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Regarding this thread:

http://www.physicsforums.com/showthread.php?t=506985

Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of the universe used to back this up when, according to that post, an open universe would not conserve energy so the total energy of the universe cannot be "zero", can it? What's the energy being defined as zero and why is that energy used to predict things about the universe when "the" energy (as the term is used in that post) is not zero?

Additionally, I found one very good explanation here for this,

http://mathoverflow.net/questions/38659/total-energy-of-the-universe/38690#38690

This seems to indicate that in flat space (the sort of space used in these zero energy universe theories) the mass can NOT be positive thus contradicting the notion that mass is positive and gravity negative and the whole thing winds up being zero. Did I interpret that correctly?

The MO question is cross-posted to Physics.SE here,

Total energy of the Universe

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2 Answers

The main reason why space overall is assumed to have an energy very close to zero is that anything else quickly leads to extreme gravitational curvature, which of course is not what we see when you look out at the stars and distant galaxies. Some curvature, sure, but not much, and even then it is mostly localized to effects such as gravitational lenses.

Even an extremely tiny amount of net energy for empty space accumulates very quickly because of the vastness of empty space. If you work out how close to zero you need to get to allow space to look as flat as we see it out to distant quasars, you unavoidably come up with a value extraordinarily close to zero energy per cubit meter of empty space.

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But on what basis do we conclude these were the conditions at the Big Bang (necessary for "zero energy universe" arguments)? And due to dark energy it's not necessary that it will remain in the current state even in the future, right? –  Ocsis2 Apr 8 '12 at 20:34
    
Never realized you commented, sorry. Easy answer though: I have no idea. To me the arguments often seem a bit backwards: The universe is very, very flat, so our theories should somehow make that fundamental to all subsequent attempts to explain it. Instead, theories postulate space as negative, matter as positive, and "somehow" it all balances out. Here's just an example of an alternative: Allow true negative energy mass, and required absolute conservation summing to zero. I don't even know if that's a theory; my point is just that it's important to choose your axiom set explicitly. –  Terry Bollinger Jun 20 '12 at 22:01
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Lawrence Krauss' argument for a flat universe having zero energy is complete nonsense.

It is essentially a rehash of the Newtonian gravity argument that an object moving at escape velocity in a gravitational field has: Kinetic energy + Potential Energy = 0 A universe which is flat may be said to be "expanding at escape velocity", hence Krauss' zero energy conclusion.

The reason it is nonsense, is as follows: - The argument only takes into account the kinetic and potential energy of particles, but NOT the energy content in their mass, which is very significant. - The "flat universe is at escape velocity" analogy is incorrect. If you have dark energy, then the universe could have any curvature and still be accelerating. (And even if the analogy held, zero energy doesn't follow !)

Finally, if you bring in relativity, energy becomes a difficult and slippery concept as you saw in the links you have (actually, I initiated that discussion, exactly due to these issues). In some sense, you could set it to whatever value you want.

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protected by Qmechanic Dec 23 '13 at 7:19

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