This month, we'll take a closer look at the cosmic microwave background. Sometimes called 'the afterglow of creation', this extremely faint microwave glow is considered one of the most convincing pieces of evidence for the Big Bang theory. But how is it that we are still observing this ancient light?
As we've seen in earlier parts of this series, space is expanding. Since the amount of matter in the Universe remains fairly constant, the increasing volume means that the average density of cosmic matter is decreasing over time. That means the density of the Universe must have been much higher in the distant past.
Right after the Big Bang, before there were any galaxies, stars or planets, elementary particles were packed almost shoulder to shoulder, and the corresponding temperature was unimaginably high. This hot, dense plasma produced energetic radiation. However, individual light particles - photons - have trouble traversing such plasma-filled environments, as they are continuously absorbed (and re-emitted) by charged particles like electrons. In other words: the hot, early Universe was opaque.
Cosmic treasure trove
Leftover radiation from the explosion is key to unlocking the Universe around us
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