galaxies where they are supermassive, containing at least a million solar masses. But some are missing, modern cosmological theories suggest the existence of black holes with masses smaller than those of stars but we have not yet observed them, and assuming they exist, we do not know how many there are in a volume of the observable cosmos in a given mass range. It all depends on what happened during the Big Bang and the nature of a new physics that was unveiled at that time.
A Hunter of Gravitational Microlenses
This animation illustrates the concept of gravitational microlensing with a black hole. When the black hole appears to pass almost in front of a background star, the light rays from the source star bend due to the warped spacetime around the foreground black hole. This acts as a virtual magnifying glass, amplifying the brightness of the distant background star. Unlike when a star or a planet is the lensing object, black holes warp spacetime so much that it significantly alters the apparent position of the distant star in the sky. © Nasa
Exploring the deepest stratum of the observable Universe and this new physics, there is a very real possibility that the famous dark matter of the standard cosmological model is partly made up of primordial black holes. They have been hunted for years, especially based on the gravitational microlensing phenomenon. When passing in front of a star observed with a telescope, especially in space, a primordial black hole, usually undetectable when isolated and not engulfing matter that heats up enough to produce X-rays in an accretion disk, will act as a lens transiently intensively shining this star.
An article recently published in the renowned journal “Physical Review D,” and freely accessible on arXiv, reveals that the future Nancy Grace Roman Space Telescope would be able to detect these black holes with this effect, distinguishing them from rogue planets in the Milky Way capable of also producing gravitational microlensing, which are already known to exist in our Galaxy.
Scheduled to be launched in the mid-2020s, the Nancy Grace Roman Space Telescope, formerly known as WFIRST, will operate as the wide-eyed cousin of Hubble. While just as sensitive as Hubble’s cameras, the telescope’s 300-megapixel wide-field instrument will image an area of the sky 100 times larger. This means that a single image from the space telescope will contain the equivalent of 100 Hubble images.
The Nancy Grace Roman Space Telescope is an infrared space telescope.The infrared telescope developed by NASA, formerly known as Wide Field Infrared Survey Telescope (WFIRST), is set to be launched in 2027 into orbit around the Earth-Sun system’s Lagrange point L2. It is named after Nancy Grace Roman, the first woman to hold an executive position at NASA and a pioneer in space-based astronomical observation. Nancy Grace Roman passed away in December 2018 at the age of 93, leaving behind a legacy that includes projects like the Cosmic Background Explorer (CoBE) satellite and the Hubble Space Telescope.
As the first head of NASA’s astronomy department, Nancy Grace Roman paved the way for space telescopes and women in science. Her contributions include the creation of the Hubble Space Telescope. The Nancy Grace Roman Space Telescope, a groundbreaking wide-field study telescope named in her honor, is scheduled for launch by May 2027, promising to expand our understanding of the Universe.
Moving on to explore the theory of primordial black holes by Hawking, let’s delve deeper into the concept. One may wonder if the Chandrasekhar limit (the threshold mass beyond which a white dwarf must gravitationally collapse, determined by the Indian astrophysicist in the early 1930s) represents the smallest possible mass for a black hole to exist in the cosmos.
While the answer is almost, the reality is that for a given mass M, concentrating it within a spherical region with a radius determined by Rs=2GM/c², the Schwarzschild radius, would be sufficient for a black hole to form.The Schwarzschild radius is the minimum radius that an object can be compressed to in order to form a black hole (where G is the gravitational constant, and c is the speed of light).
However, since it requires enormous pressures only found in astrophysics, it seems that the Landau-Oppenheimer-Volkoff limit for a neutron star is not only the stability limit for a star that has exhausted its nuclear fuel but also the minimum mass for a black hole.
But this conclusion is incorrect! Stephen Hawking demonstrated this in 1971 using the work of astrophysics and cosmology leaders Yakov Zeldovich and Igor Novikov published in 1967.
In the context of Big Bang cosmological models, it is known that the “initial” density of the observable Universe was very high, and according to equations attempting to describe the state of matter and the gravitational field near the initial cosmological singularity in classical general relativity, the Universe was very turbulent with chaotic fluctuations in its metrics and density as shown by the works of Misner (known as the mixmaster universe model) as well as Belinsky, Khalatnikov, and Lifchitz.
Under these extreme conditions, if a density fluctuation causes a given mass to shrink below its Schwarzschild radius, a mini-black hole will result. In fact, given the speed limit of interaction propagation (the speed of light), if we consider a light bubble emitted by an area the size of the Planck length at the Planck time, where these values can be approximated as zero, then such a high density of matter (or pEnergy, as a gas of photons (photons would do just as well) could lead to a gravitational collapse at time t if a mass:
M(t) = c^3t/G = 10^15 (t/10^-23) g
is inside this bubble of light with a radius of ct. If the density fluctuation occupies a region larger than this bubble, gravitational interactions haven’t had time to propagate between these different parts since the “beginning” of the observable Universe’s birth, so the overdensity doesn’t “know” it should collapse.
Mini black holes can form with mass as low as the Planck mass, Mp = 10^-5 g, and beyond, since the mass of black holes that could appear a second after the Big Bang is 10^5 solar masses.
According to the cosmological model used to describe the birth of the observable cosmos, the spectrum of density fluctuations of matter/energy will not be the same. Therefore, “the size and number of primordial black holes currently existing will be valuable indicators to set limits on the turbulence and the type of cosmological model suitable for describing the first seconds of the cosmos’ history” before the isotropization and homogenization of spacetime geometry to end up being described by slight perturbations on a Friedmann Robertson-Walker background with a cosmological constant.
This is what Stephen Hawking was the first to understand, which led to two publications before his groundbreaking discovery in 1974. He even went further because, knowing the existence of solutions describing charged black holes, he postulated that some of the cosmic radiation particles could be made up of these mini black holes and that kinds of atoms were.Atoms, with such a mini-black hole at their center, could have formed. Moreover, as early as his 1971 article, he envisioned that the majority of the mass of the observable Universe could be in the form of primordial black holes. It was while studying the properties of these mini-black holes that he discovered that they could behave like elementary particles, or like unstable hot nuclei, disintegrating by emitting what was later named Hawking radiation. In fact, as he first showed in 1974, even black holes produced by stars should be capable of evaporating by emitting this radiation. Due to the laws of quantum mechanics, black holes must indeed evaporate by emitting particles faster the smaller they are. In reality, black holes appear as black bodies emitting particles at a temperature inversely proportional to their mass. The process is faster the smaller and hotter the black hole is. A black hole the mass of Earth would radiate like a black body at a temperature of about 0.02 K. It would be colder than the cosmic microwave background radiation. It couldn’t evaporate currently but, on the contrary, it would absorb this radiation to warm up, similar to how an ice cube absorbs heat in a glass of boiling water. Incidentally, when a black hole reaches the Planck mass due to its evaporation, Hawking’s calculations break down, and a theory of quantum gravity like string theory or loop quantum gravity needs to be involved. The ultimate fate of the evaporation of a mini-black hole is one of the major unresolved problems in modern theoretical physics. Generally, as a mini-black hole approaches the Planck mass, it can be considered the ultimate elementary particle, where all high-energy physics, particles, and forces unify with space-time.