The killer black hole

The vastness of the universe resembles an empty ocean with galaxies acting as rare islands. However, this perception is deceptive. Only a small fraction of all atoms are actually found within galaxies, while the majority is believed to drift in the intergalactic medium. Similar to the roots of a massive tree, gas emanates from each galaxy, as gravity funnels fresh mass into this dense cosmic forest. In the intergalactic medium, the building blocks of creation, such as hydrogen and helium, are present, forming sheets and filaments that flow into galaxies and eventually give birth to stars. Yet, upon closer inspection, we discover that quasars are the dominant forces in shaping the universe. These immensely powerful objects, residing at the centers of some galaxies, are as small as a grain of sand compared to the Amazon River, yet they shine with the brilliance of a trillion stars. They emit massive jets of matter, reshaping the cosmos around them and even capable of extinguishing entire galaxies. But what exactly are quasars, and how do they influence the structure of the universe according to their own will?

In the 1950s, astronomers observed enigmatic radio waves emanating from various spots in the sky. These celestial objects, known as quasars, appeared as star-like dots in radio waves rather than visible light. They exhibited peculiar characteristics—some flickered, others emitted high-energy X-rays in addition to radio waves—and all were remarkably tiny. Moreover, they exhibited incredibly fast motion, moving at speeds exceeding 30% of the speed of light. The only plausible explanation was that these objects were located at immense distances, with their apparent speed being a result of the universe's expansion. Consequently, quasars were identified as the active cores of galaxies billions of light-years away, rather than mere stars. Astonishingly, quasars outshone the entire Milky Way galaxy by thousands of times, radiating with an intensity that surpassed comprehension. They were monstrous entities, unleashing unprecedented violence and energy into the void. Over a million quasars have been discovered as our sky was mapped, and they all seemed to be situated at vast distances. In astronomy, looking far away means peering into the distant past, as the light from these objects takes an immense amount of time to reach us. Quasars were particularly abundant in the early universe, reaching their peak about 10 billion years ago when galaxies and the universe itself were still in their infancy. Let's travel back in time to about 3 billion years after the Big Bang to explore the conditions prevalent at that stage.

The extraordinary luminosity and turbulence exhibited by early galaxies raise questions about their nature. The tremendous amount of light and radiation emitted by these galaxies couldn't solely be attributed to stars, as there were not enough of them to account for the observed brilliance. Additionally, since galaxies tend to grow over time through mergers, the starlight from smaller galaxies shouldn't have surpassed the brightness of any present-day galaxy. The only plausible mechanism capable of generating such vast energy outputs is the feeding of supermassive black holes. Although the exact process of their formation remains unknown, it appears that every galaxy harbors a supermassive black hole at its center. Contrary to the common perception that the light of a quasar emanates from within the black hole, it actually originates from the surrounding space—a massive swirling disk of gas known as an "accretion disk." Quasars utilize matter as fuel, much like stars do, but black holes are the most efficient engines for converting matter into energy in the entire universe. The energy released by matter falling into a black hole can be 60 times greater than that released through nuclear fusion in a

star's core. Unlike nuclear reactions, which power stars, a black hole's energy is derived from gravity. Matter falling into a black hole accelerates to nearly the speed of light before crossing the event horizon, possessing an immense amount of kinetic energy. However, once inside the black hole, this energy is retained. Thus, only matter that falls into the black hole in a specific manner allows us to observe this released energy. If matter falls straight down, the outside universe remains unaffected. However, when a substantial amount of matter spirals inwards rapidly, forming a disk, it collides and experiences friction, heating the disk to hundreds of thousands of degrees. In an area not much larger than our solar system, the core of a galaxy can emit energy many times greater than that produced by all its stars combined. This is the essence of a quasar—a supermassive black hole indulging in a feast. These black holes consume a considerable amount of matter, with typical quasars devouring anywhere from one to a hundred times the mass of the Earth in gas every minute. At the time when the universe was about a third of its current size, approximately ten billion years ago, the intergalactic medium was less dispersed. This allowed the filaments of gas around quasars to abundantly feed them, leading to the expulsion of immense amounts of light and radiation.

The brightest quasars generate powerful jets that twist the magnetic field of the surrounding matter, directing the matter into narrow cones. Resembling particle accelerators, these jets propel massive beams of matter, traversing the circumgalactic medium and forming plumes that expand to hundreds of thousands of light-years. The scale of this phenomenon is nearly incomprehensible—a minuscule region within a galaxy shaping patches of the universe extending over hundreds of thousands of light-years. However, quasars cannot sustain their feeding frenzy indefinitely, usually lasting only a few million years before their banquet ultimately devastates their host galaxy.

While the term "killing" might be an exaggeration, the influence of quasars on galaxies is profound. The intense heat generated by quasars disrupts and prevents the formation of stars, effectively transforming galaxies. Hot gas cannot condense to form stars, contrary to the common understanding that stars originate from hot, collapsing gas. In a hot gas cloud, atoms move at high speeds, resulting in powerful collisions that create pressure resisting the gravitational collapse necessary for star formation. Instead, cold gas is the ideal medium for star formation, as it readily collapses under gravity's influence. Additionally, quasars expel gas from their galaxies, depriving both themselves and their host galaxies of the raw materials required for new star formation. While this may seem unfortunate, it could potentially benefit life. An excess of stars can be hazardous, as the subsequent formation of new stars is often accompanied by massive supernova explosions that could sterilize planets. However, the interplay between different factors within the galactic environment is complex, akin to the intricacies of Earth's biosphere, where each component depends on and influences every other part. While hot phenomena like quasars and supernovae tend to drive gas out of galaxies, shockwaves and quasar jets can compress gas, temporarily giving rise to new stars. Gas that leaves the galaxy also mixes with incoming gas, recycling it back into the galactic system. In general, the moderation of these processes is crucial for our existence today.

This leads us to an intriguing question: Did the Milky Way have a quasar phase in the past? Although it remains uncertain whether every galaxy experienced a quasar phase, studying distant quasars may provide insights into the Milky Way's history. Preserving a galaxy's history is challenging, as it tends to mix away the evidence of its past, similar to how

the churning waves on a beach erode and blend sand. There is a possibility that the Milky Way was once a quasar, potentially contributing to the growth of our supermassive black hole, Sagittarius A*, which currently has a mass approximately 4 million times that of the Sun. However, the ancient history of Sagittarius A* remains unknown. Furthermore, as dormant as it may be now, Sagittarius A* could potentially transform into a quasar in the future. In a few billion years, the Milky Way will merge with the Andromeda galaxy, a process that has been observed in over a hundred "double quasars" formed during galactic collisions, where fresh gas is supplied to the central black holes. Although the outcome of this merger is uncertain, it would undoubtedly be a remarkable spectacle. Perhaps beings in the distant future will witness this event and be in awe of its grandeur. Nevertheless, there is already an abundance of fascinating phenomena to explore on Earth, at the present moment, if one possesses the knowledge to comprehend them.