MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
The Universe. Illustration depicting the Universe contained within a sphere. The size of the Universe is limited by the particle horizon - the distance travelled by light since the Big Bang. As the edge of the observable Universe is receding at the speed of light, no signal from farther away may reach us. Therefore, anything 'outside' of this boundary has no physical significance to us as we can never know anything of it. From a human perspective, this means the entirety of nature exists within this spherical shell bounded by the particle horizon.
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