Press Conference on Major Discovery - primordial B modes?

[Haelfix]

What that means is they don't have enough control of their theory to output a prediction, there are more free parameters than unknowns. In short there is no value of r or n that will either confirm or falsify Lqc at this time.

As I said in another thread the problem is that Lqc is like multi bout models of inflation. The first super bounce is followed by a standard bout of inflation. But the latter can effectively wipe out all traces of the former (this corresponds in their language to kstar < khubble), but you can't know that without making assumptions about the inflaton to begin with.

 

[exponent137]

Please, more simply for the beginning. I am not an expert.
Is Linde's theory probably based on some quantum gravity premises? What they are?

Isham wrote something about quantum gravity, as http://arxiv.org/abs/gr-qc/9310031.
If we suppose that BICEP2 is correct, can it tells something about Isham?

McAllister wrote something about quantum gravity.
http://motls.blogspot.com/2014/03/b...tational-waves.html?showComment=1395081102056
(I hope that this will not be deleted.)
He wrote more about superstrings. Is it possible to write something about quantum gravity, not connected to specific QG theories, like Isham.

Feynman's approach to quantum gravity has the problems, because of nonrenormalizability. Why this is the problem if we assume that spacetime is grained? Why we need strings, why grained spacetime is not enough?

 

[cristo]

Thanks for the info marcus.

What that means is they don't have enough control of their theory to output a prediction, there are more free parameters than unknowns. In short there is no value of r or n that will either confirm or falsify Lqc at this time.

That's exactly what I suspected.

 

[TrickyDicky]

Can anyone clarify if having a running spectral index n_s is compatible with primordial gravitational waves within the inflationary scenario?
I guess it depends on how much it tilts the spectrum, but I was under the impression that a certain amount of scale invariance is needed simply to observe coherently the primordial spectrum fluctuations (coherent acoustic peaks, etc)and in any case inflation predicts a gravitational waves spectrum almost scale invariant, how would the introduction of a running tilt affect this?

 

[exponent137]

Some things are not clear enough:
Quantum fluctuactions cause gravitational waves and these waves cause polarization of the electromagnetic waves. Does this means: any gravitational waves could produce D-mode, even not caused by Quantum fluctuations?
Does Quantum fluctuations built some different gr. waves as the classical gravitational waves?

 

[marcus]

What that means is they don't have enough control of their theory to output a prediction, there are more free parameters than unknowns. In short there is no value of r or n that will either confirm or falsify Lqc at this time.
...

In fact there are several lines of development in LQC, and results on r appear to be helping to distinguish and sort them out (assuming the observed values are confirmed).
for instance, one line of LQC development is teleparallel--Jaume de Haro is the main person on that one:
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

 

[skydivephil]

Ive often seen "matter bounce" suggested as an alternative to inflation. However in standrad LQc you go form bounce to super inflation to inflation and so on. So in this standard scenario, is it different to the phrase "matter bounce" are there different types of bounces?

 

[bapowell]

Can anyone clarify if having a running spectral index n_s is compatible with primordial gravitational waves within the inflationary scenario?
I guess it depends on how much it tilts the spectrum, but I was under the impression that a certain amount of scale invariance is needed simply to observe coherently the primordial spectrum fluctuations (coherent acoustic peaks, etc)and in any case inflation predicts a gravitational waves spectrum almost scale invariant, how would the introduction of a running tilt affect this?

Yes, sure -- they are determined by different aspects of the inflationary dynamics: GW's by the energy density and running by the shape of the potential (mostly by the third derivative, V''').

The scalar perturbation is decidedly *not* scale invariant: n_s = 1 is ruled out at several sigma.

 

[bapowell]

Ive often seen "matter bounce" suggested as an alternative to inflation. However in standrad LQc you go form bounce to super inflation to inflation and so on. So in this standard scenario, is it different to the phrase "matter bounce" are there different types of bounces?

Yes, you can have noninflationary matter bounces. In these scenarios, the perturbations are set up during the contracting phase (pass outside the shrinking horizon) and carry through the bounce. The problem is how to keep the calculation under control through the bounce which, depending on the model and perturbation variable, can become singular.

 

[bapowell]

Some things are not clear enough:
Quantum fluctuactions cause gravitational waves and these waves cause polarization of the electromagnetic waves. Does this means: any gravitational waves could produce D-mode, even not caused by Quantum fluctuations?
Does Quantum fluctuations built some different gr. waves as the classical gravitational waves?

Primordial gravitational waves produced by inflation behave just like classical gravitational waves.

 

[TrickyDicky]

Yes, sure -- they are determined by different aspects of the inflationary dynamics: GW's by the energy density and running by the shape of the potential (mostly by the third derivative, V''').

The scalar perturbation is decidedly *not* scale invariant: n_s = 1 is ruled out at several sigma.

