Future Gravitational Wave Observatories Could See the Earliest Black Hole Mergers in the Universe

In February 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) confirmed they made the first-ever detection of gravitational waves (GWs). These events occur when massive objects like neutron stars and black holes merge, sending ripples through spacetime that can be detected millions (and even billions) of light-years away. Since the first event, more than 100 GW events have been confirmed by LIGO, the Advanced VIRGO collaboration, and the Kamioka Gravitational Wave Detector (KAGRA).

Moreover, scientists have found numerous applications for GW astronomy, from probing the interiors of supernovae and neutron stars to measuring the expansion rate of the Universe and learning what it looked like one minute after the Big Bang. In a recent study, an international team of astronomers proposed another application for binary black hole (BBH) mergers: using the earliest mergers in the Universe to probe the first generation of stars (Population III) in the Universe. By modeling how the events evolved, they determined what kind of GW signals the proposed Einstein Telescope (ET) could observe in the coming years.

The study was led by Boyuan Liu, a postdoctoral researcher at the Center for Astronomy of Heidelberg University (ZAH) and a member of the Excellence Cluster STRUCTURES program. He was joined by colleagues from the ZAH and the Institut für Theoretische Astrophysik at Heidelberg University, the Cambridge Institute of Astronomy, the Institute for Physics of Intelligence, the Institut d’Astrophysique de Paris, the Centre de Recherche Astrophysique de Lyon, the Gran Sasso Science Institute (GSSI), the Kavli Institute for Cosmology, the Weinberg Institute for Theoretical Physics, and multiple universities.

From Cosmic Dark to Dawn

Population III stars are the first to have formed in the Universe, roughly 100 to 500 million years after the Big Bang. At the time, hydrogen and helium were the most plentiful forms of matter in the Universe, leading to stars that were very massive and had virtually no metals (low metallicity). These stars were also short-lived, lasting only 2 to 5 million years before they exhausted their hydrogen fuel and went supernova. At this point, the heavier elements created in their cores (lithium, carbon, oxygen, iron, etc.) dispersed throughout the cosmos, leading to Population II and I stars with higher metallicity content.

Astronomers and cosmologists refer to this period as “Cosmic Dawn” since these first stars and galaxies ended the “Cosmic Dark Ages” that preceded it. As Liu explained to Universe Today via email, the properties of Pop III stars were sensitive to the peculiar conditions of the Universe during Cosmic Dawn, which were very different from the present-day conditions. This includes the presence of Dark Matter Haloes, which scientists believe were vital to the formation of the first galaxies:

“The timing of Pop III star formation reflects the pace of early structure formation, which can teach us about the nature of dark matter and gravity. In the standard cosmology model, cosmic structure formation is bottom-up, starting from small halos, which then grow by accretion and mergers to become larger halos. Pop III stars are expected to be massive (> 10 solar masses, reaching up to 1 million solar masses, while present-day stars have an average mass of ~ 0.5 solar masses). So, many of them will explode as supernovae or become massive black holes (BHs) when they run out of fuel for nuclear fusion.”

These Pop III black holes are further believed to be where the first supermassive black holes (SMBHs) in the Universe came from. As astronomers have demonstrated, SMBHs play an important role in the evolution of galaxies. In addition to assisting in the formation of new stars and encouraging galaxy formation in the early Universe, they are also responsible for shutting down star formation in galaxies ca. 2 to 4 billion years after the Big Bang, during the epoch known as “Cosmic Noon.” The growth of these black holes and the UV radiation emitted by Pop III stars reionized the neutral hydrogen and helium that permeated the early Universe.

This led to the major phase transition that ended the Cosmic Dark Ages (ca. 1 billion years after the Big Bang), allowing the Universe to become “transparent” as it is today. However, as Liu stated, how this process started remains unclear:

“Generally speaking, Pop III stars mark the onset of cosmic evolution from a starless (boring) state to the current state with rich phenomena (reionization, diverse populations of galaxies with different masses, morphologies, and compositions, andquasars powered by accreting supermassive BHs). To understand this complex evolution, it isessential to characterize its initial phase dominated by Pop III stars.”

