# Is the total energy of the universe constant?

If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?

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Here is a related question that might be helpful physics.stackexchange.com/questions/2838/… –  Hal Swyers Oct 17 '12 at 1:32
The total energy of the universe is not well defined, so we can't even discuss whether it's constant. physicsforums.com/showthread.php?t=506985 –  Ben Crowell Oct 17 '12 at 5:36

No. The universe is dominated by dark energy, which is consistent with a cosmological constant $\Lambda$. In other words, as the universe expands, the energy density stays roughly the same. So the (energy density)*volume is growing exponentially at late times.

Although the total energy is not well defined (as the volume of the universe may be infinite), the fractional rate of growth is certainly nonzero.

You might wonder how the total energy can grow without violating energy conservation. The answer is that in general relativity, we just need $\boldsymbol{\nabla} \cdot \boldsymbol{T} = 0$, so a cosmological constant is perfectly consistent as $\boldsymbol{\nabla} \cdot \Lambda \boldsymbol{g} = 0$

For a nice explanation by Sean Carroll, see http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/energy-is-not-conserved/

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The total energy isn't just undefined because of the possibility that the universe is infinite. It's undefined for the reasons given in juanrga's answer. –  Ben Crowell May 6 '13 at 21:06

Your question is tagged as general-relativity and cosmology, and as textbooks remark (e.g. Peebles [1]) "there is not a general global energy conservation law in general relativity theory.

Therefore: ”The conclusion, whether we like it or not, is obvious: energy in the universe is not conserved” [2].

[1] Peebles P. J. E., 1993, Principles of Physical Cosmology (Princeton Univ. Press).

[2] Harrison E., 1981, Cosmology ( Cambridge University Press)

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