Cosmic defects are topological defects taking place in the context of the evolution of the universe. When the universe was very young, cosmological phase transitions occurred –the same as what happens when water turns into ice or ice into water– and it is highly likely that defects such as cosmic string and semilocal string networks arose in these transitions. Cosmic defects have been anticipated in a whole host of theories that set out to explain how the universe evolved, but it has not been possible to measure a single one. Even they cannot be measured, certain calculations can be made with respect to them to be able to determine their properties.

The researchers Joanes Lizarraga and Asier Lopez-Eiguren in the Department of Theoretical Physics and History of Science in the  UPV/EHU's Faculty of Sciences and Technology are running numerical simulations of these cosmic defects under the supervision of Prof Dr Jon Urrestilla, in collaboration with research personnel in Switzerland and the United Kingdom, among others. What these researchers are setting out to do is to determine the properties of the cosmic string networks and semilocal string networks by means of calculations made by supercomputers. "Getting to know the physics of the defects is opening up the doors to the study of the phases the universe has been through," explained the researchers. They have managed to calculate in the most exact way so far how these defects contribute towards cosmic microwave radiation. CMB (Cosmic Microwave Background) is radiation that emerged when the universe was young; it was measured for the first time about half a century ago and during the last decade more modern, more exact measurements have been made with the WMAP and Planck satellites.

Precision is their goal

Taking the standard cosmology model as the basis, they use equations derived from the theory to run computer simulations of the universe.  "We create a box that has the properties of the universe," explained Lizarraga, "and in it we solve fundamental equations to explain the theory." That way, they reproduce the evolution of the universe in the box; in other words, they produce a replica of the expanding universe. These simulations provide a lot of statistical information about the strings, and using complex calculations they can see how these strings contribute towards the CMB.

As the researchers explained, "we have used the biggest simulations run so far and so we have managed to analyse various aspects that had never been analysed with respect to the strings, and we will be able to go on analysing many others." A huge quantity of resources are needed to be able to make calculations of this size and even more if the box that simulates the universe evolves over time. But thanks to the fact that they have been able to expand the size of the simulations and have detected previously unknown setups, and what is more, because they have managed to calculate the effect that cosmological transitions exert on the defect networks, they have succeeded in considerably improving the calculation of what these strings may contribute towards the CMB. They are also developing various techniques to process the data obtained in the numerical simulations, and they have produced new tools to facilitate and develop the analysis of the data obtained in the numerical simulations of these defects.

Additional information

The PhD holder Joanes Lizarraga and the physicist Asier Lopez-Eiguren are conducting their studies under the supervision of Jon Urrestilla, lead researcher in the Department of Theoretical Physics and History of Science of the UPV/EHU's Faculty of Sciences and Technology. They are working in collaboration with various universities: University of Geneva (Switzerland), University of Sussex (United Kingdom), University of Helsinki (Finland), University of Cape Town (South Africa), University of Nottingham (United Kingdom), University of Leiden (the Netherlands) and the universities of Oporto and Lisbon (Portugal).

Bibliographical references

J. Lizarraga, J. Urrestilla, D. Daverio, M. Hindmarsh, M. Kunz, A. R. Liddle (2014). "Can topological defects mimic the BICEP2 B-mode signal?". Phys.Rev.Lett. 112.171301.

J. Lizarraga, J. Urrestilla, D. Daverio, M. Hindmarsh, M. Kunz, A. R. Liddle (2014). "Constraining topological defects with temperature and polarization anisotropies". Phys.Rev. D90.103504.

A. Achúcarro, A. Avgoustidis, A.M.M. Leite, A. Lopez-Eiguren, C.J.A.P. Martins, et al. (2014). "Evolution of Semilocal String Networks: I. Large-scale Properties". Phys.Rev. D89.063503.

Photos: Joanes Lizarraga. UPV/EHU.