One of the problems for young-Earth creationism is that we are seeing light from stars more than 6,000 light-years away, which implies that those stars pre-date the alleged date of the creation.
To get around this creationists have proposed that the speed of light used to be much higher:
The author claims that computer analysis of the measurements of light velocity recorded since 1675 shows otherwise and points to a beginning—a creation of light and a subsequent slowing down of this created light. The date of this beginning appears to support the short Biblical chronology.
(Note: I have seen this claim made in many places, and I am not concerned here with refuting the specific evidence on that page. I am merely establishing that this is a notable claim).
Edit: The article quoted above is dated 1981, although the page is still live on Answers in Genesis. Creationists have since backed off from the claim that historical measurements demonstrate that light has slowed down. Some still argue that it might have slowed down, but that scientists are ignoring this possibility. For instance the Institute for Creation Research has an article dated 2003, but still available on their website now, which seems to argue (if I understand the word-salad correctly) that this is possible. Other creationists have taken refuge in more complex theories involving a geocentric or sun-centric universe in which the speed of light or the expansion of space varies in a way that puts Earth in a privileged position such that the essential asymmetries are only invisible from the point of view of Earth.
At first sight the proposition that light has slowed seems to be an irrefutable theory. However it occurs to me that there may be astronomical phenomena which provide evidence for the speed of light at or near the time the light was emitted. An example would be a nova in a gas cloud: we would be able to see the light-sphere from the explosion growing through the cloud. Another would be a pulsar in a gas cloud, in the same way.
If we can see that light from an object 10,000 LY away was travelling at the same speed when it left the object then it would be evidence to refute the creationist argument that the speed of light might have changed in the last 10,000 years.
Is there good evidence showing whether the speed of light has slowed down over the past 6,000 years?