Probing the Early Universe

The confirmation of gravitational waves (GW) was revolutionary for astronomers, and many applications have since been proposed. In particular, scientists are eager to study the primordial GWs created by the Big Bang, which will be possible with next-generation GW detectors like the Laser Interferometer Space Antenna (LISA). As Liu explained, existing GW detectors are mostly dedicated to studying binary black hole (BBH) mergers. The same is true of detectors expected to be built in the near future. Said Liu:

“The GW emission from a BH binary is stronger when they are closer. The GW emission carries away energy and angular momentum from the system such that the two BHs will get closer over time and eventually merge. We can only detect the GW signal at the final stage when they are about to merge. The time taken to reach the final stage is highly sensitive to the initial separation of the BHs. Basically, they have to start close (e.g., less than ~ 10% of the earth-sun distance for BHs below 10 solar masses) to merge within the current age of the Universe to be seen by us.”

The question is, how do two black holes get so close to each other that they will eventually merge? Astronomers currently rely on two evolutionary “channels” (sets of physical processes working together) to model this process: isolated binary stellar evolution (IBSE) and nuclear star cluster-dynamical hardening (NSC-DH). As Liu indicated, the resulting BBH mergers have distinct features in their merger rate and properties, depending on the channel they follow. They contain valuable information about the underlying physical processes.

“Knowledge of evolution channels is necessary to extract such information to fully utilize GWs as a probe for astrophysics and cosmology,” he added.

Modeling BBH Evolution

To determine how black holes come to form binaries that will eventually merge, the team combined both channels into a single theoretical framework based on the semianalytical model Ancient Stars and Local Observables by Tracing Halos (A-SLOTH). This model is the first publicly available code that connects the formation of the first stars and galaxies to observations. “In general, A-SLOTH follows the thermal and chemical evolution of gas along the formation, growth, and mergers of dark matter halos, including star formation and the impact of stars on gas (stellar feedback) at the intermediate scale of individual galaxies/halos,” said Liu.

They also used the Stellar EVolution for N-body (SEVN) code to predict how stellar binaries evolve into BBHs. They then modeled the orbit of each BBH in their respective dark matter halos and during halo mergers, which allowed them to predict when some BBHs will merge. In other cases, BBHs travel to the center of their galaxies and become part of a nuclear star cluster (NSC), where they are subject to disruptions, ejections, and hardening by gravitational scattering. From this, they followed the evolution of internal binary orbits to the moment of merger or disruption.

Next-Generation Observatories

As Lui explained, their results had significant theoretical and observational implications:

“On the theory side, my work showed that the isolated binary evolution channel dominates at high redshifts (less than 600 million years after the Big Bang) and the merger rate is sensitive to the formation rate and initial statistics of Pop III binary stars. In fact, the majority (> 84%) of BH binaries, especially the most massive ones, are initially too wide to merge within the age of the Universe if they evolve in isolation. But a significant fraction (~ 45 – 64%) of them can merge by dynamical hardening if they fall into NSCs. These predictions are useful for the identification and interpretation of merger origins in observations.”

In terms of observational results, they found that the predicted detection of Pop III BBH mergers is not likely to be discernible by current instruments like LIGO, Advance Virgo, and KAGRA, which generally observe BBH mergers closer to Earth. “[A]ltough Pop III mergers can potentially account for a significant fraction of the most massive BH mergers detected so far (with BHs above 50 solar masses),” said Liu. “It is difficult to learn much about Pop III stars and galaxies in the early Universe from the existing data because the sample size of detected massive mergers is too small.”

However, next-generation detectors like the Einstein Telescope will be more efficient in detecting these distant sources of GWs. Once completed, the ET will allow astronomers to explore the Universe through GWs back to the Cosmic Dark Ages, providing information on the earliest BBH mergers, Pop III stars, and the first SMBHs. “My model predicts that the Einstein Telescope can detect up to 1400 Pop III mergers per year, offering us much better statistics to constrain the relevant physics.”

The paper that describes their findings recently appeared online and is being reviewed for publication in the Monthly Notices of the Royal Astronomical Society.

Further Reading: arXiv