There are roughly two answers, probably neither one is entirely satisfying. If you forget general relativity for the moment then one possible answer is that the net energy is zero. The galaxies and other stuff have just enough energy to climb out of the gravitational potential well and will reach zero average relative speed when they are infinitely far apart. So the energy didn't have to come from anywhere, it was just zero and got divided up into negative gravitational energy and kinetic energy. But what, you will ask, about the mass energy to the matter which will still exists even though infinitely dilute? Well it's almost negligble but it's still a few percent of the total mass energy, so maybe the universe will recollapse (although it doesn't look like it). The second answer is, in a curved spacetime, like our universe, if there's no time-like Killing field then there's no way to define a conserved quantity you could call "the energy of the universe". John Baez has a good write up on this. Brent Meeker On 9/8/2014 8:54 PM, Simon Plouffe wrote:
Hello,
let me rephrase the question, I know of course the concept of relative speed, even if we take into account that, at the moment of the big bang, the whole thing was going at the speed of light. Just take 1 galaxy and if we calculate the amount of kinetic energy to move a few kilometres per second , the amount of energy necessary is astronomical, how can we explain that ?, Where is the energy coming from ?
Best regards, Simon Plouffe
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