Yes, I know it is not exactly scale invariant n_s = 1, that would correspond to a pure de Sitter expansion.
My question was referring to exactly how far from scale invariance can it be, that is, my understanding is that certain basic features of what we observed in the CMB spectrum(like the existence of observable peaks at certain Δθ°) were dependent on a close-to- scale invariant power spectrum.
Or am I misunderstanding this quote from wikipedia?: "In physical cosmology, the power spectrum of the spatial distribution of the cosmic microwave background is near to being a scale-invariant function. Although in mathematics this means that the spectrum is a power-law, in cosmology the term "scale-invariant" indicates that the amplitude, P(k), of primordial fluctuations as a function of wave number, k, is approximately constant, i.e. a flat spectrum. This pattern is consistent with the proposal of cosmic inflation."

In this sense I was under the impression that the tension between BICEP2 and Planck was in part because due to the high tensor-scalar ratio observed in order to make them agree one needed to depart excesively from near-scale-invariance with a bigger than expected running of n_s, is this moreless right?

 

[bapowell]

OK, yes, now I understand. Yes, that is correct. Both the tensor and scalar perturbations contribute to the TT (temperature) spectrum at low-\ell (specifically to all \ell to the left of the central acoustic peak). The TT spectrum is remarkably low at low-\ell. Given the BICEP2 result indicating a large tensor component, that means the scalar component must be especially small. Now, if we consider a power-law spectrum with n_s = 0.96 as favored by Planck and extrapolate this spectrum to large scales (low \ell), we have too much scalar power. The problem is if we increase n_s towards scale invariance to lessen the large-scale power, we increase the small scale power beyond the very good constraints from measurements of the damping tail from probes like ACT and SPT. What is therefore needed is to add running, specifically, negative running so that n_s runs to larger values at larger scales (smaller k) and smaller values at smaller scales (larger k). This is how a large tensor component can be reconciled with the TT spectrum.

So, it's not that one needs to depart from scale-invariance -- that's already true pre-BICEP2. It's that one needs to depart from power-law -- constant n_s.

Note that there are other ways to address this issue without adding running, like incorporating neutrino masses.

 

[TrickyDicky]

OK, yes, now I understand. Yes, that is correct. Both the tensor and scalar perturbations contribute to the TT (temperature) spectrum at low-\ell (specifically to all \ell to the left of the central acoustic peak). The TT spectrum is remarkably low at low-\ell. Given the BICEP2 result indicating a large tensor component, that means the scalar component must be especially small. Now, if we consider a power-law spectrum with n_s = 0.96 as favored by Planck and extrapolate this spectrum to large scales (low \ell), we have too much scalar power. The problem is if we increase n_s towards scale invariance to lessen the large-scale power, we increase the small scale power beyond the very good constraints from measurements of the damping tail from probes like ACT and SPT. What is therefore needed is to add running, specifically, negative running so that n_s runs to larger values at larger scales (smaller k) and smaller values at smaller scales (larger k). This is how a large tensor component can be reconciled with the TT spectrum.

So, it's not that one needs to depart from scale-invariance -- that's already true pre-BICEP2. It's that one needs to depart from power-law -- constant n_s.

Note that there are other ways to address this issue without adding running, like incorporating neutrino mIasses.

Ok, I see, thanks. Even though the wikipedia quote mentioned the difference in cosmology, I guess I was still conflating the power law with the scale-invariance.
(there seems to be a typo where you must be referring to small multipoles-large scale)

 

[exponent137]

r's of BICEP2 and Planck disagree, 0,2 and 0,11. Are any explanations of this?

 

[bapowell]

They agree if you look at Planck's constraints on r when running is included.

Adding neutrino masses helps to, although I'm less familiar with this.

 

[millitiz]

I have a question. I was watching another presentation online (at Taiwan), and the presenter said that the existence of B-mode support the existence of gravitational wave, which therefore hint the existence of graviton. The confusing part for me is that, I thought classical GR would have gravitational wave solution, so I don't see where graviton really comes into the play, since it is not a necessary ingredient for generating gravitational wave.

 

[George Jones]

I have a question. I was watching another presentation online (at Taiwan), and the presenter said that the existence of B-mode support the existence of gravitational wave, which therefore hint the existence of graviton. The confusing part for me is that, I thought classical GR would have gravitational wave solution, so I don't see where graviton really comes into the play, since it is not a necessary ingredient for generating gravitational wave.

See the arXiv article

http://arxiv.org/abs/1309.5343

by Krauss and Wilczek,

and discussion about the article,

http://www.nature.com/news/how-to-see-quantum-gravity-in-big-bang-traces-1.13834

http://backreaction.blogspot.ca/2013/10/quantum-gravity-in-cosmic-microwave.html

 

[bapowell]

I have a question. I was watching another presentation online (at Taiwan), and the presenter said that the existence of B-mode support the existence of gravitational wave, which therefore hint the existence of graviton. The confusing part for me is that, I thought classical GR would have gravitational wave solution, so I don't see where graviton really comes into the play, since it is not a necessary ingredient for generating gravitational wave.

The primordial gravitational waves that give rise to the purported B-mode polarization of the CMB are special: they are generated out of the quantum vacuum by the inflationary expansion.

 

[skydivephil]

I thought MArcus and Bapowell might be interested in this paper:
http://arxiv.org/abs/1403.7623
a claim bounce prior to inflation is a better fit than just inflation. Anyone like to comment? i presume a lot of people will be trying to fit their favourite models to this data even before its confirmed.