Evidence of Eternal Inflation in the CMB?

Last week, I read on the physics arXiv blog a post titled Astronomers Find First Evidence Of Other Universes, claiming that

Our cosmos was "bruised" in collisions with other universes. Now astronomers have found the first evidence of these impacts in the cosmic microwave background.

This left me deeply puzzled because I had read the paper in question:

First Observational Tests of Eternal Inflation
By Stephen M. Feeney, Matthew C. Johnson, Daniel J. Mortlock, Hiranya V. Peiris
arXiv:1012.1995 (see here for an extended version)

yet seemed to have read something completely different out of it. So what's this all about?

Preliminaries

The cosmic microwave background (CMB) we measure today is a relic from the time when the universe was only 300,000 years old and radiation decoupled from matter. Since then, photons could travel almost undisturbed. Thus the radiation, especially the fluctuations around its mean temperature, contain valuable information about the history of the universe. The CMB temperature fluctuations have been measured with great precision by the, now completed, WMAP mission and I'm sure you've all seen their skymap.

This data from the CMB temperature fluctuations, often discussed in form of its power spectrum, has allowed us to extract parameters determining the expansion of the universe and complement other data. What we know today, among other things, is that the universe is not only big, but to excellent accuracy spatially flat. That's a feature not naturally achieved with every mode of expansion. It also requires explanation why the CMB temperature is so homogeneous and isotropic, ie essentially the same everywhere with only small fluctuations around it. The currently most widely accepted model that achieves all that easily is inflation. Inflation is basically a phase of early, very rapid expansion that succeeds in solving the problems of flatness and homogeneity (and some others in addition). Inflation then has to end at some time, so matter can form and after that the expansion of the universe proceeds in a more moderate form, allowing the structures to form that surround us today (filaments, galaxies, stars).

There are several models of inflation that differ in the detailed predictions, but the rapid expansion is what they have in common. A particular variant of inflation is called "eternal inflation." As the name says, in that case inflation does not end completely but continues eternally. The way this is thought to happen is that inflation only ends locally when a metastable "false" vacuum state decays into a "true" vacuum state and subsequently continues along a local inflation scenario that ends and results in matter formation and gives rise to a patch like our own, commonly called "bubble universe." However, the areas of false vacuum never decay away completely because they expand more quickly than they can decay. As a result, new bubble universes continue to be formed out of the false vacuum eternally.

Bubble Collisions

While eternal inflation has its proponents, the most well-known probably being Alan Guth, it hasn't been particularly popular, mostly because for what observations are concerned it's a superfluous overhead to the local inflation scenario. It increased in popularity somewhat with string theorists having to face a large number of possible vacuum states, a scenario that seems to fit nicely with the continuing creation of bubble universes that together form what's become known as the "multiverse." Still there remains the question what's it matter if we can't observe it anyway.

It turns out that there are circumstances in which we could find evidence for the existence of other bubbles because initially separate bubble universes might come to overlap during their expansion in a "bubble collision." The probability of there having been a bubble collision in our past, and that bubble collision being observable yet not fatal for the evolution of life in our universe, depends on the parameters of the model.

The Paper

That finally brings us to Feeney et al's paper. Inspired by earlier work by Aguirre et al (Towards observable signatures of other bubble universes, arXiv:0704.3473) they studied the possibility that a bubble collision in our past has left an imprint in the CMB. Their paper basically presents a particular analysis scheme for the CMB temperature fluctuations. Projected on the 2-dimensional surface of last scattering, the leftover signal would have azimuthal symmetry. They assume that a bubble collision has left a mark in the CMB that consists of a slightly different temperature in such an azimuthal patch.

They use an algorithm to analyze the temperature fluctuation that works in three steps. First, search for areas with azimuthal symmetry. Second, search for edges where the temperature makes a slight step. Third, if you've found that, look for the best parameters to reproduce what you've found. They then go on to create fake CMB fluctuations with signals of bubble collisions to quantify how well their algorithm works. The picture below, taken from Feeney et al's paper, depicts the stages of this simulation. Each quarter of the skymap is supposed to show the same area, just mirrored horizontally and vertically. The upper left part shows the patch with the temperature variation from the bubble collision without fluctuations superimposed (the Mollweide projection used to plot the map distorts the shape). The upper right part adds random fluctuations. Now the task is to get the signal back. The lower left part shows the result of looking for patches of azimuthal symmetry, the lower right one the result of looking for edges with temperature steps.

After testing out their algorithm with fake data to understand what features it is able to identify with certainty, they come to the interesting part and analyze the actual CMB data. Their algorithm doesn't find edges, but identifies 4 regions of interest whose features could possibly have been caused by bubble collisions. As the authors put it, these features are "compatible" with having been caused in that way. Two of these spots of interest btw have previously been discussed, one is the well-known CMB "cold spot," the other was identified in this paper which made use of a similar analysis as Feeney et al. It is important to emphasize though that the identification of these spots was based solely on the symmetry and they were not able to find the second identifier, the edge of the spot. For this reason the authors are careful to make clear:

"Without the corroborating evidence of a circular temperature discontinuity, we cannot claim a definitive detection [...] Azimuthally symmetric temperature modulations are not unique to bubble collisions."

Though it might be that better data from the Planck satellite will allow to extract a less ambiguous signal in the coming years, this is so far clearly no evidence for a bubble collision. Feeney et al's results are just once again evidence that there's some features in the CMB.

One also has to keep in mind that their paper already starts from the assumption that the signal of a bubble collision is of such a particular sort of merely resulting in a small temperature difference. It leaves entirely open the question how likely it is that a particular model of eternal inflation would result in such a signal that is just barely observable rather than in features entirely incompatible with what we've seen so far. It is entirely unclear to me for example what would happen if the vacuum in the other bubble or possibly even its physical constants were different from ours. It seems quite unlikely that a tiny temperature modulation is all that would come out of it. I don't think anybody has at this point a comprehensive picture of what might happen in a general bubble collision. The question is then if not it is extremely improbable that our bubble was subject to a collision and that collision, rather than wiping us out, was just nice enough to reveal itself in the upcoming Planck data.

In any case, the analysis put forward in Feeney et al's paper serves to rule out some regions of the parameter space in models that produce such an imprint in the CMB. Such constraints are always good to have. It is a nice and very straight-forward paper presenting an observer's take on eternal inflation. It's a very worthwhile analysis indeed - imagine how exciting it would be to find evidence for other universes! However, so far the evidence leaves waiting.

Update: See also one of the author's guest post at Cosmic Variance Observing the Multiverse.