No detection of gravitational radiation

[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).

 

[kimbyd]

Is none detection evidence of no gravitational radiation?

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.

 

[mfb]

Since r≥0 by definition, this implies a maximum likelihood value of r=0, or no evidence for gravitational waves.

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.

This says nothing at all about gravity waves in general.

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

 

[George Jones]

A key conclusion of the paper:

5 FUTURE PROSPECTS|PRIMORDIAL GRAVITATIONAL RADIATION = HAWKING RADIATION

In the standard calculation of the gravitational radiation in the early Universe (for example Maldacena & Pimentel [2011]), in calculating the graviton propagator one uses the Bunch-Davies vacuum (Bunch & Davies 1978), which is equivalent the Gibbons and Hawking thermal vacuum (Gibbons & Hawking 1977), which includes Gibbons and Hawking thermal radiation (Gibbons & Hawking 1977), which is Hawking radiation (Hawking 1974) from the causal horizon in the early Universe. If such gravitational radiation were found, it would constitute a conrmation of the Hawking (1974) mechanism. This gravitational radiation is produced by a quantum process quite different from the gravitational waves recently discovered by LIGO (e.g. LIGO and Virgo Collaborations 2016), which is in the classical gravitational wave domain.

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.

 

[wolram]

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.

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

 

[bapowell]

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

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.

 

[kimbyd]

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

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

 

[wolram]

Thanks bapowell iI did not know thees models existed.