The James Webb Space Telescope (JWST) has elicited well-deserved plaudits for its ability to peer far back into the early days of our universe, only a few hundred million years after the Big Bang that birthed it. Webb’s infrared vision lets it see light from some of the first galaxies, light that has been stretched far beyond visible wavelengths by the expansion of our cosmos.
https://pbs.twimg.com/media/FaPyRUeWQAEe-PY?format=jpg&name=4096×4096
One such galaxy, provisionally dubbed Maisie’s Galaxy, if confirmed, would be the youngest yet, dating from 300 million years after the beginning. This means its light is 13.5 billion years old. That should mean that it is 13.5 billion light years away, right? Nope. Wrong.
When we move out to cosmological distances, we start to get a disconnect between time and distance. This is not so in the neighborly confines of our solar system. When we read that images from a Mars rover take thirty minutes to reach Earth, we understand that this is due to the finite speed of light. Mars and the rover are thirty light minutes away.
But at cosmological distances, where we are talking about galaxies whose light took billions of years to reach us, the expansion of the universe complicates matters. This doesn’t apply in the solar system. The Sun’s gravity is a much stronger force than expansion, and keeps the planets in their orbits.
The light from Maisie’s Galaxy was emitted 13.5 billion years ago and has taken 13.5 billion years to reach us. In that intervening time, the universe has continued to expand. Maisie’s Galaxy is now (with several simplifying assumptions, so don’t take this as gospel truth) nearly 32 billion light years away.
Its light-travel-time distance is 13.5 billion light years. Its co-moving distance is 32 billion light years.
https://universe-review.ca/I02-06-expansion.jpg
There is an additional factor that makes JWST’s feats even more impressive. The angular size of the galaxy—how large it appears in the sky—was set for us when the universe was much smaller, and the galaxy occupied more of the heavenly view. This spreads its already sparse light, diminished by its long passage, over more area, making it even dimmer and harder to see.
If this seems to violate your common sense, consider that common sense is based on experience. Unless you are a professional cosmologist or a space nerd, such matters are not part of your experience. Nevertheless, they are real, and further evidence that we live in a wondrous cosmos!
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