The End of the Dark Ages: First Light
JWST will be a powerful time machine with infrared vision that will peer back over 13.5 billion years to see the first stars and galaxies forming out of the darkness of the early universe.
Why is a powerful infrared observatory key to seeing the first stars and galaxies that formed in the universe? Why do we even want to see the first stars and galaxies that formed? One reason is... we haven't yet! The microwave COBE and WMAP satellites saw the heat signature left by the Big Bang about 380,000 years after it occurred. But at that point there were no stars and galaxies. In fact the universe was a pretty dark place.
The Early Universe
After the Big Bang, the universe was like a hot soup of particles (i.e. protons, neutrons, and electrons). When the universe started cooling, the protons and neutrons began combing into ionized atoms of hydrogen (and eventually some helium). These ionized atoms of hydrogen and helium attracted electrons turning them into neutral atoms - which allowed light to travel freely for the first time, since this light was no longer scattering off free electrons. The universe was no longer dark! But we still don't really know what the universe's first light, created by sources (stars) that fused these hydrogen atoms into more helium, looked like.
JWST will be able to see back to about 200 million years after the Big Bang. But why do we need infrared light to see what was going on in the early universe?
Imagine light leaving the first stars and galaxies nearly 13.6 billion years ago and traveling through space and time to reach our telescopes. We're essentially seeing these objects as they were when the light first left them 13.6 billion years ago. Because the universe is expanding, the farther back we look, the faster these objects (like the first stars and galaxies) are moving further away from us, which means that their light is being shifted towards the red. Their light is what we call "redshifted."
Redshift means that light that is emitted by these first stars and galaxies as visible or ultraviolet light, actually gets shifted to redder wavelengths by the time we see it here and now. For very high redshifts (i.e., the farthest objects from us), that visible light is generally shifted into the near- and mid-infrared part of the electromagnetic spectrum. For that reason, to see the first stars and galaxies, we need a powerful near- and mid-infrared telescope, which is exactly what JWST is!
Watch how JWST's ability to look farther into space than ever before will bring newborn galaxies into view: