Quote:
|
Originally Posted by Fr. Wayne
OK that's a fair upper limit for parts. Does it have any lower limit?
|
Yes, and that is an excellent question!
Below about 5 Mpc—I am not sure of the exact distance—the Hubble relation “doesn’t work,” due to local motions. Case in point, the Andromeda galaxy (M31) is about 1 Mpc away, and would be receding at 72 km/sec per the Hubble expansion, but instead is coming at us at 100 km/sec (and gaining!)
Contraction and expansion in the cosmos are complementary, in the same way that erosion and sedimentation are complementary in geology. Every square foot of the earth’s surface is generally either in the process of being eroded-away or “sedimented”-in. On the surface, the two processes mix and match endlessly. Yet below sea-level, there is net sedimentation, and above sea-level, there is net erosion.
Likewise, in the cosmic dance, expansion and contraction intermingle endlessly. I already mentioned the space between the earth and moon is expanding at 100 ppt/yr. On the other hand, the space between earth and man-made satellites contracts (they spiral back down). The height of geosynchronous orbit (GESO), about 24,000 miles above the equator, is the dividing line or “inflection point” between expansion and contraction. Below GESO, satellites spiral in, above it, they spiral away. Right at GESO, they are at a quasi-static orbit, but eventually—without adjustment—a satellite at GESO will drift into one bin or the other: expansion or contraction.
All astronomical systems are rife with such inflection points, of all character, where things either fall in or fall away. As with the “sea-level” on earth that provides the dividing line between net-erosion and net-expansion, this 5 Mpc dimension, while not as clear-cut as the shoreline, nonetheless provides a conceptual divide: within a 5 Mpc “volume” there is net-contraction; beyond it, there is net expansion. So this 5 Mpc distance represents the “minimum” below which, as you asked, the Hubble relation does not apply.