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In the quest to understand the fundamental laws of nature, cosmology and particle physics have become increasingly intertwined. Theories beyond the Standard Model (BSM) often predict new particles or interactions with observable consequences for the large-scale structure and evolution of the Universe. Our new paper, “CosmoBit: A GAMBIT module for computing cosmological observables and likelihoods,” introduces a powerful open-source tool designed to bridge the gap between these two fields, enabling truly comprehensive global analyses of new physics. This work by The GAMBIT Cosmology Workgroup, including our own Will Handley, marks a significant step towards a unified framework for testing fundamental theories against the full spectrum of experimental data.

The Challenge of Siloed Analyses

Historically, the tools for cosmological and particle physics analyses have been developed in parallel. Cosmologists use sophisticated packages like CosmoMC (astro-ph/0205436) and MontePython (1210.7183) to compare models against data from the Cosmic Microwave Background (CMB), supernovae, and large-scale structure. Meanwhile, particle physicists employ global fitting frameworks like GAMBIT (1705.07908) to combine results from colliders, direct and indirect dark matter searches, and flavour physics.

This separation creates a major hurdle. A BSM theory, such as one involving new dark matter candidates or modified neutrino properties, makes predictions for both domains. A robust test of such a theory requires a joint analysis that simultaneously considers all relevant constraints. CosmoBit was developed to make this possible within the powerful, modular environment of GAMBIT.

A Unified Framework for Cosmological Inference

CosmoBit functions as the dedicated cosmology module for the Global and Modular BSM Inference Tool (GAMBIT). By leveraging GAMBIT’s sophisticated dependency resolution and statistical sampling engines, it allows researchers to perform combined cosmological and particle physics global fits for the first time.

The power of CosmoBit lies in its modularity and its ability to act as a central hub for a suite of well-established, public codes. Rather than reinventing the wheel, it provides robust interfaces to:

  • Boltzmann Solvers: It seamlessly integrates with CLASS (1104.2933) to compute theoretical predictions for CMB and matter power spectra.
  • Likelihood Packages: It can utilize the official Planck likelihood code (plc) for state-of-the-art CMB analysis, as well as the extensive library of late-time cosmological likelihoods (e.g., BAO, SNe Ia) from MontePython.
  • Specialized Physics Codes: It interfaces with AlterBBN for Big Bang Nucleosynthesis calculations, MultiModeCode for computing primordial power spectra from inflationary models, and DarkAges to model exotic energy injection from dark matter annihilation or decay.

This architecture enables CosmoBit to explore a vast landscape of physical scenarios, from the standard ΛCDM model to extensions involving inflation, dark radiation, modified neutrino properties, and dark matter interactions.

Probing New Physics: An Illustrative Example

To demonstrate its capabilities, the paper presents a detailed analysis of neutrino physics in non-standard cosmological scenarios. The standard cosmological model, as constrained by data from experiments like Planck (1807.06209), places tight limits on the sum of neutrino masses. However, these limits can be relaxed if other cosmological assumptions are altered.

The CosmoBit analysis explores a scenario where both the neutrino temperature and the number of effective relativistic species ($\Neff$) are allowed to vary. The results reveal a significant degeneracy: a decrease in the neutrino temperature can be compensated by introducing additional “dark radiation” to keep $\Neff$ close to its standard value. This has a profound impact on the constraints on neutrino masses. Since the energy density of non-relativistic neutrinos is sensitive to their temperature, a lower neutrino temperature significantly weakens the cosmological bound on their total mass.

This example showcases the power of a true global fit. By simultaneously varying parameters across different physical domains, CosmoBit uncovers complex relationships that would be missed in separate analyses, opening up previously constrained regions of parameter space for new physics. As we enter an era of precision cosmology, tools like CosmoBit will be essential for interpreting the wealth of new data and uncovering the next standard model of our Universe.

Will Handley

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