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Observational Manifestations of Primordial Objects in the Early Universe Through the Hydrogen Subordinate Lines

This paper proposes a new mechanism where primordial massive compact objects at redshifts $z \sim 100$ –$1000$ emit few-eV photons that populate excited hydrogen levels, creating observable circular Fraunhofer-like absorption or emission features in the cosmic microwave background via subordinate hydrogen lines in the radio wavelength range.

Original authors: Viktor K. Dubrovich, Yury N. Eroshenko, Stanislav I. Shirokov

Published 2026-08-18

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CC BY 4.0

Original authors: Viktor K. Dubrovich, Yury N. Eroshenko, Stanislav I. Shirokov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

To understand the universe as it was in its earliest moments, astronomers face a profound challenge: the period known as the "Dark Ages." This era began shortly after the Big Bang, when the universe cooled enough for hydrogen atoms to form, but before the first stars and galaxies ignited to light up the cosmos. During this time, the universe was filled with a vast, dark sea of neutral hydrogen gas, permeated by the faint, cooling afterglow of the Big Bang itself, known as the cosmic microwave background. Because there were no bright stars to illuminate this gas, and because the light from that distant epoch has stretched into wavelengths invisible to optical telescopes, this era has remained largely hidden from direct view. Scientists have long hoped to find a way to peer into this darkness, not by seeing the objects themselves, but by detecting how those objects might have disturbed the surrounding gas and the ancient light passing through it.

In a new study, researchers propose a specific method to detect massive, invisible objects from this ancient era by looking for subtle shadows they cast on the cosmic microwave background. The team suggests that if massive primordial black holes or similar exotic objects existed during the Dark Ages, they would have emitted radiation capable of exciting hydrogen atoms in their vicinity. This excitation would cause the hydrogen to absorb specific frequencies of the background radiation, creating a distinct, ring-like pattern of absorption that could be detected by modern radio telescopes. Rather than seeing the object directly, astronomers would see a "Fraunhofer-like" absorption spectrum—a dark ring in the radio sky—revealing the presence of a massive, hidden entity that has been invisible until now.

The researchers focus on a specific window of time in the universe's history, roughly between 100 and 1,000 times the current age of the universe, a period when the first massive structures were beginning to form. In the standard model of cosmology, the universe during this time was a uniform, dark fog of gas. However, if a massive object, such as a primordial black hole, existed within this fog, it would have acted as a powerful source of energy. As matter fell toward such an object, it would heat up and emit radiation, including ultraviolet light. This light would travel outward, striking the surrounding hydrogen atoms.

Normally, the hydrogen atoms in the early universe would remain in their lowest energy state, or ground state, because the background radiation of the universe was too weak to push them to higher energy levels. The researchers calculate that the radiation from a massive object, however, would be strong enough to pump a significant number of these atoms into higher, "subordinate" energy levels. This process is similar to how a specific type of light can make certain materials glow, but in this case, the effect is to create a population of excited atoms that would not otherwise exist in such numbers.

Once these atoms are excited, they interact with the cosmic microwave background in a unique way. Because the gas surrounding the object is slightly cooler than the background radiation itself, the excited hydrogen atoms act as a filter. They absorb specific frequencies of the background radiation as it passes through the cloud of gas surrounding the object. This creates a shadow, or an absorption line, in the spectrum of the cosmic microwave background. The researchers describe this as a "Fraunhofer-like" absorption, a term borrowed from the dark lines seen in the spectrum of the Sun, which are caused by cooler gases absorbing specific colors of light. In this cosmic scenario, the "Sun" is the entire background of the universe, and the "cooler gas" is the cloud surrounding the hidden massive object.

The study identifies that these absorption features would appear in the radio part of the electromagnetic spectrum. This is a crucial detail because, while the light from the early universe has been stretched by the expansion of the cosmos, the specific transitions between these high energy levels of hydrogen land in a frequency range that modern radio telescopes can detect. The researchers point out that these signals would appear as circular or ring-shaped features in the sky, centered on the location of the massive object. The size of these rings would be determined by the distance the radiation from the object could travel before the gas became too thin to interact, a region that could span tens of thousands of light-years.

