I Don't Know Where Else To Ask This

Ask questions, find resources, browse the virtual shelves.
artart
Asternaut
Posts: 2
Joined: Wed Oct 06, 2010 12:36 pm

I Don't Know Where Else To Ask This

Post by artart » Wed Oct 06, 2010 1:00 pm

Gents,

Over the past year or so, I've been viewing APOD daily. While I can't site a specific reference, there have been suggestions that elements of the heavens are colliding/interacting/transforming/etc. into new or changed entities. But...if I understand the "Big Bang" theory, the universe is constantly expanding such that everything is more distant from all other entities than when I began typing this. How can things be farther apart and yet colliding at the same time? (Submitted by an Engineer).

User avatar
bystander
Apathetic Retiree
Posts: 20758
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Re: I Don't Know Where Else To Ask This

Post by bystander » Wed Oct 06, 2010 1:20 pm

Gravity dominates interactions at interplanetary, interstellar, and even intergalactic scales within clusters of galaxies. Expansion doesn''t dominate until much larger distance scales. See: Wikipedia: Metric expansion of space

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18354
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: I Don't Know Where Else To Ask This

Post by neufer » Wed Oct 06, 2010 1:21 pm

artart wrote:Gents,

Over the past year or so, I've been viewing APOD daily. While I can't site a specific reference, there have been suggestions that elements of the heavens are colliding/interacting/transforming/etc. into new or changed entities. But...if I understand the "Big Bang" theory, the universe is constantly expanding such that everything is more distant from all other entities than when I began typing this. How can things be farther apart and yet colliding at the same time? (Submitted by an Engineer).
Things are colliding but only on a galactic scale or smaller.

Expansion of the universe is negligible (i,e, less than 1 km/s) on such scales.
Art Neuendorffer

User avatar
Ann
4725 Å
Posts: 11520
Joined: Sat May 29, 2010 5:33 am

Re: I Don't Know Where Else To Ask This

Post by Ann » Wed Oct 06, 2010 3:10 pm

Think of it like this. The Andromeda Galaxy is only about two million light-years away from us. The Milky Way and Andromeda are both rather big and massive galaxies, and they are close enough that their mutual gravity holds them in a steady grip. Right now they are dancing around each other, and they - we - will probably collide a couple of billion years from now.

But consider the Virgo Cluster. The Virgo Cluster contains many galaxies that are bigger and more massive than either the Andromeda Galaxy or the Milky Way. But the galaxies in the Virgo Cluster are also, on average, about 60 million light-years away from us. That is too much for the gravity of the Virgo Cluster to get a real "hold" on us. We are not "connected" to the Virgo Cluster through any mutual gravitation. Therefore, the Virgo Cluster is "sailing away" from us as the universe expands.

Ann
Color Commentator

artart
Asternaut
Posts: 2
Joined: Wed Oct 06, 2010 12:36 pm

Re: I Don't Know Where Else To Ask This

Post by artart » Wed Oct 06, 2010 3:29 pm

Gee Golly Gosh! Your wonderful answers just about blow my mind. It all sounds good but pardon me if I don't follow it all. I wish Dick Feynman was around; wonder what sort of an answer he would give us.

Thanks!!

User avatar
BMAONE23
Commentator Model 1.23
Posts: 4076
Joined: Wed Feb 23, 2005 6:55 pm
Location: California

Re: I Don't Know Where Else To Ask This

Post by BMAONE23 » Wed Oct 06, 2010 4:43 pm

See if this helps.
Remember the old Vibrating football games
Click to play embedded YouTube video.
you hit the juice and the players moved about the playing field.
Now imagine a playing field that is slightly convex. When you hit the juice, the players would all move away from each other "Expansion". Now if you tie strings to several groups of players (not string theory... but to represent gravity) then when you hit the juice, bound groups will move together and bang into each other while as a whole unbound groups will move away from each other.

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18354
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: I Don't Know Where Else To Ask This

Post by neufer » Wed Oct 06, 2010 10:17 pm

Ann wrote:
Think of it like this. The Andromeda Galaxy is only about two million light-years away from us. The Milky Way and Andromeda are both rather big and massive galaxies, and they are close enough that their mutual gravity holds them in a steady grip. Right now they are dancing around each other, and they - we - will probably collide a couple of billion years from now.
NO to the "mutual gravity holds them in a steady grip" and
NO to the "dancing around each other."

The Andromeda Galaxy is currently careening towards the Milky Way at a whopping 300 km/s (0.1% c).

This pure kinetic velocity is much faster than either the Hubble expansion rate (55 km/s)
or the Andromeda/Milky Way escape velocity (~65 km/s) and it dominates both.

Since the Andromeda Galaxy seems to be odd man out so far as local velocities are concerned
I can only surmise that Andromeda is an interloper that was accidentally hurdled
this way by dynamical interactions with other more distant galaxies.

It's more like the Koozebanian galley-oh-hoop-hoop mating finale than
all that Koozebanian "dancing around each other" that goes on prior to that.
Click to play embedded YouTube video.
Art Neuendorffer