If the universe is expanding and everything is moving away from everything else, why do galaxies collide?

Answer 1

Saying that "everything is moving away from everything else" is an over-simplification. As several above have correctly said, while the universe on the whole is expanding, "small" areas within the universe (clusters of galaxies) are gravitationally bound to each other - they orbit each other and sometimes collide into each other.

Our solar system is not expanding. Out galaxy is not expanding. And here's some fun trivia - the Milky Way Galaxy and the Andromeda Galaxy are on a collision course! They will collide in 3 to 5 billion years (I forget exactly when).

Answer 2

Several of the answers above are quite good, as is the question.

A somewhat different way of looking at this question. The part of the Universe we can see (that is, the 13.7 billion light-years out to the Cosmic Microwave Background) is remarkably uniform on large scales (bigger than, about 10 million light years), but it is not uniform on smaller scales. It is also remarkably well balanced between expansion and contraction---although the whole Universe is quite definitely expanding on large scales, it is, in some sense "just barely" expanding. The relatively small regions of the Universe that are just a little more dense than the average are contracting. The largest such regions have collapsed to form clusters of galaxies. Within the clusters, the individual galaxies are bound to all the other galaxies in the cluster, and can collide with them. Collisions between galaxies are actually quite frequent, and it is likely that all large galaxies have collided with several smaller galaxies.

Answer 3

The Universe is expanding as a whole and everything does have a radial (outward) velocity along with the Universe. However, there are also transverse velocities which are at right angles to the radial velocities. The overall effect of the radial and transverse velocities of any object can be thought of as along the hypotenuse of a triangle, the other two sides being the radial and transverse velocities. This means that objects do not in general only move in the direction the Universe is expanding. Thus it is likely that, given enough time, objects could encounter each other and collide even without the effect of gravity. This would then amplify the effect, making collisions more likely and to happen sooner.

Think of an ice skater travelling in a straight line. If the skater slides an object over the ice at an angle from him/her, it will move not along a parallel line to the skater but along a sloping path. Two skaters moving along parallel paths could release objects that may, or may not, collide depending on the angles the objects were released at.

Incidentally, White Tiger of God's argument that the laws of Thermodynamics do not allow for the possibility of a Big Bang is false. There is a way for the Big Bang to have happened without violating these laws but the ideas involve quantum dynamics which are not really suited to discussion here.

Answer 4

The universe is expanding, but everything is not moving away from everything else. On a very large scale, on average, things are moving apart as space expands, but on a small, local scale, gravity still pulls things together, for example, colliding galaxies.

Answer 5

The expansion applies to the universe as a whole, but smaller systems of masses can interact within that expansion and actually resist that expansion. Thus the galaxies themselves do not expand, nor do the gatactic clusters. But the large-scale clusters are receding from each other. Within the clusters, galaxies follow their own gravitational interactions which work independently of universal expansion.

Answer 6

One reason is gravity, and another is the random order of chaos, which dictates all number of possibilties. to wit. Visualize this: After the "big bang" some particles have different velocities than the velocities of other particles, and therefore following up from behind the entire mass, they collide, thus forever more changing their direction and speed and imparting those same changes to whatever they collide with. And this will factor on, even though immediately after the explosion all particles were radiating outward. They are still radiating outward from the epicenter, but collision push them onto different courses and gravity attracts them together. And all this at varying rates, therefore.....

Answer 7

It is a good question. There is no perfect solution in astronomy. My hunch is that the universe expansion is not happening. They are misreading the data. The red shift may be caused by other causes Not due to the galxies moving away.

Answer 8

On a brilliant scale, each thing is shifting remote from us. that is shown by ability of the pink shift. On a smaller scale, our galaxy, the Andromeda galaxy and distinctive smaller galaxies interior of sight are appropriate by ability of gravity and are not shifting away, yet are in orbit around one yet another! that is something of the universe that is increasing! interior of our team of galaxies, some are shifting away slightly, others (alongside with the Andromeda) are shifting in direction of us slightly. those that are shifting in direction of us tutor a blue shift! related to colliding Galaxies, those are purely appropriate by ability of gravity interior the comparable way that we are appropriate to the Andromeda galaxy. they are nonetheless shifting remote from us nevertheless, purely no longer from one yet another! that is believed that our galaxy will collide with the Andromeda in some million years! i'm hoping this solutions your question.

Answer 9

Gravity.

Any two bodies close enough together will be attracted to one another by their gravitational pulls.

Not everything is moving away from everything else in a straight line, hence why some bodies get close enough together for a collision to occur.

Answer 10

Thermodynamics.

All though one of the people here said he is an engineer he needs to account for the fact that there is no friction in space, thus you remain at the same speed forever unless you are stop by something else.

Secondly the Laws of Thermodynamics do not allow any space for the argument of the Big Bang.

Thus there is a designer involved in everything.



1) 1st Law of thermodynamics: Energy can not be destroyed or created. It can only be transformed.

2) 2ND Law of thermodynamics: All things have to have an equilibrium.

3) 3rd Law of thermodynamics: All matter and existence decays into chaos.