4
$\begingroup$

Alan Guth's cosmic inflation theory posits that about 10^-38 second before the Big Bang which led to our particular universe, a tiny patch of space doubled in size more than 100 times, from sub-atomic size to about 1cm. This "inflation" now ends, the potential energy of the scalar field in this pebble-sized infant universe converts to a hot soup of particles and radiation, and what we call the Big Bang is this hot soup expanding.

The theory explains the start and the end of inflation thus: Permeating that tiny patch was a scalar field, the potential energy curve of which happened to have a peculiar shape which caused negative pressure, and negative pressure caused gravity to be repulsive rather than attractive, hence the inflation. Like a ball perched on a hilltop, that potential energy is not stable, and at some point it would roll down the curve, this potential energy loss turns into matter and radiation, ending the inflation phase.

Now, during the inflation phase, space expanded at a constant energy density, therefore energy appears to be macically created, potentially violating energy conservation. In videos of his talks about inflation, professor Alan Guth explains that the universe's total energy was actually conserved to a net of zero or very low value, because the above (positive) energy was exactly balanced by the negative energy of gravity. I can understand that attractive gravity has negative energy, but shouldn't repulsive gravity have positive energy?

$\endgroup$
  • 2
    $\begingroup$ What is "repulsive gravity"? $\endgroup$ – ACuriousMind Oct 20 '16 at 21:54
  • $\begingroup$ More on repulsive gravity. $\endgroup$ – Qmechanic Oct 20 '16 at 22:11
  • 2
    $\begingroup$ therefore energy appears to be macically created, potentially violating energy conservation Not true. Energy is always conserved locally in GR, and there is never any global conservation. Inflation isn't an exception to either of these statements. See physics.stackexchange.com/questions/2838/… $\endgroup$ – Ben Crowell May 6 '18 at 0:09
  • 1
    $\begingroup$ Guth explains that the universe's total energy was actually conserved to a net of zero or very low value This is speculative and currently cannot be expressed in any rigorous way by any viable physical theory. That's because we don't currently have any theory that can talk about the total energy of the universe or whether such a thing would be conserved. $\endgroup$ – Ben Crowell May 6 '18 at 0:10
  • 1
    $\begingroup$ The phrasing of this discussion in terms of repulsive gravity and positive or negative energy basically doesn't work. If Guth describes it that way in a popular lecture, that's him trying to get across the general flavor of the ideas. It's not something that can be made rigorous. GR says that the source of gravitational fields is the stress-energy tensor, not a scalar density of mass-energy. The kinds of things that Guth is describing to laypeople as "positive or negative energy" are actually described in terms of energy conditions: en.wikipedia.org/wiki/Energy_condition $\endgroup$ – Ben Crowell May 6 '18 at 0:13
-1
$\begingroup$

I do not think you will get a definite answer to this question. So, I will throw some speculation into it.

There is no repulsive gravity. That does not mean attractive gravity can not be turned around into repulsion.

Take an example of repulsion explained in terms of exchange of (virtual) particles. Two bodies can throw particles to one another and this would enable them to get away from one another - repulsion. This is simple to understand in terms of conservation of momentum.

Now try to explain attraction in form of particle exchange! The particles have to take a turned around path. Basically, it is repulsion turned around into attraction in this case.

I am not trying to say this is how electromagnetic attraction/repulsion work. I am just stating one of the mainstream explanations of it.

Same way, natural outcome of "set of various tensors" is attraction. Any repulsion has to be this attraction turned around. Basically, it would be inversion of curvature of space.

Nature just by itself, without intelligent intervention, is not capable of doing this inversion. That is why we have had no natural scenarios/objects to observe and experiment with this inverted curvature. This is unlike other fields of physics like electromagnetism where nature does provide us objects and scenarios to experiment with and formulate those fields.

All scientists including Einstein formulated one way curvature. Because that is only one available to us for experimenting and formulating.

So, nature by itself is not capable of this inversion. Big Bang (inflation), & supernova explosions may be possible exceptions to this. We may not fully understand what goes on in there.

Dark energy is possibly an inversion of space curvature that escaped (ran away) at the time of inflation and spread into the universe, and was later overridden at local levels by local curvatures.

So, IMHO, no repulsive gravity, only attractive gravity turned around.

$\endgroup$
  • $\begingroup$ An inversion of spatial curvature--analogized to the sort of effects one sees after some unusually violent vehicular collisions--had also occurred to me before I formulated my own answer (or half-answer, which I guess is why yours got butted ahead of it), but I'd like to see you address that provision for negative gravitational mass which I had described in it, as, frankly, her credentials at Canada's Perlmutter Institute outweigh any of your own that I can see. Also, Guth's book "The Inflationary Universe" describes no collision--except a matter/antimatter one--among the possibilities. $\endgroup$ – Edouard Feb 20 '18 at 15:52
  • $\begingroup$ (A matter / antimatter collision, as an inflationary source, was ruled out by Guth due to the lack of any bright spot in the sky, although I think the OP would be aware of that.) $\endgroup$ – Edouard Feb 20 '18 at 15:57
  • $\begingroup$ Repulsive gravity can occur near domain walls. $\endgroup$ – probably_someone Aug 16 '18 at 18:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.