NASA/WMAP
What types of phases could the universe have inhabited in the distant past? We know that the universe was hot and dense in its youth, and has subsequently been cooling as it expands. Just like H2O can exist as liquid water, ice, and steam, it is possible that the constituents of our universe can exist in different phases, and that transitions between these phases could have occurred in our past. In many cases, such cosmological phase transitions give rise to imperfections known as topological defects, which can survive until today, and possibly be detected through cosmological observations. Recently my colleagues Stephen Feeney, Hiranya Peiris, and Daniel Mortlock and I performed a search for a particular type of topological defect known as cosmic textures in Cosmic Microwave Background data from the WMAP satellite.
Before I describe our results, here is a little more on the background. Topological defects arise in theories where symmetries are broken as the universe cools. Symmetries abound in theories of high energy physics. For example, the standard model of particle physics can be specified by its symmetries: this determines the types and number of particles and the interactions between them. Underlying symmetries give rise to the unification of forces, and are a pillar of modern physics. To illustrate the types of symmetries I am referring to, here is a simple example: consider a theory with three interacting particles that have rotational symmetry in field space (a space where the axes are labeled by these three types of particles). Just as one can have rotational symmetry in real three dimensional space, one can have rotational symmetry in field space. There are many combinations of the three particles and their interactions that look identical. Such a symmetry of the field space is known as a global symmetry.
In the early universe, there might have been a set of such fields which possess this rotational symmetry. At each point in space, there will be a different combination of the fields, since all combinations are equivalent. However, as the universe cools, this symmetry can be broken, making the different combinations of fields no longer equivalent. Knots in the field configuration form at locations in space where the field combinations change. These knots carry energy, and are known as cosmic textures.
Until as recently as the mid-nineties, cosmic textures (along with cosmic strings) were a leading candidate for producing the seeds of galaxy formation. The predictions for cosmic textures were explored in seminal papers by Turok and Spergel. The main competing theory was
cosmic inflation, where the density fluctuations are produced by quantum fluctuations. Density fluctuations induce temperature fluctuations in the Cosmic Microwave Background radiation (CMB), and these two theories are distinguished by their predictions for the statistical properties of these fluctuations. One of the most important results of the WMAP satellite was to definitively rule out topological defects as the main seeds of structure formation; the data came down strongly in support of the inflationary hypothesis.
However, there is still the very real possibility that textures or other topological defects might be lurking in the background, making up only a small component of the density fluctuations. This idea has strong theoretical support from particle physics, because of the importance of symmetry breaking. In this scenario, the name of the game is to look for individual textures as opposed to studying the overall statistical properties of the CMB: the main component of the fluctuations now acts as a ``noise" which must be removed in order to uncover the presence of individual textures.
So, are there any features in the CMB which look like they might be described by cosmic textures? A series of important papers by Cruz et. al. suggested that the answer might be yes. The temperature fluctuations in the CMB are characterized by their statistics. Given a theory of the statistics, it is therefore possible to ask just how unlikely it is to find a feature of a particular type. There is one particularly famous feature, the CMB Cold Spot, which seems very anomalous. In addition, the shape and size of the Cold Spot is well-described by the shape and size expected of a cosmic texture.
This lead Cruz et. al. to conjecture that there is evidence for cosmic textures in the CMB. This result was predicated on a very small portion of the available CMB data: the Cold Spot occupies roughly one percent of the observed area on the CMB sky. If the cold spot was a texture, the theory of how textures are formed predicts that there should be other textures visible in different directions on the sky. This was recognized by Cruz et. al. However, an analysis that tests the predictions of the theory on the full sky is extremely challenging due both to the volume of data and to the huge number of possible arrangements of textures on the sky.
Stephen, Hiranya, Daniel, and I were able to overcome these difficulties, and devise an algorithm that can test the theory of cosmic textures using all of the available CMB data. This work built on our previous search for the signatures of cosmic bubble collisions. Our strategy was to use a two-step method where we first identify promising candidate signatures (such as the Cold Spot) and then directly compare the probability for a theory with cosmic textures to a theory without, given the data. We concluded that when you take all the data into account, the data do not warrant augmenting the standard cosmological model with cosmic textures. However, there are some interesting hints that there could be a weak signal. We'll have to wait for better data to see for sure.
Fortunately, we won't have to wait long since we will be able to perform an improved analysis on data from the Planck satellite when it becomes public in early 2013. At worst, this will yield much stronger constraints on theories that predict cosmic textures. At best, we may find evidence for textures, which can then be corroborated with data on the polarization of the CMB from Planck when it becomes available in a few years. This is a very exciting time for cosmology, as the flood of new data is allowing us to test fundamental physics in ways that were never possible before. I look forward to seeing whether textures, or something else, may be lurking in the background!