Hunting Dark Matter with the Universe’s First Light

Author: Denis Avetisyan

An upcoming 21cm cosmology experiment, Hongmeng, promises to sharpen our search for interactions between dark matter and ordinary matter.

The study establishes increasingly stringent upper limits on the cross-section for interactions between dark matter and baryonic matter-represented by the parameter $σ\_0$-with projections indicating that a five-year mission will significantly refine these bounds, potentially surpassing current constraints derived from cosmic microwave background observations, though sensitivity remains dependent on galactic population parameters.

This paper details how the Hongmeng experiment can improve constraints on dark matter-baryon scattering cross-sections by a factor of 21, utilizing Fisher forecast techniques and careful foreground removal strategies.

The unexpectedly deep 21cm absorption signal reported in 2018 remains a puzzle for standard cosmological models, prompting exploration of dark matter interactions as a potential explanation. This paper, ‘Sensitivity of Hongmeng 21cm experiment on scattering dark matter’, investigates the capacity of the forthcoming Hongmeng lunar orbiting satellite to constrain scattering between dark matter and baryons, a process capable of influencing the intergalactic medium and deepening the 21cm signal. Through detailed forward modeling and Fisher forecasting, we demonstrate that Hongmeng can improve current limits on the dark matter-baryon scattering cross-section by a factor of 21 over a five-year mission, potentially reaching a prospective upper limit of $4 \times 10^{-43}$ cm² for dark matter masses between 0.1 and 0.4 GeV-but how will these refined constraints shape our understanding of dark matter’s fundamental properties and its role in the early universe?

The Echo of Creation: Unveiling Cosmic Dawn

The universe transitioned from a homogenous plasma to a structured cosmos during a period known as Cosmic Dawn, and a key to unlocking its secrets lies in the 21-centimeter radio emission from neutral hydrogen. Before the first stars ignited, this hydrogen permeated the universe, absorbing and re-emitting background radiation at a specific wavelength – 21 centimeters – creating a faint, pervasive signal. Detecting this signal offers a unique probe of the universe’s infancy, potentially revealing details about the initial distribution of matter, the formation of the first structures, and the nature of dark matter and dark energy. Unlike observations of the cosmic microwave background which capture the universe in its earliest moments, the 21cm signal evolves over time, providing a three-dimensional map of the universe during this crucial epoch of reionization and structure formation, essentially allowing scientists to “see” the universe as the first stars and galaxies began to emerge from the darkness.

The universe’s earliest light isn’t visible in the traditional sense; instead, a subtle radio emission – the 21cm signal – holds the key to understanding cosmic dawn. This signal originates from neutral hydrogen atoms, which permeated the universe before the first stars ignited. However, its inherent weakness presents a formidable challenge to astronomers. The signal is easily overwhelmed by foreground contamination – pervasive radio emissions from our galaxy, distant galaxies, and even terrestrial sources. Distinguishing the faint cosmological whisper from this cacophony of noise requires sophisticated data analysis techniques and exceptionally sensitive instruments. The difficulty isn’t merely about detecting a weak signal; it’s about precisely isolating it from sources orders of magnitude brighter, a task akin to hearing a pin drop amidst a rock concert. Consequently, a clear detection remains elusive, hindering detailed investigations into the conditions that characterized the universe’s formative years.

The pursuit of understanding cosmic dawn is significantly hampered by the difficulty in separating the desired cosmological signal from overwhelming foreground emissions. Radio telescopes, while powerful, detect not only the faint 21cm radiation from neutral hydrogen in the early universe, but also a multitude of other radio sources – galactic synchrotron emission, extragalactic sources, and even terrestrial interference. These foregrounds are orders of magnitude stronger, effectively masking the subtle variations that encode information about the universe’s infancy. Current data analysis techniques, while sophisticated, struggle to accurately model and subtract these contaminants, introducing uncertainties that limit the ability to confidently identify and interpret the true 21cm signal. Consequently, crucial details about the first stars, galaxies, and the reionization epoch remain elusive, hindering progress in unraveling the mysteries of the universe’s formative years.

