Viability after BICEP2 of LQC matter bounce

[marcus]

I'm hoping that David Horgan will create some supplemental material for this, as he has for several other LQG and CDT papers. The pedagogical supplements help one read and understand the papers. In any case, here's the recent deHaro paper AND the previous one that lays out the LQC matter bounce idea.

http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(Submitted on 25 Mar 2014)
The CMB map provided by the Planck project constrains the value of the ratio of tensor-to-scalar perturbations, namely r, to be smaller than 0.11 (95% CL). This bound rules out the simplest models of inflation. However, recent data from BICEP2 is in strong tension with this constrain, as it finds a value r=0.20^+0.07_−0.05 with r=0 disfavored at 7.0σ, which allows these simplest inflationary models to survive. The remarkable fact is that, even though the BICEP2 experiment was conceived to search for evidence of inflation, its experimental data matches correctly theoretical results coming from the matter bounce scenario (the alternative model to the inflationary paradigm). More precisely, most bouncing cosmologies do not pass Planck's constrains due to the smallness of the value of the tensor/scalar ratio r≤0.11, but with new BICEP2 data some of them fit well with experimental data. This is the case with the matter bounce scenario in the teleparallel version of Loop Quantum Cosmology.
4 pages, 1 figure

Jaume is the CATALAN form of the name which would be "Jaime" in Spanish. Catalan is a separate language from Spanish which is spoken in Catalonia (the region around Barcelona). The previous paper, on which this one is based is:

http://arxiv.org/abs/1309.0352
Cosmological perturbations in teleparallel Loop Quantum Cosmology
Jaime Haro
(Submitted on 2 Sep 2013 (v1), last revised 18 Nov 2013 (this version, v3))
Cosmological perturbations in Loop Quantum Cosmology (LQC) are usually studied incorporating either holonomy corrections, where the Ashtekar connection is replaced by a suitable sinus function in order to have a well-defined quantum analogue, or inverse-volume corrections coming from the eigenvalues of the inverse-volume operator.
In this paper we will develop an alternative approach to calculate cosmological perturbations in LQC based on the fact that, holonomy corrected LQC in the flat Friedmann-Lemaître-Robertson-Walker (FLRW) geometry could be also obtained as a particular case of teleparallel F(T) gravity (teleparallel LQC). The main idea of our approach is to mix the simple bounce provided by holonomy corrections in LQC with the non-singular perturbation equations given by F(T) gravity, in order to obtain a matter bounce scenario as a viable alternative to slow-roll inflation.
In our study, we have obtained an scale invariant power spectrum of cosmological perturbations. However, the ratio of tensor to scalar perturbations is of order 1, which does not agree with the current observations. For this reason, we suggest a model where a transition from the matter domination to a quasi de Sitter phase is produced in order to enhance the scalar power spectrum.
http://inspirehep.net/record/1252057?ln=en
4pages, published in JCAP 1311 (November 2013)

David Horgan's other stuff (commentary helpful to reading other QG research papers) is here:
http://quantumtetrahedron.wordpress.com/2014/03/25/a-curvature-operator-for-lqg-by-alesci-assanioussi-and-lewandowski/
(this is what he has for the recent Alesci, Assanioussi, Lewandowski paper on LQG curvature operator)

 

[marcus]

I should also mention the related work of Sergei Odintsov.
CV: http://www.ice.csic.es/files/odintsov/sergei_odintsov_briefcv.pdf
publications: http://inspirehep.net/author/profile/S.D.Odintsov.1
For example these recent papers:
http://arxiv.org/abs/arXiv:1402.7114
Universe acceleration in modified gravities: F(R) and F(T) cases
Kazuharu Bamba, Sergei D. Odintsov
(Submitted on 28 Feb 2014)
We review recent progress on cosmological issues and theoretical properties of modified gravity theories. In particular, we explicitly explore the conformal transformation, the Starobinsky inflation, and a unified scenario of inflation and late time acceleration in F(R) gravity and F(T) gravity (extended teleparallel gravity). Furthermore, we examine neutron stars and the hyperon problem in F(R) gravity. Moreover, for loop quantum cosmology (LQC), the natures of finite-time future singularities in F(T) gravity are presented. In addition, we investigate F(T) gravity theories from the Kaluza-Klein (KK) and Randall-Sundrum (RS) theories.
14 pages

