If matter and energy can't be created or destroyed does this mean that at the start of the big bang the amount of kinetic energy would be the same as it is today? If this is the case, can we measure how much kinetic energy there is our entire universe?

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    $\begingroup$ Kinetic energy can be converted into many other forms of energy. As such, there's no reason to expect that the total amount of kinetic energy should be the same. $\endgroup$ Jun 26, 2018 at 15:33
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    $\begingroup$ Related: physics.stackexchange.com/q/2838/2451 and links therein. $\endgroup$
    – Qmechanic
    Jun 26, 2018 at 15:45
  • $\begingroup$ K.E. depends on the reference frame. That's why it makes no sense to say "total kinetic energy" without specifiying a frame. Energy is thus "free", but "work" (differences of energy" is not (and it's expensive). $\endgroup$
    – FGSUZ
    Jun 26, 2018 at 18:02

1 Answer 1


Kinetic energy can be converted into potential energy (consider a stone thrown straight up as it slows down and momentarily hangs in the air), and other forms of energy can become kinetic (consider igniting an explosive, converting chemical energy into kinetic energy).

Fukugita and Peebles did an ambitious inventory of the current energy content of the universe, and they estimated that the kinetic energy of the intergalactic medium contributes $10^{−8.0\pm 0.3}$ of the total energy density. There might be a few other components here, like kinetic energy in cosmic rays (likely less) and perhaps in condensed systems (presumably, due to the virial theorem, on par with gravitational binding energy, maybe $10^{-6}$ for galaxies and $10^{-4.9}$ for stars). But this distribution is not constant over time.


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