The pursuit of the 21cm signal represents a fundamental quest to unveil the universe’s formative years, as this faint emission carries information about the conditions present before the first luminous stars and galaxies emerged. Detecting and accurately interpreting this signal allows scientists to map the distribution of neutral hydrogen-the dominant form of matter in the early universe-and trace the evolution of density fluctuations that ultimately seeded cosmic structure. By meticulously analyzing the signal’s subtle variations, researchers aim to determine the epoch when the first stars ignited, reionized the universe, and initiated the complex processes that led to the cosmos observed today. Successfully isolating this cosmological signature promises to resolve long-standing questions regarding the nature of dark matter and dark energy, and provide an unprecedented glimpse into the birth of structure in the universe.

Five years of observations can constrain astrophysical and dark matter parameters, with tighter limits on Population II, feedback, and velocity dispersion when Population III parameters are fixed to their fiducial values, as demonstrated by the 68% and 95% confidence intervals shown in the contours.

A Lunar Sanctuary: Silencing the Static

The Hongmeng Experiment employs a lunar-orbiting platform to mitigate radio frequency interference (RFI) originating from Earth. Terrestrial sources, including both human-generated transmissions and natural atmospheric phenomena, dominate the ultra-low frequency radio spectrum. By positioning the observatory in lunar orbit, specifically avoiding the near side of the Moon which is subject to significant terrestrial RFI, the experiment drastically reduces this foreground noise. This strategic placement allows for the detection of the faint cosmological signal at 21cm, which would otherwise be obscured by terrestrial radio emissions. The orbital parameters are optimized to maintain a consistently shielded view of the sky, further minimizing interference and maximizing observational sensitivity.

The Hongmeng Experiment’s lunar orbit provides a uniquely advantageous observational platform for ultra-low frequency radio astronomy. Terrestrial radio transmissions, as well as atmospheric effects, create significant interference at these wavelengths, obscuring cosmological signals. By establishing an observatory in a stable orbit around the far side of the Moon, the experiment effectively shields its instruments from these foreground sources. This positioning minimizes the impact of both direct radiation and reflections from the Earth, its atmosphere, and even the near side of the Moon, allowing for the detection of exceedingly faint signals that would otherwise be undetectable from Earth-based observatories. The specific orbital parameters are optimized to maintain continuous visibility of the target sky regions and to ensure long-duration, uninterrupted observations.

Radio frequency interference (RFI) from terrestrial sources, including human technology and atmospheric phenomena, constitutes a significant foreground contaminant in 21cm cosmology observations. The far side of the Moon provides a uniquely shielded environment, dramatically reducing this interference due to the lunar body blocking direct terrestrial emissions. This reduction in foreground noise directly improves the signal-to-noise ratio for detecting the faint cosmological 21cm signal, which is expected to be several orders of magnitude weaker than typical terrestrial RFI. Consequently, observations from this location enable the detection of subtle variations in the 21cm signal that would otherwise be obscured, facilitating more precise measurements of the early universe’s properties and large-scale structure.

The Hongmeng Experiment focuses on detecting the global 21cm signal, a faint radio emission originating from neutral hydrogen during the cosmic dawn and epoch of reionization. This signal carries information about the large-scale structure and evolution of the universe when the first stars and galaxies formed, between approximately 150 million and 1 billion years after the Big Bang. Analyzing the power spectrum and statistical properties of the 21cm signal allows researchers to map the distribution of neutral hydrogen and infer the properties of early structures, including the formation of the first stars, galaxies, and black holes. The global average signal, in particular, provides constraints on the timing and duration of reionization, as well as the temperature and ionization state of the intergalactic medium during this crucial epoch. Precise measurement of this signal requires minimizing foreground contamination and maximizing sensitivity at ultra-low frequencies, which is the primary motivation for a lunar-based observatory.

