A key idea in science is
consilience; widely diverse areas of knowledge turn out to have connections and provide mutual support for each other in various ways. This applies across the board; linguistics, biogeography, geology, psychology, and everything else. It does not mean that the distinctions between scientific domains is meaningless; it means that since they all work in the one world there is often overlap in some of the problems.
To press your analogy: if scientists are like detectives, then they may all apply similar methods, and they may share their sources; but they still keep their case files clearly distinct!
That creationism has problems with both evolution and cosmology is not an adequate justification for combining them. On that basis we should include geology, linguistics, archaeology, history, philosophy, and a few other fields of study as well! Actually, I do think we should pay much more attention to astronomy generally in the whole creationism debate; it is not merely the Big Bang that causes them difficulties. Just the size of our own galaxy is inconsistent with young earth creationism.
Sometimes evolution and cosmology are singled out for combination because of a metaphysical notion of progress. Ironically, this is regarded as a misleading by many mainstream cosmologists and biologists. Evolution has no ordained direction, as far as we can tell. The concept of a grand ladder of nature that celebrates a march of progress all the way from primordial quark-gluon soup to humanity is an anthropomorphic notion that is more misleading than useful, in my opinion.
The unity of knowledge encompasses all the fields of science; but we can identify different domains of interest. There is nothing at all as far as I can tell in biological evolution that gives a useful insight into cosmology, or vica versa. There may be analogies; but these are as often misleading as they are illuminating. The technical details are totally distinct.
Philosophically there is an interesting distinction as well. Big bang cosmology attracts a lot of criticism -- most of it incompetent -- from individuals who perceive it as a kind of creationist notion, due to the notion of a beginning of time. A number of the big bang critics are atheists or agnostics with a strong philosophical aversion to origins of any description.
Cheers -- Sylas
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