Hubble sees primarily ultraviolet and visible light and lacks the capability to explore the periods when the first luminous objects formed - known as the Cosmic Dark Ages. To see this we need a telescope exquisitely sensitive to infrared light. As Heinz points out, because the Universe is still expanding, the farther away something is the faster its receding from us. Because light travels at a finite speed, the expansion stretches lights wavelength (red shifts) into the infrared. The further away an object, the more red shifted it is into the infrared spectrum. Webb was designed to address this issue. Its 20 foot primary mirror and infrared wavelength sensitivity enables us to look further back in time than ever before, almost to the cusp of the Big Bang. A time machine, in effect.
Webb is a marvellous machine. It will explore not only the frontiers of cosmology but is also designed to "sniff" the atmospheres of exoplanets, perhaps detecting the chemistry that makes life possible.
Expect many more revelations.
