In the graph, the galaxy emitted the light when the universe was 2.5 billion years old. The redshift of those photons allow us to calculate when they were emitted, what the distance of the galaxy was at that time, and what the present-day distance of the galaxy is. Suppose that we're now observing photons from a distant galaxy. Over time, our galaxy moves on the black vertical line, and we're currently located at the black dot: the current age of the universe is 13.8 billion years. The horizontal axis shows the distance to us, and the vertical axis is the cosmic time. The graph below shows the path of a photon in an expanding universe (based on the Standard Model of cosmology and the latest data). A photon from a distant source has to overcome the expansion of the universe in order to reach us. And only now that light has reached and overtaken us. It looks like as if there was a race between our point running away from those galaxies (with the expansion of universe and space) and the light that was emitted at that time. If this is so, what is the theoretical limit we can see far back in the past? Therefore we will never see the light that old (close to the time of Big Bang) no matter how powerful the telescope. But if that is so, then wouldn't it put a limit on the oldest light we can see, no matter how powerful the telescope is (even it is more powerful than the James Webb Space Telescope)? This should be expected, because at the time just after the Big Bang, the light emitted by all objects must have already overtaken all other objects, including the location of earth. If so, why is it that it is only now, after 13.2 billion years later the light from those galaxies has reached us? Or in other words, are we sure that the light we are seeing from those galaxies indeed travelled for 13.2 billion years?
The point at which Earth exists now must have been quite close to those galaxies back then. Today, with the help of Hubble Space Telescope, we are able to capture the light of galaxies emitted at that time. 13.2 billion years ago the universe was rather small, having started only half a billion years ago.
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