Evidence for the Existence of First Ever Stars in the Universe

Please give me a reference that explains how the expansion of space-time affects the density of matter. I have never found one or gotten an answer from Anton or others. Either the answer to the question is obvious and people ignore my query, or the question is irrelevant or the question is malformed. Can you help me resolve the point. I genuinely want to know. Please help me.

Thanks, Anton. The beginnings of the universe are becoming clear-er. We still have much to learn, much to tease out of the little data we can access. Very interesting work, from both teams. And, of course, yourself

How large was the Universe at the time these Pop 3 stars? The Pop 3 stars existed when the universe was 800 million years old. The expression "when the Universe was 800 million years old" means that the Universe has been expanding for 800 million years. How large was the Universe when the Universe was 800 million years old?