We report a discovery of a quasar at z = 4.96 \pm 0.03 within a few Mpc of the quasar SDSS 0338+0021 at z = 5.02 \pm 0.02 .
The newly found quasar has the SDSS i and z magnitudes of \approx 21.2 , and an estimated absolute magnitude M _ { B } \approx - 25.2 .
The projected separation on the sky is 196 arcsec , and the redshift difference \Delta z = 0.063 \pm 0.008 .
The probability of finding this quasar pair by chance in the absence of clustering in this particular volume is \sim 10 ^ { -4 } -10 ^ { -3 } .
We conclude that the two objects probably mark a large-scale structure , possibly a protocluster , at z \approx 5 .
This is the most distant such structure currently known .
Our search in the field of 13 other QSOs at z \hbox { \hbox to 0.0 pt { \raise 1.8275 pt \hbox { $ > $ } } \lower 2.795 pt \hbox { $ \sim$ } } % 4.8 so far has not resulted in any detections of comparable luminous QSO pairs , and it is thus not yet clear how representative is this structure at z \approx 5 .
However , along with the other evidence for clustering of quasars and young galaxies at somewhat lower redshifts , the observations are at least qualitatively consistent with a strong biasing of the first luminous and massive objects , in agreement with general predictions of theoretical models .
More extensive searches for clustered quasars and luminous galaxies at these redshifts will provide valuable empirical constraints for our understanding of early galaxy and structure formation .