In this decade, we anticipate a complete window for observing the Universe with advanced multimessenger survey instruments for electromagnetic radiation, cosmic rays, neutrinos and gravitational waves.

The evolution of the Universe is largely shaped by gravity, giving rise to large scale structure in filaments and voids down to galaxies and their constituents. The associated radiative phenomena indicate an “arrow of entropy” that points to scales generally less than 1 Mpc, where we find interactive and transformative processes such as galaxy mergers, active galaxies, supernovae and gamma-ray bursts (GRBs). On these scales, the Transient Universe serves as a cosmic beacon in the era of reionization to the present. Thus, entropy appears to be increasing, from an initially low value at the birth of the Universe as conjectured by Penrose, with conceivably jumps in some of the brightest and most extreme transient events.

Multimessenger astronomy aims at the measurement of physical and astronomical parameters across various observational windows, in and beyond the electromagnetic spectrum. It promises a probe of gravity with the potential to discover the relationship between large structure formation by dark matter, galaxy formation, star formation and their end products, to unravel the astronomical origin and physical mechanism giving rise to active galactic nuclei, core-collapse supernovae (CC-SNe) and GRBs.

Ultra-high energy cosmic rays (UHECRs) and cosmological GRBs stand out as the most relativistic transient events that may be telling us about gravitation in the strongly nonlinear regime in the spacetime around black holes.