A fresh look at linear cosmological constraints on a decaying dark matter component

Abstract: We consider a cosmological model in which a fraction $f$ of the Dark Matter (DM) is allowed to decay in an invisible relativistic component, and compute the resulting constraints on both the decay width (or inverse lifetime) $\Gamma$ and $f$ from purely gravitational arguments. We report a full derivation of the Boltzmann hierarchy, correcting a mistake in previous literature, and compute the impact of the decay --as a function of the lifetime-- on the CMB and matter power spectra. From CMB only, we obtain that no more than 3.8 % of the DM could have decayed in the time between recombination and today (all bounds quoted at 95 % CL). We also comment on the important application of this bound to the case where primordial black holes constitute DM, a scenario notoriously difficult to constrain. For lifetimes longer than the age of the Universe, the bounds can be cast as $f\Gamma < 6.3\times10^{-3}$ Gyr$^{-1}$. For the first time, we also checked that degeneracies with massive neutrinos are broken when information from the large scale structure is used. Even secondary effects like CMB lensing suffice to this purpose. Decaying DM models have been invoked to solve a possible tension between low redshift astronomical measurements of $\sigma_8$ and $\Omega_{\rm m}$ and the ones inferred by Planck. We reassess this claim finding that with the most recent BAO, HST and $\sigma_8$ data extracted from the CFHT survey, the tension is only slightly reduced despite the two additional free parameters, loosening the bound to $f\Gamma < 15.9\times10^{-3}$ Gyr$^{-1}$. The bound however improves to $f\Gamma < 5.9\times10^{-3}$ Gyr$^{-1}$ if only data consistent with the CMB are included. This highlights the importance of establishing whether the tension is due to real physical effects or unaccounted systematics, for settling the reach of achievable constraints on decaying DM.

• The paper provides tighter constraints on dark matter decay, showing up to 3.8% of DM may have decayed from recombination to today based on CMB data.
• The analysis uses linear cosmological models and CMB lensing to break degeneracies between decaying DM and massive neutrinos.
• The study underscores the need for refined multi-component DM models and improved datasets to resolve current cosmological tensions.

Overview of Decaying Dark Matter Cosmological Constraints

The paper "A fresh look at linear cosmological constraints on a decaying dark matter component" critically examines how a portion of dark matter (DM) may decay into invisible relativistic particles, generally referred to as "dark radiation," and explores the implications of such decay on cosmologies. Authored by Vivian Poulin, Pasquale D. Serpico, and Julien Lesgourgues, this study revises prior findings by addressing past inaccuracies and derives tighter cosmological constraints using recent datasets, including Planck 2015 data.

Main Findings and Numerical Results

The study presents constraints on the DM decay from the Cosmic Microwave Background (CMB) and matter power spectra, placing rigorous limits on the DM's decay width ($\Gamma$) or inverse lifetime, and the fraction of decaying DM ($f_{\rm dcdm}$). The authors find that:

1. Up to 3.8% of the DM could have decayed from recombination to today, based solely on CMB data.
2. For lifetimes surpassing the Universe's age, the bounds tighten to $\Gamma < 6.3\times10^{-3}$ Gyr$^{-1}$.
3. Even subtle effects such as CMB lensing help break potential degeneracies between decaying DM and massive neutrinos, elucidating that neutrino mass constraints remain robust with decaying DM inclusions.

Practical and Theoretical Implications

This work extends constraints on DM decay broadening potential applications across numerous cosmological scenarios including multi-component DM models and primordial black holes (PBH) as DM candidates. The latter is of particular interest given challenges in constraining black hole mergers potentially releasing gravitational waves as dark radiation. The findings indicate that if all DM composed primordial black holes experienced mergers akin to known gravitational wave events, constraints on decay fractions would be violated, putting the primordial black hole hypothesis in jeopardy unless mass reprocessing is negligible.

Future Developments in AI and Astrophysics

The analytical methodologies applied here reflect broader trends in computational astrophysics, where sophisticated AI could further refine constraints and model interactions within dark sectors. As machine learning techniques evolve, particularly in anomaly detection and pattern recognition in sparse datasets, we might anticipate richer frameworks capable of examining nonlinear interactions over cosmic timescales to enhance constraint precision.

Conclusion

In synthesizing recent data with improved theoretical frameworks, the study successfully provides more stringent bounds on DM decay parameters, though only a model complexity shift—a more extensive parameter application—has demonstrated potential statistical improvement. This suggests that while valuable, a simple decaying fraction DM model cannot alone bridge gaps hinted by imminent observational discrepancies. The paper underscores the necessity of further precise data integration and theoretical elasticity before meaningful strides towards resolving major cosmological tensions can unfold.