eli5: Does expansion of the universe not cause reduction in mass and energy? How does does the mass and energy of an expanding universe remain constant and not reduce or diffuse?

Question

eli5: Does expansion of the universe not cause reduction in mass and energy? How does does the mass and energy of an expanding universe remain constant and not reduce or diffuse?

In: 279

Answer 1

The amount of matter/energy *does* stay constant and it also *does* get more diffuse because it spreads out to fill a greater volume. Both are true.

The only way the universe could expand and yet remain at the same density is if more matter/energy was being constantly created. That was Sir Fred Hoyle’s Steady State theory. But the theory he mockingly named “The Big Bang” turns out to have been the better description, and retained the name, ironically.