Yes, because these events may have been much more frequent in the early universe http://www.nature.com/scitable/blog/postcards-from-the-universe/a_possible_ans

Yes, because these events may have been much more frequent in the early universe http://www.nature.com/scitable/blog/postcards-from-the-universe/a_possible_answer_to_fermi8217s http://www.nature.com/scitable/blog/postcards-from-the-unive... http://arxiv.org/abs/astro-ph/9901322 http://arxiv.org/abs/astro-ph/9901322