Einstein's biggest lesson in general relativity is that space itself is not flat, unchanging, and absolute. It is woven together with time into one fabric: space-time. This tissue is continuous, smooth and becomes bent and deformed in the presence of matter and energy. Everything that exists in this space-time moves along a path determined by the curvature of space-time, and its movement is limited by the speed of light. But what if there are defects in the fabric itself? This is not science fiction, but a really existing idea in theoretical physics. Associated with it are high-energy relics like domain walls, cosmic strings and monopoles. Ethan Siegel from Medium.com tried to answer the question, what could be their origin, properties and how they will get along with the ordinary universe.
It turned out that getting a defective Universe is not so difficult mathematically.
The gravitational behavior of the Earth in orbit is not due to invisible gravitational gravitation, but is better described by the Earth freely falling through curved space in the presence of the Sun. Even in this case, the curvature of space will be too small and there will be no defects in it.
Try to represent the space as best you can. What does it look like? Will it be empty, smooth, and mostly uniform? Do you also think that the only deviations from this state will be associated with the presence of masses and quanta of energy? This is a good approach that physicists usually take. On a large scale, space will be a three-dimensional grid, the only deviations in which will be small regions of spatial curvature of small magnitude, creating the gravitational force that we know well. Space in this configuration will be in the state of least energy.
The fabric of space-time ripples and warps due to mass. As far as we know, space never folds into itself or bends.
But what about excited states? Or others? To make it easier, let's subtract two spatial dimensions and leave one: the line. The line can be straight, open and endless, or closed, like a loop. In both cases, they will be lines in a state of least energy. What would a high energy state be like? Imagine taking your line and hanging it like a string. Now make a knot on the string, as if you were tying laces. A string without a knot will represent a one-dimensional space in the lowest energy state; a string with one knot will represent a one-dimensional space in the first excited state. This node will be a 0-dimensional topological defect.
Now you can do some interesting things with the line containing the node. You can tie another knot in the same way and get two topological defects instead of one. But if you tie a knot in the opposite direction (that is, make the same loop, but otherwise cross the ends before tossing and tightening), this knot will be topologically opposite to the original knot. If you very carefully align both knots (original and opposite), it turns out that they can untie each other and return the line to a state of low energy.
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Two types of these zero-dimensional effects - knot and anti-knot - have physical analogies in our universe: magnetic monopoles. The node corresponds to an isolated magnetic north pole; anti-node to the isolated south magnetic pole. If you combine them, they annihilate, like matter and antimatter, and return the fabric of space-time to a low energy state. Since monopoles are just point particles, they will behave like ordinary matter, not much different from the electrical monopoles (positive and negative electric charges) that exist in our universe.
The concept of a magnetic monopole emitting lines of a magnetic field in the same way that an isolated electric charge would emit lines of an electric field
So let's get back to our 3D universe. Now you can imagine not only point defects, but also high-dimensional defects:
- Cosmic strings: when a one-dimensional line in some way permeates the entire observable universe
- Domain walls: when a two-dimensional plane with different properties from one side to the other passes through the universe
- Space textures: when an area of three-dimensional space is twisted into knots
So, we have monopoles (0-dimensional), strings (1-dimensional), walls (2-dimensional) and textures (3-dimensional) - all kinds of defects that arise from different mechanisms of the same class: when symmetry is broken.
Differences between universes, one created according to standard cosmology (left) and one created with a significant network of topological defects (right), give rise to a variety of large-scale structures. Our observations are enough to rule out cosmic strings and domain walls as the dominant component of the modern universe.
Symmetry breaking is a serious matter in physics. Each existing symmetry corresponds to a stored value, so if the symmetry is broken, that value is no longer stored. Monopoles can be produced by breaking spherical symmetry; strings can be produced by breaking angular or cylindrical symmetry; violation of discrete symmetry can create domain walls. Other defects are a little more difficult to find, but they often come into play when it comes to scenarios with additional dimensions. But the first three - in particular, monopoles, cosmic strings, and domain walls - are of particular interest to cosmology.
We know that everything is not limited by the Standard Model and there are many extensions and additions that can have interesting observable consequences. One of them is the idea of the Grand Unification, when electromagnetic, weak and strong nuclear forces combine at high energies. The idea behind the unification is that all three forces of the Standard Model, and perhaps even gravity at high energies, could be combined into a single structure. This would not only lead to the emergence of new particles and interactions, but would also allow the emergence of magnetic monopoles. The lack of magnetic monopoles in the observable Universe is often cited as proof of cosmic inflation and that the Universe will never get hot enough after the end of inflation to restore the symmetry of the Grand Unification Theories.
If the symmetry that ensures the Grand Unification were broken, a colossal number of magnetic monopoles would appear. But they are not in our Universe; if cosmic inflation took place after this symmetry was broken, at least one monopole would have to remain within the observable universe
Cosmic strings and domain walls could appear during phase transitions, if they really existed, soon after the end of inflation. There may be some extremely high-energy symmetries formed in early times, when broken, such defects appear. Cosmic strings and domain walls - one or a whole network - would have to leave a signature in the large-scale structure of the Universe, textures would appear in the CMB, and monopoles would have to be created through direct experiments. Some physicists point to the magnetic monopole discovered on February 14, 1982, as proof of cosmic inflation: there was one monopole in the observable universe, and we saw it!
In 1982, an experiment led by Blas Cabrera, armed with eight turns of wire, recorded a change in the flux of eight magnetons: readings indicating a magnetic monopole. Unfortunately, at the time of discovery, no one was nearby and no one was able to reproduce the result, as well as find another monopole
And if monopoles behave like matter, cosmic strings, domain walls or cosmological textures will seriously affect the expansion of the Universe. Cosmic strings will behave like spatial curvature, limited to about 0.4% of the total energy density, and domain walls will create a form of dark energy that accelerates the expansion of the universe so slowly that it will not even be noticeable. Cosmological textures will have the same effect as the cosmological constant, but our observable universe will have to be limited to one single defect to explain our observations.
Various components of the energy density of the Universe and when they could manifest in full force. If cosmic strings or domain walls existed in any quantity, they would seriously affect the expansion of our universe.
Monopoles, strings, walls, textures and other defects should be super heavy if they existed. Monopoles would be the most massive particles ever discovered (100 trillion times more massive than a top quark). Strings, walls, and textures would become seeds for large-scale structures, pulling material together and forming other structures that we can easily see with modern telescopes, surveys, and CMB data. Current constraints tell us that such structures do not exist in abundance, and they would hardly account for more than a few percent of the total energy budget of space.
To date, there is no evidence that our universe is defective, other than that single observation of a magnetic monopole 35 years ago. Although we cannot refute their existence (only limit), we need to keep our ears on top and be prepared not only for their possible detection, but also for any other additions to the Standard Model that are not prohibited by physics. In most cases, if they don't exist, then there must be something suppressing their existence. Lack of evidence does not indicate the absence of the phenomenon. However, and about the availability too.
Ilya Khel