Dark Sectors with Mass Thresholds Face Cosmological Datasets

Interacting dark sectors may undergo changes in the number of their relativistic species during the early universe, due to a mass threshold <math display="inline"><mi>m</mi></math> (similar to changes in the Standard Model bath), and in doing so affect the cosmic history. When such changes occur close to recombination, i.e., for <math display="inline"><mrow><mi>m</mi><mo>∼</mo><mo stretchy="false">(</mo><mn>0.1</mn><mi>–</mi><mn>10</mn><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math>, the stringent bound on the effective number of neutrino species, <math display="inline"><msub><mi>N</mi><mi>eff</mi></msub></math>, can be relaxed and the value of the Hubble expansion rate <math display="inline"><msub><mi>H</mi><mn>0</mn></msub></math> inferred from cosmic microwave background (CMB) observations raised. We search for such sectors (with and without mass thresholds) in the latest cosmological datasets, including the full-shape (FS) of BOSS DR12 galaxy power spectrum. We perform a detailed analysis, accounting for the choice of prior boundaries and additionally exploring the possible effects of dark sector interactions with (a fraction of) the dark matter. We find <math display="inline"><mi mathvariant="normal">Δ</mi><msub><mi>N</mi><mi>eff</mi></msub><mo>≤</mo><mn>0.55</mn><mo stretchy="false">(</mo><mn>0.46</mn><mo stretchy="false">)</mo></math> at 95% confidence level (CL) with (without) a mass threshold. While a significantly larger Hubble rate is achieved in this scenario, <math display="inline"><msub><mi>H</mi><mn>0</mn></msub><mo>=</mo><mn>69.0</mn><msubsup><mn>1</mn><mrow><mo>-</mo><mn>1.1</mn></mrow><mrow><mo>+</mo><mn>0.66</mn></mrow></msubsup></math>, the overall fit to <math display="inline"><mrow><mi>CMB</mi><mo>+</mo><mi>FS</mi></mrow></math> data does not provide a compelling advantage over the <math display="inline"><mi mathvariant="normal">Λ</mi><mi>CDM</mi></math> model. Furthermore, we find that dark matter interactions with the dark sector do not significantly improve the (matter fluctuations) <math display="inline"><msub><mi>S</mi><mn>8</mn></msub></math> tension with respect to the <math display="inline"><mi mathvariant="normal">Λ</mi><mi>CDM</mi></math> model. Our work provides model-independent constraints on (decoupled) dark sectors with mass thresholds around the eV scale.