Out of the Darkness + Webb will be able to see back to when the first bright objects (stars and galaxies) were forming in the early universe. Credit: STSci

Why Infrared?

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 combining 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 opaque! However, it would still be some time (perhaps up to a few hundred million years post-Big Bang!) before the first sources of light would start to form, ending the cosmic dark ages. Exactly what the universe's first light (ie. stars that fused the existing hydrogen atoms into more helium) looked like, and exactly when these first stars formed is not known. These are some of the questions Webb was designed to help us to answer. See also our Q&A with John Mather about the Big Bang.

Shifted Light

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. By the time this light reaches us, its color or wavelength has been shifted towards the red, something we call a "redshift." Why? In this particular case, it's because when we talk about very distant objects, Einstein's General Relativity comes into play. It tells us that the expansion of the universe means it is the space between objects that actually stretches, causing objects (galaxies) to move away from each other. Furthermore, any light in that space will also stretch, shifting that light's wavelength to longer wavelengths. This can make distant objects very dim (or invisible) at visible wavelengths of light, because that light reaches us as infrared light.

+ Webb will be able to see back to about 100 million - 250 million years after the Big Bang. But why do we need to see infrared light to understand the early universe? Because light from these objects is shifted to the red. Credit: Aleš Tošovský + Electromagnetic Spectrum Characteristics Credit: NASA

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 Webb is!