As the mass of universe is not constant, it means that its linear momentum is not constant. But thinking in another way, is the universe subjected to a net external force? But what is external for the universe? The universe itself comprises of everything, so shouldn't every interaction be internal?

As every interaction is internal, there must be no external force, and the linear momentum of the universe must be constant!

Two different aspects give two different results. I am quite confused. Does the universe violate law of conservation of linear momentum?

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    $\begingroup$ Possible duplicate by OP: physics.stackexchange.com/q/487769/2451 $\endgroup$ – Qmechanic Oct 17 '19 at 8:45
  • $\begingroup$ @Qmechanic It was about Big Bang. So are you saying that I should delete it? $\endgroup$ – Shreyansh Pathak Oct 17 '19 at 8:51
  • $\begingroup$ @Unique Why do you think that the mass of the universe is not constant ? If we include energy with mass (using $E=mc^2$ equivalence) then every physical process that we know of conserves mass/energy in a closed system. $\endgroup$ – gandalf61 Oct 17 '19 at 10:28
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    $\begingroup$ how could you possibly define momentum of the universe to begin with? $\endgroup$ – Umaxo Oct 17 '19 at 12:04
  • $\begingroup$ @Umaxo I define the linear momentum of the universe to be the sum of the individual momenta of each and every body $\endgroup$ – Shreyansh Pathak Oct 25 '19 at 7:44

The momentum of an object of growing mass can remain constant if the velocity of its centre of mass is zero. If the universe were growing in mass isotropically, and if its constituent parts were accelerating isotropically, then its overall momentum could remain zero (ie constant).


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