I No detection of gravitational radiation

1. Jul 24, 2017

wolram

Is none detection evidence of no gravitational radiation?

arXiv:1707.06755 [pdf, other]
Reanalysis of the BICEP2, Keck and Planck Data: No Evidence for Gravitational Radiation
J. Richard Gott III (Princeton University), Wesley N. Colley (University of Alabama in Huntsville)
Comments: LaTeX (MNRAS style), 10 pages
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

A joint analysis of data collected by the Planck and BICEP2+Keck teams has previously given r=0.09+0.06−0.04 for BICEP2 and r=0.02+0.04−0.02 for Keck. Analyzing BICEP2 using its published noise estimate, we had earlier (Colley & Gott 2015) found r=0.09±0.04, agreeing with the final joint results for BICEP2. With the Keck data now available, we have done something the joint analysis did not: a correlation study of the BICEP2 vs. Keck B-mode maps. Knowing the correlation coefficient between the two and their amplitudes allows us to determine the noise in each map (which we check using the E-modes). We find the noise power in the BICEP2 map to be twice the original BICEP2 published estimate, explaining the anomalously high r value obtained by BICEP2. We now find r=0.004±0.04 for BICEP2 and r=−0.01±0.04 for Keck. Since r≥0 by definition, this implies a maximum likelihood value of r=0, or no evidence for gravitational waves. Starobinsky Inflation (r=0.0036) is not ruled out, however. Krauss & Wilzcek (2014) have already argued that "measurement of polarization of the CMB due to a long-wavelength stochastic background of gravitational waves from Inflation in the early Universe would firmly establish the quantization of gravity," and, therefore, the existence of gravitons. We argue it would also constitute a detection of gravitational Hawking radiation (explicitly from the causal horizons due to Inflation).

2. Jul 26, 2017

kimbyd

No. It's evidence that what is known as the "tensor to scalar ratio" is low. There are some non-inflation models which predict this, though there are also a number of inflation models that have little to no tensor fluctuations.

If there aren't any of the fluctuations which produce these specific gravity waves in the early universe, then it's disappointing in some respects because it removes a possible window into the nature of the early universe.

This says nothing at all about gravity waves in general.

3. Jul 26, 2017

Staff: Mentor

No evidence for gravitational waves in the cosmic microwave background at the sensitivity of BICEP2. They can be there, just too small to be seen up to now.
Several experiments plan to improve the sensitivity by more than a factor 10 in the next years.

This is independent of the direct detection of gravitational waves. It is also independent of other indirect tests, like binary pulsars.

Gravity waves are something else (e. g. water waves).

4. Jul 26, 2017

George Jones

Staff Emeritus
A key conclusion of the paper:

If this conclusion is accepted by the physics community, if such gravitational radiation were found, and if Hawking is still alive when this happens, then he would possibly be in line for a Nobel prize.

Last edited: Jul 26, 2017
5. Jul 27, 2017

wolram

Thanks for the replies , can you tell me which models have little to no tensor fluctuations?

6. Jul 27, 2017

bapowell

A useful criterion is the Lyth bound: $\Delta \phi/ M_{\rm Pl} \propto \sqrt{r/0.01}$. It tells us that, generically, in models where the inflaton field variation is smaller than the Planck scale, the tensor/scalar ratio is suppressed. Concave models, like so-called "hill top" inflation of the form $V \sim 1 - \phi^n$ near the maximum, have small field variations for acceptable values of $n_s$, and hence, small $r$. Starobinsky's $R^2$ inflation is in this category. Hybrid inflation is another class of models with suppressed tensor/scalar ratio because $\Delta \phi$ can be made arbitrarily small. In contrast, traditional convex models of the form $V \sim \phi^n,$like Linde's "chaotic" inflation, generate a comparatively large amplitude of gravitational waves because in these models field variations are greater than the Planck scale.

7. Jul 27, 2017

kimbyd

I believe both the ekpyrotic and bounce models have essentially no tensor perturbations.

8. Jul 28, 2017

wolram

Thanks bapowell iI did not know thees models existed.