Is Gravitational Energy Truly Negative in a Zero-Energy Universe?

[InvalidID]

http://en.wikipedia.org/wiki/Zero-energy_universe

The idea relies on negative gravitational energy, but isn't gravitational energy simply negative from convention?

http://en.wikipedia.org/wiki/Potent...ion_where_gravitational_energy_is_negative.3F

 

[Drakkith]

Yes it is only by convention. See the following.

From here: http://en.wikipedia.org/wiki/Potent...ion_where_gravitational_energy_is_negative.3F

Why choose a convention where gravitational energy is negative?

As with all potential energies, only differences in gravitational potential energy matter for most physical purposes, and the choice of zero point is arbitrary. Given that there is no reasonable criterion for preferring one particular finite r over another, there seem to be only two reasonable choices for the distance at which U becomes zero: r=0 and r=\infty. The choice of U=0 at infinity may seem peculiar, and the consequence that gravitational energy is always negative may seem counterintuitive, but this choice allows gravitational potential energy values to be finite, albeit negative.

The singularity at r=0 in the formula for gravitational potential energy means that the only other apparently reasonable alternative choice of convention, with U=0 for r=0, would result in potential energy being positive, but infinitely large for all nonzero values of r, and would make calculations involving sums or differences of potential energies beyond what is possible with the real number system. Since physicists abhor infinities in their calculations, and r is always non-zero in practice, the choice of U=0 at infinity is by far the more preferable choice, even if the idea of negative energy in a gravity well appears to be peculiar at first.

The negative value for gravitational energy also has deeper implications that make it seem more reasonable in cosmological calculations where the total energy of the universe can meaningfully be considered; see inflation theory for more on this.

 

[InvalidID]

Yes it is only by convention. See the following.

From here: http://en.wikipedia.org/wiki/Potent...ion_where_gravitational_energy_is_negative.3F

But the idea of a zero-energy universe relies on negative gravitational energy, so if I changed the convention, then the zero-energy universe idea stops making sense?

 

[Mordred]

This is a description of false vacuum inflationary model. In essence the gravity is the balancer to make the sum of total forces/energy= zero. The other factor not mentioned is vacuum energy between the true vacuum/false vacuum. This model has numerous problems which led to other inflationary models in the following sequence. old inflationary model, new inflationary model. eternal inflationary model, chaotic inflationary model. etc. the list goes on. The main problem with A.Guth's model has to do with runaway inflation. It never stops inflating. Some models proposed pocket universes from this model. Severla multiverse models also use it.
As stated its main problem has never been settled. However if you wish to study it start with false vacuum/true vaccuum. Its a model that is not as popular as it once was. Hope that helps

 

[InvalidID]

Oh, well that went over my head. It's okay though as I probably can't understand it at this point in time.

If it's an unpopular model, I wonder why he has mentioned it in this video: http://youtu.be/D6XAkVA7RmY?t=4m2s I suppose it's because he believes that it is true.

 

[Mordred]

This recent threada covers it fairly well, Their is numerous articles and explanations in the thread including a few alterate models.

https://www.physicsforums.com/showthread.php?t=674090.

quicke explanation i copied from the above thread.

Here is a quick guideline on process.
Key point in order for this model to work is that energy density must balance with zero energy. Gravity being considered as negative energy.
Rapid expansion occurs this creates a false vacuum. This false vaccuum. To maintain energy conservation energy is borrowed. I can't recall what the model states its borrowed from but if I recall its borrowed from gravity.
With that energy quantum tunneling occurs from virtual particles. Some of the virtual particles tunnel to the true vacuum. Leaving real particles.

It should be noted that virtual particles are created in a large variety of sources. Cosmological horizons. =Unruh radiation. Blackholes is Hawking radiation. Schwinger particle production is electromagnetic disturbences. Parker radiation is due to expansion.
All of the above are various blackbody radiation.

There are countless other particle production methods.
What they all boil down to is a vacuum is never empty.
False vacuum being the lowest energy state has quantum fluctuations described by Heisenburg uncertainty principle. Those fluctuations in turn create virtual particles. Those virtual particles in the right circumstances become real particles.
Throughout out all this for this model the energy density must stay equal to zero with gravity and vacuum energy as part of the balancers.
However even if the energy density isn't zero the various particle production methods describe above are all still valid.
Sounds crazy however their is tons of research and models that support this ultimate free lunch.