By Chris Lee, Ars Technica

From a certain perspective, the Universe looks as smooth and uninteresting as a billiard ball—the smoothest billiard ball ever made. What do I mean by this? The radiation from the Big Bang, now so deeply red-shifted that it is microwave radiation, looks pretty much the same no matter where we look. This cosmic microwave background (CMB) is so smooth that the WMAP satellite, designed to look for lumps in this background, had to have unbelievable sensitivity to successfully see any. One consequence of this smooth background is that the observable Universe had to have undergone a period of very rapid expansion, referred to as inflation.

In current mainstream models, where dark energy and inflation are strapped on like a sort of prosthetic, there is just one Universe, and being alone, it can’t collide with anything. But in models derived from string theory, dark energy and inflation turn up naturally, which is nice. The catch is that, in these models, our Universe might not be alone.

We don’t know how many bubbles of inflationary universes might have seeded in some sort of over-arching background called a “false vacuum.” This is a part of a larger string theory problem: it is so generic that the number of universes and the values of fundamental constants in those universes are largely unconstrained. One way to narrow the field is to look for evidence of these other bubbles. This is exactly what a group of researchers has done.

Looking into other universes?

WAAAAAIT, I hear you cry. Surely, by definition, we cannot see other universes. You would be correct. But in a Multiverse that is crammed with bubble universes, they may sometimes collide. When they do so, they will stretch space-time at the location of the collisions. This would leave an imprint on the CMB. Essentially, the temperature of the Universe would be slightly cooler on one side of the collision boundary than the other. This changes the exact microwave frequencies of the CMB, which should show up in our maps. Finally, a collision between two spherical bubbles results in a circular feature, so we have a specific shape to look for as well.

So the highly simplified answer is to build a chain of data processing tools that scans CMB maps, looking for circular features within a certain size range, and, depending on how hard the collision was, a discontinuity in the temperature. Sounds easy, right?

Umm, no, not really. Finding circular features is very easy in real data, simply because they will occur by chance and the way the instrument gathers data might even create such features. Certainly, any map that involves stitching data together will have discontinuities, so a naive search will turn up many many hits.

To get around this, the researchers used a tool developed by the WMAP folk that takes a computer generated CMB and passes that through a model of the WMAP instrumentation and data processing steps. This tool allowed the researchers to figure out exactly how many features that looked like collisions would be found, independent of whether they were, in fact, collisions or not—turns out you get about ten of those.

This same tool also allowed the researchers to test the sensitivity of their own chain of analysis. After finding out what the natural background of false positives was, they generated additional CMB data that included collisions of various strength. These backgrounds were run through the WMAP model to generate a map with all the warts that are present in real measurements. That was passed through their own analysis to see if they could find the collisions. After a bit of optimization work, they figured their tools were as sensitive as possible, and didn’t generate too many false positives.

Hidden in this is a whole bunch of statistics. We don’t have a good model for inflation or for collisions between universes. If we find a particular collision with a certain strength, what is the probability that this is actually a collision and not a false positive? It is not simply enough to say, “we expect ten false positives, therefore everything above ten is gravy.” Collisions in certain parts of the sky are more likely to be false. Collisions with certain parameters are more likely to be real, while others are more likely to be noise. And, in between these statistics is a whole lot of stretching to allow for the fact that we know very little about inflation. All in all, it is a very intimidating problem.

You’ve strung us out long enough now, what is the answer?

The real WMAP data turned up 14 potential collisions. Of those, all but four were eliminated as almost certainly being false positives. The remaining four were in a region of the sky where the chances of a false positive was high. These statistics led to the conclusion that our Universe has not collided with any other universes. This places an upper limit on the density of bubble universes, and, hopefully, provides some insight into how string theory should be modified so that it provides a model of this Universe.

The story doesn’t end there, though. First of all, the four false positives may yet turn out to be real—they have some internal consistency that is unlikely (but not impossible) for a false positive. Now that the analysis pipeline has been developed, it can also be applied to data from the Planck satellite, which has more sensitivity and better spatial resolution than the WMAP data.

No doubt, we will be hearing more about this in the future. Along with it, we will find revised and refined models of the Universe coming our way.

Image: Physical Review Letters

Source: Ars Technica

Citation: “First Observational Tests of Eternal Inflation.” By Stephen M. Feeney, Matthew C. Johnson, Daniel J. Mortlock, and Hiranya V. Peiris. Physical Review Letters, Vol. 107, Iss. 7, August 8, 2011. DOI: 10.1103/PhysRevLett.107.071301

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