# Conservation of energy.

1. Jun 6, 2009

### flawless86

Hi,

The first law of thermodynamics states that energy can neither be created nor destroyed. Therefore the total energy of the universe is the same in any given process. But how can the universe so be expanding ? Does its amount of energy not increase during the expansion ?

Best regards...

2. Jun 6, 2009

### Janus

Staff Emeritus
It really just a transformation of one energy type to another. For example, if you throw a rock into the air, it starts with a lot of kinetic energy. As it climbs, it slows down, losing kinetic energy but gaining gravitational potential energy. The total energy of the rock, potential energy + kinetic energy, stays the same.

The universe is like that. As it expands, it trades off one form of energy for another. Its a little more complicated, as you have both gravitational energy, kinetic energy, and apparently "dark energy" to deal with. One opposes the expansion, one is due to the rate of expansion and the other drives the expansion, The principle stays the same, any amount one of them increases is balanced by a decrease in another so the total stays the same.

3. Jun 6, 2009

### Wallace

If you think just about matter, there is no problem. As the Universe expands the density of matter decreases, conserving the total amount of matter in any given 'co-moving' region (co-moving means a region that stays the same size if you 'factor out' the overall expansion).

For other types of energy there is the interplay between kinetic and potential energy as suggested by Janus. You have to be careful though. In fact the normal Newtonian 'Energy' of the Universe is not conserved as the Universe expands. You have to realise that in general relativity the conservation laws are expressed in the fully 4 dimensional form (3 space + time). A quantity analogous to energy is conserved, even if energy in the everyday Newtonian sense is not.