Hi, I've been wondering about some things; (sorry, introduction is about astronomy, but rest is physics) The future of the universe is determined by the quantity of mass. Lets take the example of the big crunch, in which gravity will essentially get the upperhand. Now, One pointed out to me that if mass is equivalent to energy through the famous E=mc² theory, then when we assume that if everything changes into the form of energy (all mass into energy), what will this have as implications to that big crunch? I did some research and i found that essentially, in Einsteins theory of general relativity, mass is needed to form gravitational fields (or the distortion of space-time). Apart from other terms as quantum gravity, which implies gravitons and possibly gravity waves, I thus wonder whether pure energy (like photons, with mass 0) has an implication on gravity (can it make gravitational fields equivalent to those of mass) without the need of the pure energy to convert into solid mass Thanks!