From Patrick Johnson
Discussing the “horizon problem” of the uniformity of the universe, you say “Look across space from one edge of the visible universe to the other…the two edges are nearly 28 billion light years apart and our universe is only 14 billion years old” (19 March, p 31).
This reflects the popular fallacy that the distance from our galaxy to the farthest observable sources of electromagnetic radiation can be no more than 14 billion light years if our universe is only 14 billion years old. It does not take into account that the expansion of the universe involves the expansion of space itself.
By the time the photon reaches us, the actual distance from its source is greater than the distance it would have travelled if space itself had not been expanding. Aside from this technicality, I agree with the gist of what you say about the “horizon problem”.
From Kevin Jones
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Why is the horizon problem a problem? Assuming that the universe did begin with the big bang, and continues to expand to this day, you have radiation travelling 14 billion light years in each direction, with a total distance across the universe of twice that, at 28 billion light years.
Pierceton, Indiana, US
Michael Brooks writes:
• The answer is that the big bang didn’t cause an explosion outwards from a point. Space and time came into being with the big bang, and space has been expanding everywhere ever since.
The 28 billion light years defines the width of the visible universe: we can see that two points separated by this distance seem to have the same temperature, but we can’t understand how.
Portland, Oregon, US
