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The speed of light
YECs have a problem with mainstream astronomy. If the Universe truly is as big as real astronomers say it is, and the Earth truly is as young as YECs want it to be, it shouldn't be possible to see anything in the night sky that's more than a few thousand light-years away from Earth. In reality, most of the things we do see in the night sky are at least tens of thousands of light-years from Earth. Some YECs address this problem by attempting to argue that the Universe is actually not so big as real scientists say it is; others (most prominently Barry Setterfield) argue that light used to travel at near-infinite velocity, and that the speed of light has been continually decaying since the moment of Creation.
The problem with YEC "c decay" theories is not that mainstream science dogmatically rejects any and all hint that lightspeed might not be constant. Instead, the problem is that real scientists have looked for evidence of past changes in lightspeed, and the evidence is consistent with changes at a rate of no more than a tiny fraction of a percent per billion years. Since the YEC version of "c decay" explicitly requires the speed of light to have altered by several orders of magnitude (i.e., far too much change) within a paltry few thousand years (i.e., far too little time), real scientists conclude that Creationist "c decay" theories simply aren't supported by the evidence.