Two months ago, I showed a video of a new computer simulation that fairly accurately recreates the evolution of the universe from around 400 million years after the Big Bang to the present, and also linked to the paper by its creators.
While that was visually arresting, non-physicists might have had a hard time interpreting what they were seeing. The journal Nature, where the paper was published, has produced a video that explains more and gives a commentary as the universe evolves. It is pretty good, though the way it depicts the Big Bang, while dramatic, reinforces some of the popular misconceptions of what it was like.
The volume that the simulation encompasses is large enough that it contains 41,416 galaxies at z=0 (i.e., the present) and what it does is put in the input values that we know about what the universe consisted of 12 million years after the Big Bang, the laws we know about how the constituents interact, and then watches it evolve. As the authors say, “The initial conditions for structure formation in the Universe are tightly constrained from measurements of anisotropies in the cosmic microwave background radiation.”
Here are some quotes from the paper that describe what they researchers did and why their result is new and significant.
The authors describe the limits of previous simulations.
No single, self-consistent simulation of the Universe was able to simultaneously predict statistics on large scales, such as the distribution of neutral hydrogen or the galaxy population of massive galaxy clusters, together with galaxy properties on small scales, such as the morphology and detailed gas and stellar content of galaxies.
Simulating the formation of realistic disk galaxies, like our own Milky Way, has remained an unsolved problem for more than two decades. The culprit was an angular momentum deficit leading to too high central concentrations, overly massive bulges and unrealistic rotation curves.
They describe what enabled them to overcome those problems.
Rapid advances in computing power combined with improved numerical algorithms and more faithful models of the relevant physics have allowed us to produce a simulation (named Illustris) that simultaneously follows the evolution of dark matter and baryons in detail. Starting approximately 12 million years (Myr) after the Big Bang, our simulation tracks the evolution of more than 12 billion resolution elements in a volume of (106.5 Mpc)3 up to the current epoch (redshift z=0)
Our calculation therefore overcomes the problems of previous hydrodynamic simulations which either did not cover a large enough portion of the Universe to be representative, lacked adequate resolution, or failed to reach the present epoch.
The paper goes on to describe what they achieved.
Unlike previous attempts, we find a mix of galaxy morphologies ranging from blue spiral galaxies to red ellipticals, with a hydrogen and ‘metal’ (that is, all elements other than hydrogen and helium) content in good agreement with observational data. At the same time, our model predicts correctly the large-scale distribution of neutral hydrogen, and the radial distribution of satellite galaxies within galaxy clusters. Our results therefore demonstrate that the Λ cold dark matter (ΛCDM) model can correctly describe the variety of observational data on small and large scales in our Universe.
The fact that our calculation naturally produces a morphological mix of realistic disk galaxies coexisting with a population of ellipticals resolves this long-standing issue. It also shows that previous futile attempts to achieve this were not due to an inherent flaw of theΛ CDM paradigm, but rather due to limitations of numerical algorithms and physical modelling.
Of course, we never solve all the problems and there are yet outstanding issues.
Although our simulation provides a significant step forward in modeling galaxy formation by reproducing simultaneously many disparate observations on large and small scales, there are still outstanding problems. One such problem lies in the formation of low-mass galaxies: our simulation tends to build up the stellar mass of low-mass galaxies below M*≈1010M⊙ too early, resulting in stellar populations that are too old, with mean ages a factor of two to three larger than observed.
It will clearly be challenging to test new schemes that also directly treat the stellar radiation fields with statistically meaningful samples of galaxies, as this poses extremely high computational demands that go significantly beyond what was achieved in the present work. Nevertheless, such new generations of large-scale high-resolution hydrodynamic simulations might become feasible within the next decade.
It never ceases to amaze me that although we humans have existed for just a tiny fraction of time in just a tiny speck of the universe, we have managed to recreate almost our entire history.