Vector Dark Matter from Inflationary Fluctuations (1504.02102v1)

Abstract: We calculate the production of a massive vector boson by quantum fluctuations during inflation. This gives a novel dark-matter production mechanism quite distinct from misalignment or thermal production. While scalars and tensors are typically produced with a nearly scale-invariant spectrum, surprisingly the vector is produced with a power spectrum peaked at intermediate wavelengths. Thus dangerous, long-wavelength, isocurvature perturbations are suppressed. Further, at long wavelengths the vector inherits the usual adiabatic, nearly scale-invariant perturbations of the inflaton, allowing it to be a good dark matter candidate. The final abundance can be calculated precisely from the mass and the Hubble scale of inflation, H_I. Saturating the dark matter abundance we find a prediction for the mass m = 10^-5 eV (10^14 GeV/H_I)^4. High-scale inflation, potentially observable in the CMB, motivates an exciting mass range for recently proposed direct detection experiments for hidden photon dark matter. Such experiments may be able to reconstruct the distinctive, peaked power spectrum, verifying that the dark matter was produced by quantum fluctuations during inflation and providing a direct measurement of the scale of inflation. Thus a detection would not only be the discovery of dark matter, it would also provide an unexpected probe of inflation itself.

• The paper introduces a novel gravitational mechanism where inflationary quantum fluctuations generate massive vector bosons as dark matter.
• It provides a precise calculation showing that a vector boson mass of ~10⁻⁵ eV from a high inflationary Hubble scale (~10¹⁴ GeV) can account for the observed dark matter.
• The model naturally suppresses isocurvature perturbations by inheriting adiabatic fluctuations, aligning with cosmic microwave background observations.

Overview of "Vector Dark Matter from Inflationary Fluctuations"

The paper "Vector Dark Matter from Inflationary Fluctuations" proposes a novel mechanism for dark matter (DM) production involving massive vector bosons generated by quantum fluctuations during inflation. This mechanism is distinct from prevalent DM production theories, such as the misalignment mechanism or thermal production, and offers a compelling connection between the inflationary epoch and the genesis of DM.

Summary of Key Findings

• Novel Production Mechanism: The authors calculate the production of massive vector bosons via quantum fluctuations during inflation, positing a method for DM generation that is entirely gravitational, requiring no additional interactions beyond gravity. These vector bosons acquire a peaked power spectrum, distinguishing their production from that of scalars or tensors, which typically exhibit scale-invariance. The abundance produced is solely a function of the vector boson's mass and the Hubble scale during inflation.
• Suppressing Isocurvature Perturbations: A significant feature of this mechanism is its suppression of dangerous, long-wavelength isocurvature perturbations, allowing these vectors to be viable DM candidates. The spectrum of the produced vectors is adiabatic and nearly scale-invariant at long wavelengths, inheriting adiabatic perturbations from the inflaton while naturally suppressing isocurvature fluctuations. This aspect appeals to observational constraints, especially those from the cosmic microwave background (CMB).
• Precise Abundance Calculation: The paper provides an exact calculation of the DM abundance in terms of the vector mass and the inflationary Hubble scale. For instance, the research derives that a vector boson mass of about $10^{-5}$ eV occurs when $H_I$ is at the high end ($\sim 10^{14}$ GeV), potentially saturating the observed DM abundance. This precision offers a stark contrast to scalar-based production where similar calculations are often constrained by initial conditions or coupling constants.
• Implications for High-Scale Inflation: This work has implications for high-scale inflation—detectable through forthcoming cosmological observations like those related to primordial gravitational waves. The appropriate mass range of vectors aligns well with these scenarios, providing a testable prediction linking particle physics and cosmology.

Implications and Future Directions

The paper offers strong implications for theoretical and experimental physics, including:

• Direct Detection: Suggests the potential for direct detection of these vector bosons through kinetic mixing with photons, within the relevant mass range. This emphasis on laboratory detectability bridges theoretical work with feasible experimental efforts, such as using electromagnetic resonators as outlined in recent proposals.
• Enhancing Gravitational Wave Studies: If gravitational wave observatories determine a high inflation scale, this could further constrain vector mass predictions, potentially confirming or, if not observed, challenging the assumptions and calculations within this work.
• Astrophysical Consequences: The distinct power spectrum suggests unique DM structures at astrophysically small scales. Future studies could investigate the survival and observational consequences of these structures within galactic dynamics, giving way to potential new insights into the nature of DM distribution and interaction on small scales.

Conclusion

The paper "Vector Dark Matter from Inflationary Fluctuations" presents a compelling case for vector bosons as viable DM candidates, with a thorough analysis grounded in inflationary physics. It emphasizes a direct connection between early-universe cosmology and present-day DM phenomena, offering a framework for novel theoretical exploration and experimental verification. This work could significantly advance our understanding of DM and inflation, unraveling new avenues for both cosmological and particle physics research.