PTAs do not detect gravitational waves in the same way as observatories such as LIGO, Virgo, or KAGRA. Instead of using kilometer-scale laser interferometers to sense waves with frequencies of hundreds of hertz, PTAs turn the Galaxy into a vast detector. They observe millisecond pulsars—rapidly rotating neutron stars whose radio pulses arrive with extraordinary regularity. A passing gravitational wave slightly stretches and compresses spacetime, changing the apparent arrival times of those pulses by tiny amounts. When dozens of pulsars are monitored over many years, correlated timing variations can reveal a stochastic background: not one isolated cosmic collision, but the combined signal of countless unresolved sources. International PTA collaborations have reported compelling evidence for such a background at nanohertz frequencies, where the leading explanation is a population of orbiting supermassive black-hole binaries.

These binaries are expected to form when galaxies merge and bring their central black holes together. As the two black holes orbit, they emit gravitational radiation and gradually lose energy, causing their separation to shrink. The most powerful sources in the nanohertz band are expected to contain black holes with total masses of roughly a billion times that of the Sun or more. Yet this explanation immediately raises a difficult question. If the Universe’s largest black holes grew from smaller ancestors, how could those initial seeds become so massive so quickly? Observations by the James Webb Space Telescope and the Chandra X-ray Observatory have identified unexpectedly massive black-hole candidates at very high redshifts, intensifying the debate over whether ordinary stellar remnants could have assembled the earliest giants rapidly enough.

In their Physical Review D study, Ghodla and Ilie examine whether the origin of those seeds could influence the gravitational-wave background billions of years later. Their analysis follows the subsequent evolution of black holes formed through two proposed early-Universe channels. The first is direct collapse, in which enormous clouds of primordial gas avoid fragmenting into ordinary stars and collapse almost directly into black holes. The second involves supermassive Dark Stars, hypothetical primordial objects powered not mainly by nuclear fusion but by energy released through interactions involving dark matter. In the WIMP dark-matter scenario explored by the researchers, dark-matter heating could support a large, relatively cool and extended star while it continues to draw in gas. Such an object might grow to a mass of a million Suns or more before ultimately collapsing into a massive black-hole seed.

The importance of these two channels lies not only in the size of their seeds, but also in their expected abundance. A larger initial black hole can reduce the amount of subsequent growth required to reach the masses observed in the modern Universe. However, a sufficiently numerous population of massive seeds could also produce too many later mergers, creating a gravitational-wave background stronger than the one measured by PTAs. The researchers therefore model the halos hosting the early seeds, track the cosmic growth and merger histories of their black-hole descendants, and calculate the gravitational-wave spectrum expected from the resulting binary population. Their calculations indicate that supermassive Dark-Star remnants with a number density of approximately 10^-3 per cubic megaparsec could make a substantial, potentially dominant contribution to the observed nanohertz signal.

The direct-collapse scenario examined in the study produces a markedly weaker background. The reason is primarily demographic: the direct-collapse black holes considered by the researchers are expected to be far rarer, with characteristic number densities near 10^-6 per cubic megaparsec. Even if individual seeds are massive, too few of them would eventually find partners, form binaries, and merge in sufficient numbers to generate a strong stochastic signal. By contrast, a more abundant population of Dark-Star remnants would provide many more opportunities for descendant black holes to enter galactic nuclei, pair up after galaxy mergers, and radiate gravitational waves. The difference illustrates how a gravitational-wave background can depend not only on the mass of the sources, but also on the population statistics and environments in which those sources formed.

The study also turns PTA measurements into a possible census of objects that existed when the Universe was less than a few hundred million years old. For the models considered, seed densities in the approximate range of 10^-2 to 10^-1 per cubic megaparsec would begin to overproduce the measured gravitational-wave background. The exact limit depends strongly on the dark-matter halo masses associated with the seeds, because the halo environment affects black-hole growth, galaxy assembly, merger rates, and the eventual mass distribution of the binaries. In this sense, PTAs could constrain a population at redshifts greater than 10 even though the mergers responsible for most of the signal take place much later, after billions of years of cosmic evolution.

