Can Milky Way and Andromeda collision reconcile with an Expanding Universe with galaxies spreading away from each other like "raisins in a loaf"?
I understand that our local galaxy group is considered "gravitationally bound" and therefore exempt from the expansion from each other ((, but we don't seem to have other galaxies collected into their own "local groups" of gravitationally bound clusters, so are we saying we're somehow unique? Is there a trick of perception taking place?)) <---edit:this is wrong
While objects cannot move faster than light, this limitation applies only with respect to local reference frames and does not limit the recession rates of cosmologically distant objects.
It seems to me that if we can perceive at cosmological distance something that cannot exist, perhaps we are falsely observing an expanding universe. Maybe everything IS gravitationally bound and we're just seeing expansion because... Relativity?
The short simple version is that if galaxies happen to be close enough to each other they fall toward each other faster than the space between them expands. That's what "gravitationally bound" means.
Dont we see other galaxy groups though? Im no astronomer, but I do recall the universe having some degree of structure above the scale of individual galaxies, with groups and clusters of them forming larger groups or filaments surrounding voids of space with fewer galaxies in them.
I imagine any explanation of the expansion of the universe for people that are not themselves studying astronomy is going to be simplified in a way that gives the average person the basic idea but not the complete picture to avoid confusion when explaining the concept. Ive not studied astronomy, but I did get most of the way through a physics degree, and know that at least there, a lot ideas are explained in that sort of way to people without much knowledge of the subject, especially the more confusing concepts. I wouldnt be surprised if thats the case for most fields of science. For a different example as an analogy, its common knowledge that you cant move faster than light (ignoring the whole expanding spacetime stuff), but it isnt always explained why this is the case, leading to questions from some people like "what happens if I fly a spaceship to the speed of light, and then turn on the rockets to try to go faster?" which have easy answers or just dont make sense as a question if one has had the behavior of objects at high speed explained, but which seem reasonable enough questions to ask if all youve been told is that the speed of light is just some cosmic speed limit. People cant reasonably blame you for finding an incomplete explanation you've been given, well, incomplete, and then asking questions that come to mind as a result.
(Do note I'm not an astrophysicist, so this may be a bit wrong, but I think the main part of it is right.)
Not exactly. Everything in the universe is constantly drifting away from everything else. The reason it is pretty much only visible at the scale of galactic clusters is that literally every force in the universe overpowers this expansion, unless the distances between the objects are truly absurd, in the range of millions or billions of light years.