Sky at Night Magazine - - CONTENTS - by Chris Lin­tott CHRIS LIN­TOTT co­p­re­sents The Sky at Night

This rev­e­la­tion about the LMC might help solve a long-stand­ing cos­mo­log­i­cal mys­tery, as pre­dict­ing the num­ber of satel­lite gal­ax­ies that sur­round gal­ax­ies like the Milky Way is some­thing of a chal­lenge. These small sys­tems are be­lieved mostly to have formed early, and so how many there are is very sen­si­tive to de­tails of the make up of the Uni­verse, es­pe­cially to what we as­sume about the amount of dark mat­ter and how it be­haves.

Most as­tronomers be­lieve that, al­though it is mys­te­ri­ous, there is much more dark than or­di­nary mat­ter, but for years mod­els with plenty of dark mat­ter have pre­dicted more satel­lite gal­ax­ies than we see. More sen­si­tive sur­veys have found fainter sys­tems, but this new re­sult may sug­gest they’ve had more com­plex lives than we would oth­er­wise ex­pect. If merg­ers of satel­lites are com­mon, we need to do more than just count them – we need to un­der­stand their his­to­ries too.

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