There may be “cracks” in the universe: cosmic strings that can’t be seen from Earth

Beijing time on December 4th, the time may have “cracks” in the air, but human telescopes can not see them. These ancient cracks, if they do exist, are remnants of the very short period of time after the Big Bang, when the universe had just moved from a hotter, stranger state to the colder, more familiar state we see today. A new theory says the big cooling, which physicists call “phase changes,” doesn’t occur in the universe at the same time, and in some places it starts earlier than elsewhere.

These cracks in the universe, if they exist, are likely remnants of the very short time after the Big Bang.

Cooler parts of the universe form bubbles and expand in space until they encounter other bubbles. Eventually, all space changed and the original universe disappeared.

However, this ancient state of high energy may exist at the boundary between bubbles, where the cooling regions of the space-time structure meet and do not fully fuse together, creating cracks. Some physicists believe that in cosmic microwave background radiation (CMB), we may still find some evidence of the existence of cracks or imperfections known as “cosmic strings”. However, a new paper points out that the evidence is too weak to distinguish it from noise.

Oscar Hernandez, a physicist at Montmacier University in Canada and one of the paper’s authors, says cosmic strings are hard-to-imagine objects, but in a familiar world, there are analogies. He said, “Have you ever walked across an icy lake?” Do you notice cracks in the frozen lake? The lake is still strong, there’s nothing to be scary, but there are cracks. “

The formation of these cracks is similar to the phase-change process of cosmic strings. “Ice is phase-altered water,” Hernandez said. Water molecules begin to connect into pieces, usually in a hexagonal shape. Now, imagine you’re laying a perfect hexagonal tile on the lake. If someone on the other side of the lake starts laying tiles, your chances of lining up your tiles are basically zero. “

On an icy lake, long cracks can form at imperfect intersections. In the interwoven structure of space-time, if the basic physics theory is correct, it may form cosmic strings. The researchers believe that there are fields in space that determine the behavior of fundamental forces and particles. The initial phase changes in the universe produced these fields.

“There may be a field related to a particle, and in a sense it must ‘choose a frozen and cooled direction’. And because the universe is really big, in different parts of the universe, the field will choose a different direction,” Hernandez said. Then when the universe cools down, there will be discontinuous lines, and there will be energy lines that cannot be cooled. “

Today, these intersections will appear in the form of infinitely fine lines of energy, passing through space. Hernandez points out that the discovery of these cosmic strings is significant because they will be new evidence that physics is bigger and more complex than current models allow.

Currently, the most advanced particle physics theory is called the standard model. The model includes quarks and electrons that make up atoms, as well as other more exotic particles, such as the Higgs boson and neutrinos.

However, despite many strange phenomena, most physicists still believe that the standard model is incomplete. The researchers have come up with a number of ideas to extend the model, such as supersymmetric particles and superstring theory. Superstring theory holds that all particles and forces can be interpreted as tiny, multidimensional “string” vibrations. It is worth noting that the “string” in superstring theory is different from the cosmic “string”. Since there are not many metaphors available, sometimes physicists in different fields reuse the same one.

“A lot of the standard model extensions that people are very optimistic about — such as superstring theory and many other theories — naturally lead to cosmic chords that are produced by the (post-Big Bang) bulge,” Hernandez said. “And if we can find these cosmic strings, that would be an exciting discovery.

Since 2017, there has been a lot of interest in finding cosmic strings in the cosmic microwave background, Hernandez and other researchers wrote in a paper published November 18 in the arXiv database.

Some researchers, including Hernandez, have argued that convolutional neural networks, a powerful pattern-recognition software, would be the best tool for finding evidence of cosmic string structures in the cosmic microwave background. ‘Assuming a perfect, noise-free cosmic microwave background radiation map, even if its energy level (or “tension”) is very low, we might find cosmic strings from the computer that runs the neural network,’ they wrote in a 2017 paper.

However, in this new 2019 paper, they discuss the subject again, and point out that in reality, it is almost certainly impossible to provide neural networks with sufficiently clear cosmic microwave background data to detect these potential cosmic strings. Other brighter microwave sources can blur the cosmic microwave background and make it difficult to completely separate. Even the best microwave instruments today are not perfect, with limited resolution, and their recording accuracy fluctuates randomly with pixels. Together, the researchers wrote, they found that all of these factors combined would result in a degree of loss of information that no current or planned way to document and analyze the cosmic microwave background could overcome the problem. This way of finding cosmic strings doesn’t work.

But they also write that doesn’t mean it’s over. One of the new methods of finding cosmic strings is based on measuring the expansion of the universe in various directions in the ancient part of the universe. Hernandez said the method, known as “21 cm intensity mapping,” does not rely on studying the motion of individual galaxies or on accurate images of the cosmic microwave background. Instead, it is based on measuring the average rate at which hydrogen atoms leave The Earth in all parts of the deep space.

It is called “21 cm intensity mapping” because hydrogen atoms release electromagnetic energy at a wavelength of 21 cm. There are no observatories on Earth that can implement this method. The researchers say they hope to find clearer evidence of cosmic strings in the data once the observatories with sufficient detection capabilities are on the line, which will not be far from the discovery.

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