# I Andromeda and the cosmological redshift

1. May 11, 2018

### Ken G

I understand, you don't want to feel like you are telling them everything you know, you want to feel like you are measuring what you say and deciding what is worth saying and what will only confuse. Should you explain what a coordinate is, and teach them that language only has meaning when you couple it to the referents that give it meaning, or do you let them continue to think that "words just mean stuff" without the referents? What are they ready to know?

2. May 12, 2018

### JMz

FYI, its speed relative to the center of the MW is about 200 km/s, not 200,000, according to NASA. (More like 100, actually.)

3. May 12, 2018

### PAllen

One way to try to make this question independent of coordinates is consider a given scenario in different possible universes. For example:

1) We have two observers whose round trip light signal times are comparable to Milkyway to Andromeda, and the initially have no mutual red or blue shift (in terms of an FLRW cosmology, this means that one of them would have a small so called peculiar velocity).

2) One of the observers triggers an explosion of locally known characteristics.

3) We can ask about the peak blueshift from said explosion observed by the other observer for the following cases:

a) Flat spacetime (i.e.Minkowski spacetime)
b) an expanding universe with neither accelerating nor decelerating expansion
c) a universe with accelerating expansion
d) a universe with deceleration expansion.

Then, I believe, the following are true:

(a) and (b) will show the exact same maximal blueshift

(c) will show slightly less maximal blueshift than (a) or (b)

(d) will show slightly more maximal blueshift that (a) or (b).

One way to understand the possibly surprising equivalence of (a) and (b) for this case is that the maximal non-accelerating expansion case (without cosmological constant) of FLRW metrics is the so called Milne cosmology which is actually just (a) in funny coordinates.

4. May 13, 2018

### Orodruin

Staff Emeritus
I am sorry, but I am not sure what you want to say by these examples. You have different universes and in each one you have the same sort of ambiguity. It is unclear to me how you would "match" the universes to each other, they are different universes.

5. May 15, 2018

### alantheastronomer

Taking the local Hubble constant to be approximately 70 km/sec/Mpc and the distance between the Milky Way and Andromeda to be approximately 80 kpc, we get a recessional velocity due to the expansion of the universe of 5.6 km/sec. The velocity of approach as measured by blueshift is about 130 km/sec. So without the expansion of the universe we should have been seeing an approach velocity of 135.6 km/sec.

6. May 15, 2018

### JMz

Distance to M31 is 800 kpc, right? So we would get the earlier 55 km/s estimate.

7. May 15, 2018

### alantheastronomer

OMG you're absolutely right, I misplaced a decimal point! Sorry!

8. May 19, 2018

### JMz

'S OK. I have never in my life misplaced a decimal point. Or a minus sign. Hah! ;-)

9. May 21, 2018

### nikkkom

This is logically flawed: "without the expansion of the universe" there would be no Milky Way, no Andromeda. No galaxies at all.

Now, you can look at the current situation of where these two galaxies are and how they move, disregarding their deep past. In this case, "expansion of space" or lack thereof is simply a choice of coordinates. You can choose one where "space expands" or one where it does not. Both are equally valid. Physical observables, such as relative velocity of these two galaxies, are independent from coordinate choice. Ergo, "we should have been seeing" the same approach velocity.

Physically observable changes are only seen for the case where expansion of space is not constant - in particular, when it is increasing, as appears to be the case. But this (1) has very little effect on the distance scale of Andromeda;
and (2) the effect is very small anyway - if you want to directly measure (as opposed to inferring it by comparing observed data to cosmological models w/o dark energy) that expansion of space is increasing, by showing that distant galaxies' redshifts are increasing [actually, that "they decrease slower than they should"] - this would require very precise and very long-baseline measurements. IIUC such measurements are not within our capabilities yet.

Last edited: May 21, 2018