Thread: Doppler shift

I have seen a number of television programs that suggest that our galaxy will eventually collide with another. If this is true, why aren't astronomers seeing a doppler shift into the blue color of the spectrum.

Additionally there have been reports of galaxys colliding with each other. If the expanding universe model is correct, how can a collision occur? Is one galaxy moving faster than the other? Additionally, as stated above wouldn't there be a slight difference in doppler shift colors between the two galaxys?

Pelkey

I think its all based on weather a group of galaxies is with in range of another galaxy to be able to interact with each other gravitationally, so thus two galaxies moving in the same direction can attract each other. I think the term used is local gravity against the universal driving force.

didnt a collision just happen with our galaxy and another?

Our galaxy has already collided with another in the past. We collided with the dwarf galaxy Sagittarius. It was only a partial collision, but is expected to really happen in about 100 million years.

Some nearby galaxies are moving in our general direction, and the light from these galaxies is indeed blueshifted. The vast majority of observable galaxies, however, are farther away and not gravitationally bound to ours; the light from all of them is redshifted due to the cosmic expansion, regardless of their local velocity.

Bodies that are close enough together and, together, massive enough can 'overcome' the general expansion of space & remain gravitationally bound. Examples are our solar system & our galaxy (or any galaxy); thus, neither is expanding. Similarly, galaxies are gravitationally bound into clusters & superclusters. You usually hear something like: the distances between all the galaxies are increasing; it would probably be better to say that the distances between the galaxy clusters (or superclusters) are increasing.

If 2 galaxies are colliding, and we aren't seeing the collision "from the side", so that their local velocities toward or away from us are different, there will indeed be a difference in redshift between the 2, though it may or may not be large enough to be observable.