My understanding is that yes, overall, in aggregate, large bundles of matter in the universe are moving away from each other. But the largest bundles are galaxy clusters, which are made up of galaxies held together by mutual gravitation, so there is some force countering general expansion in that region. While overall, space is expanding and things are moving away from each other, it does not necessarily mean that each individual galaxy is moving away from every other one as a rule. I'm not sure as to why they are on a collision course, but it is perfectly consistent with expansion. You're free to look it up (as you might have, its been a month) but this is my off the cuff understanding.

[Delete] @Baden

Read. Learn.

[Delete] @Baden

The Big Bang is absolute nonsense, and I was happy when noted quantum scientist Erik Verlinde said philosophically he didn't buy into the Universe all came about as a result of various Big Whoosh out v of no where. His own work follows more along the lines of the universe as developing from quantum entanglement of information - a totally new approach thank goodness, one that makes sense scientifically as opposed to the religious overtones of the current theory.

The Big Bang is so bad at correlating with current observations, that scientists have to make up (yes, the Sci Fi aspect of current science rears its head yet again), "invisible" dark energy and dark matter that make up 70% and 25% of the universe respectively in order to account for anti-gravity forces (I thought they told us that the universe is curved space time?) This article will give you a glimpse at how torturous it has become for scientists to hold on to their myth that they have been teaching as the Scientific Genesis for decades.

https://www.nationalgeographic.com/science/space/dark-matter/

The average direction across all galaxies is away from one another. However that does not mean that every single pair has to be moving apart. Each galaxy has its own idiosyncratic motion, called its 'peculiar velocity', which is measured as deviation from the velocity it would have if it followed the average of all galaxies. Two galaxies near one another can have peculiar velocities towards one another giving a convergence speed that is bigger than the divergence that would arise if they both followed the overall average, so they collide.

Something that might help is to consider a child's helium balloon that she has lost hold of and is floating up into the sky. As it rises, the outside pressure reduces so the balloon expands (eventually it will burst!) so the average motion of helium atoms in the balloon is away from one another. But individual helium atoms in the balloon have big peculiar velocities and are still banging into one another like anything - just like some galaxies.

Andromeda is just doing the Local Motion (as in, C'mon baby, do the locomotion -- see below).

Galaxies do collide every now and then, but it won't be a cataclysmic event. The stars in galaxies are a long ways apart from each other. Think of two large groups of people colliding head on where each person is 600 feet from each other person. It's possible that the two groups could pass through each other without any collisions at all.

The Milky Way and the Andromeda galaxy are both spinning (slowly) so they will mix into each other, slowly, and then they will continue on as before, probably moving away from everything else till hell freezes over.

I haven't decided what to do with the black holes in the two galactic centers. What do you think? Should we have them merge?. I haven't decided on what to do with the big hump in the middle of Andromeda, either. Any suggestions?

Meanwhile, let's all do the locomotion.

It's strange that we have to make do with the line of best fit. I guess there's a lot of uncertainty in astronomy. I have nothing against the Big Bang but if there are alternative theories I don't mind.

(Y) Thanks

LOL. I like the song. Thanks

(Y)

It's strange that we have to make do with the line of best fit. — TheMadFool

The issue is that there is no fit at all. The Big Bang was totally fabricated so that science had their own Genesis story to be taught in textbooks. And when the Big Bang starts to fall apart completely, science comes up with another whopper by declaring that 95% of the universe is completely invisible, but just enough is there to keep the Big Bang myth going. 95%!!! They just made up a whole universe out of thin air!!!

Now why do you think scientists are so reluctant to admit they are totally wrong and just made the whole thing up ?

if there are alternative theories I don't mind. — TheMadFool

Are you familiar with the steady state theory (Fred Hoyle)?

Just read it. It seems to have many holes but it's an alternative alright.

What about the Big Bang Theory? Does it have holes that need patching up?

understanding astrophysical theory is above my pay grade, let alone finding holes in it.

understanding astrophysical theory is above my pay grade, let alone finding holes in it. — Bitter Crank

Strange isn't it that we look for the best scientific theory to help explain our world and on the other hand we have the adage ''too good to be true''.

One explanation could be that galaxies acquire their own motion which results in this type of collision. However, isn't the global expansion greater than the local motions? — TheMadFool

Let's assume the observation data that galaxies in general are moving away from each other is true and as well Andromeda is moving closer to Milky way.

Now the galaxies are in clusters called local group, so it is the local group which is moving away from each other. Within the local group due to due to more local influences, movement of the individual galaxies might be different. That's one way this can be explained.

Now why do you think scientists are so reluctant to admit they are totally wrong and just made the whole thing up ? — Rich

The basis of modern science is on observation data, formal logic and assumptions (which can be true or false). Note that it is not too different from study of philosophy

The big bang theory has been based similarly, which answers most of the questions, not all. There is no other better theory which explains the observation data, of there the scientific community will be the first to dump the big bang theory and embrace the better version.

Are you familiar with the steady state theory (Fred Hoyle)? — Bitter Crank

Steady state theory was once proposed by Einstien, later researched by Fred Hoyle et.al., has many more loopholes than the big bang theory. It does not confirms to observational data, hence most scientific communities are not keen to adopt instead of big bang.

Right. I first heard about...50 years ago, and the steady state theory hasn't had any traction since then.

1. The Big Bang began 13.8 billion years ago

2. Galaxies are moving away from each other because space is expanding. One frequent analogy is that of colored spots on an expanding balloon. As the balloon blows up the spots on it move away from each other.

3. The Andromeda galaxy is on a collision course with our galaxy, the Milkyway.

How is this possible?

According to 2, ALL galaxies should be getting farther and farther away from each other but the Andromeda galaxy is coming towards the Milkyway.

In addition there's evidence that such inter-galactic collisions have/are occured/occuring in the universe.

One explanation could be that galaxies acquire their own motion which results in this type of collision. However, isn't the global expansion greater than the local motions? — TheMadFool

Firstly we can only measure the radial velocity of Andromeda.
That means that we have no way of measuring its velocity on the left-right or up-down axes.
Only the forwards-backwards axis yields the blue-shift or red-shift.
It is thus unlikely that it will collide with the Milky Way.

Secondly, the velocities only of the most distant galaxies are moving away from us.
So there are small fluctuating amounts of velocity beyond the majority of velocities
which obey the expansion principle.