A paradigm shift in the standard model of cosmology took place about two decades ago with the discovery of the accelerated expansion of the universe using Type Ia supernovae as distance indicators. The discovery of "dark energy" was possible through repeated observations of small patches of the sky, a few weeks apart. Today, a revolution in the science involving astronomical transients at optical wavelengths is taking place: we are able to study nearly the entire sky in the search for rare cosmic events, even with characteristic time scales of hours or even minutes.
The intermediate Palomar Transient Factory (iPTF) - soon to be upgraded to the much larger Zwicky Transient Facility (ZTF) - is leading this exciting development. Among the most spectacular recent findings by iPTF rank the detection of a multiply-imaged lensed Type Ia supernova. Another area of great interest is the search for electromagnetic counterparts of gravitational wave signals from mergers of compact objects.