How to Simulate Colliding Galaxies

Obtain the necessary hardware. For realistic simulations, typical home computers may not be up to the task. Big-name institutions such as NASA simulate galaxy collisions. A supercomputer called "Blue Horizon" was used to simulate the collision between our Milky Way and the Andromeda galaxies. ScienceDaily points out that at over 1 trillion (10^12) calculations per second, the simulation took four days. Assuming exactly 1 trillion calculations per second, and exactly four days, the simulation entailed (10^12) x 3600 x 24 x 4 = 3.456 x 10^17 calculations. Assume a laptop has 2 GHz of processing power. 3.546 x 10^17 calculations would take 3.456e17/(2e9) = 1.728 x 10^8 seconds. This is 48000 hours, which is 5 years, 5 months and 21 days.

Account for the central black hole. They are at the core of many spiral and elliptical galaxies. Orbiting stars, dust and other matter typically surrounds central black holes. Galactic black holes are incredibly dense. From a distance, they are point-like heavy masses that anchor galaxies. Matter, and other black holes, can orbit and eventually fall into a galactic black hole. During collision, central black holes from each galaxy spiral inward toward each other. When the black holes collide, there is an enormous energy release. It signals the galaxies have merged.

Consider time limitations. Galaxy collision is not "quick," like a car collision. On the other hand, galaxies do not "collide" indefinitely. Further back than about 13 billion years, galaxies did not exist. Stars have a finite lifetime. At the longest, a star may shine for about 5 to 6 billion years. Visible galaxies cannot have collided too long ago, as they would not have existed yet. Nor could they collide sufficiently late (or for a sufficiently long time), since their stars would eventually burn out.

Factor in dark matter according to the best accounts presently available. This part is especially tricky since nobody can claim to know what dark matter is, how it behaves and if it even exists. Dark matter may be just moons, planets, asteroids, comets, burnt-out stars and dust. It may be black holes. Dark matter may be subatomic particles yet unknown to researchers. Dark matter may be the gravitational effect of "normal matter" in a parallel universe. In that parallel universe, parallel humans may be scratching their heads over "their" dark matter, which is just "our" normal matter affecting them through gravity. Dark matter may be an illusion due to a faulty understanding of gravity on vast enough time and distance scales. It may be some mix of all of those phenomena.

Remember that most of a galaxy is empty space. When two galaxies collide, actual collisions take place occasionally between stars, nebulae and the central black holes. In their entirety, galaxies are not solid objects like bricks. Galaxy collisions are similar to merging clouds.