Inflation predicts that the observed perturbations should be in thermal equilibrium with each other (these are called adiabatic or isentropic perturbations). This structure for the perturbations has been confirmed by the Planck spacecraft, WMAP spacecraft and other cosmic microwave background (CMB) experiments, and galaxy surveys, especially the ongoing Sloan Digital Sky Survey. These experiments have shown that the one part in 100,000 inhomogeneities observed have exactly the form predicted by theory. There is evidence for a slight deviation from scale invariance. The spectral index, n s is one for a scale-invariant Harrison–Zel'dovich spectrum. The simplest inflation models predict that n s is between 0.92 and 0.98 . This is the range that is possible without fine-tuning of the parameters related to energy. From Planck data it can be inferred that n s =0.968 ± 0.006, and a tensor to scalar ratio that is less than 0.11 . These are considered an important confirmation of the theory of inflation.