▁the ▁distance ▁scale ▁var ies ▁very ▁much ▁in ▁different ▁areas . ▁There ▁is ▁no ▁map ▁projection ▁in ▁which ▁the ▁distance ▁between ▁any ▁two ▁points ▁on ▁Earth , ▁measured ▁along ▁the ▁great ▁circle ▁ge odes ics , ▁is ▁directly ▁proportional ▁to ▁their ▁distance ▁on ▁the ▁map ; ▁such ▁accuracy ▁is ▁possible ▁only ▁with ▁a ▁glo be . ▁ ▁Met ric ▁tens ors ▁ ▁In ▁differential ▁geometry , ▁the ▁back bone ▁mathematics ▁for ▁general ▁relativ ity , ▁a ▁metric ▁tensor ▁can ▁be ▁defined ▁which ▁precisely ▁character izes ▁the ▁space ▁being ▁described ▁by ▁explaining ▁the ▁way ▁distances ▁should ▁be ▁measured ▁in ▁every ▁possible ▁direction . ▁General ▁relativ ity ▁necessarily ▁inv okes ▁a ▁metric ▁in ▁four ▁dimensions ▁( one ▁of ▁time , ▁three ▁of ▁space ) ▁because , ▁in ▁general , ▁different ▁reference ▁frames ▁will ▁experience ▁different ▁intervals ▁of ▁time ▁and ▁space ▁depending ▁on ▁the ▁in ert ial ▁frame . ▁This ▁means ▁that ▁the ▁metric ▁tensor ▁in ▁general ▁relativ ity ▁rel ates ▁precisely ▁how ▁two ▁events ▁in ▁spac etime ▁are ▁separated . ▁A ▁metric ▁expansion ▁occurs ▁when ▁the ▁metric ▁tensor ▁changes ▁with ▁time ▁( and , ▁specifically , ▁whenever ▁the ▁spatial ▁part ▁of ▁the ▁metric ▁gets ▁larger ▁as ▁time ▁goes ▁forward ). ▁This ▁kind ▁of ▁expansion ▁is ▁different ▁from ▁all ▁kinds ▁of ▁exp ans ions ▁and ▁explos ions ▁commonly ▁seen ▁in ▁nature ▁in ▁no ▁small ▁part ▁because ▁times ▁and ▁distances ▁are ▁not ▁the ▁same ▁in ▁all ▁reference ▁frames , ▁but ▁are ▁instead ▁subject ▁to ▁change . ▁A ▁useful ▁visual ization ▁is ▁to ▁approach ▁the ▁subject ▁rather ▁than