VLBI20-30: a scientific roadmap for the next decade -- The future of the
  European VLBI Network
  This white paper describes the science case for Very Long Baseline
Interferometry (VLBI) and provides suggestions towards upgrade paths for the
European VLBI Network (EVN). The EVN is a distributed long-baseline radio
interferometric array, that operates at the very forefront of astronomical
research. Recent results, together with the new science possibilities outlined
in this vision document, demonstrate the EVN's potential to generate new and
exciting results that will transform our view of the cosmos. Together with
e-MERLIN, the EVN provides a range of baseline lengths that permit unique
studies of faint radio sources to be made over a wide range of spatial scales.
  The science cases are reviewed in six chapters that cover the following broad
areas: cosmology, galaxy formation and evolution, innermost regions of active
galactic nuclei, explosive phenomena and transients, stars and stellar masers
in the Milky Way, celestial reference frames and space applications. The
document concludes with identifying the synergies with other radio, as well as
multi-band/multi-messenger instruments, and provide the recommendations for
future improvements. The appendices briefly describe other radio VLBI arrays,
the technological framework for EVN developments, and a selection of spectral
lines of astrophysical interest below 100 GHz. The document includes a glossary
for non-specialists, and a list of acronyms at the end.
