ETHOS - An Effective Theory of Structure Formation: From dark particle physics to the matter distribution of the Universe

Abstract: We formulate an effective theory of structure formation (ETHOS) that enables cosmological structure formation to be computed in almost any microphysical model of dark matter physics. This framework maps the detailed microphysical theories of particle dark matter interactions into the physical effective parameters that shape the linear matter power spectrum and the self-interaction transfer cross section of non-relativistic dark matter. These are the input to structure formation simulations, which follow the evolution of the cosmological and galactic dark matter distributions. Models with similar effective parameters in ETHOS but with different dark particle physics would nevertheless result in similar dark matter distributions. We present a general method to map an ultraviolet complete or effective field theory of low energy dark matter physics into parameters that affect the linear matter power spectrum and carry out this mapping for several representative particle models. We further propose a simple but useful choice for characterizing the dark matter self-interaction transfer cross section that parametrizes self-scattering in structure formation simulations. Taken together, these effective parameters in ETHOS allow the classification of dark matter theories according to their structure formation properties rather than their intrinsic particle properties, paving the way for future simulations to span the space of viable dark matter physics relevant for structure formation.

ETHOS - An Effective Theory of Structure Formation

The paper "ETHOS - An Effective Theory of Structure Formation: From dark particle physics to the matter distribution of the Universe" by Cyr-Racine et al. introduces the ETHOS framework, designed to bridge dark matter microphysical theories with observable structure formation in the Universe. The framework aims to translate the complexities of particle dark matter interactions into effective parameters that influence cosmological simulations, thereby facilitating the exploration of a wide array of dark matter models without individually simulating each specific scenario.

The ETHOS framework builds on the recognition that diverse dark matter models can yield similar cosmological structures, provided they are characterized by comparable effective parameters. These parameters affect pivotal quantities such as the linear matter power spectrum and the self-interaction transfer cross section of nonrelativistic dark matter. Specifically, the framework defines a mapping from the parameters of dark matter particle physics models to these effective parameters, streamlining the incorporation of complex dark sector physics into structure formation simulations.

Framework and Methodology

The ETHOS framework provides a methodical approach to map microphysical theories to cosmological parameters. This mapping involves:

1. Characterizing Interactions: The framework considers dark matter that may self-interact or interact with other particles (like dark radiation). The interactions are quantified by parameters such as the dark matter coupling to relativistic components and self-scattering interaction cross sections.

2. Mapping Parameters: Parameters related to dark matter microphysics (e.g., particle masses, coupling constants) are translated into effective parameters that influence the linear matter power spectrum and structure formation. This includes terms that affect the drag opacity between dark matter and dark radiation, alongside sound speed calculations for dark matter temperature evolution.

3. Simulation Input: These effective parameters serve as inputs for cosmological simulations. By simulating ETHOS parameters instead of individual models, researchers can effectively constrain many dark matter paradigms simultaneously.

Key Findings and Implications

The ETHOS framework enables a model-independent analysis of dark matter by understanding how variations in microphysics translate into observable cosmological phenomena. Some significant points and implications include:

• Non-Relativistic Assumptions: The approach concentrates on non-relativistic dark matter and leverages this to simplify the calculation of structure formation outcomes.

• Handling Complex Interactions: ETHOS systematically captures the effects of complex dark matter interactions, including those involving dark radiation and self-scattering, which are pivotal in addressing current discrepancies in galactic and sub-galactic scales.

• Intermittent Simulations: By covering effective parameter spaces, ETHOS reduces the need for multiple iterations of extensive simulations on distinct particle models, thus conserving computational resources.

The findings presented suggest ETHOS could significantly expand its exploratory scope to include warm or decaying dark matter scenarios, potentially offering insights into a broad spectrum of dark matter-related phenomena.

Prospects for Future Developments

This framework sets the stage for further investigation into the parameter space of dark matter interactions. Future developments could include:

• Inclusion of Additional Particle Models: ETHOS can be extended beyond current interaction types, potentially encompassing radiative or decaying dark matter models.

• Improved Simulations: Utilizing ETHOS can lead to high-resolution simulations that might offer solutions to enigmas like the missing satellite problem or core-cusp issues in galaxy profiles.

Overall, the ETHOS framework prioritizes efficiency and model generality, which broadens the horizons for studying dark matter’s role in structure formation and cosmological evolution. Its application could drive significant advances in our understanding of both particle physics and cosmological interactions.