The calculations further support the conclusion that binaries with total black-hole masses above about 10^9 solar masses dominate the predicted PTA signal. Lower-mass systems can be numerous, but their individual gravitational-wave emission is substantially weaker, and their combined contribution at nanohertz frequencies is comparatively limited. This mass dependence offers an important physical explanation for why early massive seeds are so relevant. If the first black holes were born with enough mass—or grew rapidly enough—to become members of extremely massive binary systems, their descendants could leave a measurable imprint on today’s pulsar timing data. If instead the early Universe produced mostly small seeds, later growth and mergers would have to build the largest black holes, potentially changing both the amplitude and the shape of the gravitational-wave background.

The implications extend beyond one proposed type of primordial star. A successful explanation of the PTA signal must be tested against observations of galaxy populations, black-hole masses, quasar activity, dark matter, and the timing data themselves. Future PTA observations should improve the measurement of the background’s amplitude and spectral shape, while larger pulsar samples and longer observing campaigns may help identify departures from the simplest population models. At the same time, JWST and other observatories will continue searching for black holes and luminous galaxies at cosmic dawn. If the gravitational-wave background is stronger or structured in a way that favors an abundant population of massive descendants, it could provide indirect evidence that supermassive Dark Stars once existed—even if the original stars are too distant and faint to observe directly.

The proposed scenario is therefore a striking example of how the Universe can preserve ancient history in unexpected forms. A dark-matter-powered object that vanished at the beginning of cosmic time might be impossible to see today, yet its collapsed remnant could grow inside a galaxy, merge with another supermassive black hole, and contribute to spacetime vibrations detected by pulsars in the present-day Milky Way. “Pulsar timing arrays are usually thought of as probes of supermassive-black-hole binaries in the relatively recent Universe,” Cosmin Ilie said. “What our work shows is that the signal may also contain information about how the ancestors of those black holes formed at cosmic dawn.” If future measurements confirm the connection, the nanohertz sky could become a new archaeological record of the first black-hole seeds—and a powerful test of the exotic physics that may have shaped the earliest luminous objects in existence.

Subject of Research: Early supermassive black-hole seeds, supermassive Dark Stars, direct-collapse black holes, and the nanohertz gravitational-wave background detected by Pulsar Timing Arrays.

Article Title: Reconstructing PTA measurements via early seeding of supermassive black holes

News Publication Date: 19-Aug-2026

Web References: https://doi.org/10.1103/hvfd-8fkr

References: Ghodla, S. and Ilie, C., “Reconstructing PTA measurements via early seeding of supermassive black holes,” Physical Review D, published 17 August 2026.

Image Credits: Ghodla and Ilie, Physical Review D (2026).

Keywords

Supermassive black holes, Dark Stars, direct-collapse black holes, gravitational waves, Pulsar Timing Arrays, nanohertz astronomy, cosmic dawn, dark matter, black-hole seeds, galaxy mergers, James Webb Space Telescope, astrophysics

For the first time, DR20 includes optical spectra obtained with the BOSS spectrograph at Las Campanas Observatory in Chile, complementing observations made at Apache Point Observatory in New Mexico. The two facilities give SDSS-V access to both hemispheres, allowing astronomers to survey a wider and more representative portion of the sky. By combining this coverage with robotic fiber-positioning technology, researchers can observe thousands of objects efficiently and return repeatedly to selected targets whose brightness or spectral behavior changes over time.

One of the release’s most powerful components is SPIDERS, short for SPectroscopic IDentification of eROSITA Sources. The program links optical spectra from SDSS with X-ray detections made by the eROSITA space telescope. Approximately 200,000 X-ray sources now have optical identifications and measured redshifts, which reveal how far away they are and how much energy they produce. This creates the largest and most uniform spectroscopic follow-up sample of X-ray sources assembled to date, turning a map of high-energy points of light into a three-dimensional survey of the energetic Universe.

