▁embedding ▁in ▁hyp ers pace ▁is ▁required ▁for ▁an ▁expansion ▁to ▁occur . ▁The ▁visual izations ▁often ▁seen ▁of ▁the ▁universe ▁growing ▁as ▁a ▁b ubble ▁into ▁nothing ness ▁are ▁mis leading ▁in ▁that ▁respect . ▁There ▁is ▁no ▁reason ▁to ▁believe ▁there ▁is ▁anything ▁" out side " ▁of ▁the ▁expand ing ▁universe ▁into ▁which ▁the ▁universe ▁exp ands . ▁ ▁Even ▁if ▁the ▁overall ▁spatial ▁extent ▁is ▁infinite ▁and ▁thus ▁the ▁universe ▁cannot ▁get ▁any ▁" lar ger ", ▁we ▁still ▁say ▁that ▁space ▁is ▁expand ing ▁because , ▁locally , ▁the ▁characteristic ▁distance ▁between ▁objects ▁is ▁increasing . ▁As ▁an ▁infinite ▁space ▁grows , ▁it ▁remains ▁infinite . ▁ ▁D ens ity ▁of ▁universe ▁during ▁expansion ▁Despite ▁being ▁extremely ▁dense ▁when ▁very ▁young ▁and ▁during ▁part ▁of ▁its ▁early ▁expansion ▁- ▁far ▁dens er ▁than ▁is ▁usually ▁required ▁to ▁form ▁a ▁black ▁hole ▁- ▁the ▁universe ▁did ▁not ▁re - collapse ▁into ▁a ▁black ▁hole . ▁This ▁is ▁because ▁commonly - used ▁calculations ▁for ▁gravit ational ▁collapse ▁are ▁usually ▁based ▁upon ▁objects ▁of ▁relatively ▁constant ▁size , ▁such ▁as ▁stars , ▁and ▁do ▁not ▁apply ▁to ▁rapidly ▁expand ing ▁space ▁such ▁as ▁the ▁Big ▁Bang . ▁ ▁Effect s ▁of ▁expansion ▁on ▁small ▁scales ▁The ▁expansion ▁of ▁space ▁is ▁sometimes ▁described ▁as ▁a ▁force ▁which ▁acts ▁to ▁push ▁objects ▁apart . ▁Though ▁this ▁is ▁an ▁accurate ▁description ▁of ▁the ▁effect ▁of ▁the ▁cosm ological ▁constant , ▁it ▁is ▁not ▁an ▁accurate ▁picture ▁of ▁the ▁phenomen on ▁of ▁expansion ▁in ▁general