Expert: Tom Whiting - 1/20/2010

Question
What is the radial velocity of our sun relative to the galactic core?  More generally, are galaxies expanding, contracting, or fixed in size?

Answer
Hi Frank,
Galaxies, in fact all matter, are relatively fixed in size for the time being (see last paragraph)... it's the spacetime continuum that is expanding at the Hubble constant of about 71 Km/sec per Megaparsec. (The more distant galaxies are just going along for the ride, like an un-manned canoe going down a mountain stream, going faster and faster (as the expansion is slowly accelerating).

Since that expansion is...Per Megaparsec, that's 32.6 million light years, the actual
space expansion in our "lowly, small" 100,000 ly diameter Milky Way galaxy is hardly even measureable, let alone catastrophic.

I've read somewhere that at 30,000 ly from the center, the Solar System takes something like
250 million years to make one orbit around the Milky Way. As C = 2 pi r, that's about 188,000 ly that we cover in 250 million years.  I'll let you figure out the arithmetic there, but I've
read that the Solar System is traveling something like about 130 miles per second relative to the galaxy center.  (We are cutting "new ground in new space" every second, never to return
to where we were even last week)... because the galaxy is orbiting the Virgo Supercluster at
about 600 miles per second, but that's a different story.

Eventually in trillions of years, maybe a tad sooner, the spacial expansion will become so
great that even galaxies will be ripped apart, then Solar Systems, then even individual
molecules and atoms.  But that's a long way off..."The BIG RIP"...so don't lose any sleep over
it because we've only just begun at 4.6 billion years ago for the Solar System and "only" 13.7 billion years ago since the initial Big Bang... with a Universe total lifespan in the trillions
of years. The original H/He mix from the BB is still 97%, as only 3% of all normal matter is the heavy elements, (Lithium #3 to Uranium #92). As song says, We've only just begun.
Clear Skies,
Tom Whiting
Erie, PA

PS...notice that the first question has nothing to do (is not relative to)...your second question.