Challenging the Isotropic Cosmos – Tarun Souradeep

Two weeks ago at AIMS we hosted the first Cape Town Astronomy and Cosmology Colloquium of the year. These colloquia are intended to bring together the Cape Town astronomy community to hear distinguished local and visiting scientists give general interest talks aimed at a wider audience.

Currently visiting AIMS we have Prof. Tarun Souradeep from IUCAA (Inter-University Centre for Astronomy and Astrophysics), and he was kind enough to talk to us about the future potential for testing our assumptions of an isotropic cosmology.

Tarun began with an overview of the amazing progress that has been made in CMB experiments over the last decades, explaining how satellites from COBE to PLANCK and beyond have transformed our view of the early universe. He showed us that the microwave background is like a gigantic, plasma IMAX screen, located at the furthest reaches of the observable universe (14Gpc away and 14Gyr in the past). What we see on this screen are small fluctuations of around 70 micro K on a background of around 2.725K.

Not only is there temperature data available, by looking at the polarization of the CMB we can extract even more information about the early universe. This is a particular hope for future experiments.

Tarun then outlined how the statistical analysis of the CMB is carried out using a spherical harmonic decomposition of the temperature data. The various peaks seen as the harmonic coefficients are plotted against their multipole can be used to probe cosmological parameters. For example the position of the first peak can be used to infer that we live in a spatially flat universe, and the height of that peak depends on the fraction of baryons in the cosmological fluid.

As a result of the sorts of considerations above, the parameters of the universe are very well constrained if we assume that we live in Lambda CDM (cold dark matter with a cosmological constant) universe that’s isotropic and homogeneous. The next challenge is to see how well these constraints hold if we attempt a non-parametric (or rather, a less constrained) fit of the data.

One important possibility to consider is that we may live in a non-statistically-isotropic universe. To try at judge this possibility from the CMB involves including the directional information when carrying out a spherical harmonic decomposition. Such information can give us insight in global topology, anisotropy and symmetries.

Many papers have examined the apparent anomalies in the WMAP data, with varying results. But, by looking at statistical isotropy more generally, it is possible to consider many types of anomaly simultaneously.

Tarun then discussed different techniques for studying the breakdown of statistical isotropy; for example studying the symmetry of iso-contours of correlations around a point can be used to search for the breakdown of statistical isotropy

The last part of Tarun’s talk was spent discussing the work Tarun and his collaborators have been doing to use the bipolar power spectrum (BiPS) and a bipolar spherical harmonic decomposition (BiPoSH) to study the breakdown of statistical in a systematic way. One particular example that Tarun discussed was the use of the BiPS to study the topology of the cosmos.

Tarun finished his talk with a discussion of some other ways in which an analysis with BiPOSH could tell us about violations of statistical isotropy in the cosmos.

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