[Paper Review] Gravity waves goodbye

J. P. Zibin, D. Scott+1 more|arXiv (Cornell University)|Apr 18, 1999

Cosmology and Gravitation Theories3 citations

TL;DR

This paper argues that the detection of primordial gravitational waves via cosmic microwave background (CMB) anisotropies is now highly unlikely, due to strong observational constraints and theoretical preferences for small tensor-to-scalar ratios. It concludes that current data and leading inflationary models favor scalar perturbations overwhelmingly, making a detectable stochastic gravitational wave background in the CMB improbable with existing sensitivity limits.

ABSTRACT

The detection of a stochastic background of long-wavelength gravitational waves (tensors) in the cosmic microwave background (CMB) anisotropy would be an invaluable probe of the high energy physics of the early universe. Unfortunately a combination of factors now makes such a detection seem unlikely: the vast majority of the CMB signal appears to come from density perturbations (scalars) - detailed fits to current observations indicate a tensor-to-scalar quadrupole ratio of T/S < 0.5 for the simplest models; and on the theoretical side the best-motivated inflationary models seem to require very small T/S. Unfortunately CMB temperature anisotropies can only probe a gravity wave signal down to T/S \sim 10% and optimistic assumptions about polarization of the CMB only lower this another order of magnitude.

Motivation & Objective

• To assess the likelihood of detecting a stochastic background of long-wavelength gravitational waves in the cosmic microwave background (CMB).
• To evaluate the constraints on the tensor-to-scalar quadrupole ratio (T/S) from current CMB observations.
• To examine whether future CMB polarization measurements could improve detection sensitivity to gravitational waves.
• To determine whether the best-motivated inflationary models allow for a detectable gravitational wave signal.

Proposed method

• Analyzing CMB temperature anisotropy data to constrain the tensor-to-scalar ratio (T/S) in the simplest cosmological models.
• Using detailed fits to current observational data to estimate the upper bound on T/S.
• Evaluating the potential improvement in gravitational wave detection sensitivity from CMB polarization measurements.
• Assessing theoretical expectations from leading inflationary models, which predict very small T/S ratios.
• Comparing observational limits (T/S ~ 10%) with theoretical predictions to assess detectability.
• Concluding that even optimistic polarization assumptions only reduce the detection threshold by one order of magnitude.

Experimental results

Research questions

• RQ1 What is the current upper bound on the tensor-to-scalar quadrupole ratio (T/S) from CMB temperature anisotropy data?
• RQ2 Can CMB polarization measurements significantly improve the sensitivity to primordial gravitational waves?
• RQ3 How do the predictions of the best-motivated inflationary models compare with observational constraints on T/S?
• RQ4 Is a detectable stochastic gravitational wave background in the CMB still viable given current data?
• RQ5 To what extent do scalar perturbations dominate the CMB signal compared to tensor (gravitational wave) contributions?

Key findings

• Current CMB observations constrain the tensor-to-scalar quadrupole ratio to T/S < 0.5 in the simplest models.
• CMB temperature anisotropies alone can only probe gravitational wave signals down to T/S ~ 10%.
• Even with optimistic assumptions about CMB polarization, the detection threshold is only improved by one order of magnitude, reaching T/S ~ 1%.
• Theoretical models that are best-motivated by high-energy physics predict very small tensor-to-scalar ratios, making detection unlikely.
• The vast majority of the CMB signal arises from density perturbations (scalars), not gravitational waves (tensors).
• The combination of observational and theoretical constraints renders the detection of a primordial gravitational wave background in the CMB highly improbable.