The transient and variable universe

Astronomy is undergoing a dramatic revolution in terms of our ability to monitor the time-variability of the Universe in a continuous way using new facilities coupled with fast computers. This opening of the temporal domain is transforming our knowledge of how the Universe evolves, particularly for objects which are undergoing explosive change, such as supernovae and gamma-ray bursts (GRBs). These explosive events can release enormous amounts of energy both in electromagnetic radiation and in non-electromagnetic forms such as neutrinos and gravitational waves. They lie at the frontier our understanding of the laws of physics under the most extreme conditions.

Current observing facilities enable the sky to be monitored fairly continuously in real-time over large areas and in electromagnetic radiation, gravitational waves, and neutrinos, capturing the temporal behavior of the Universe in a way previously unattainable. Example electromagnetic facilities include the LOFAR radio telescope, the Pan-STARRs optical facility, and the Swift and Fermi high-energy satellites. Non-electromagnetic facilities are also now observing, particularly the Advanced LIGO and Virgo gravitational-wave observatories, which recently found their first sources, and the Antares and IceCube neutrino experiments. The data from all these facilities and their immediate predecessors have already opened the temporal domain, but are just a foretaste of what is to come.

Many of the previously developed theories have come under intense pressure by new observational results, such as the highly variable emission seen at late times in GRBs, the discovery of kilonovae associated with neutron star mergers and the still unexplained fast radio bursts (FRBs). Theoretical models predict a variety of exotic explosions and stellar mergers, together with their multiple signatures across the electromagnetic spectrum. Theory also predicts that some will be accompanied by gravitational waves, neutrinos, and high-energy particle emission.

In the next decade, the number of transients found will increase by several orders of magnitude as more powerful facilities come on-line, in particular the Large Synoptic Survey Telescope (LSST) and the Square Kilometer Array (SKA) in the electromagnetic domain. This evolution will be accompanied by the rapid progress in the sensitivity of all-sky detectors neutrinos (KM3NeT and IceCube) and gravitational waves (Advanced LIGO and Virgo).