Want to pursue a second PhD
I'm finalizing my PhD in structural engineering and was thinking about getting an another PhD in mechanical engineering. After I have discussed this with my adviser, I do not think that it was the right move for me (mainly due to age [I'm 29 now, no family yet and number of publication is high in my case]). I know a friend who was getting his PhD in structural engineering while getting an MS in Math. He likes it! 

It is a personal decision. If you are doing this to compensate for what do you think is not a "satisfactory work" during you PhD, look at the facts! you graduate, got couple of publications and a faculty position! You can always self-improve, explore a different reach area or engage in interdisciplinary field. If you measuring successes by the number of papers, then, more papers will come with time! You do not need a "degree" to be satisfied. However, if space-time is something that you are very interested in, then the heart wants what the heart wants. 

I found this the other day while surfing the internet that might help you; "At least for a while, in your heart of hearts you'll be confident only that the university will soon enough discover its error in awarding you the PhD, and at some point will brand you (in public no doubt) the fraud you know you are.  This too will pass as you come to realize that students are extremely gullible and many of your colleagues are even greater frauds than you.  That is to say, you know more than you think you do, and students and the people you work with will appreciate the range and depth of your knowledge and abilities if you let them." The Transition from Graduate Student to Assistant Professor