without resorting to scalar fields

[marcus]

"...without resorting to scalar fields"

http://arxiv.org/astro-ph/0703566

Co-authored by Parampreet Singh, one of the experts in Quantum Cosmology (gauged by publication trackrecord and citations by other scholars, see:
https://www.physicsforums.com/showthread.php?p=1368143#post1368143 )

from page 6 of the paper

"Loop Quantum Cosmology...is known to modify the equation of state of ordinary matter, thereby permitting a solution of the horizon problem, without resorting to scalar fields."

This is not news, it's been known for quite some time. That's not the point of the paper, which concerns ways to explain the observed structure formation---the statistical features of observed clumping.

The comment about the horizon problem not requiring a specially cooked-up "inflaton" field is not the main burden of the article, it is just tossed out as a motivating remark.

What the authors are pointing out is that "inflaton" scenarios look different when there is only one main puzzle they are needed for, than when there are several puzzles. If some of the other puzzles are addressed naturally by the cosmological model, without resorting to putting in an exotic field by hand, it makes the scenarios less compelling.

And one is then tempted to see if the remaining puzzle of structure formation can also be addressed without assuming an exotic field. This is what the authors try to do, in fact, and they show that it is a tough problem but (i would judge) not entirely hopeless.

 

[marcus]

natural QC inflation doesn't give 60 e-foldings

Bojowald states the problem succinctly on page 14 of this paper
http://arxiv.org/abs/0705.4398

"While these effects can be used in inflationary scenarios which require negative pressure at early times [references], the effects quickly subside once the universe has expanded to some size very small compared to its current size. In fact, such a quantum geometry epoch of inflation does not last long enough to provide all 60 e-foldings required for successful structure formation...."

In other words, quantum geometry with no exotic extras gives SOME inflation naturally, and that combined with other features can address certain problems, but it is not ENOUGH inflation to provide for the usual structure formation story.
For that story, you need expansion by a factor of e⁶⁰, and you don't get that much from the quantum geometry of its own accord.
So LQC people regularly assume an additional scalar field inflation episode, like everybody else, when the subject comes up.

But since there seems to be only one major obstacle to dispensing with scalar field altogether, it is tempting to take up the challenge of explaining structure formation in some other way----and this is what the earlier paper, that Parampreet Singh co-authored, proposes to do.