Expert: Philip Stahl - 2/11/2006

Question
Hi Philip.  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
Hello.

The Milky Way and Andromeda are two members of what is called "the Local Group" - comprising about 20-22 separate galaxies, gravitationally bound as a cluster. At the same time, it implies that - at any time - any members of the gravitationally bound group may be moving toward or away from each other. In the first case, "blue shifts" would be obvserved in their spectra. (e.g. the defined lines, say for hydrogen are shifted toward the short wavelength end ot the spectrum)

In the second case, "red shifts" would be observed.

Outside the Local Group, however, all external conglomerates (e.g. other galaxy clusters) all exhibit *red shifts*. This means that they are undergoing recession, *away* from us.

Thus, the Hubble expansion of the universe which you referenced, only applies to galaxy clusters as a whole - and not to the individual members galaxy motions within each cluster. Hence, the motion of one galaxy toward another in a particular cluster would not be contradictory to the Hubble expansion.

On one further point, since the Big Bang was an "explosion" of time, as well as space, there is no way a priori to affirm a "center" for the explosion, relative to the universe as a whole. That is, there is absolutely no way for us to observationally discern any "geometric center".

The best analogy to the cosmological expansion is probably that of a balloon which is being inflated, and which has ink dots on its surface. From the viewpoint of any one of the ink dots - all the others would be seen as moving from it. Hence, there is no privileged position or real "center" - nor can any be deduced by any methods currently available.