An inflationary gravitational wave background consistent with BICEP2 isdifficult to reconcile with a simple power-law spectrum of primordial scalarperturbations. Tensor modes contribute to the temperature anisotropies atmultipoles with $l\lesssim 100$, and this effect --- together with a prior onthe form of the scalar perturbations --- was the source of previous bounds onthe tensor-to-scalar ratio. We compute Bayesian evidence for combined fits toBICEP2 and Planck for three nontrivial primordial spectra: a) a runningspectral index, b) a cutoff at fixed wavenumber, and c) a spectrum described bya linear spline with a single internal knot. We find no evidence for a cutoff,weak evidence for a running index, and significant evidence for a "broken"spectrum. Taken at face-value, the BICEP2 results require two new inflationaryparameters in order to describe both the broken scale invariance in theperturbation spectrum and the observed tensor-to-scalar ratio. Alternatively,this tension may be resolved by additional data and more detailed analyses.
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机译：与BICEP2一致的膨胀引力波背景很难与原始标量摄动的简单幂律谱调和。张量模式在$ l \ lesssim 100 $时会导致多极点的温度各向异性，而这种效应-与标量摄动形式的先验-一起是张量与标量比先前界限的来源。我们为三个非平凡的原始光谱计算了BICEP2和Planck的组合拟合的贝叶斯证据：a）运行光谱指数，b）固定波数的截止，c）由带有单个内部结的线性样条曲线描述的光谱。我们没有发现中断的证据，也没有发现运行指数的证据，而发现“破裂”频谱的重要证据。以面值计，BICEP2结果需要两个新的通货膨胀参数，以便描述扰动谱中的破碎标度不变性和观测到的张量与标量比。或者，可以通过其他数据和更详细的分析来解决此问题。
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