Title: Primordial Black Holes and the First Stars
Authors: Julia Monika Koulen, Stefano Profumo, Nolan Smyth
First Author’s Institution: Department of Physics, University of California Santa Cruz
Status: Published in Physical Review Journals [closed]
Connecting Primordial Black Holes and the First Stars
Galaxies are composed of gas, stars, and central supermassive black holes, all embedded within a halo of dark matter. Understanding the origins of these components is key to unraveling how the Universe has evolved since the Big Bang. One theory, illustrated in Figure 1, proposes that the early Universe was filled with small, primordial black holes, formed from tiny density fluctuations shortly after the Big Bang. These primordial black holes would have potentially shaped the formation of large-scale cosmic structures and the growth of supermassive black holes through their gravitational influence. Where they clustered, the first stars would form, making primordial black holes a unique dark matter candidate that can bridge the gap to baryonic matter—the matter we can directly observe.
Figure 1: Schematic illustration of the role of primordial black holes in cosmic structure formation. Clusters of primordial black holes would serve as sites for the first stars to form, and over billions of years, these regions could give rise to galaxies. Image credit: ESA.
The first generation of stars, known as “Population III” stars, triggered the Epoch of Reionization—the period when the Universe transitioned from darkness to light. This paper considers a theoretical scenario: if the early Universe were filled with primordial black holes, how would they influence the formation of the first stars? Furthermore, is it possible to predict how massive and abundant these primordial black holes were?
Insights from Computer Simulations
Figure 2: The formation of stars, as suggested by the maximum hydrogen number density nH , over time, as measured through redshift. The authors run simulations with primordial black holes of varying mass and abundances (colored lines), compared to a run without primordial black holes (dotted black curve). Figure 1 in the original paper.
To test this hypothesis, the authors ran a large-scale cosmological simulation of structure formation in the early Universe, following the evolution of dark matter and gas under the standard cold dark matter model until the gas cooled and condensed enough to form the first Population III stars. They then re-simulated selected regions of interest at higher resolution, this time including primordial black holes of different masses mPBH and abundances fPBH. In the simulations, the primordial black holes could both interact gravitationally and accrete matter, releasing radiation into their surroundings. These two effects—gravity and radiative feedback—play opposing roles in star formation: whereas gravitational interactions can create overdensities that accelerate the collapse of gas, heating from accretion-driven feedback can suppress or delay the onset of star formation.
Figure 2 shows the hydrogen number density in the simulation regions over time as a proxy for the formation of Population III stars, for different masses and abundances of primordial black holes. More massive primordial black holes, of up to mPBH ~ 104 M⊙, lead to earlier and more rapid growth of hydrogen number densities in the simulations, regardless of their abundance. Therefore, more massive black holes in large numbers can significantly amplify star formation in the early Universe, likely via their gravitational influence. This effect persists in simulations with lower-mass black holes, down to mPBH ~ 10 M⊙, at sufficiently high abundances. However, a decrease in abundance leads instead to a delay in Population III star formation. This indicates that, for lower-mass black holes, radiative heating dominates over gravitational effects, leading to the suppression rather than the promotion of star formation.
Constraining the Properties of Primordial Black Holes
The authors then provide a framework for constraining the masses and abundances of primordial black holes based on the average hydrogen number densities required for Population III stars to form at different redshifts. The limits in Figure 3 can be used to complement data from gravitational wave observations and cosmic microwave background measurements, and could also be applied to cosmic epochs beyond the early Universe when subsequent generations of stars and galaxies form. In the future, expanding these simulations as well as including more realistic black hole mass distributions and more detailed gas physics, could lead to even stronger constraints.
Figure 3: Constraints on the masses and abundances of primordial black holes to form Population III stars, at different redshifts. The colored lines represent the upper limits for mPBH and fPBH . Figure 3 in the original paper.
This work presents a novel approach to testing theories of primordial black holes by examining their potential influence on the formation of the first stars. By comparing simulated star formation times with their expected occurrence, the authors show how future observations of the early Universe could further constrain the properties of primordial black holes.
Astrobite edited by Alexandra Masegian.
Featured image credit: “This artist’s concept takes a fanciful approach to imagining small primordial black holes. In reality, such tiny black holes would have a difficult time forming the accretion disks that make them visible here.” (from NASA Goddard Space Flight Center)
