micrOMEGAs 7: Beyond standard cosmology

This paper introduces micrOMEGAs 7, a major software upgrade that extends dark matter calculations beyond standard cosmology by enabling user-defined modifications to the Hubble expansion and entropy evolution, while also incorporating updated experimental constraints and improved treatments for sub-GeV dark matter and indirect detection.

The Gist

Imagine the universe as a giant, expanding balloon. For decades, scientists have used a specific set of rules to calculate how much "dark matter" (the invisible stuff holding galaxies together) should be left over from the Big Bang. These rules assumed the balloon was filled with a specific type of gas (radiation) that cooled down in a predictable way.

The paper you provided introduces micrOMEGAs 7, a major software upgrade that acts like a "universal calculator" for dark matter. It's not just a better calculator; it's a calculator that can now handle scenarios where the universe didn't follow the standard rules.

Here is a breakdown of what this new version does, using simple analogies:

1. Rewriting the Rules of the Universe's Expansion

The Old Way: Imagine a race where runners (dark matter particles) are trying to pair up and disappear. In the standard model, the track (the universe) expands at a steady, predictable speed. If the runners pair up too slowly, they survive; if they pair up too fast, they vanish. The software used to assume the track always expanded the same way.

The New Way (micrOMEGAs 7): This version realizes that in some theories, the track might expand much faster or slower than expected, or the runners might be injected onto the track late in the race by a mysterious "coach" (a decaying field).

• The Analogy: Think of the universe's expansion as a river. The old software assumed the river always flowed at a constant speed. The new software allows you to say, "What if the river suddenly flooded (expanding faster)?" or "What if a dam broke and dumped a bunch of new swimmers into the river (entropy injection)?"
• Why it matters: It lets scientists calculate how much dark matter would exist in these weird, non-standard scenarios, such as a universe that was dominated by heavy, slow-moving matter for a while before becoming the radiation-filled place we see today.

2. Handling the "Tiny" Dark Matter

The Old Way: The software was great at calculating heavy dark matter (like a bowling ball), but it struggled with very light dark matter (like a ping-pong ball) because the physics gets messy at low energies.

The New Way: The update includes a special "microscope" for light dark matter.

• The Analogy: If heavy dark matter is like a car crashing into a wall, light dark matter is like a feather hitting a cloud. The new version understands that when these tiny particles collide, they don't just break into smaller pieces; they turn into specific, light particles called "mesons" (like pions and kaons). The software now has a dedicated manual for how these specific particles behave, ensuring the math is accurate even for the tiniest dark matter candidates.

3. Checking Against Real-World Evidence

The software is like a detective that checks its theories against three main crime scenes (experiments):

• The Cosmic Microwave Background (CMB): This is the "baby picture" of the universe. The new version checks if the dark matter model would have left a fingerprint on this baby picture by heating up the early universe too much. It's like checking if a suspect's alibi fits the timeline of a crime scene.
• Direct Detection (The "LZ" Experiment): This is like trying to catch a ghost by seeing if it bumps into a wall. The software now includes the very latest results from the LZ experiment (a giant tank of liquid xenon). It has been updated to be more realistic, accounting for the fact that dark matter might have "electromagnetic" properties (like a tiny magnet) that change how it hits atoms.
• Indirect Detection (The "Fermi-LAT" Telescope): This looks for the "smoke" left behind when dark matter particles collide and destroy each other in space. The software now uses updated maps of "dwarf galaxies" (small, dark-matter-heavy galaxies) to see if the predicted smoke matches what telescopes actually see.

4. The Collider "Z'" Check

Particle colliders (like the Large Hadron Collider) smash particles together to create new ones. The new software has a specific tool to check if a hypothetical particle called a Z' (a heavy cousin of the Z boson) has been ruled out by recent data from the CMS experiment. It acts like a quick "pass/fail" test for theories involving this specific particle.

Summary

In short, micrOMEGAs 7 is a massive upgrade to the tool scientists use to predict the amount of dark matter in the universe.

• It stops assuming the universe always expanded the same way.
• It gets much better at handling very light dark matter.
• It updates its "checklist" with the latest data from telescopes, underground detectors, and particle colliders.

It allows researchers to ask "What if?" questions about the early universe and get reliable answers, even if the universe behaved in ways that are very different from our standard textbook description.

