SIERRA NEGRA, Mexico — Skinny dogs, rattlesnakes and burros roaming the ridges of this long-dormant volcano know something is afoot, though there hasn't been an eruption here in centuries. What causes locals to stir, and scientists to salivate, is a 17-story monster of a telescope being assembled atop this 15,000-foot peak. It is a giant time machine, probing the origins of the universe with an antenna dish the size of a baseball infield. It looks like a space-age windmill, with two U-shaped steel arms to hold up a giant satellite dish. It will be the largest millimeter-wave radio telescope in...