Into the Void: Black holes in the early universe

Surprisingly, some of the brightest things in the universe are not necessarily found at Yale.

Quasars, even those that are billions of light years away, are some of the “brightest beacons” in the universe. Yet how can quasars radiate so much energy that they can be seen from Earth? One explanation is that at each quasar’s center is a growing supermassive black hole (SMBH).

Black holes feed from accretion disks: disks of matter formed by particles that collect around the singularity. But what observable evidence do we have for this? “During the process of falling in, [particulate matter] is heated to temperatures of 107 to 108 Kelvin,” says Priyamvada Natarajan, Professor of Astronomy and Physics at Yale. “Some fraction of the rest mass energy of [this matter] gets converted into radiation.” This hyper-efficient energy transformation is empirically understood in Einstein’s famous equation: E=mc2. This radiation is observed as electromagnetic waves between visible light and X-rays, and it allows scientists to infer the presence of SMBHs as highly luminous quasars.

Using the quasar’s luminosity, astrophysicists can accurately determine a black hole’s mass. Some quasars are so bright that they indicate that their powering black holes are almost a billion times larger than the sun. But, despite their intimidating masses, black holes actually have a limit on their growth. Black holes reach a “sweet spot”—an intricate balance between the pull of gravity on particles and the push of the disk’s radiation. This limit, known as the Eddington limit, places an upper bound on how big black holes can get in a certain period of time. But within five hundred million years (a relatively short time) of the big bang, SMBHs have been observed with masses surpassing this limit.

Tal Alexander, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel, and Priyamvada Natarajan recently published research in Science to provide a different mechanism for black hole growth—one that accounts for super-Eddington accretion, therefore explaining the existence of SMBHs. One of the Eddington limit’s constraints is that only the black hole’s gravity is significant. In the early universe, the first structures to form were likely extremely gas-rich star clusters. In such an environment, light black hole ‘seeds’ (one to ten solar masses) can zigzag through dense cold gas, pulled by the more massive stars’ gravity. As they random walk through the gas rich star, cluster black hole ‘seeds’ subsonically sweep up gas preventing the formation of an accretion disk. In this new configuration, the black hole takes in matter akin to a wind—accreting more mass faster than by having an accretion disk. After their calculations, Alexander and Natarajan found that the accretion rate for this geometry is super-Eddington.

This mechanism explains how the first quasars were formed. Not only does it offer a new black hole accretion model, but it also challenges previous accretion models. Most importantly, however, the mechanism provides an explanation for the SMBHs seen in the early universe.


Cover Image: An artist’s rendition of accretion lines from a black hole’s accretion disk, courtesy of WikiCommons