An important and long-standing puzzle in the history of modern physics is the gross inconsistency between theoretical expectations and cosmological observations of the vacuum energy density , by at least 60 orders of magnitude , otherwise known as the cosmological constant problem .
A characteristic feature of vacuum energy is that it has a pressure with the same amplitude , but opposite sign to its energy density , while all the precision tests of General Relativity are either in vacuum , or for media with negligible pressure .
Therefore , one may wonder whether an anomalous coupling to pressure might be responsible for decoupling vacuum from gravity .
We test this possibility in the context of the Gravitational Aether proposal , using current cosmological observations , which probe the gravity of relativistic pressure in the radiation era .
Interestingly , we find that the best fit for anomalous pressure coupling is about half-way between General Relativity ( GR ) , and Gravitational Aether ( GA ) , if we include Planck together with WMAP and BICEP2 polarization cosmic microwave background ( CMB ) observations .
Taken at face value , this data combination excludes both GR and GA at around the 3 \sigma level .
However , including higher resolution CMB observations ( “ highL ” ) or baryonic acoustic oscillations ( BAO ) pushes the best fit closer to GR , excluding the Gravitational Aether solution to the cosmological constant problem at the 4– 5 \sigma level .
This constraint effectively places a limit on the anomalous coupling to pressure in the parametrized post-Newtonian ( PPN ) expansion , \zeta _ { 4 } = 0.105 \pm 0.049 ( +highL CMB ) , or \zeta _ { 4 } = 0.066 \pm 0.039 ( +BAO ) .
These represent the most precise measurement of this parameter to date , indicating a mild tension with GR ( for \Lambda CDM including tensors , with \zeta _ { 4 } = 0 ) , and also among different data sets .