Vector dark matter, inflation and non-minimal couplings with gravity

We propose a cosmological dark matter production mechanism in the form of a longitudinal massive vector boson. We build upon the work of Graham et.al. including non-minimal couplings of the massive vector with gravity, developing a well motivated set-up from an effective field theory perspective. We carefully track the dynamics of vector field in passing from inflation to radiation dominated universe to show that the late time abundance of longitudinal modes -- excited initially by the quantum fluctuations during inflation -- can provide the observed dark matter abundance for sufficiently weak non-minimal coupling and wide range of vector masses $5 \times 10^{-7} \lesssim m\, [{\rm eV}] \lesssim 5 \times 10^{3}$. The final abundance of dark matter depends on two parameter, the vector mass and its non-minimal coupling with gravity. We discuss experimental venues to probe this framework, including the production of a stochastic gravitational wave background. The latter is especially interesting, as the same mechanism that generates dark matter can potentially lead to the production of gravitational waves in the LISA frequency band, through the second-order effects of large dark matter iso-curvature perturbations at small scales. We take a first step in this direction, identifying the potential information that gravitational wave experiments can provide on the parameter space of dark matter within this scenario.Comment: 25 pages plus appendices, 9 figures. Links to codes are provided in the tex

Consistency conditions and primordial black holes in single field inflation

We discuss new consistency relations for single field models of inflation capable of generating primordial black holes (PBH), and their observational implications for CMB $\mu$-space distortions. These inflationary models include a short period of non-attractor evolution: the scale-dependent profile of curvature perturbation is characterized by a pronounced dip, followed by a rapid growth leading to a peak responsible for PBH formation. We investigate the squeezed and the collapsed limits of three and four point functions of curvature perturbation around the dip, showing that they satisfy consistency relations connecting their values to the total amplification of the curvature spectrum, and to the duration of the non-attractor era. Moreover, the corresponding non-Gaussian parameters are scale-dependent in proximity of the dip, with features that again depend on the amplification of the spectrum. For typical PBH scenarios requiring an order ${\cal O}(10^7)$ enhancement of the spectrum from large towards small scales, we generally find values $f_{\rm NL}^{\rm sq}\,=\,{\cal O}(10)$ and $\tau_{\rm NL}^{\rm col}\,=\,{\cal O}(10^3)$ in a range of scales that can be probed by CMB $\mu$-space distortions. Using these consistency relations, we carefully analyze how the scale-dependence of non-Gaussian parameters leads to characteristic features in $\langle \mu T \rangle$ and $\langle \mu \mu \rangle$ correlators, providing distinctive probes of inflationary PBH scenarios that can be tested using well-understood CMB physics.Comment: 27 Pages + Appendices, 15 Figure

Inflation and Primordial Black Holes

We review conceptual aspects of inflationary scenarios able to produce primordial black holes, by amplifying the size of curvature fluctuations to the level required for triggering black hole formation. We identify general mechanisms to do so, both for single and multiple field inflation. In single field inflation, the spectrum of curvature fluctuations is enhanced by pronounced gradients of background quantities controlling the cosmological dynamics, which can induce brief phases of non--slow-roll inflationary evolution. In multiple field inflation, the amplification occurs through appropriate couplings with additional sectors, characterized by tachyonic instabilities that enhance the size of their fluctuations. As representative examples, we consider axion inflation, and two-field models of inflation with rapid turns in field space. We develop our discussion in a pedagogical manner, by including some of the most relevant calculations, and by guiding the reader through the existing theoretical literature, emphasizing general themes common to several models.Comment: 58 Pages, 19 Figures + Appendices, this version: the discussion on the threshold is clarified with a footnote, an extensive list of references added, some typos are correcte

New constraints on axion-gauge field dynamics during inflation from PlanckPlanck and BICEP/Keck data sets

We present new constraints on spectator axion-${\rm U}(1)$ gauge field interactions during inflation using the latest $Planck$ (PR4) and BICEP/Keck 2018 data releases. This model can source tensor perturbations from amplified gauge field fluctuations, driven by an axion rolling for a few e-folds during inflation. The gravitational waves sourced in this way have a strongly scale-dependent (and chiral) spectrum, with potentially visible contributions to large/intermediate scale $B$-modes of the CMB. We first derive theoretical bounds on the model imposing validity of the perturbative regime and negligible backreaction of the gauge field on the background dynamics. Then, we determine bounds from current CMB observations, adopting a frequentist profile likelihood approach. We study the behaviour of constraints for typical choices of the model's parameters, analyzing the impact of different dataset combinations. We find that observational bounds are competitive with theoretical ones and together they exclude a significant portion of the model's parameter space. We argue that the parameter space still remains large and interesting for future CMB experiments targeting large/intermediate scales $B$-modes.Comment: 26 pages + appendix, 11 Figures. Accepted for publication in JCAP. Updated reference