Galaxies over time
Galaxies today are vastly different in size and shape from the early galaxies that have been seen by the Hubble Space Telescope. The JWST will trace the evolution of the earliest galaxies, showing how they evolve over time.
The JWST will also be a uniquely powerful tool in the study of the effects, and perhaps reveal the nature of, the mysterious dark components of the Universe: Dark Matter and Dark Energy. It is known that these make up more than 90% of the mass of the Universe, but their nature is as yet unidentified.
It is relatively easy for modern astronomers to 'see' Dark Matter, albeit indirectly: the effects of its gravitational pull are clearly observable in many distant galaxies from measurements of the velocity of stars at the edges of spiral galaxies. Cosmologists have inferred the existence of Dark Energy from the latest studies on the geometry of the Universe.
Although the JWST, like the other telescopes, can only observe luminous objects, it will be able to detect subtle distortions in the shapes of the most distant galaxies caused by the gravitational deflections of masses that cannot be seen directly. In this way it will be possible to infer how this Dark Matter is distributed.
Knowledge about the relationship between dark and luminous matter lies at the heart of understanding the evolution of galaxies like our own. In addition, the total density of Dark Matter and Dark Energy decides the global geometry of the Universe and its final fate: will it expand indefinitely or collapse at some distant future?