A review of Neil deGrasse Tyson's latest, "Space Chronicles: Facing the Ultimate Frontier".
Please circle my space science page.
The short answer is that from the vantage point of Earth, the observable Universe appears as a sphere with a radius of 46 billion light years.
It's based on the amount of expansion of space that has occurred between us and the furthest observable objects. Without an expanding Universe, those furthest objects would be 13 billion light years away, but the space behind a photon traveling towards us from that object has expanded to 46 billion light years. I'm currently on my phone, but will try to dig up some papers to share tomorrow.
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