What constitutes a fact for you? It's a theoretical fact even though it's been detected in an analog experiment. I suspect that will be the only way we can actually detect Hawking radiation. William Unruh is the author of the paper revealing the success of an analog experiment. At least one of the authors. The summary gives a more realistic picture than me. Pretty cool esperiment. http://arxiv.org/abs/1008.1911 Actually that is only one of several claims to have detected Hawking radiation in analog experiments. Always be lots of skepticism by those who think analog experiments are somehow fake.