Galactic Dynamics and the spread of Galactic Civilisations

Sammanfattning: Abstract The idea of the spread of life through the galaxy is, in most minds, considered science fiction. Yet, scientists argue that most galaxies have had more than enough time for civilisation development and space travel. In this thesis I simulate how civilisations can spread through the galaxy via close encountering stars, assuming they travel at sub-relativistic speeds. A defined region around the solar neighborhood, populated by Sun-like stars, is simulated over time by tracking the motion of stars using the epicycle approximation. The systems of stars include suitable planets for life development (abiogenesis) where each step of the planet's evolution process is represented by a certain evolution index. The indices are governed by comparing a random number to a probability distribution, representing the probability for a planet to evolve. The probability distribution is in turn extended by including a steepness parameter N, where N = 0 gives a Poisson distribution while N > 1 produces a distribution that is more tightly peeked. The model keeps track of close encountering stars, which allow for civilisations to colonize a passing, habitable planet. On average, a star's nearest neighbor is at a distance of 36, 47, and 82 pc for simulations with 1000, 600, and 200 stars respectively. By changing the probability distribution that governs the chance of index increase, the first technologically developed civilisation was found after 0.27, 3.65, or 4.8 Gyr, depending on the steepness parameter of the probability distribution. The number of colonized planets over time followed an exponential increase in the beginning with a fall-off close to the saturation point of the number of stars. For star counts of 200, 600, and 1000 stars, allowing only one civilisation to travel while the others remain stationary on their home planets, the galaxy became dominated by the single spreading civilisation only when considering 1000 stars with a steepness parameter of N = 3. In this case, 89 per cent of the planets were colonized by the single civilisation. For 200, 600 and 1000 stars and N = 0, the stationary civilisations dominated, colonizing 99 %, 99.3 % and 99.7 % of the planets respectively. For N = 3, 85 % of the 200 planets and 66 % of the 600 planets were dominated by the stationary civilisations after 12 Gyr. This implies that decreasing the mean distance between stars (by including more stars) in addition to including the steepness parameter, made it possible for a single civilisation to colonize the galaxy faster than the rate of which stationary civilisations develop.