▁Cop ern ican ▁Princi ple ▁was ▁not ▁truly ▁tested ▁on ▁a ▁cosm ological ▁scale ▁until ▁measurements ▁of ▁the ▁effects ▁of ▁the ▁cos mic ▁mic row ave ▁background ▁radiation ▁on ▁the ▁dynamics ▁of ▁distant ▁ast ro phys ical ▁systems ▁were ▁made . ▁A ▁group ▁of ▁astronom ers ▁at ▁the ▁European ▁Southern ▁Observ atory ▁noticed , ▁by ▁meas uring ▁the ▁temperature ▁of ▁a ▁distant ▁inter gal actic ▁cloud ▁in ▁thermal ▁equilibrium ▁with ▁the ▁cos mic ▁mic row ave ▁background , ▁that ▁the ▁radiation ▁from ▁the ▁Big ▁Bang ▁was ▁demon stra b ly ▁war mer ▁at ▁earlier ▁times . ▁Un iform ▁cool ing ▁of ▁the ▁cos mic ▁mic row ave ▁background ▁over ▁bill ions ▁of ▁years ▁is ▁strong ▁and ▁direct ▁observ ational ▁evidence ▁for ▁metric ▁expansion . ▁ ▁T aken ▁together , ▁these ▁phen omena ▁over wh el ming ly ▁support ▁models ▁that ▁rely ▁on ▁space ▁expand ing ▁through ▁a ▁change ▁in ▁metric . ▁It ▁was ▁not ▁until ▁the ▁discovery ▁in ▁the ▁year ▁ 2 0 0 0 ▁of ▁direct ▁observ ational ▁evidence ▁for ▁the ▁changing ▁temperature ▁of ▁the ▁cos mic ▁mic row ave ▁background ▁that ▁more ▁b izar re ▁constru ctions ▁could ▁be ▁ruled ▁out . ▁Until ▁that ▁time , ▁it ▁was ▁based ▁purely ▁on ▁an ▁assumption ▁that ▁the ▁universe ▁did ▁not ▁behave ▁as ▁one ▁with ▁the ▁Mil ky ▁Way ▁sitting ▁at ▁the ▁middle ▁of ▁a ▁fixed - metric ▁with ▁a ▁universal ▁explos ion ▁of ▁galaxies ▁in ▁all ▁directions ▁( as ▁seen ▁in , ▁for ▁example , ▁an ▁early ▁model ▁proposed ▁by ▁Mil ne ).