Structure on the largest scale


Just as I was leaving my physics studies to begin my theological ones, word began to trickle out among astronomers and cosmologists that something very unexpected was being observed at largest imaginable scale.

Cosmologists had always imagined that when you zoomed out to the scale of the Universe (a la the Powers of Ten movie that I used to regularly show on the first day of one of my astronomy classes) that the relative clumpiness of matter at the small scale would smooth out into an homogeneous and isotropic one. If you imagine that the Universe began in a cosmic explosion then it’s hard to see how anything else might have arisen.

And yet, back in the mid-eighties, people were talking about a cosmic void in the direction of the constellation Boötes. There was a huge zone that had no galaxies within its bounds. There was no way to imagine how such a thing came to be. And then another void was discovered. And another.

Eventually people started plotting the distribution of galaxies in three dimensions rather than two. And they discovered that the voids were much more common:

“But only in the last ten years or so have astronomers discovered that galaxies themselves form into a far larger structure. The 100 billion galaxies that we know about are woven into a wispy web-like arrangement consisting of dense compact clusters, elongated filaments and sheet-like walls, amid large near-empty void regions.

This structure has become known as the Cosmic Web and one of the great challenges in modern cosmology is to accurately model and simulate it.

That’s turning out to be tricky.

One of the important features of the Cosmic Web is that its structures range over many orders of magnitude. And since the largest structures, such as the wall-like features, are formed out of the smaller ones such as filaments and clusters, it’s crucial that any model can handle the relationship between them at all these scales.”

Lately there’s a new idea about how to explain this surprising structure – which still violates one of the primary principles of Cosmology, cosmic homogeneity, though not the more fundamental principle of cosmic isotropy (which is based in the Copernican principle).

Enter Rien van de Weygaert and Willem Schaap at the University of Groningen in the Netherlands. These guys have developed a way of modeling structures over many scales without the unnatural smoothing that other approaches use.

Their trick is to think of galaxies as points in 3D space and to fill the space between them with tetrahedra. These tetrahedra must be constructed in such a way that, if a sphere were inflated inside each one until it touched the sides, there would be no galaxies inside each sphere.

This is known as a Delauney tessellation. What’s special about Delauney tessellations is that as the scale gets larger, there are rules for combining the tetrahedra into larger ones. These rules are special because they are reversible, meaning that the important features of the original structure can be reconstructed when you zoom in again.

That makes it much easier to simualate the feedback between structures on various scales.

Read the full article here.

So yay. A new field of mathematics to master. And a new tool to think about how the non-fractal structure of the Universe came to be.

And tonight is the Longest night of the year. Something to think about as you walk outside in the silence of the solstice night and look out into the dark sky peering to the edge of the Universe.

The Author

Episcopal bishop, dad, astronomer, erstwhile dancer...


  1. Scott says

    Some years ago, I seem to remember, “chance theory” said something similar, i.e., when expecting random distribution, one finds instead grouping in one (k)uadrant of the “universe”. For what it’s worth. Now explain constellations. Is order all about perception, or does order exist purposefully to be discovered?

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