The discussion revolves around Alan Guth's concept of 'gravitationally repulsive material' in the context of cosmic inflation. Participants explore the theoretical underpinnings of this material, its implications for the early universe, and its relationship with concepts like vacuum energy and dark energy. The conversation touches on high-energy physics, general relativity, and the nature of matter during inflation.
Participants express a range of views on the nature of gravitationally repulsive material and its implications, with no clear consensus on the mechanisms or theories involved. Multiple competing ideas are presented, reflecting the complexity and ongoing debate in the field.
The discussion highlights limitations in current understanding, particularly regarding the assumptions underlying various theories and the unresolved nature of high-energy physics in relation to gravitational repulsion and inflation.
Can you explain why they should be relevant (and in case of dark matter, why repulsing) at very high energy densities?M. Bachmeier said:Dark Matter... Dark Energy...
It has nothing to do with "high energy" and it does not happen for just any kind of matter -- it is a special kind of energy, namely an energy density with quantum numbers of the vacuum (usually referred to as "vacuum energy"). Vacuum energy is a very unusual kind of thing; for one, it has a constant density. This means that it does not dilute as the universe expands -- it is persistent. It is *this* property that results in a negative pressure. High densities of vacuum energy are believed responsible for the early bout of ultrarapid expansion called inflation; in much, much lower densities it might also be implicated in the present-day accelerated expansion of the universe.Herbascious J said:Hi and thank you for the detailed information. I'm looking for a further description of "a condition theoretically possible under GR and particle physics". I would like to know why such high energies create negative pressure (and repulsive gravity) in matter. Is there some basic principle within particle theory that suggests negative pressure at high energies for matter?? Any input is appreciated.
Just to be clear, while this is definitely a line of thought that many theorists have considered to be quite enticing, nobody has yet come up with a specific idea that is really compelling. There are multiple ideas for how this could be, but they tend to be rather ad-hoc.Chronos said:Affirming bapowell, current vacuum energy may be remnants of the original scalar field responsible for inflation. It was too feeble to become noticeable until several billion years ago. @M Bachmeier, you would be well advised not to indulge in such discussions based solely on 'instinct'. You will notice contributors here regularly offer references to published papers in support.
Inflation comes to end when the inflaton field finds itself in the vacuum. The field retains some potential energy even in the vacuum, which it dissipates by oscillating about the minimum of the potential (sort of like water sloshing about in a bucket). These oscillations manifest themselves as inflaton particles which subsequently decay into hot radiation, effectively resulting in a hot big bang from which conventional, post-inflationary expansion picks up.Herbascious J said:At what point is matter introduced? Shouldn't there be some kind of matter like energy during inflation that will decay into particles?
We use the Einstein-Cartan-Kibble-Sciama (ECKS) theory of gravity which naturally extends general relativity to include the spin of matter. The torsion of spacetime generates gravitational repulsion in the early Universe filled with quarks and leptons, preventing the cosmological singularity: the Universe expands from a state of minimum but finite radius... Thus the ECKS gravity provides a compelling alternative to speculative mechanisms of standard cosmic inflation...
Gravitational repulsion induced by torsion, which becomes significant at extremely high densities, prevents the cosmological singularity...
In 2011, Nikodem Poplawski showed that the Einstein-Cartan theory eliminates the general-relativistic problem of the unphysical singularity at the Big Bang.[7] The minimal coupling between torsion and Dirac spinors generates a spin-spin interaction which is significant in fermionic matter at extremely high densities...
Does anyone have any resources which explain the nature of Alan Guth's 'gravitationally repulsive material'
In order to achieve this inflationary period, it is necessary to introduce a new scalar field into the menageris of known [ and conjectued physical particle fields. As far as I am aware, this field is not taken to be directly related to other known fields of physics but is introduced solely to obtain an inflationary phase in the early universe. It is sometime releferred to as a "Higgs' field but itdoes not seem to be the ordinary one.
..there are some theories such as supergravity and ECSK theories which employ a non zero torsion that plays a significant role...see Note 19.10 and ppg 31.3
...ECSK in which torsion is introduced and considered to describe a direct gravitational effect of density of spin...
