Dark matter from scalar field fluctuations

Abstract: Dark matter (DM) may have its origin in a pre-Big Bang epoch, the cosmic inflation. Here, we consider for the first time a broad class of scenarios where a massive free scalar field unavoidably reaches an equilibrium between its classical and quantum dynamics in a characteristic time scale during inflation and sources the DM density. The study gives the abundance and perturbation spectrum of any DM component sourced by the scalar field. We show that this class of scenarios generically predicts enhanced structure formation, allowing one to test models where DM interacts with matter only gravitationally.

• The paper proposes that dark matter originates from scalar field fluctuations during inflation, establishing a new equilibrium state.
• It quantifies dark matter abundance using a free scalar field Lagrangian, ensuring consistency with CMB isocurvature constraints.
• The study highlights enhanced small-scale structure formation, offering a testable alternative to traditional misalignment mechanisms.

Dark Matter from Scalar Field Fluctuations

The paper "Dark matter from scalar field fluctuations" explores a novel class of scenarios wherein dark matter (DM) originates from scalar field dynamics during cosmic inflation. The research is conducted by Tommi Tenkanen and explores the intersection of dark matter physics and cosmological inflation theory.

The study posits that DM may emerge due to a massive free scalar field achieving an equilibrium between classical and quantum dynamics during inflation. This mechanism potentially leads to the establishment of a DM density. The research offers a comprehensive quantification of DM abundance as well as the perturbation spectrum for a DM component sourced by a scalar field. Crucially, it is demonstrated that these scenarios predict enhanced structure formation, thereby allowing for the testing of models where DM interacts predominantly or solely gravitationally with other matter.

The paper scrutinizes the requirements necessary to meet cosmological observations, particularly with regard to DM isocurvature perturbations. Compliance with stringent limits derived from the Cosmic Microwave Background radiation is essential. Utilizing scalar fields, which are abundant in extensions of the Standard Model, the paper suggests they may naturally set initial conditions for non-thermal DM production post-inflation.

For a more technical insight, a Lagrangian is presented, defined by the equation $$\mathcal{L}_{\rm DM}= \frac{1}{2}\partial^\mu\chi\partial_\mu\chi - \frac{1}{2}m^2\chi^2$$, where $\chi$ denotes a scalar field decoupled from radiation and minimally coupled to gravity. The paper reasons that scalar fields could inherently reach an equilibrium state during inflation, circumventing specific initial conditions.

This work also contrasts its findings with classical approaches, including the misalignment mechanism, and examines dynamics such as the quantum stochastic equilibrium attained by the field. By applying the principles explained for scalar field dynamics, the study finds a parameter space where the typical field value after inflation can explain the observed DM abundance without conflicting with CMB constraints on isocurvature.

The implications of this theoretical framework are significant, especially in terms of enhancing our understanding of small-scale structure formation in the universe. The consideration of scalar fields as potential DM constituents enriches the theoretical possibilities for what constitutes DM and under what conditions it forms. This paradigmatic approach may influence future models or experiments that probe the gravitational interactions of DM with standard model particles and could delineate new pathways for understanding the role of scalar fields in the broader context of cosmological structure formation.

In sum, by proposing a model where DM can originate from scalar field fluctuations during inflation, the paper contributes to the broader discourse on identifying cosmic inflation conditions that might lead to the DM density observed today. Future research could extend these insights, exploring more complex interactions or multi-field scenarios, and how these affect DM density and distribution in the universe.