We propose a GeV-scale self-interacting dark matter (SIDM) candidate within a
dark $U(1)_D$ gauged extension of the Standard Model (SM), addressing
small-scale structure issues in $\Lambda$CDM while predicting an observable
contribution to $\Delta N_{\rm eff}$ in the form of dark radiation. The model
introduces a fermionic DM candidate $\chi$ and a scalar $\phi$, both charged
under an unbroken $U(1)_D$ gauge symmetry. The self-interactions of $\chi$ are
mediated by a light vector boson $X^\mu$, whose mass is generated via the
Stueckelberg mechanism. The relic abundance of $\chi$ is determined by thermal
freeze-out through annihilations into $X^\mu$, supplemented by a non-thermal
component from the late decay of $\phi$. Crucially, $\phi$ decays after the Big
Bang Nucleosynthesis (BBN) but before the Cosmic Microwave Background (CMB)
epoch, producing additional $\chi$ and a dark radiation species ($\nu_S$). This
late-time production compensates for thermal underabundance due to efficient
annihilation into light mediators, while remaining consistent with structure
formation constraints. The accompanying dark radiation yields a detectable
$\Delta N_{\rm eff}$, compatible with Planck 2018 bounds and within reach of
next-generation experiments such as SPT-3G, CMB-S4, and CMB-HD.

Este artículo explora los viajes en el tiempo y sus implicaciones.

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