Dark Photon Stars: Formation and Role as Dark Matter Substructure

Abstract: Any new vector boson with non-zero mass (a dark photon' orProca boson') that is present during inflation is automatically produced at this time from vacuum fluctuations and can comprise all or a substantial fraction of the observed dark matter density, as shown by Graham, Mardon, and Rajendran. We demonstrate, utilising both analytic and numerical studies, that such a scenario implies an extremely rich dark matter substructure arising purely from the interplay of gravitational interactions and quantum effects. Due to a remarkable parametric coincidence between the size of the primordial density perturbations and the scale at which quantum pressure is relevant, a substantial fraction of the dark matter inevitably collapses into gravitationally bound solitons, which are fully quantum coherent objects. The central densities of these dark photon star', orProca star', solitons are typically a factor $10^6$ larger than the local background dark matter density, and they have characteristic masses of $10^{-16} M_\odot (10^{-5}{\rm eV}/m)^{3/2}$, where $m$ is the mass of the vector. During and post soliton production a comparable fraction of the energy density is initially stored in, and subsequently radiated from, long-lived quasi-normal modes. Furthermore, the solitons are surrounded by characteristic `fuzzy' dark matter halos in which quantum wave-like properties are also enhanced relative to the usual virialized dark matter expectations. Lower density compact halos, with masses a factor of $\sim 10^5$ greater than the solitons, form at much larger scales. We argue that, at minimum, the solitons are likely to survive to the present day without being tidally disrupted. This rich substructure, which we anticipate also arises from other dark photon dark matter production mechanisms, opens up a wide range of new direct and indirect detection possibilities, as we discuss in a companion paper.

Overview of "Dark Photon Stars: Formation and Role as Dark Matter Substructure"

The paper titled "Dark Photon Stars: Formation and Role as Dark Matter Substructure" delves into the complex phenomenon where dark photon condensates emerge as substantial features within the architecture of dark matter. The authors explore a theoretical framework suggesting that vector bosons—specifically, dark photons—which obtain mass during the inflationary epoch could play a significant role as components of dark matter. Utilizing both analytical and numerical methods, the study proposes that these particles lead to diverse and dense substructures within the cosmos, including the formation of gravitationally bound objects known as "dark photon stars" or "Proca stars."

Key Findings

• Production During Inflation: The paper explores how a massive vector boson, present during the inflationary period, can emerge as a constituent of dark matter. This occurs due to the primordial fluctuations inherent to inflationary dynamics. The longitudinal mode of these vectors effectively behaves as a scalar field and acquires a scale-invariant power spectrum. Contrarily, the transverse modes remain unproduced.

• Dark Matter Substructure: A particularly striking result of their analysis is the formation of dense dark matter solitons—dark photon stars—driven by quantum wave mechanics and gravitational collapse. These solitons exhibit densities significantly higher than the ambient dark matter density and possess masses contingent on the mass of the vector boson.

• Hierarchical Structure Formation: The study indicates that these dark photon stars are enveloped by 'fuzzy' halos. Furthermore, lower-density 'compact' halos emerge from the collapse of larger-scale overdensities. This sequential folding into gravitationally bound states generates a hierarchy within dark matter structures, from solitons to larger compact halos.

• Detection Implications: The paper suggests these dark matter substructures open new avenues for experimental detection. The solitons, due to their oscillatory quasi-normal modes and high densities, propose unique signatures in observational data, whether through gravitational microlensing or indirect detection. Additionally, this rich substructure may also offer insights if interactions with the Standard Model occur, broadening potential search methodologies.

Implications and Future Research

The findings of this study suggest profound implications in the understanding of dark matter composition and behavior. The vector bosons considered could be pivotal in forming substructures that defy current expectations of dark matter distribution and dynamics. On a theoretical level, this framework aligns with string theory predictions of bosonic fields. Practically, it envisions new phenotypical opportunities in cosmic observations, potentially shifting paradigms in dark matter research methodologies.

Future research directions will involve a deeper exploration of dark photon interactions, stability of solitonic structures, and potential coupling effects with ordinary photons—in particular, probing such configurations' survival through cosmic epochs to validate their viability in contemporary observational scenarios. The potential observational signals detailed, including gravitational waves emanating from dark photon star oscillations, might drastically augment the methodologies and instruments deployment for dark matter detection.

Conclusion

In summary, the paper provides an extensive and rigorous analysis of dark photon star formation, setting a stage for dark matter studies considerably enriched by quantum mechanics insights. It contributes a pivotal framework for understanding dark matter's substructure, compelling further theoretical, empirical, and observational inquiries into a deeply intricate aspect of cosmology. The implications extend beyond merely filling the dark matter puzzle, confronting cosmologists and physicists with the necessity to intertwine quantum and gravitational principles more directly in their paradigms.