The N-body Cosmology code called GADGET,
written by Volker Springel at the Max-Planck Institute, Garching, Germany, simulates the creation of structure in the Universe.
Exercise 2.2: Building Your Own Universe
The N-body Cosmology code called GADGET,
written by Volker Springel at the Max-Planck Institute, Garching, Germany, simulates the creation of structure in the Universe.
The program starts with a box millions of light years on a side, with a random spread of matter in it -- represented by a random sprinkling of points. As each pulls on the other, structure starts to form, and eventually the initially random points form sheets and filaments.
If you have access to a good plotting program, such a Matlab, then it is worth your while to explore the output of GADGET for yourself. If this is the case, see the submission and retrieval forms below. You can submit up to three (3) runs per day. The program will run overnight on machines in the Astronomy Department.
The next day, use the retrieve form to get the output from the previous night's run. The output files will have names such as Orion021007_003.dat. "Orion" is the group label you select on the Submit form below. "021007" is the date, e.g., 2002, Oct. 7. "003" is the time slice; a typical run returns 000, 001, 002, 003, and 004 time slices that represent evolution of the structure of the universe.If you dot not have access to such a plotting program, or if you just want to take a look at pre-generated output, see the plots below. Each set of four plots is for a given time slice (000 being the "youngest" and "004" being the oldest representations of the evolutionary stage of the simulated universe). The first plot shows a 3-D representation of the simulation and the second, third, and fourth show x-, y-, and z-plane slices through the 3-D data. The spatial slices are handy for comparing these simulations with the Las Campanas redshift plots from Ex. 2.1, which was also in space slices, not data that filled a volume such as your simulation does.
Time slice 000:
Time slice 001: Time slice 002: Time slice 003: Time slice 004:
Submit a request to run GADGET
The three parameters determine what sort of universe you will simulate: Omegamatter , OmegaLambda
, and the Initial Redshift. The meaning of these is as follows.Omega is a measure of the density the universe relative the "critical" density. The critical density is the density for which the universe will expand forever, reaching zero expansion velocity only at infinite time. So, Omega = 0 is an empty universe (boring!), 0 < Omega < 1 is an "open" universe (the expansion goes on for infinite time), Omega = 1 is a critical universe (described above), and Omega > 1 is a closed universe, that is, a universe that, at some finite point in time, stops expanding, turns around, and ends in a "big crunch".
Omegamatter is a measure of the relative density of the universe in matter (stars, gas, VW Bugs, etc.). If there were only matter in the universe, then Omega = Omegamatter. This, however, is not the case. Current observations indicate that gravity is not the only game in town. As it turns out, there seems to be a fundamental repulsion in the universe that counteracts gravity! OmegaLambda is a measure of the relative mass density of the vacuum state that leads to this repulsion. If there were no matter, only repulsion, in the universe (obviously, this is not the case), then Omega = OmegaLambda. In general, neither Omegamatter nor OmegaLambda = 0, and so Omega = Omegamatter + OmegaLambda.
Redshift is a measure of how far away an object is from Earth. An object with a higher redshift is further away, and, since the speed of light is finite (large, albeit, but not infinitely so), the further away an object is, the further back in time we observe it (think about that for a minute). So, the Initial Redshift simply sets the physical size of the simulation.
Click here for the definition of comoving coordinates. Also scroll down to the "Comoving Coordinates" section on this page about the expansion of the universe.
Your group ID is a label that lets you identify which simulation is the one you submitted. Three submissions (one for each ID) are possible per day. A request submitted today will be ready for pickup tomorrow by following the "Retrieval" link. Please note that as this is a public utility. Someone else might have submitted a run for some Group for a given date. You don't want to overwrite that, so, before submitting, follow the "Retrieve" link to see if data is ready for pickup for any of the IDs.
The key points to keep in mind about the exploration are: