If space in the universe is expanding, then why is the Andromeda galaxy on a collision course with ours?

Question

I have watched enough specials on PBS and the Science channel to know that the universe is expanding and that everything is supposedly moving away from everything else in the universe. However, this does not jive with the reported fact that our nearest neighboring galaxy (Andromeda) is on a collision course with our own galaxy (the Milky Way) and that galactic collisions are commonplace throughout the observable universe. How do astronomers maintain the expanding universe model in the face of this evidence?

Answer 1

Space is expanding. But within that expanding space objects have their own relative motion with respect to each other. If the region of space is small enough the rate of expansion is not enough to dominate the gravitationally induced relative motion. The expansion can be seen between galactic clusters but within a galactic cluster it is not sufficiently large to overwhelm the gravitationally induced relative motions which the member galaxies undergo.

Answer 2

The Andromeda galaxy (M31) is just one (albeit one of the three largest) of 36 galaxies that form the local group and engage in a "dance" around the center of mass. In turn it is just one of many groups within the Virgo Supercluster. Both the Milky Way and Andromeda galaxies have their own satellite system of smaller galaxies. During this "dance" galaxies collide. There is good photographic evidence that Andromeda has had earlier collisions. Expansion does not effect things like solar systems, galaxies or local groups. At what level of organization it does play a role, I'm not sure. Hopefully a professional like Angela S will be able to tell you.

Answer 3

1. Each galaxy has its own orbital path.

2. Space itself is expanding equally everywhere.



So some simple maths.



The orbit of Andromeda is bringing it closer to the Milky Way. Why? Because its movement is much faster than the expansion of space between it and us. One cancels out the other and the difference allows it to approach closer.



More distant galaxies are inevitably moving away from us, because the expansion of space exceeds the speed of their orbital paths, even when their orbital path is facing in our direction.



The expansion of space is also cumulative. So the farther away a galaxy is from us, the more the distance between it and us grows. Few galaxies are close enough for their orbits to make a difference.



Andromeda is relatively close-by (our nearest galactic neighbour), and so is one of the very few galaxies whose orbit can outpace the expansion of space.



Galaxies can and do collide. Some parts of space are much denser than others, and where galaxies are close together there is more chance that their orbits will bring them into collision.

Answer 4

Space, in general, is expanding. What is happening here with the Andromeda and Milky Way can be compared to what is happening in our own Solar System. The Solar System is bound together gravitationally and so there are collisions, like asteroids with planets. The galaxies are also in clusters, bound together by gravity, and hence collisions in galactic clusters also. Nevertheless, space, outside of gravitationally bound systems, is expanding.

Answer 5

The Milky Way (our galaxy) and Andrameda galaxies are 2 galaxies of about 30 galaxies that are considered the local group. These 30 galaxies are all gravitationally linked. They are all traveling through the universe together and over the next few trillion years will all combine with each other to make a larger galaxy with trillions and trillions of stars.



Most galaxies in the universe are gravitationally linked with other galaxies in their area.

Answer 6

The Universe is only expanding on the large scale (that is beyond the Local Cluster). In the Local Cluster the Milky Way and Andromeda are the two largest so they are attracting each other.

Answer 7

The Universe is expanding but certain locations have different rates at which they are expanding. Imagine a room filled with people and they all ran away from the center of the room. There would be people bumping into each other all over the place. Key thing here is direction. They are all leaving the center of the room, they are not all going in the same direction though. My thoughts. Excellent question!

Answer 8

Over distances of a few million light years, the acceleration of gravity is strong enough to overcome the expansion. Galaxy clusters remain gravitationally bound together. Only over distances of hundreds of millions of light years or more does the expansion of space become the dominant motion.



Based on observations that show the rate of expansion is increasing, though, some cosmologists think that eventually everything, even atoms, will be ripped apart.

Answer 9

That's like asking why two kids fighting on the school bus keep colliding even though the bus is "expanding" away from the school...



Galaxies are tiny little specks of matter compared with the vastness of the universe, there's no conflict in picture one or two of these flecks bumping into each other.

Answer 10

correct bacause its not expanding things are moving in all directions not outward from a single point the big bang is a myth adopted as a means of explaning the origen of the universe hubbles measurments did not include all moving things only the ones that showed a certain light shift to measure the speed of there movement away from us.

the flaw in this assumption is due to the lack of consideration given to an "SMA" sigularity mass acceleration.

****the big bang is a myth the re release of matter can can be attributed to an "sma" wich is a singularity mass acceleration. that occures when two black holes orbit each other and as they accelerate they reach near light speed there mass then expands and as it goes past the event horison matter is cast off in a centrifical spin as the singularity breacks downthese particles would be lacking the elements that makeup radiation as radiation was castoff when they went into the black hole so the particles would be primed to absorb radiation again to form matter. this is ofcourse assuming that einstiens theory of relativity is correct. and that aproching light speed causes a change in mass i guess the ecape velocity of a black hole is more than just the speed of light. but that doesnt take into account the expansion of the mass of the singularity there may be at some point in this expansion period a moment of physical contact between the two black holes that moment may be very dramatic if the singularities have different charges or very benighn if the charges are similar.the differance being a big bang or a long spray. i am not versed in quantum mechanics especially when it comes to employing to forms of infinity.

**this is not an explanation of the start of the univers this is an explanation of a phase of matter regeneration in the universe that is infinet in its existance or static as some would call it.

and may only occur once every 15 billion years. or even more often in unkown regions of the universe.it may be that subordinate laws are changed as these newprticles emerge.

like plants change to coal and then to diamond after variing degrees of presure and time. and helium is made from hydrogen in a star no one can denie the presure of a black hole is intense and more than enough to change an eliment but can it change on a quantum level. perhaps the subordinate laws only apply to matter from the most recent "sma".in other atomic stucture may be different like electons that orbit a nucleous with rightangle turns or that pass though the center of the nucleous in a stait line back and forth then allowing for different types of bonds and propertys.

Answer 11

There are two answers to your question.

First, some galaxies are moving faster than others, causing these collisions.

Second, the Milky Way is one of the largest Galaxies in its region, so it has a significantly larger gravitational pull than others, causing it to attract entire galaxies to itself.

Answer 12

space is expanding,its form expansion is not the normal and usual expansion we are both used to,its expansion is an expansion which allows what you said is happening between our galaxy and the Andromeda galaxy

Answer 13

2 objects that are expanding in size can still be heading towards each other. The two ideas aren't mutually exclusive. If you draw 4 dots on a balloon and blow into it, the balloon and the dots all get bigger - if the dots were galaxies, I don't see what's stopping them colliding AND expanding.