Modeling the Entire Universe: Blog 37 - The Last Blog

From the very first week of Astronomy 16 to now, from estimating the earth's rotation to modeling the geometry of the universe, we have made it to the last homework blog post of the introductory astronomy sequence. And what better way to wrap up our exploration of all things big and small in the universe than to model the entire universe? 

This week in astronomy, we explored how small density changes in the early universe can determine the overall structure of our universe today. As we have seen in the blog posts, these ideas take a lot of math, and we have barely scratched the surface of all the math that can be done to guess at the entirety of the universe, most of which we cannot see. So what better way to tackle such a large problem than with powerful computers modeling the universe?

The Illustris Simulation models the dark matter, baryonic matter, and dark energy to show the evolution of the universe, or at least a part of it that fits into a 75 Mpc/h box. The simulation goes from a redshift of 127 (the early universe) to a redshift of zero (present day).

The simulation starts off with a view of the dark matter density of the universe, which is not evenly distributed. Not coincidentally, the dark matter demonstrates a similar pattern to to the dense clumps and connecting filaments associated with galaxy clusters. Using the "Spatial Query on Click" feature, we can explore a 400 kpc/h radius of the simulation and return information on up to 20 subhalos. 

Dark Matter Density of a 75 Mpc/h box

Analyzing a random selection of subhalos in an over dense region, we can see that low mass halos are more frequent than high mass halos. Additionally, on average, 9% of the mass of these halos is stellar mass, meaning that this mass is significantly composed of something other than visible matter.

A histogram plotting the frequency of halo masses in log(M) with bins of 2.5.

A histogram plotting the frequency of halo masses in log(M) with bins of width 0.5.

Beyond these snapshots in time, we can also explore our universe over time. This video speeds up the formation of the universe in both dark matter and gas temperature. Just from this one short video, we can learn a lot of about our early universe and its evolution.

The simulation starts off rather "dark" with the universe being dominated by dark matter until about z = 9 (about half a billion years after the big bang), when the very first hints of gas appear. This early period is called the "Dark Ages," when gas is neutral. The gas goes from blue to having some green around z=6 (less than a billion years after the big bang), symbolic of the "Epoch of Reionization," the official end of the Dark Ages when gas becomes increasingly ionized. This seems to be around the same time the first stars begin to form.

The early universe (z \( \approx\) 6). Dark matter is prominent and spread out while there is little prominence of hot gas.

Stars then begin to form rapidly, with growth even accelerating at times. The most rapid star growth then occurs around redshifts of 2.3 to about 0.6. Around this time we also being to see large bursts of energy in the gas temperature simulation.

The universe closer to today (z \( \approx\) 1). Dark matter has concentrated into dramatic over dense regions and star formation is occurring. 

Clear structures form throughout the simulation with smaller structures combining to form larger ones, rather than large objects breaking up. This hints at a trend of increasing gravity and decreasing gas pressure over time. With this increase in gravity, we seem the formation of over dense regions of dark matter and hot gas connected by filaments. As we explored in previous blog posts, this distribution is not uniform due to inconsistencies in pressure in the early universe that have been magnified with the passage of time. Theses dense regions collapse in on themselves due to the high levels of gravity found in dark matter.

The universe today (z=0) with many dense regions of dark matter and hot gasses.

These structures are not only found on a large scale, but are also surprisingly similar to a simulations at smaller scales. On both scales, dark matter and gas density tends to be clustered into regions connected by filaments. The dark matter is much more closely limited to these filaments. This follows along with our earlier observation that ionized gas forms as a consequence of dark matter. However, the gas is at a higher energy and is less massive, less confined by gravity, and therefore less limited to the filaments. On the smaller scales, all types of matter seem to be less limited to these filaments, most likely due to the smaller scales of mass and gravity.

Dark Matter Density                                  Gas Density        

Most of the medium or large galaxies are found in clusters and not in the field and within those galaxies, gas is densest towards the middle of the galaxies. This lines up exactly with what we learned at the beginning of the semester, when we explored the structure of the Milky Way. And within one of these over dense regions of the universe lies a galaxy cluster containing a very familiar spiral galaxy, with an average-sized star being orbited by an averaged-sized planet filled with astronomers looking up and out at our universe today.

Sources:
http://www.illustris-project.org/explorer/
http://earthsky.org/space/dark-matter-hairs-filaments-streams-gary-prezeau