In universes with significant curvature or cosmological constant , cosmic microwave background ( CMB ) anisotropies are created very recently via the Rees-Sciama or integrated Sachs-Wolfe effects .
This causes the CMB anisotropies to become partially correlated with the local matter density ( z < 4 ) .
We examine the prospects of using the hard ( 2-10 keV ) X-ray background as a probe of the local density and the measured correlation between the HEAO1 A2 X-ray survey and the 4-year COBE-DMR map to obtain a constraint on the cosmological constant .
The 95 % confidence level upper limit on the cosmological constant is \Omega _ { \Lambda } \leq 0.5 , assuming that the observed fluctuations in the X-ray map result entirely from large scale structure .
( This would also imply that the X-rays trace matter with a bias factor of b _ { x } \simeq 5.6 \Omega _ { m } ^ { 0.53 } . )
This bound is weakened considerably if a large portion of the X-ray fluctuations arise from Poisson noise from unresolved sources .
For example , if one assumes that the X-ray bias is b _ { x } = 2. , then the 95 % confidence level upper limit is weaker , \Omega _ { \Lambda } \leq 0.7 .
More stringent limits should be attainable with data from the next generation of CMB and X-ray background maps .