Most of the X-ray sources in this sample are active galactic nuclei, powered by material spiraling into supermassive black holes at the centers of galaxies. As gas falls inward, it forms a hot accretion disk and can generate intense radiation across the electromagnetic spectrum. X-rays often arise from a high-temperature region known as the corona, located above the disk. Because X-rays can reveal the central engine even when visible light is obscured, they provide astronomers with an important way to identify black holes hidden behind thick clouds of gas and dust.

The combined SDSS and eROSITA data are already reshaping estimates of how black holes grew through cosmic time. Astronomers use the X-ray luminosity function—a statistical census of black holes divided by brightness and distance—to track the population from the nearby Universe to redshifts approaching six. At those distances, the light began its journey when the Universe was less than a billion years old. The survey finds that extremely luminous quasars were more common in the early Universe than many previous studies suggested, indicating that enormous black holes were able to gain mass at remarkable speeds during cosmic dawn.

The observations also reveal a substantial population of black holes that optical surveys alone fail to detect. Dust and gas can absorb ultraviolet and visible radiation, concealing an active nucleus from conventional searches. Yet even X-ray observations do not capture the entire story. When researchers calculate the total mass accumulated by supermassive black holes over cosmic history, they find that black holes selected through soft X-ray emission account for only a minority of the growth seen in the local Universe. The result implies that roughly 70 to 90 percent of supermassive black-hole growth occurred behind heavy obscuring material or during phases in which X-ray emission itself was strongly suppressed.

DR20 also follows quasars as they change. The Black Hole Mapper’s Reverberation Mapping program repeatedly observes selected targets and measures the time delay between variations in the accretion disk and the response of the broad-line region, a rapidly moving cloud of gas surrounding the black hole. Because the delay reflects the size of this region, and the gas velocity can be measured from broadened emission lines, researchers can estimate black-hole masses using a form of geometric physics. The method offers a way to weigh distant black holes that cannot be directly imaged.

Another time-domain effort, the All-Quasar Multi-Epoch Spectroscopy program, monitors tens of thousands of quasars over multiple observing epochs. Its data can reveal changing outflows, possible pairs of orbiting supermassive black holes and “changing-look” quasars that unexpectedly switch between active and apparently dormant states. Some spectra proved so unusual that they disrupted established automated analysis pipelines. Scientists therefore visually inspected the most challenging observations, creating a valuable catalog of rare and abnormal objects that could expose physical processes missing from standard models of quasar activity.

The scale and flexibility of the project are made possible by SDSS-V’s Robotic Focal Plane System, installed on both the Sloan Foundation Telescope at Apache Point Observatory and the du Pont Telescope at Las Campanas Observatory. Hundreds of robotic positioners arrange optical fibers across the focal plane, directing light from selected galaxies, stars and quasars into the BOSS spectrographs. The system can be reconfigured rapidly, supporting repeated observations and Target of Opportunity campaigns when transient or rapidly changing sources demand immediate attention. All DR20 Black Hole Mapper products are publicly available through SDSS archive services, including spectra, catalogs, X-ray counterparts, visual classifications and analysis tools. Researchers, educators and the public can explore the data through web interfaces, SQL queries and Python-based notebooks, making this vast record of cosmic black-hole activity accessible far beyond the original collaboration.

Subject of Research: Supermassive black holes, quasars, active galactic nuclei, X-ray sources and cosmic black-hole growth.

Article Title: SDSS Data Release 20 Reveals Hidden Growth and Unexpected Abundance of Early-Universe Black Holes

Web References: SDSS Data Release 20; SDSS Data Release 20 announcement

References: Merloni, A., Lamer, G., Liu, T., et al. (2024). The SRG/eROSITA all-sky survey: First catalog of X-ray sources (eRASS1). Astronomy & Astrophysics, 682, A34. ADS Abstract; Publisher Full Text

Image Credits: SDSS-V, Scott Anderson, University of Washington

Keywords

SDSS-V, Data Release 20, Black Hole Mapper, supermassive black holes, quasars, active galactic nuclei, eROSITA, SPIDERS, X-ray astronomy, reverberation mapping, cosmic dawn, galaxy evolution, astronomical spectroscopy