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As I understand, from watching the Discovery Channel, the total amount of energy in the universe is zero. As such, people like Hawking explain that the universe can be created out of nothing because... well it actually is nothing. That's fine, but if that is true today I guess it also was true shortly after the Big-Bang. This period is usually described as being extremely hot and dense and at the very moment of the Big-Bang the energy density is described as being infinite. How can this be true as the total amount of energy at that time was actually zero? Somehow the description hot and dense doesn't match very well with zero energy. What am I missing here?

Qmechanic
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Henk Kuyer
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  • Discussion: http://chat.stackexchange.com/rooms/17149/did-you-read-this-book – Sensebe Nov 19 '14 at 14:42
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    Note that the total energy of the universe isn't a particularly well-defined thing if you take General Relativity at face value -- the metric is time-dependent, so you don't have a time translation symmetry, so the magic is broken. – Zo the Relativist Nov 19 '14 at 23:16
  • Related: http://physics.stackexchange.com/q/136860/10389 – Pacerier May 27 '15 at 08:34

2 Answers2

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In Zero-Energy Model, negative energy associated with Gravity counterbalances positive energy associated with matter, photons, etc. So, No, Big Bang wasn't cold. You are just looking at partial picture (you just ignored Gravity).

This is what Zero-Energy Model says:

With traditional Big Bang model (which doesn't contain Inflation), the universe started out with zero energy (but hot) and continue to exist with zero energy till today.

With Inflatory Big Bang model, energy from gravitational curvature transferred to matter (and other positive matter contributors) in order to make the total energy of the universe zero, if it wasn't already zero. All several phases of traditional Big Bang are shifted to post Inflation (making end of Inflation as starting point of Big Bang evolution) containing the total energy of phases zero as that of traditional Big Bang model. Inflation and post-Inflation phases were hot. For before Inflation, the model provides nothing, but as for temperature, most physicists believe that the universe was hot. There are cold Big Bang models in existence which exploit this lack of information. But, even with cold Big Bang models, total energy of post-Inflation universe is zero (till today).

Note: Zero-Energy Model may not be correct as it's more or less a kind of hand-waving model. In non zero energy case, it shouldn't be problem seeing hot Big Bang, I suppose. In reality, energy of universe isn't absolute or constant and should not be definable, smashing Zero-Energy Model.

Earth is a Spoon
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  • Let me rephrase the question a bit: after the big bang but before inflation, was the net energy of the universe zero or not? And after inflation? You see, I still have the feeling that if you rewind the history of the universe the positive energy of matter and the negative energy of space are squeezed together and you end up with a little bit of nothing. Which is the big bang. – Henk Kuyer Nov 19 '14 at 21:26
  • @Henk I rephrased the answer fully and removed unnecessary parts. – Earth is a Spoon Nov 20 '14 at 05:57
  • With traditional Big Bang model (which doesn't contain Inflation), the universe started out with zero energy No, see http://physics.stackexchange.com/questions/2838/total-energy-of-the-universe –  Nov 20 '14 at 06:00
  • @Ben I am talking about the Zero-Energy Model, of course. – Earth is a Spoon Nov 20 '14 at 06:02
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In the standard homogeneous cosmological models the total energy in an expanding volume is zero. This is true for positive, negative or zero curvature and it must take into account the gravitational energy (which is negative), dark energy, matter and heat. Since the gravitational energy is negative the heat can be positive and increasing as you go back towards the big bang.

However, "can be" is not the same as "must be". The other possibility is that the energy in the vacuum during the period of inflation counterbalances the negative gravitational energy. The universe before or during inflation could then have been cold and heat would have been released at the end of inflation when the vacuum collapses. Heat would also be generated by matter annihilation. Again this is only one of many possibilities. We can't say if the universe started hot or cold. we only know that it heated at some point very early on. Either possibility is consistent with zero total conserved energy.

Philip Gibbs - inactive
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  • How on the holy Earth can you say Big Bang cold.. Have you forgotten the chaotic motion of particles? – Earth is a Spoon Nov 20 '14 at 04:59
  • @SachinShekhar I guess for the second paragraph, there are no particles in the inflation period, just field excitations which have no definable temperature. – anna v Nov 20 '14 at 05:12
  • @anna The inflationary epoch is a consequence of the nuclear force breaking away from the weak and electromagnetic forces that it was unified with at higher temperatures. So, inflation isn't possible without high temperature. – Earth is a Spoon Nov 20 '14 at 05:29
  • @SachinShekhar No . it is the epoch before 10^-32 seconds here http://en.wikipedia.org/wiki/Big_Bang#mediaviewer/File:History_of_the_Universe.svg , where there are no particles yet, but the inflaton homogenizes the cosmic soup of positive and negative energies in this case. – anna v Nov 20 '14 at 05:34
  • @anna You are mixing traditional and Inflatory Big Bang models. – Earth is a Spoon Nov 20 '14 at 05:50
  • In the standard homogeneous cosmological models the total energy in an expanding volume is zero. No, there is nothing "standard" about this claim. The standard interpretation of GR is that we can't measure the total energy-momentum in a region of space in a cosmological spacetime, if the region is cosmological in size. See http://physics.stackexchange.com/questions/2838/total-energy-of-the-universe –  Nov 20 '14 at 05:57
  • @SachinShekhar that is the context of this answer. The old BB model cannot explain the homogeneity of the cosmic microwave background radiation so is not in the picture any longer. – anna v Nov 20 '14 at 07:03
  • @SachinShekhar I did not say the big bang is cold. I said we dont know if the beginning was hot or cold. You cant refute this by citing inflation lore because there is still no complete theory of inflation, let alone experimental verification. The inflationary state or pre-inflationary state does not have to be hot if it is metastable. – Philip Gibbs - inactive Nov 20 '14 at 08:10
  • @Ben Crowell, I use the word standard to refer to the cosmological model, not that the energy is zero. However the fact that the energy in the volume is zero is also generally accepted. My answer to the question you link is the only correct one but even Lubos says the energy is zero in his answer (albeit for the wrong reasons). – Philip Gibbs - inactive Nov 20 '14 at 08:44