...It might well be the case that there are fundamental asymmetries inherent in natures laws and that the symmetries that we see are often merely approximate features that do not persist right down to the deepest levels...
...so that the torsion can be coupled to the intrinsic angular momentum (spin) of matter, much in the same way in which the curvature is coupled to the energy and momentum of matter. In fact, the spin of matter in curved spacetime requires that torsion is not constrained to be zero but is a variable in the principle of stationary action...
Einstein–Cartan theory has been historically overshadowed by its torsion-free counterpart and other alternatives like Brans–Dicke theory because torsion seemed to add little predictive benefit at the expense of the tractability of its equations...
Yes, absolutely. Even if we could build a particle accelerator capable of such a feat, however, it would be much much harder to actually produce inflation: inflation doesn't just require inflatons, it also requires that the value of the inflaton field be nearly constant and of an appropriate value across a region of space (a very small region of space, but not a single point either).Herbascious J said:I have a question; It seems Guth is saying that energy from this inflation scalar field decays into particles. If it was possible to recreate sufficiently high energies (I realize it's not), could we convert matter back into 'inflatons'?
[ If it was possible to recreate sufficiently high energies (I realize it's not), could we convert matter back into 'inflatons'?/QUOTE]
Nobody really knows. Maybe a quantum theory of gravity or a GUT will provide insights. Inflations are theoretical rather than observed as is the inflationary scalar field. All we know for sure, I think, is that such a model matches observations.
Also, keep in mind inflation is a glued on appendage to the front end of our cosmological model, the FLRW model. It's there not because of first principles but because it provides a fit to cosmological observations. And it does NOT explain the origin of the big bang of whatever kind of bang one hypothesizes: it starts about 10-37 seconds AFTER the big bang...close to the Planck regime...but still separated from it:
http://en.wikipedia.org/wiki/History_of_the_universe#Summary
In the mainstream view of particle [matter] creation discussed in these forums, it seems current theory more closely aligns with some sort of an initial quantum perturbation, a negative pressure or equivalently repulsive gravity, inflationary expansion [a dynamic geometry] coupled with cosmological horizons.
In fact, just what causes particles to appear is not all that clear:
What is a particle?
https://www.physicsforums.com/showthread.php?t=386051
Here are excerpts [abbreviated by me] about particle/matter ambiguity that I like:
Marcus:The trouble with the particle concept is that one cannot attribute a permanent existence; It only exists at the moment it is detected. The rest of the time there is a kind of spread out thing---a cloud---a wave---a field---something that is less "particular", something that cannot be detected.
Meopemuk: According to scientific method we are not allowed to speculate about things that cannot be registered/observed/verified. If we do use such unobservable things (e.g., wave functions, quantum fields, etc) in our formalism we should keep in mind that these are mathematical tools unrelated to the physical world……
Stoer: ....with the concept of particles alone you are not able to calculate anything beyond classical physics: No quantum mechanics, no atoms, no spectra, no nuclei, no nucleons, no quarks, ... they all rely on unmeasurable concepts like wave functions, field operators, Hilbert spaces, etc. You can't register them, but have to live with them if you want to do physics...
Stoer:...Inertial and accelerating observers do not agree on the number of particles they observe because the typical definition of a "particle" relies on the definition of a vacuum state which has to be (at least asymptotically) flat; acceleration spoils this concept because acceleration is equivalent to gravity and therefore undermines the basis of special relativistic quantum field theory. ...
Quantum gravity isn't necessary. If the inflaton field exists, then inflatons can be produced in sufficiently high-energy collisions, full-stop. This must necessarily be the case, because if it weren't, then inflatons could not decay into standard model particles. Which they had to to produce our universe.Naty1 said:Nobody really knows. Maybe a quantum theory of gravity or a GUT will provide insights.
If the inflaton field exists, then inflatons can be produced in sufficiently high-energy collisions, full-stop.