The Intergalactic Web: A Cosmic Thermometer

The intensity of the 21cm signal-a spectral line emitted by neutral hydrogen-is inversely proportional to the spin temperature $T_s$ of the IGM. $T_s$ represents the effective temperature at which the spin states of hydrogen are in thermal equilibrium, and is influenced by both the kinetic temperature $T_K$ of the gas and the intensity of the cosmic microwave background (CMB). A colder IGM, with $T_K$ lower than the CMB temperature, results in a stronger 21cm signal due to a larger population difference between the spin states. Conversely, heating from sources like early galaxies increases $T_K$, diminishing the population difference and weakening the 21cm signal. Furthermore, the shape and width of the 21cm power spectrum are directly affected by the distribution of temperatures within the IGM, making accurate temperature modeling crucial for interpreting observations and extracting cosmological information.

X-ray radiation from early galaxies primarily heats the Intergalactic Medium (IGM) through the transfer of energy to IGM electrons, increasing the gas temperature and broadening the 21cm signal. Conversely, Ultraviolet (UV) radiation from these same galaxies is the dominant mechanism for ionizing the IGM, specifically hydrogen atoms, leading to the creation of free electrons and impacting the neutral hydrogen fraction. The intensity of X-ray radiation is proportional to the star formation rate in early galaxies and the abundance of X-ray binaries, while UV radiation’s effectiveness is dictated by the luminosity and spectral shape of young, massive stars. These processes are wavelength-dependent; X-ray photons deposit energy efficiently due to their high energy, whereas UV photons have sufficient energy to ionize hydrogen at $13.6$ eV. The balance between heating from X-rays and ionization from UV radiation determines the thermal and ionization state of the IGM, directly influencing the observed 21cm signal.

Population III and Population II galaxies represent the dominant sources of ionizing and heating radiation for the Intergalactic Medium (IGM) during the Epoch of Reionization. Population III stars, being massive and hot, emitted copious amounts of ultraviolet (UV) radiation capable of ionizing neutral hydrogen in the IGM. As Population III stars evolved and were replaced by Population II stars, the primary source of ionizing radiation transitioned, though Population II galaxies also contribute significantly through their own UV output. Furthermore, both stellar populations emit X-ray radiation, directly heating the IGM and altering its thermal state. The spectral characteristics and luminosity of these galaxies, determined by their star formation rates and metallicities, directly impact the degree of ionization and temperature of the surrounding IGM, influencing the observed 21cm signal.

Accurate interpretation of the 21cm signal relies on a comprehensive understanding of how radiation sources – specifically Population III and Population II galaxies – interact with the Intergalactic Medium (IGM). These galaxies contribute both ionizing and heating radiation, altering the IGM’s thermal and ionization state and thus modifying the properties of the 21cm emission. Variations in the intensity and spectral characteristics of this radiation directly affect the amplitude and shape of the 21cm power spectrum. Consequently, precise modeling of this interplay is crucial for extracting cosmological parameters – such as the amplitude of density fluctuations, $ \sigma_8 $, and the running spectral index, $n_s$ – from 21cm observations. Errors in accounting for the radiation-IGM interaction will introduce systematic uncertainties in these parameter estimations.

The intergalactic medium temperature and corresponding likelihood are influenced by SDM parameters, with color representing the chi-squared value and hatched regions indicating consistent cooling or heating between redshifts of 11 and 46.

Unveiling the Invisible: Constraining the Dark Universe

Fisher analysis serves as a crucial predictive tool in cosmology, allowing researchers to forecast how well future 21cm observations will constrain various cosmological parameters. This statistical method doesn’t rely on analyzing actual data, but instead utilizes the predicted covariance matrix of the observable quantities to estimate the achievable precision on parameters like dark matter properties and the expansion rate of the universe. By mathematically modeling the expected signal and noise, Fisher analysis identifies which parameters are most sensitive to 21cm measurements and quantifies the expected uncertainties. This pre-experiment assessment is invaluable for optimizing observational strategies, guiding instrument design, and ultimately maximizing the scientific return from complex cosmological surveys, offering a robust framework for interpreting results and comparing different cosmological models before substantial observational effort is expended.