http://arxiv.org/abs/1402.3071
http://inspirehep.net/record/1281221
On R+αR² Loop Quantum Cosmology
J. Amorós, J. de Haro, S.D. Odintsov
(Submitted 13 Feb 2014)
Working in Einstein frame we introduce, in order to avoid singularities, holonomy corrections to the f(R)=R+αR² model. We perform a detailed analytical and numerical study when holonomy corrections are taken into account in both Jordan and Einstein frames obtaining, in Jordan frame, a dynamics which differs qualitatively, at early times, from the one of the original model. More precisely, when holonomy corrections are taken into account the universe is not singular, starting at early times in the contracting phase and bouncing to enter in the expanding one where, as in the original model, it inflates. This dynamics is completely different from the one obtained in the original R+αR² model, where the universe is singular at early times and never bounces. Moreover, we show that these holonomy corrections may lead to better predictions for the inflationary phase as compared with current observations.
22 pages, 5 figures

This exposition of and commentary on teleparallel gravity by Aldrovandi and Pereira
http://www.ift.unesp.br/users/jpereira/tele.pdf
can almost serve as an introductory graduate-level textbook.
But I'm wondering--are their prohibitive costs to using this variant of GR? It is claimed to be equivalent to GR, but seems at first sight to be radically different.

 

[marcus]

More thoughts about LQC after BICEP2:
http://www.math.columbia.edu/~woit/wordpress/?p=6782&cpage=1#comment-208604
==quote Smolin==
The effect of loop quantum gravity on models of inflation have been studied by several authors, by adding an inflaton field and potential to loop quantum cosmology models. A recent detailed study is http://arxiv.org/abs/1302.0254 by Agullo, Ashtekar and Nelson. Some of the standard picture is unaffected but there is one crucial modification to the predictions which is that the usual relationship between r and the tensor tilt is modified (see Figure 9 and eq. 5.18). This is interesting given that apparently one possible way to resolve some of the tension between the Planck and Bicep1&2 data could be to have a blue tensor tilt, which would contradict the usual inflationary prediction.
==endquote==
The cited equation 5.18 is on page 39, and Figure 9 comes on page 41, after some discussion.
==quote Agullo Ashtekar Nelson page 39==
Since this expression holds independently of the inflaton potential and relates two independent observable quantities, it serves as a test of the standard scenario. Forthcoming observation of the effect of tensor perturbations in the CMB will provide a test of this relation. Does LQC modify this relation? We have seen that the tensor-to-scalar ratio remains unmodified. However, the tensor spectral index is modified because of the pre-inflationary evolution. The LQC tensor spectral index is obtained from the tensor power spectrum after averaging...
==endquote==
Here follows equation 5.17 for the LQC tensor spectral index n_t, which is modified by the pre-inflation LQC bounce, and, derived from that, equation 5.18 for the LQC tensor to scalar ratio r_LQC.

Figure 9 shows a striking difference between LQC prediction of the tensor scalar ratio (solid line) and the standard inflation model prediction (dashed green curve) at low modes k.

The above comment by Smolin calls attention to the AA&N paper and goes on to mention something else that could be looked for in light of the BICEP2 findings.

==quote Smolin==
There are also implications from loop quantum gravity for parity odd effects in the tensor modes, coming from the existence of an analogue of the QCD theta parameter in the gravitational action: the Immirzi parameter. There is the exciting possibility that this parameter could be measured by observing primordial B-T and B-E correlations, which sec 8.2 of the BICEP2 paper hints may be possible. This was proposed in http://arxiv.org/abs/0806.3082 and developed in http://arxiv.org/abs/1108.0816, http://arxiv.org/abs/1104.1800 and http://arxiv.org/abs/1007.3732.
==endquote==