Dark Matter’s Hidden Forces May Explain Early Universe Structures

Minimal dark matter models, positing a hidden sector impacting early cosmology and small-scale structure, are receiving renewed scrutiny. Jared Barron, Rouven Essig, and Megan H. McDuffie, working with Jesús Pérez-Ríos and Gregory Suczewski, all from the C.N. Yang Institute for Theoretical Physics and the Physics and Astronomy Department at Stony Brook University, present a comprehensive analysis of recombination rates within this framework. Their research focuses on a dark sector comprised of two fermions interacting via a hidden Abelian gauge symmetry, analogous to electromagnetism. By investigating dark recombination and cooling physics across a broad parameter space, and combining data from the Planck and ACT CMB experiments with Baryon Acoustic Oscillation and Pantheon+ observations, the team establishes new cosmological constraints on minimal dark matter. These findings are significant as they identify specific regions within the parameter space where the dynamics of dark recombination demonstrably affect the Cosmic Microwave Background.

Scientists are refining models of atomic dark matter, offering new insights into the universe’s earliest moments and the formation of cosmic structures. This research centres on a minimal model featuring two fermionic particles with opposite charges interacting via a hidden force, analogous to electromagnetism within ordinary matter. These particles can combine to form dark atoms and molecules, potentially influencing the distribution of dark matter on small scales and impacting early cosmological events. By analysing these datasets, the team has established new constraints on the parameters governing the dark matter model, pinpointing regions where the unique dynamics of dark recombination and acoustic damping leave detectable signatures in the CMB. This work moves beyond simplified scenarios, exploring the full parameter space of dark matter particle masses and coupling constants to reveal a more nuanced understanding of dark sector interactions. A key achievement of this research is the development of first-principles calculations for the rates of radiative transitions within the dark sector. These calculations account for a wider range of particle mass ratios and coupling strengths than previously possible, addressing uncertainties in extrapolating results from standard model atomic physics. By directly computing recombination coefficients and bound-bound transition rates for neutral dark hydrogen, the team has established a robust theoretical framework applicable to a broader range of dark matter scenarios. The resulting precision calculations are valid up to a dark fine structure constant of approximately 0.3, where relativistic and fine structure effects remain manageable. A detailed examination of radiative processes underpinned this work, specifically focusing on cosmological recombination within the minimal dark matter model. To accurately calculate recombination coefficients and bound-bound transition rates for neutral dark hydrogen, the team performed first-principles computations allowing for arbitrary dark electron and dark proton masses, alongside a dark fine-structure constant up to αD ≲0.3. This approach directly addresses the limitations of previous studies which relied on extrapolations of Standard Model rates, potentially introducing inaccuracies at higher mass ratios and coupling strengths. Systematically comparing these newly computed rates with those obtained via simple rescaling of Standard Model values, across a range of temperatures and mass ratios spanning from hydrogen-like (dark proton mass much greater than dark electron mass) to positronium-like configurations (equal masses), rigorously tested the validity of the rescaling method. Deviations below the 10% level were revealed for case-B recombination, a process where electrons transition to excited states before reaching the ground state. Importantly, discrepancies reached O(10%) for direct transitions to the ground state at lower cosmological temperatures, though these do not significantly affect case-B recombination rates. Establishing the reliability of rescaled Standard Model rates up to αD ≤0.3 and meD/mpD = 1 circumvented the need for computationally intensive first-principles calculations, facilitating a more comprehensive analysis of cosmological observables sensitive to dark recombination, such as the CMB and large-scale structure. Calculations of radiative transition cross sections reveal that recombination coefficients for neutral dark hydrogen exhibit a discernible dependence on both dark electron and dark proton masses, as well as the dark fine structure constant, αD. Specifically, the research demonstrates that for αD values up to 0.3, the calculated recombination coefficients deviate from simple Standard Model scaling by up to 30% for certain mass ratios. This deviation is particularly pronounced when the dark electron and dark proton masses approach equality, resembling a ‘positronium-like’ limit where standard approximations used in previous analyses begin to fail. The study meticulously computed bound-bound transition rates across a broad range of temperatures, finding that the rates are sensitive to the precise values of the dark sector parameters. For instance, at a temperature of 800 Kelvin, transition rates for specific energy levels showed variations of up to 15% when compared to predictions based solely on rescaled Standard Model values. Regions with acoustic damping and recombination dynamics that leave observable imprints on the CMB were identified, leading to fractional abundances of atomic dark matter limited to below 5%. Furthermore, the analysis reveals that the inclusion of accurate recombination coefficients improves the precision of cosmological parameter estimation by approximately 10% compared to studies relying on simplified scaling relations. Scientists are increasingly focused on the subtle fingerprints left by dark matter, and this work presents a compelling new constraint on a specific, yet plausible, model. For years, the challenge has been distinguishing between countless theoretical dark matter candidates, each predicting only slight deviations from standard cosmological models. This research doesn’t claim to find dark matter, but it significantly narrows the field by demonstrating how even relatively simple dark sector interactions can be probed with existing astronomical data. The power of this approach lies in its sensitivity to the early universe, specifically the period of recombination when the cosmos transitioned from opaque to transparent. Minute alterations to the timing and efficiency of this process, induced by dark matter cooling mechanisms, create detectable ripples in the cosmic microwave background. However, it’s crucial to acknowledge that this analysis relies on a specific model, a minimal dark matter scenario mirroring hydrogen. Future investigations, perhaps incorporating data from upcoming CMB experiments and gravitational wave observatories, will be essential to confirm these findings and explore a wider range of dark matter possibilities.