As in you don't need to know anything more about the inflaton field to know it can be produced in a particle accelerator than the fact that inflation ended, and the end of inflation produced standard model particles.Naty1 said:Chalnoth:
What does "full stop" mean?
Right. This is what normally happens when we produce massive particles in particle accelerators: they decay. We mostly know about them from their decay products. The recent Higgs detection, for instance, stems from combinations of outgoing particles (typically muons and photons) whose combined energies are around 125GeV.Herbascious J said:There is a line of thought that has hung me up on all of this. Perhaps my understanding is off. I believe that particles carry with them aspects like charge, spin, lepton number, baryon number, and even mass, etc. To me this is what distinguishes matter from other forms of energy. Although these attributes can be annihilated (anti-particle collisions) and reduced to pure energy, it seems that any field that gives rise to matter, should in some aspect contain these attributes in some form. Otherwise, shouldn't the energy of the inflation field simply decay into photon radiation? I apologize for my ignorance here, I'm stepping well out of my current grasp of things.
I believe that particles carry with them aspects like charge, spin, lepton number, baryon number, and even mass, etc. To me this is what distinguishes matter from other forms of energy.
Otherwise, shouldn't the energy of the inflation field simply decay into photon radiation?
Although these attributes can be annihilated (anti-particle collisions) and reduced to pure energy, it seems that any field that gives rise to matter, should in some aspect contain these attributes in some form.
Chalnoth said:If the inflaton couldn't decay into other particles, it would still be around in large numbers.
Quantum mechanically, a photon isn't really that distinct from other forms of matter, except for the fact that it has no mass. It's just one (of many) quantum-mechanical fields. It doesn't form solid objects because of the lack of charge and mass, but then neutrinos don't form solid objects either.Herbascious J said:Ok, so (according to theory) the inflaton decays into the particles we observe. Does this make the inflaton a form of matter (unlike a photon)?
Chalnoth said:Though I suppose the particle it has most in common is the Higgs, as the Higgs is not only a boson, but is a scalar boson and also has mass.
Yes, it would act as a force carrier. Obviously this force has to be extremely weak, or else we would have detected its effects. Scalar fields produce forces that are rather like gravity, but with a different impact on light compared to non-relativistic matter.Herbascious J said:Ok, interesting. So does this mean the inflaton is theoretically a force carrier? And like the higgs it has mass.
Yes, the negative pressure is extremely well-understood. This is down to the dynamics of how the energy density of the field changes as the universe expands. When the field is in the right sort of configuration, the expansion actually acts to slow down the change in energy density so much that it acts as if it has a nearly constant energy density. As the space-time curvature is proportional to energy density, and as the expansion rate is a manifestation of space-time curvature, nearly constant energy density means nearly constant expansion rate.Herbascious J said:It seems to me that it should be gravitationally attractive. Is there any insight into why this field is repulsive? Is there some negative pressure associated with it? Thanks everyone, I think I'm getting more clear on all of this.
Does anyone have any resources which explain the nature of Alan Guth's 'gravitationally repulsive material'?
...What creates repulsive gravity is negative pressures. That’s the feature of the cosmological constant and also of states of scalar fields dominated by their potential energy, which is the way conventional inflation works. Certainly the most plausible explanation for acceleration today, and for inflation early in the universe, was that the universe contains materials that have negative pressures.
One important thing we’d love to know about dark energy is whether or not the energy density is constant over time, as it would be if it were a cosmological constant. Or, it could vary with time — in which case, our best explanation would be that it’s energy of a slowly evolving scalar field that fills all of space...
To learn about higher temperatures physicists turn to accelerators….
CERN will allow us to study the conditions in the universe at just 5 x 10^-15 seconds after the bang…. The accelerator experiments provide crucial information, but a high energy beam is not exactly the same as the hot gas we believe filled the early universe. The particles in an accelerator all have essentially the same energy while the particles of a gas have a range of energies. In addition, the particles in an accelerator beam are much further apart, on average, than the particles of the early universe. Accelerator experiments show us what happens when two particles collide at high energy, but we must rely on theoretical calculations to infer how the temperature and pressure of a hot dense gas expands.