The authors emphasize that this method offers a way to detect objects that are otherwise impossible to see. Traditional telescopes cannot see the objects themselves because they are too distant and too faint, and the light they emit is shifted into wavelengths that are blocked by Earth's atmosphere or are simply too weak to detect. However, by looking for the shadow these objects cast on the cosmic background, astronomers can infer their existence. The researchers calculate that the depth of this absorption shadow would be significant enough to be detected by planned large-scale radio telescopes, such as the Square Kilometre Array or the upgraded Giant Metrewave Radio Telescope.

The paper also addresses potential obstacles to this detection. One concern is whether the gas around these objects would be heated too much by the radiation, which would reduce the contrast between the gas and the background radiation. The researchers show that at the redshifts they are considering, the gas remains cool enough relative to the background radiation for the absorption effect to be strong. They also rule out other competing signals, such as the thermal Sunyaev-Zel'dovich effect or free-free emission, showing that these would be too weak to mask the absorption signal they are looking for.

The study suggests that if such massive primordial objects exist, they would leave a distinct fingerprint on the universe that we can now hope to find. The researchers do not claim to have found these objects yet, but they provide a concrete roadmap for how to look for them. They argue that the necessary conditions for this effect to occur are plausible, even if the objects themselves are rare and exotic. The existence of such objects would provide direct evidence for the formation of massive black holes in the very early universe, a topic that remains one of the great mysteries of cosmology.

The authors note that the detection of these signals would be a major breakthrough, offering a glimpse into the "Dark Ages" that has never been possible before. It would allow scientists to test theories about how the first structures in the universe formed and whether primordial black holes played a role in that process. The study concludes that the effect is within the reach of current and future technology, provided that astronomers know exactly what to look for. By tuning their radio telescopes to the specific frequencies where these absorption lines should appear, and by scanning the sky for the characteristic ring-shaped shadows, the scientific community may soon be able to illuminate the darkest corners of our cosmic history.

This approach represents a shift in how astronomers might explore the early universe. Instead of trying to see the faint, distant light of the first stars or galaxies, they propose looking for the absence of light in a very specific pattern. It is a method that turns the entire cosmic background into a screen, against which the shadows of the universe's first massive objects can be projected. If successful, this technique could reveal a population of objects that have been hidden in plain sight, waiting for the right kind of observation to bring them into focus. The researchers have laid out the physical mechanism, the expected signal, and the tools needed to find it, turning a theoretical possibility into a tangible target for the next generation of astronomical discovery.

Technical Summary: Observational Manifestations of Primordial Objects in the Early Universe Through the Hydrogen Subordinate Lines

Problem Statement
The "Dark Ages" of the Universe (redshifts $z \approx 1100$ to $z \approx 30$ ) represent a period where the Universe was largely devoid of bright, directly observable sources, rendering standard optical and X-ray astronomy ineffective. While the 21 cm line of neutral hydrogen offers a potential probe, the detection of massive primordial compact objects (such as Primordial Black Holes, PBHs) remains a significant challenge. Existing models often assume a homogeneous distribution of energy sources (e.g., decaying dark matter), failing to account for isolated, massive radiation sources that could locally alter the state of the intergalactic medium. Furthermore, direct observation of high-redshift objects is hindered because their primary emission lines (e.g., Lyman- $\alpha$ ) are redshifted into the far-infrared, a range blocked by Earth's atmosphere and difficult to access from the ground.

Methodology
The authors propose a mechanism for detecting massive compact objects by observing their interaction with the surrounding neutral hydrogen and the Cosmic Microwave Background (CMB), rather than observing the objects directly. The core of the methodology involves:

