First Law still works with an expanding universe

[shunyadragon]

Source: http://www.scientificamerican.com/article/energy-can-neither-be-created-nor-destroyed/?&WT.mc_id=SA_SPC_20140808

The law of conservation of energy, also known as the first law of thermodynamics, states that the energy of a closed system must remain constant—it can neither increase nor decrease without interference from outside. The universe itself is a closed system, so the total amount of energy in existence has always been the same. The forms that energy takes, however, are constantly changing.

. . . .

Modern cosmology has offered up new riddles in energy conservation. We now know that the universe is expanding at a faster and faster rate—propelled by something scientists call dark energy. This is thought to be the intrinsic energy per cubic centimeter of empty space. But if the universe is a closed system with a finite amount of energy, how can it spawn more empty space, which must contain more intrinsic energy, without creating additional energy?

It turns out that in Einstein’s theory of general relativity, regions of space with positive energy actually push space outward. As space expands, it releases stored up gravitational potential energy, which converts into the intrinsic energy that fills the newly created volume. So even the expansion of the universe is controlled by the law of energy conservation.

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[klaus54]

Interesting!

Confirms my decision to have taken freshman physics.

BTW, what's "positive" energy?

K54