Expert: Jayendra Upadhye - 2/11/2006

Question
Hi Jayendra.  I have red that the Milky Way and Andromeda galaxies are headed towards eachother (at about 300,000 miles per hour) and that they will collide in about five billion year's time.

My question is, if the universe began at an explosion at a single point, shouldn't everything be radiating out from that point and hence away from eachother?

How could it be that two galaxies are actually headed towards eachother?

Thanks for your consideration and time,

Nick

Answer
Hi Nick,
You are correct.
The Andromeda (M31 as 31st object on messier's catalog) is part of the "local" group of galaxies consisting of
1 - Milky way
2 - Large magellanic cloud which is a an irregular satellite galaxy pearticularly ricj in red and blue giants
3 - small magellanic cloud
4 - Andromeda galaxy
5 - its 1st satellite galaxy
6 - iys second satellite galaxy

All these actually share the expansion of the universe implying they are gravitationally bound and show almost same red shift to other objects in the universe. ie distant observers looking at the milky way will see the same red shift for all 6 objects.

Together this cluster of galaxies is headed towards the virgo super cluster of galaxies.

Coming to your main question, well..it is TRUE all things in the universe MUST go away from all other things..BUT GRAVITY overcomes this motion for objects in "close" proximity. (mark the quotes!)

the expanse of this local group will tell you what "close" means!

That is the reason why stars can form! else all gas and dust would have dispersed uniformly throughout the universe and we would be seeing a density fo 1000 2000 atoms per cubic meter or even less! and no stars and everything would be dark and devoid of life!

Gravity "sort of" forces an exception to happen to the fissiparous tendency of the bang and encourages agglomeration of matter, which leads to star formation which in the end leads to life!

So you know which "true" gods to worship now..:)

It is this force coupled with small lumpiness or anisotropy of the initial bang that has led to the presentday spongiform large scale structure of the universe.

A universe filled with huge voids bridged by thin bridges of streamers made up of galaxies of all kinds.

The virgo and coma clusters represent one such bridge we happen to see "at close range".

Jayen