Population Enhancement: The authors calculate the population of highly excited hydrogen levels ( $n \ge 3$ ) in the vicinity of a massive object (modeled as an accreting PBH). They posit that while the background CMB radiation is insufficient to excite these levels due to the Wien tail suppression (Boltzmann factor $\sim 10^{-20}$ at $z=1000$ ), a localized source emitting few-eV photons (e.g., from an accretion disk extending to large radii) can significantly boost the population of levels $n=2, 3, 4$ and higher.
Radiative Transfer and Optical Depth: The study derives the relative population of excited states ( $n_n/n_H$ ) by balancing the excitation rate from the central source against the de-excitation rate driven by the CMB. They estimate the optical depth ( $\tau$ ) of the resulting subordinate transitions (radio recombination lines) in a spherical shell around the object.
Thermal Considerations: The authors analyze the temperature difference between the gas and the CMB. At $z > 150$ , Compton scattering keeps the gas temperature close to the CMB temperature, suppressing absorption. However, they argue that at slightly lower redshifts ( $z \sim 150$ ) or due to adiabatic heating in compressed gas, a temperature difference sufficient to create observable absorption features may exist.
Observational Feasibility: The paper calculates the expected angular size, line depth, and frequency of these transitions when redshifted to the present day. It evaluates the sensitivity requirements for future radio telescopes (e.g., SKA, uGMRT) to detect these features.

Key Contributions and Results

Mechanism of "Excitation Radio Lines" (ERLs): The paper introduces the concept of "Excitation Radio Lines," distinct from standard recombination lines. These are subordinate hydrogen transitions ( $n \to n'$ where $n, n' \gg 1$ ) that become optically thick ( $\tau \sim 1$ ) due to the external radiation field of a compact object, which boosts the population of high- $n$ levels by 8–10 orders of magnitude compared to thermal equilibrium.
Spectral-Spatial Distortions: The interaction results in a Fraunhofer-like absorption spectrum in the CMB. Because the gas temperature is slightly lower than the CMB temperature (or due to the specific population inversion dynamics), the object appears as a circular absorption feature (or emission, depending on the exact thermal state) against the CMB background.
Redshifted Observability: A critical finding is that while Lyman- $\alpha$ at $z \sim 1000$ shifts to the far-infrared ( $\sim 122 \, \mu\text{m}$ ), subordinate transitions (e.g., $5 \to 6$ , $6 \to 7$ ) shift into the millimeter and centimeter radio bands ( $\sim 1\text{--}2 \, \text{cm}$ at $z \sim 1000$ ). This makes them accessible to ground-based radio telescopes.
Quantitative Estimates:
- Optical Depth: For a PBH with mass $10^8 M_\odot$ and luminosity $10^{44} \, \text{erg s}^{-1}$ , the optical depth in the $5 \to 6$ transition is estimated to be $\tau \sim 3$ (or $\sim 0.3$ with turbulent broadening), confirming the feasibility of detection.
- Line Depth: The expected absorption depth is approximately 10% of the continuum ( $\Delta T/T \sim 0.1$ ).
- Angular Size: The characteristic angular size of the absorbing region is estimated at $\sim 6\text{--}10$ arcseconds, which is resolvable by modern and planned interferometers.
- Integration Time: Calculations suggest that detecting a 10% absorption line with a system temperature of $\sim 100 \, \text{K}$ requires integration times on the order of seconds to hours, depending on beam dilution and source brightness.

Significance and Claims
The paper claims that this mechanism offers a novel, indirect method to detect massive primordial compact objects (such as PBHs or early quasars) during the Dark Ages, a regime previously considered observationally inaccessible. By shifting the observational window from the blocked far-infrared to the accessible radio spectrum, the authors argue that planned large radiotelescopes (SKA, uGMRT) could detect these objects.

The significance lies in the potential to:

Confirm the existence of massive primordial black holes or other exotic compact objects formed before the first stars.
Probe the thermal history and structural formation of the Universe at redshifts $z \sim 100\text{--}1000$ .
Provide a practical observational avenue that complements existing 21 cm studies, specifically targeting the "Little Red Dots" (LRDs) observed by JWST at lower redshifts and extrapolating their properties to the early Universe.

The authors maintain a modest tone, acknowledging that the estimates are order-of-magnitude calculations dependent on specific accretion disk models and that the actual number of such sources in the observable Universe is unknown. They emphasize that the effect relies on the existence of sufficiently luminous, isolated sources and that the proposed method is most effective precisely in the obscured conditions that make traditional detection difficult.

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