The convergence of 21cm cosmology and Cosmic Microwave Background (CMB) observations offers a powerful synergy for probing the nature of dark matter. While the CMB provides a snapshot of the early universe, revealing its initial conditions and composition, 21cm observations trace the subsequent evolution of cosmic structure, particularly the distribution of neutral hydrogen gas. By jointly analyzing data from both sources, researchers can break degeneracies in parameter estimation and obtain tighter constraints on dark matter properties, such as its interaction strength with ordinary matter. This combined approach allows for a more holistic understanding of dark matter’s influence on the universe, moving beyond the limitations of either technique in isolation and promising significant advancements in unraveling this fundamental component of the cosmos.

The Hongmeng Experiment is poised to dramatically refine the search for interactions between dark matter and ordinary matter, specifically baryons. Through high-precision measurements of the 21cm signal – a faint radio emission from neutral hydrogen – the experiment will enable rigorous tests of diverse dark matter models. Current constraints on the dark matter-baryon scattering cross-section, a key indicator of interaction strength, are expected to be improved by a factor of 21. This leap in sensitivity will allow scientists to probe smaller interaction strengths and lower dark matter masses, potentially revealing the nature of this elusive substance and its role in the universe’s evolution. The projected limits, reaching $σ_0≲4×10^{-43}$cm² for dark matter masses below 0.1 GeV, promise a substantial advancement in understanding the composition and fate of the cosmos.

The Hongmeng Experiment anticipates a substantial refinement in the understanding of dark matter interactions, specifically aiming to constrain the dark matter-baryon scattering cross-section to a value of $σ_0≲4×10^{-43} \text{cm}^2$ for dark matter particles with masses below 0.1 GeV. This represents a considerable improvement over existing limits and will allow for rigorous testing of various dark matter models. Such precise measurements are crucial because the nature of dark matter-and how it interacts (or doesn’t) with baryonic matter-directly influences the formation and evolution of large-scale structures in the universe. By pinpointing the strength of this interaction, the experiment promises to illuminate the fundamental composition of the cosmos and provide deeper insights into its past, present, and ultimate fate.

The pursuit of dark matter’s interactions, as detailed in this paper regarding the Hongmeng experiment, reveals a certain audaciousness in the face of the unknown. It is a grand attempt to quantify something fundamentally elusive. As James Maxwell observed, “The true value of any theory lies not in its ability to predict, but in its ability to explain.” This experiment, focused on refining the limits of dark matter-baryon scattering cross-sections, embodies that sentiment. The potential for a factor of 21 improvement, while technically impressive, subtly underscores how little is definitively known. Black holes are the best teachers of humility; they show that not everything is controllable, and neither are the depths of the cosmos. Theory is a convenient tool for beautifully getting lost.

Where Do We Go From Here?

The prospect of a factor of 21 improvement in constraints on dark matter-baryon scattering cross-sections, as detailed by this work regarding the Hongmeng 21cm experiment, is not a triumph of prediction, but a measure of how little is truly known. Current limits, however stringent, are born of assumptions-assumptions concerning the nature of dark matter, the details of reionization, and the efficacy of foreground removal techniques. The Hongmeng experiment, while promising, will merely refine the contours of this ignorance, pushing the boundary of what remains unexplained.

Any interpretation of non-detections, or even positive signals, requires rigorous consideration of systematic uncertainties. The Fisher forecast methodology, while valuable, rests on Gaussian approximations and assumes full knowledge of cosmological parameters. To claim definitive constraints on dark matter requires a careful accounting for astrophysical foregrounds – a task that may prove as elusive as the dark matter itself. Schwarzschild and Kerr metrics describe exact spacetime geometries, but tell little of what lies within the event horizon, or beyond the reach of observation.

The pursuit of dark matter is, ultimately, a search for the limits of knowledge. Each refinement of the scattering cross-section-each narrowing of the parameter space-is not a step toward understanding, but a recognition of the vastness of what remains unknown. Any discussion of quantum singularity requires careful interpretation of observables. The true value of experiments like Hongmeng lies not in what they reveal, but in their capacity to expose the fragility of current theoretical frameworks.

Original article: https://arxiv.org/pdf/2512.05056.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

The Echo of Creation: Unveiling Cosmic Dawn

A Lunar Sanctuary: Silencing the Static

The Intergalactic Web: A Cosmic Thermometer

Unveiling the Invisible: Constraining the Dark Universe

Where Do We Go From Here?

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