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Cooking the Cosmos

First Things First: Initializing the Cosmos

All cosmology simulations start with tiny fluctuations in what was essentially a uniform distribution of matter and energy at the earliest moments of creation. How these fluctuations evolve depends on what cosmologists assume about their origins. Two schools of thought predominate:

Gaussian Approach

Inflation,the stupendous expansion of space immediately following the Big Bang "blows up" minute, spontaneous fluctuations in the mass-energy field to macroscopic dimensions. These density fluctuations are later further amplified by gravity.

Because the fluctuations are purely random, cosmologists should be able to detect a range of fluctuations over the range of clusters to superclusters of galaxies; the spectrum of fluctuations represent variations in matter density. These fluctuations are described by a power spectrum, which plots the strength of the fluctuations against a length scale.

The shape of the predicted power spectrum depends on the nature of matter in the universe. Fortunately, cosmologists have observational data, in the form of background radiation measurements, that help define the properties of the power spectrum.

Non-Gaussian Approach

This approach to structure formation predicts that density fluctuations arise from so-called phase transitions occurring after inflation and the separation of gravity from the other fundamental forces. According to this view, the phase transition accompanying the separation of the nuclear and electromagnetic forces causes defects, much like the cracks in ice cubes that form when water undergoes the transition of liquid to solid. These cosmic defects, which can take the form of zero-dimensional points, one-dimensional strings, or two-dimensional sheets, trapped leftovers of the earlier, denser universe. Enormously dense, these defects could have seeded the large-scale structures we see today.

Most of the research being conducted in the Grand Challenge Cosmology Consortium assumes a Gaussian model for generating cosmologically-significant density fluctuations. Also assumed is that large-scale structure evolved from the "bottom-up"--that is, small-scale structures such as galaxies formed first, only later merging to form the vast sheets and filaments that we observe today.

There is another approach to the question of large structure formation, however: the "top-down" theory, proposed by the late Russian physicist Yakov Zel'dovich in the 1970s. The top-down theory proposes that large-scale density fluctuations caused vast, pancake-like structures to form first. The pancakes eventually fragmented into galaxies and galaxy clusters.

During the 80's this "top-down"theory fell out of favor, but it may be enjoying something of a renaissance with a model, recently described by cosmologists at the Universities of Hawaii and Toronto, which marries top-down with bottom-up theory. The marriage allows for hierarchical clustering at smaller scales but with these taking place within the much larger scales proposed by the Zel'dovich scenario.

Cosmic Cookery

Anyone who has made a cheese soufflé knows that the difference between a lovely, light concoction and a sodden lump of eggs, milk and cheese lies in just the right mix of ingredients combined with exacting technique.

The same holds true when "cooking" the cosmos--digitally, that is. Introduce too much "hot" dark matter into the model and structures emerge too late. Excessive "cold" dark matter causes galaxies to form too soon. If you want to compare your simulations to what's observable, you'd better calibrate the amounts of cold and dark matter, and throw in some ordinary, baryonic matter as well.

And for picking out details of galaxy formation, don't forget to factor in the right physics.

Recipes for a Digital Universe
Adding the Right Physics

What then is the secret mix? What does it take to "cook" the cosmos so it comes out just right? For the past decade, cosmologists have been experimenting with codes containing three layers of complexity. First, they constructed codes that employed only pure dark matter. The next codes added gas dynamics,the motions of ordinary baryonic matter -- primarily the hydrogen and helium that formed in the first 100 seconds after the Big Bang. More powerful computers have allowed cosmologists to add the essential physics of galaxy formation itself to the mix. In short, they're trying compute the works!

Pure Dark Matter
Adding Gas Dynamics
The Works!

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Copyright, (c) 1995: Board of Trustees, University of Illinois

NCSA. Last modified 10/6/95.