The James Webb Space Telescope will rewrite cosmic history — if it works

By | December 5, 2021

Natalie Wolchover writes:

To look back in time at the cosmos’s infancy and witness the first stars flicker on, you must first grind a mirror as big as a house. Its surface must be so smooth that, if the mirror were the scale of a continent, it would feature no hill or valley greater than ankle height. Only a mirror so huge and smooth can collect and focus the faint light coming from the farthest galaxies in the sky — light that left its source long ago and therefore shows the galaxies as they appeared in the ancient past, when the universe was young. The very faintest, farthest galaxies we would see still in the process of being born, when mysterious forces conspired in the dark and the first crops of stars started to shine.

But to read that early chapter in the universe’s history — to learn the nature of those first, probably gargantuan stars, to learn about the invisible matter whose gravity coaxed them into being, and about the roles of magnetism and turbulence, and how enormous black holes grew and worked their way into galaxies’ centers — an exceptional mirror is not nearly enough.

The reason no one has seen the epoch of galaxy formation is that the ancient starlight, after traveling to us through the expanding fabric of space for so many billions of years, has become stretched. Ultraviolet and visible light spewed by the farthest stars in the sky stretched to around 20-times-longer wavelengths during the journey here, becoming infrared radiation. But infrared light is the kind of atom-jiggling light we refer to as heat, the same heat that radiates from our bodies and the atmosphere and the ground beneath our feet. Alas, these local heat sources swamp the pitiful flames of primeval stars. To perceive those stars, the telescope with its big perfect mirror has to be very cold. It must be launched into space.

The catch is that a house-size mirror is too large to fit in any rocket fairing. The mirror, then, must be able to fold up. A mirror can only fold if it’s segmented — if, instead of a single, uninterrupted surface, it’s a honeycomb array of mirror segments. But in order to collectively create sharp images, the mirror segments, after autonomously unfolding in space, must be in virtually perfect alignment. Spectacularly precise motors are needed to achieve a good focus — motors that can nudge each mirror segment by increments of half the width of a virus until they’re all in place. [Continue reading…]

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