How Smooth is the Universe?

“A cloud is made of billows upon billows upon billows that look like clouds. As you come closer to a cloud you don’t get something smooth, but irregularities at a smaller scale.” -Benoit Mandelbrot

It isn’t just the clouds that appear smooth, but aren’t if you zoom in close. In fact, it isn’t just the mathematical curiosity known as the Mandelbrot set that’s full of irregularities and ever increasing complexity as you zoom in.

It turns out that the entire Universe itself has these properties.

“Excuse me,” you might say. “My Universe is certainly not smooth.”

And of course, you’d be right. All you have to do is look into your own backyard.

(Image credit: NASA.)

Because a cubic meter of Earth weighs a bit more than a couple of tonnes. But a cubic meter off of the Earth? A cubic meter of interplanetary space? Well, that weighs practically nothing!

In other words, when we talk about smoothness of the Universe, we’re talking about how different one place is from another in terms of “amount of stuff,” or density. We call this property homogeneity, or “sameness” in different locations.

Our Solar System is incredibly inhomogeneous, in fact. The densest places have something like 10³⁰ times as much stuff as the least dense places. But when we talk about the Universe as a whole, we need to look at scales significantly larger than just our paltry Solar System.

In fact, you might imagine that if you look on much larger scales, maybe the Universe is significantly smoother. Rather than look at Solar System-sizes, maybe we need to look at galactic scales?

(Image credit: Adam Block/NOAO/AURA/NSF.)

And it turns out that, on these scales (a million light years or so), the Universe is still very inhomogeneous. If you took a bubble a million light years in diameter and put it around a very dense region (like a large galaxy), and compared it with an identical bubble placed around the emptiness of intergalactic space, the densities you’d find would indeed be very different.

But they’re only different by about a factor of a million. Yeah, that’s right. Only a million. When you consider that looking at an Earth-sized bubble gave a factor of 10³⁰ difference, a million (10⁶) doesn’t seem so big, does it?

So what if we start looking at very, very large scales?

(Image credit: 2 degree Field Galaxy Redshift Survey.)

Above is a map of galaxies, on the scale of about a billion light years. Once you start looking on scales this large, one region of space doesn’t appear much different than another. In fact, on scales this size, every region of the Universe has roughly the same density. Take a look at how much the above image resembles this generic simulation of structure in the Universe.

(Image credit: M. White, University of California, Berkeley.)

At this level, you can put a bubble a billion light years in diameter around the densest spot in our Universe and one around the least dense spot in our Universe, and the two regions will differ by less than 0.1%.

On the largest scales observable, our theory predicts that one region will differ from another by just a few parts in 100,000, consistent with the minuscule fluctuations we observe in the Cosmic Microwave Background.

(Image credit: WMAP team / NASA.)

In fact, because we understand the physics of how structure forms, we can look at a region of space, count the galaxies in that space, and (so long as we properly account for bias) test just how homogeneous our Universe is. Whether or not our predictions match up with theory gives us yet another cross-check of both our standard cosmological model,

as well as a test of the theory of gravity upon which we base our model: General Relativity.

Recently, a scientific finding has been gaining a bit of buzz, as well it should when an observation doesn’t quite match what we predict. One of the best types of indicators we’ve been using for our observations of large-scale structure are known as Luminous Red Galaxies, which are bright, abundant, and easily identifiable.

(Image credit: Galaxy Zoo, from the AEGIS survey.)

But something may be amiss with these LRGs. Shawn Thomas and his colleagues have found structures in the Universe stretching over three billion light years, containing an overabundance of LRGs from what our theory predicts! In other words, from looking at these LRGs, the Universe, on these very large scales, is less smooth than we expect! Previous studies — for instance, with the Sloan Digital Sky Survey — had seen observations in line with predictions, but there are a number of interesting possibilities for what these structures mean.

(Image credit: V. Springel et al., MPA Garchina, and the Millenium Simulation.)

We could be looking, specifically, at a region of particularly sharp density contrast. It’s statistically unlikely, but certainly plausible.

We could also be seeing an effect of bias; perhaps we do not correctly understand how Luminous Red Galaxies form given the overall concentration of matter.

(Image credit: Paul and Liz Downing, of Messier 4.)

Finally, there could be a contamination of the sample by nearby red stars, leading many galaxies that are luminous but not red to be mis-identified as LRGs.

I greatly prefer any of these to any of New Scientist’s wild speculations, but the upcoming SDSS data release should actually decide the issue.

(Image credit: SDSS-III.)

It’s always amazingly interesting when an observation in the Universe surprises us, and while it’s almost always an anomalous effect that is capable of being explained by what is currently known, every once in a while nature surprises us, and gives us the opportunity to learn something new about the way the Universe works! (A very interesting critique of this paper and its interpretations were recently put forth by Peter Coles, which is worth reading for those of you interested in further details.)

Is this anomaly that we’re seeing now merely something that will be easily explained away, or are we on the precipice of finding a hole in something as major as our understanding of gravity? Fortunately, we should know the answer in just a few months; stay tuned. In the meantime, enjoy thinking about the possibilities!

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