Technical Summary

Technical Summary: micrOMEGAs 7 – Beyond Standard Cosmology

Problem Statement
The search for particle dark matter (DM) requires computational tools capable of predicting relic densities and scattering rates across diverse theoretical frameworks. Historically, tools like micrOMEGAs have relied on the standard cosmological assumption of a radiation-dominated early universe with conserved entropy and a fixed relationship between the Standard Model (SM) plasma temperature and the Hubble expansion rate. This framework becomes inadequate for scenarios motivated by inflationary reheating, hidden-sector dynamics, or models involving late-decaying fields, where the early universe may follow a complex thermal history. Furthermore, existing treatments of sub-GeV DM often lack precision regarding non-perturbative QCD effects (hadronic annihilation), and direct detection routines have not fully incorporated effective electromagnetic couplings or the latest experimental limits from the LZ collaboration.

Methodology
The authors present micrOMEGAs 7, a major upgrade that generalizes the treatment of Boltzmann equations to accommodate non-standard cosmological histories. The core methodology involves:

1. Generalized Background Evolution: The code solves coupled Boltzmann equations for the energy density of a decaying field $\phi$ (which may represent an inflaton or other heavy component) and the entropy density of the SM thermal bath. This allows for user-defined modifications to the Hubble expansion rate ($H$) and entropy evolution. The framework introduces parameters for the equation of state ($w$) of the decaying field and a scaling parameter ($\alpha$) governing the temperature evolution of the SM bath ($T \propto a^{-\alpha}$) during reheating.
2. Non-Thermal DM Production: The DM number density evolution equation is extended to include source terms from the decay of $\phi$ into DM particles, alongside standard annihilation and semi-annihilation processes. This enables the calculation of relic densities in scenarios such as low-temperature reheating, early matter domination, and kination.
3. Sub-GeV Hadronic Treatment: For DM masses below the GeV scale, the code implements a dedicated framework for annihilation into light mesons via a scalar mediator. This utilizes three parameterizations: leading-order chiral perturbation theory ($\chi$PT-LO), energy-dependent scalar form factors constrained by dispersion relations, and an alternative dispersive construction.
4. Direct and Indirect Detection Updates:
   • Direct Detection: The code incorporates a recast of recent LZ results (2022 and 2024 analyses) and extends the elastic scattering routine to include photon exchange interactions, accounting for dipole moments (spin-1/2) and quadrupole moments (spin-1).
   • Indirect Detection: New tables for particle spectra (photons, positrons, antiprotons, neutrinos) are added, extending the mass range down to 0.5 GeV for quark pair annihilation and to the meson mass for meson pair annihilation. The code integrates Fermi-LAT limits from dwarf spheroidal galaxies and updated CMB constraints (Planck) on energy injection during recombination.
5. Collider Constraints: A new routine implements 95% C.L. limits on $Z'$ mediators based on CMS dilepton resonance searches using 140 fb$^{-1}$ of Run 2 data.

Key Contributions and Results

• Non-Standard Cosmology: The code now supports a generalized background where the Hubble parameter receives contributions from early matter domination or kination, and entropy is injected via late decays. Validation against independent numerical solutions shows excellent agreement for various $w$ and $\alpha$ choices.
• Sub-GeV Precision: The implementation of scalar-meson interactions provides a consistent description of sub-GeV DM phenomenology, with updated spectra tables extending down to 0.5 GeV.
• Experimental Constraints:
  • LZ Recast: The code reproduces LZ5T limits for spin-independent scattering with high fidelity, though the implementation is noted as conservative (less restrictive than the official LZ limit for masses below 20 GeV).
  • CMB-ION: The code calculates the effective energy deposition fraction ($f_{eff}$) for CMB constraints, showing agreement within a few percent with previous literature (Ref. [24]).
  • Fermi-LAT & CMS: Limits from dwarf spheroidal galaxies and CMS dilepton searches are now directly accessible within the package.
• Direct Detection Extensions: The inclusion of photon exchange diagrams allows for the computation of direct detection signals for DM with electromagnetic moments, a feature previously unavailable for loop-induced interactions.

Significance
The authors claim that micrOMEGAs 7 provides a unified and robust tool for exploring early-universe histories that were previously difficult to treat within a single numerical code. By lifting the assumptions of radiation domination and entropy conservation, the package enables reliable relic density calculations in scenarios where the standard freeze-out or freeze-in pictures are distorted. The integration of updated experimental constraints (LZ, Planck, Fermi-LAT, CMS) and the improved treatment of light DM and electromagnetic couplings ensure consistent comparisons between cosmology, astrophysics, and collider probes across the viable parameter space. The release represents a significant step toward a comprehensive framework for testing particle DM models in both standard and non-standard cosmological settings.