Let me get this straight, the Big Bang was the start of the universe and it has been expanding ever since. But somewhere in that mess the galaxies formed as clumps of mass gravitated together. But Hubble found that all stars were red shifted.
So, how is that gravity acted to pull matter together but red shift tells us that all matter is moving apart?
Gravity pulls things together over (relatively) small distance scales; the expansion of the Universe happens over much larger scales.
IRCC the force of gravity that holds the galaxy together is stronger than the force exerted by the expansion of spacetime.
Say you had a couple of magnets sitting on a sheet of rubber, stuck to each other. If you stretch out the rubber, the magnets won’t move apart, even though their universe is expanding.
So it contracting? What about Hubble’s red shift?
All stars aren’t redshifted. What Hubble measured was the redshift of what was known as “spiral nebulae”. These were thought to be part of the Milky Way, as, well, no one knew there were other galaxies. Hubble found that the distances to these nebulae are vast, they are “island universes” in their own right (that is, other galaxies), far away from our galaxy, they mostly all appear to move away from us, and the further away they are, the faster they are moving. This lead to the realizations that 1. the universe is huge, dude, and 2. it’s expanding. Local stars within our galaxy can be redshifted or blueshifted depending on whether they’re moving away from us or towards us. Of course, all galaxies aren’t actually redshifted either, as galaxies also attract each other and form clusters. For instance, Andromeda is about to whack into us.
Hubble’s constant has a value of about 71 kilometers per second per megaparsec.
That is, if you look at two objects a megaparsec apart, on average they’ll be receding from each other at 71 kilometers per second.
A megaparsec happens to be 3.26 million light years.
The milky way is about 100,000 light years across.
Over that distance, the Hubble constant gives a recessional velocity of 2.18 km per second.
Compare that number to the gravitationally bound velocities of the earth and sun:
Earth’s average orbital velocity about the sun is is 47.9 kilometers per second
The sun’s average orbital velocity about the center of the Milky way is about 220 km/s.
Recession due to universal expansion is peanuts within the confines of a galaxy.
Maybe. It’s moving roughly toward us (or we towards it), but we don’t have the sophistication to accurately measure its transverse motion at this time.
Current estimates of Galactic escape velocity are running at between 460 km/sec and 560 km/sec.
Again, the rate of Hubble expansion within a galaxy, 2.1 km/sec, pales beside this number.
Andromeda Smandormeda, there are two other galaxies smacking into us right this minute! maybe three! Yeah, sure, the’re just “little” galaxies. That’s what they tell you, anyway. Next thing you know, your entire galactic arm is getting stretched out into the great cosmic void. Whadda you going to do now, eh?
Tris