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JWST released the deepest highest resolution view of the infrared universe earlier this week, with people amazed at the distant galaxies, gravitationally lensed by the galaxy cluster SMACS 0723. The light from one of those galaxies comes from 13.1 billion years ago. However, people were surprised to know that the galaxy is not 13.1 billion light-years away. It is actually much further away – 30 billion light-years. So what gives?
Looking deeper into the universe means looking back in time. That is due to the finiteness of the speed of light. Take, for example, Betelgeuse. The red giant is the right shoulder of the constellation of Orion, located around 550 light-years away (with a fair bit of uncertainty). That means that the light that we see now (or the great dimming of two years ago) actually happened half a millennium ago.
As a rule of thumb, for galactic distances, you can turn how long light took to get here into light-years. This also works for intergalactic distances in the very local universe. So, the light from the Andromeda galaxy has traveled for 2.5 million years, and the large spiral galaxy is indeed 2.5 million light-years away.
Once things start being a few billion light-years away, the equivalence doesn’t work anymore. Let’s take SMACS 0723, the cluster at the center of the JWST image. Its light comes from 4.6 billion years ago – but its actual distance from us today is almost one billion light-years more. This is due to the expansion of the Universe.
As the light travels, the universe is slowly but surely expanding. When it comes to large intergalactic distances, the universe has been expanding for a long time – and that adds up. When we get to objects whose light comes from a time very close to the beginning of the universe, we get these enormous distances.
This brings us to another important question: How big is the visible universe? The furthest objects we can see in any direction are around 46.5 billion light-years away, which makes the whole universe 93 billion light-years across.
Well, more or less, because these measurements depend on cosmological properties of the universe – and currently, we are in a bit of a pickle when it comes to that. Different methods to measure the expansion rate of the universe give different numbers, which then affect all the other measurements.
But this is science, after all: the constant process of refining our methods, our models, and our measurements.
