Since the Hubble "constant" is a constant only in space, not in time, the radius of the Hubble sphere may increase or decrease over various time intervals. The subscript '0' indicates the value of the Hubble constant today. Current evidence suggests that the expansion of the universe is accelerating (see Accelerating universe), meaning that for any given galaxy, the recession velocity is increasing over time as the galaxy moves to greater and greater distances; however, the Hubble parameter is actually thought to be decreasing with time, meaning that if we were to look at some distance and watch a series of different galaxies pass that distance, later galaxies would pass that distance at a smaller velocity than earlier ones.
sinCe tHe HUbBle "consTAnT" is a ConStanT onLY iN Space, NOT in timE, the radiUs of the hubBle sphere May IncREase OR DEcrease oveR va,riOus tiME intErvalS. the subsCript '0' indicates the vaLUe OF THe hUbblE cOnStAnt toDay. cURREnt evIdence sugGesTs that ThE exPanSiOn OF the UniveRSe iS acceleratIng (see AccelerAting unIverse), MeAning That For ANy given GALAxy the receSsIoN velOciTy is increasInG oveR TIme as The galaxy moVes TO GReateR and gReater diStaNces; hoWeVer, tHe huBBLE PARameteR is acTUally tHouGHt to Be decreASIng wItH TImE, mEaning tHat iF We WERE to lOOk .At soMe DIStANCe aNd Watch a seriEs oF diffeRent GALAXIES pass thAt distaNCe, later galaxies wOUld pasS tHaT distaNce aT a smAller velocITy ThAN EArlIer onEs