Friedmann Equation in Era of Multi-Messenger Astronomy: Neutrinos, Gravitational Waves and Cosmic Rays

Abstract

The Friedman equation is the cornerstone of modern cosmology, governing the evolution of the universe and providing us with insights into its expansion, energy composition, and large-scale structure. Over the past few decades, the advancement of multi-messenger astronomy integrating data from neutrinos, gravitational waves, and cosmic rays which have greatly enriched the understanding of the universe. Neutrinos from the early universe contribute to the radiation density, and continuous observations tighten the constraints on their mass and interactions even more. Gravitational waves generated by cosmic events such as black hole mergers and cosmic inflation contribute to the energy budget of the universe, thereby refining the models of cosmic expansion and structure formation. Cosmic rays, especially the ultra-high-energy particles detected by observatories like the Pierre Auger Observatory, contribute to the energy density and affect galaxy formation and intergalactic heating.