Correlation between galaxies and QSO in the SDSS:na signal from   gravitational lensing magnification?
  We report a detection of galaxy-QSO cross-correlation w_GQ in the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR) over 0.2-30 arc-minute scales. We cross-correlate galaxy samples of different mean depths r'=19-22 (z_G =0.15-0.35) with the main QSO population (i'<19.1) at mean z_Q=1.6. We find significant positive correlation in all cases except for the faintest QSOs, as expected if the signal were due to weak lensing magnification. The amplitude of the signal on arc-minute scales is about 20%. This is a few times larger than currently expected from weak lensing in LCDM but confirms, at a higher significance, previous measurements by several groups. When compared to the galaxy-galaxy correlation w_GG, a weak lensing interpretation indicates a strong and steep non-linear amplitude for the underlaying matter fluctuations: sigma=400 on scales of 0.2 Mpc/h, in contradiction with non-linear modeling of LCDM fluctuations. We also detect a normalized skewness (galaxy-galaxy-QSO correlation) of S_3=21 +/-6 at z=0.15 (S_3= 14 +/- 4 at z=0.35), which several sigma low, as compared to LCDM expectations. These observational trends can be reconciled with lensing in a flat Lambda universe with sigma_8=1, provided the linear spectrum is steeper (n = 1) than in the LCDM model on these small (cluster) scales. Under this interpretation, the galaxy distribution traces the matter variance with an amplitude that is 100 times smaller: ie galaxies are anti-bias with b=0.1 on small scales, increasing to b=1 at 10 Mpc/h. 
Enrique Gaztanaga (IEEC/CSIC)