Press Conference on Major Discovery - primordial B modes?

[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.