A question.

[Braincandy]

Was watching a Christopher Hitchens debate.. In his opening he was talking about how all galaxies are moving away from each other, the "red spectrum". Then shortly after he spoke of the Andromeda galaxy on a collision course with us. How?

 

 

-BC

[toastywombel]

Was watching a Christopher Hitchens debate.. In his opening he was talking about how all galaxies are moving away from each other, the "red spectrum". Then shortly after he spoke of the Andromeda galaxy on a collision course with us. How?

 

 

-BC

 

Well, the statement, "all galaxies are moving apart" is somewhat misleading. By galaxies, I think that Mr. Hitchens might be referring to galaxies/ galactic clusters/ and or local groups.

 

It is also good to keep in mind the galaxies are not moving apart as much as the space between them is increasing.

 

When we look at the night sky, the majority of distant galaxies/galactic clusters/local groups are red-shifted. Galactic clusters are galaxies that are gravitationally bound to each other. Andromeda is part of our local galactic cluster. This is from wikipedia,

 

"There is, as yet, no way to know whether the possible collision is definitely going to happen or not. The radial velocity of the Andromeda galaxy with respect to the Milky Way can be measured by examining the Doppler shift of spectral lines from stars in the galaxy, but the transverse velocity cannot be directly measured. Thus, while it is known that the Andromeda galaxy is getting closer to the Milky Way by about 120 km/s, there is no way to tell whether it is going to collide or miss. The best indirect estimates of the transverse velocity indicate that it is less than 100 km/s.[4] This suggests that the dark matter halos, although possibly not the actual disks, of the galaxies will collide. A future European Space Agency spacecraft, the Gaia mission, expected to launch in the Spring of 2012, is intended to measure the positions of stars in the Andromeda galaxy with sufficient precision to pin down the transverse velocity.[1]"

 

http://en.wikipedia.org/wiki/Andromeda–Milky_Way_collision

[benedictusk]

What toastywombel said is correct, but if you still don't understand, this experiment might clear things up (you probably shouldn't actually do it to avoid making a mess, but I'm pretty sure you'll understand just by reading the procedure)

1. Get a jar of marbles

2. Open it

3. Flip it upside down with your hand covering the opening

4. Move your hand away from the opening and let the marbles fall

The marbles will start to spread out, getting farther away from the center and, in general, farther away from each other. However, some marbles will collide.

[Braincandy]

So basically gravity wins at close range?

[Radical Edward]

So basically gravity wins at close range?

 

basically yes.

 

The analogy I like to use (rather than a balloon expanding) is letting a loaf of bread rise. at the start, the loaf of bread is all nicely compact, but as the yeast starts metabolizing, the bread starts to expand. There are two basic forces at work - the expansion of the bread due to the yeast and the molecular forces of the molecules of the dough, pulling it back together. On average, the expansion wins (over long distances) and any two bits of bread are moving away from one another. On very short distances however (half a millimetre say) the tension of the bread molecules holds the bread together, so you get bubbles. When a bubble forms, some bits of bread will be pulled closer together.

 

how does that work for you?