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Wednesday, 4 December 2019

If they exist, cosmic strings would be much harder to detect than expected

Following the Big Bang, the Universe has undergone several phase transitions that have resulted in broken symmetry of the physical laws. According to some cosmological models, some of these breaks in symmetry would have resulted in the formation of particular cosmic structures at the meeting point of unstable regions of the Universe; these structures are called topological defects, and cosmic strings are one of them. Cosmologists initially thought that signatures of these linear energy structures could be detected in the cosmic microwave background. However, recently, physicists have shown that these signatures would be too weak for their detection to be possible.

Cosmic strings are hard to imagine, according to Oscar Hernández, a physicist at McGill University in Montreal. " Have you ever walked on a frozen lake? Have you noticed any cracks in the ice? It's still pretty solid, but cracked, "he says. These cracks are formed by a phase transition process similar to that of cosmic strings.

Topological defects predicted by physics beyond the Standard Model

Imperfect meeting points on the surface of a frozen lake form long cracks. In the structure where space and time intersect, they form cosmic strings, if the underlying physics is correct.

Researchers believe that in space, some fields determine the behavior of forces and fundamental particles. The first transition phases of the Universe gave birth to these fields. Today, these points of intersection between fields would appear as infinitely thin lines of energy across space.

Several simulations have shown, if they exist well, the distribution of the cosmic strings during the evolution of the Universe. These topological defects are provided by many theoretical models. Credits: Nature

Most physicists think that the standard model is incomplete. " Many extensions of the standard model naturally lead to cosmic strings after inflation. So, what we have is an object that is predicted by many models. Therefore, if they do not exist, all these models are excluded.

Cosmic strings: they would be impossible to detect in the cosmic microwave background

Hernández and Razvan Ciuca of Marianopolis College in Westmount, Quebec, had previously argued that a convolutional neural network - a powerful type of pattern search software - would be the best tool for locating string evidence in the CMB.

Assuming a perfect and noise-free CMB card, they wrote in a separate article in 2017, a computer using this type of neural network should be able to find cosmic strings even if their energy level (or "voltage") is remarkably low.

But in this new article published on the arXiv server , they showed that in reality, it is almost impossible to provide enough CMB data for the neural network to detect these potential strings. Other brighter microwave sources obscure the CMB and are difficult to disentangle completely. Even the best microwave instruments are imperfect, with limited resolution and random fluctuations in the accuracy of recording from one pixel to another.

They found that all these factors, and more, added to a level of information loss that no current or planned CMB recording and analysis method could ever overcome. This method of chase cosmic strings is a dead end. This does not mean that everything is lost, however.

A new method for detecting cosmic strings

A new method based on measurements of the expansion of the universe in all directions, in old parts of the Universe, could work. This method - called 21-centimeter intensity mapping - does not rely on the study of individual galaxy motions or on accurate CMB images.

Instead, it is based on measurements of the rate at which hydrogen atoms move away from the Earth, on average, in all parts of deep space. This method should be able to provide sufficiently constrained data to restart the hunt for cosmic strings.


Information Theoretic Bounds on Cosmic String Detection in CMB Maps with Noise
Razvan Ciuca1  Oscar F. Hern´andez1,2†

1Department of Physics, McGill University, 3600 rue University, Montr´eal, QC, H3A 2T8, Canada

2Marianopolis College, 4873 Westmount Ave.,Westmount, QC H3Y 1X9, Canada


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