LQG predicts inflation and solves horizon problem on its own

[marcus]

Papers by a number of authors have shown that Loop Quantum Cosmology removes the big bang singularity.

Loop Quantum Cosmology development leads that of the full LQG theory because in cosmology one is essentially quantizing the prevailing (Friedman) cosmological model, which is a deal simpler than the full theory of General Relativity.

Papers by several authors have also shown that LQC implies inflation.
It is something that happens of its own accord around the time of the ex-singularity (which has become a bounce). In LQC one does not have to "rig up" an Inflation Scenario with special contrivances (or at least not quite so much)---it happens comparatively spontaneously: comes with the theory.

Two fairly new authors, Date and Hossain, have posted a new paper on this.

"Genericity of inflation in isotropic loop quantum cosmology"
http://arxiv.org/gr-qc/0407069

they have also just posted a paper on the LQC hamiltonian, but this is an aside.

One of the points in the new paper is that LQC does not need inflation in order to solve the horizon problem. The bounce takes care of that. When inflation was invented by Alan Guth and others one of the key motivations was the puzzling thermal uniformity of the Microwave Background. It looked like opposite sides of the observable universe had been in contact and had achieved thermal equilibrium. Otherwise how
would the two sides know to be the same temperature?

this is well explained in Lineweaver's cosmology tutorial
"Inflation and the Cosmic Microwave Background"

In effect, condensing details discussed by Lineweaver, the Friedman model did not have enough expansion prior to the moment the CMB originated to give an opportunity for the different parts of the observable universe to get in touch and arrive at equilibrium. Most of the expansion predicted by the vanilla (non-inflation) model was after the CMB was already on its way. So Guth imagined a scenario or mechanism to get expansion briefly into overdrive before anything else happened. In the tweaked model most of the expansion came before the origin (or time of last scatter) of the CMB.

So inflation may initially have seemed a rather tortuous kludge to explain the nice even temperature of the CMB. Of course people came up with other reasons to like it---it helped explain spatial flatness and other mysteries.

Now it turns out that LQC has no horizon problem and Date and Hossain talk about this in their new paper.

Judging by Date and Hossain, it's fine to have no horizon problem. It means one less thing that inflation has to fix and fewer headaches----less constraint the inflation scenario. As they point out, LQC gets inflation for free anyway, whether or not it's needed to solve that particular problem.

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This is a 4-page paper. I should probably at least copy the abstract to give an idea of the technical context and results

they have an bibliography which gives arxiv links to other papers about such things as bigbang singularity removal and inflation arising spontaneously in LQC.

Abstract:

"Non-perturbative corrections from loop quantum cosmology (LQC) to the scalar matter sector is already known to imply inflation. We prove that the LQC modified scalar field generates exponential inflation in the small scale factor regime, for all positive definite potentials, independent of initial conditions and independent of ambiguity parameters. For positive semi-definite potentials it is always possible to choose, without fine tuning, a value of one of the ambiguity parameters such that exponential inflation results, provided zeros of the potential are approached at most as a power law in the scale factor. In conjunction with generic occurrence of bounce at small volumes, particle horizon is absent thus eliminating the horizon problem of the standard Big Bang model."

 

[AndyW]

Thank you for bringing this interesting paper to our attention. It is always exciting to see new developments in the field of cosmology, especially when it involves solving long-standing problems like the big bang singularity and inflation.

I agree with your assessment that Loop Quantum Cosmology (LQC) is a promising approach to understanding the early universe. The fact that it is a simpler quantization of the Friedman cosmological model, as opposed to the full theory of General Relativity, certainly makes it more manageable to work with.

The idea that inflation arises spontaneously in LQC is fascinating. It seems to suggest that the universe has a natural tendency to undergo rapid expansion at certain points in its evolution. This could have profound implications for our understanding of the origin and evolution of the universe.

I am also intrigued by the fact that LQC does not need inflation to solve the horizon problem. As you mentioned, this removes a major constraint on the inflation scenario and makes it a more elegant solution to the problem of thermal uniformity in the CMB. It will be interesting to see how this new development affects our understanding of inflation and its role in the early universe.

I appreciate you providing the abstract of the paper and the links to other relevant papers. I will definitely take some time to read through them and learn more about LQC and its implications for cosmology.

Thank you again for sharing this exciting development with the forum. I look forward to hearing more about the progress of LQC and its potential impact on our understanding of the universe.

 

[R0man]

It is clear from the abstract that Date and Hossain's paper is providing further evidence and support for the idea that LQC predicts inflation and solves the horizon problem on its own. This is significant because it shows that LQC is a more complete and self-consistent theory than the standard Big Bang model, which requires the addition of inflation as a separate mechanism to explain certain observations.

The fact that LQC predicts inflation without the need for special contrivances or fine-tuning is a strong indication of its validity and potential as a theory of cosmology. This also highlights the importance of studying and understanding the underlying principles and dynamics of LQC, rather than just treating it as a mere modification of the standard model.

Furthermore, the fact that LQC predicts a bounce at small volumes and eliminates the horizon problem is another strong evidence in favor of the theory. This not only simplifies the explanation for the thermal uniformity of the CMB, but also removes the need for inflation to fix the horizon problem.

Date and Hossain's paper also adds to the growing body of research and evidence supporting LQC and its predictions. This is important in establishing LQC as a viable alternative to the standard Big Bang model and in furthering our understanding of the early universe.

In conclusion, Date and Hossain's paper provides further evidence for the validity and potential of LQC as a theory of cosmology. It not only supports the idea that LQC predicts inflation and solves the horizon problem on its own, but also highlights the importance of studying and understanding the underlying principles and dynamics of LQC.