The Telescope As A Time Machine
Astronomy is history. Because light takes time to travel from one place to another, we see objects not as they are now but as they were at the time when they released the light that has traveled across the universe to us. Astronomers can therefore look farther back through time by studying progressively more-distant objects.
The chief difficulty in employing this "time machine" to observe the cosmos during its past epochs arises from the fact that distant objects appear fainter than closer ones. We must therefore capture and analyze the light from progressively dimmer objects as we push farther back into the past. Specialized instruments are needed to study our nearest neighbor, the Andromeda Galaxy, which is 2 million light-years away. But this companion of the Milky Way shines a million times more brightly than a similar galaxy seen at a distance of 2 billion light-years! This comparison gives a sense of how difficult it will be to obtain images of objects formed close to the Big Bang era more than 10 billion years ago.
During the past few years, the Hubble Space Telescope has obtained long-exposure images that reveal the faintest objects ever detected. Some of these objects are galaxies seen during their early developmental stages when they were rich in young, hot, and very luminous stars. To peer still farther back through time, to reach the era when stars first began to shine, astronomers need a telescope that can detect extremely low intensities of infrared light. Astronomers need sensitivity in the infrared part of the spectrum because the light from these young stars in distant galaxies, even though emitted as visible light, has been stretched by the expansion of the universe to appear to us as infrared light.
The Hubble Space Telescope can observe the shortest-wavelength portion of the infrared domain, but its 2.4-meter mirror is too warm and too small to detect the faint glow from the most distant young galaxies. To observe galaxies in their earliest epochs, the survey report recommends a new, advanced- technology telescope designed to work best in the infrared part of the spectrum. In an orbit a million miles from Earth, this telescope will become so cold that its own infrared glow will be insignificant compared with the light from the distant galaxies, something an earthbound telescope could never achieve. Also, being above Earth's veil of air allows us to see radiation that cannot penetrate it, and guarantees the sharpest images the telescope can deliver, free from the turbulence in Earth's atmosphere that handicaps telescopes on the ground.
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