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No matter where you look in the universe, in any direction, your line of sight will eventually run into some type of matter or radiation. The Earth is embedded in the solar system, with planets, moons, rocky and icy bodies, dust, and plasma particles permeating our environment. Beyond our own backyard are stars, gas, and dust strewn throughout the Milky Way, and at even greater cosmic distances are galaxies, quasars, and the matter in the intergalactic medium. If you somehow manage to pick a line of sight that doesn’t run into any of those, you’ll still encounter something: the cosmic microwave background, which is thought to be the leftover radiation from the early stages of the hot Big Bang.
And yet, no matter what we observe in any direction, two properties will correspond to whatever object we see:
- We are seeing the object not as it is today, but as it was a finite amount of time ago: when it emitted the light that’s now striking our eyes.
- That object is currently a specific distance away from us; if we could somehow “freeze” time and measure the distance between ourselves and that object, we’d get a certain value.
You might think that these two properties — time and distance — would be equal. A star whose light arrives after a journey of 10 years is 10 light-years away; a galaxy whose light arrives after a 100-million-year journey is 100 million light-years away; light from the Big Bang that arrives after a 13.8-billion-year journey has the emitting location 13.8 billion light-years away.
But that isn’t true at all — and the expanding universe is to blame.
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Artist’s logarithmic scale conception of the observable universe. Galaxies give way to large-scale structure and the hot, dense plasma of the Big Bang at the outskirts. This ‘edge’ is a boundary only in time. (Credit: Pablo Carlos Budassi; Unmismoobjetivo/Wikimedia Commons)
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James' World 2 
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