Editing Primordial fluctuations (section)

===Tensor modes===
{{main|Gravitational wave}}

The presence of primordial [[tensor]] fluctuations is predicted by many inflationary models.  As with scalar fluctuations, tensor fluctuations are expected to follow a power law and are parameterized by the tensor index (the tensor version of the scalar index). The ratio of the tensor to scalar power spectra is given by
:<math>r=\frac{2|\delta_h|^2}{|\delta_R|^2},</math>
where the 2 arises due to the two polarizations of the tensor modes. 2015 [[cosmic microwave background|CMB]] data from the [[Planck (spacecraft)|Planck satellite]] gives a constraint of <math>r<0.11</math>.<ref name=":0">{{cite journal|page=1 |title=Planck 2015 results. XX. Constraints on inflation|journal=Astronomy & Astrophysics|volume=594|arxiv = 1502.02114|doi = 10.1051/0004-6361/201525898|year = 2016|last1 = Ade|first1 = P. A. R.|last2=Aghanim|first2=N.|author2-link=Nabila Aghanim|last3=Arnaud|first3=M.|last4=Arroja|first4=F.|last5=Ashdown|first5=M.|last6=Aumont|first6=J.|last7=Baccigalupi|first7=C.|last8=Ballardini|first8=M.|last9=Banday|first9=A. J.|last10=Barreiro|first10=R. B.|last11=Bartolo|first11=N.|last12=Battaner|first12=E.|last13=Benabed|first13=K.|last14=Benoît|first14=A.|last15=Benoit-Lévy|first15=A.|last16=Bernard|first16=J.-P.|last17=Bersanelli|first17=M.|last18=Bielewicz|first18=P.|last19=Bock|first19=J. J.|last20=Bonaldi|first20=A.|last21=Bonavera|first21=L.|last22=Bond|first22=J. R.|last23=Borrill|first23=J.|last24=Bouchet|first24=F. R.|last25=Boulanger|first25=F.|last26=Bucher|first26=M.|last27=Burigana|first27=C.|last28=Butler|first28=R. C.|last29=Calabrese|first29=E.|last30=Cardoso|first30=J.-F.|display-authors=29|bibcode=2016A&A...594A..20P|s2cid=119284788}}</ref>