Cosmic Strings: The Mysterious ‘Seeds’ of the Universe

Extremely heavy and thin as hair, these astonishing strands of matter are known as cosmic strings. Scientists believe that in the history of the Universe, they played the role of “seeds” from which galaxies and their giant filamentary clusters grew. Who scattered these “seeds”? Have they survived to the present day, and is it worth looking for them in space? And, more generally, how much can we trust cosmological models? Are scientists mistaken in predicting such “oddities” in the cosmos?

To reconcile the experimental data on the large-scale structure of the Universe, we have to hypothesize the existence of cosmic strings with astonishing, even contradictory properties: they must be thin and, at the same time, unusually heavy and extremely stable formations. Can such objects exist in nature, and how did they form in the turbulent ocean of primordial, hot plasma?

The answer to these questions, as is often the case today, came from the opposite end of science—from the physics of the microworld. This is not surprising, as in the depths of matter, scientists encounter the same colossal densities and energies as in the furnace of the “primary explosion.” The cosmos and the microworld—the largest and the smallest—merge here. It’s not for nothing that they say opposites converge!

A couple of decades ago, while studying the interactions of elementary particles in accelerators, physicists concluded that at very high energies, exactly like those in the first moments of the “primary explosion,” a single universal force acts in matter. Gravity, electromagnetism, and other known types of interactions are just its various manifestations, similar to how electricity and magnetism are two specific manifestations of a single electromagnetic force field. Figuratively speaking, gravity and other types of forces are low-energy “tails” of the universal force, into which it splits as the Universe cools down.

Calculations say that the gravitational field was the first to split off. This cataclysm, which changed the face of the Universe, occurred in the first moments of its life. Traces of those ancient events have largely been erased, and it is very difficult to detect them today—most of them are currently beyond our experimental capabilities.

The next cataclysm erupted when the component of the universal force responsible for interactions within atomic nuclei split off. This happened when the Universe had already expanded to very large sizes (In its “early childhood,” it, like us, grew incomparably faster than it does today!) Its distant regions no longer had time to exchange interactions, and the splitting process in each of them occurred in its own way—independently of each other.

This is similar to how events unfolded in remote areas a couple of hundred years ago when there was no telephone or internet, and it took weeks to exchange information. Therefore, what happened in one area during a week had no effect on the events that took place in others at the same time. In scientific terms, these events remained causally unconnected. However, at the junctions of remote areas, in border regions, residents had to decide which course of events, if they were different, they should prefer for themselves.

Similarly, uncertainty arose in causally unconnected regions in the cooling Universe. At their junctions, nature also “didn’t know” which way of splitting it should choose and, so to speak, froze in confusion—like the legendary Buridan’s ass, which starved to death, unable to decide which of two identical haystacks, the left or the right, it should eat first.

To help the reader visualize all this more clearly, here are a couple of other simple examples. The first is when the temperature in the mountains changes and snow avalanches descend from their peaks. They descend independently from different sides, and if this happens simultaneously, a snow cap remains on the peak that “didn’t have time to think” where to slide. The second example is the solidification of molten matter. If you look at the surface of a solidified bar with high magnification, you can also distinguish areas where crystallization occurred differently, while a thin layer of matter with a destroyed crystal lattice remained in the gaps.

This is what happened when the Universe cooled. It also left behind boundary walls and string-like filaments, consisting of “primordial matter” with an unsplit interaction. However, it would be more correct to say —f rom a “highly stressed homogeneous vacuum field,” which is what space was back then.

Compared to the modern vacuum, it would have looked like an extremely dense, very heavy medium. And despite the fact that the walls and strings with the “primordial vacuum” sealed inside were incredibly thin — about 10⁻²⁸ centimeters (that’s as many times smaller than a centimeter as a centimeter is smaller than the radius of the visible part of the Universe), their mass was colossal. A tiny piece of this “canned vacuum” film would weigh more than an entire galaxy.

Catastrophes come in different scales—volcanic eruptions, tsunamis, earthquakes. But imagine the “crystallization” of the entire space at once. A cosmic cataclysm as a result of which the world becomes completely different, and the remains of the old world float in the form of heavy and very thin films and filaments.

What Happened Next?

And this is what happened next. Under the action of gravitational attraction, huge forces of surface tension immediately arose in the boundary walls, which in a matter of moments tore them into pieces. Those pieces, in turn, also almost instantly contracted, forming “black holes.”

Science fiction writers who compose space odysseys can rest easy—their fabulous starships are not in danger of crashing into a relict wall of canned primordial matter. All such walls have long since disintegrated!

A different fate befell cosmic strings. The weight of the primordial matter sealed within them is also enormous. Ten linear meters of such a thin string with a diameter of 10⁻²⁸ centimeters would weigh as much on our scales as our neighbor, the Moon. That’s a lot, but still almost thirty orders of magnitude less than a piece of a wall. And the strings survived—they withstood it. Gravity could not tear them into pieces and turn them into “black holes.”

The filaments of super-heavy primordial matter, sealed in the strings, pierced the hot substance of the young Universe like spiderwebs with their strong gravitational field. Some parts of them attracted and accelerated others. The strings twisted and moved. If you sequentially, like in a movie, watch the computer-drawn pictures of such a string’s movement, it very much resembles the behavior of an angry snake coiling up into rings in a tank of water.

As a result of interactions between their intersecting parts and collisions with oncoming strings, numerous breaks occurred. Daughter strings, in turn, broke themselves and broke up oncoming ones. In addition, as they accelerated and collided, the strings emitted powerful gravitational waves. The latter is similar to how electrons, accelerated in an X-ray tube, collide with the substance of the screen and produce a beam of hard electromagnetic rays. By emitting gravitational waves, the strings “thinned out” and gradually melted away.

Generally speaking, according to Einstein’s theory, all the bodies around us emit gravitational waves, but their intensity is extremely low, and they cannot be noticed even with the most sensitive instruments. It’s a different matter for very heavy bodies with enormous accelerations. Then the radiation becomes very strong and carries away a significant part of their mass.

As a result of these processes, all very long strings disappeared, and many short ones “melted.” Their remnants are scattered throughout the cosmos. However, the remaining ones had a decisive influence on the fate of our world. Ultimately, it is to them that life and intelligence owe their appearance.

Galactic Continents of the Universe

When the young Universe became less fiery, which happened at the age of about ten thousand years, and the velocities of the particles of the nuclear plasma that filled it could no longer resist the inexorable gravitational attraction of the still-unmelted strings, the condensation of matter began. Around small strings, groups of several galaxies formed; around larger ones, their large clusters. And the more matter gathered, the stronger the forces pulling it together became. The Universe entered the prime of its life.

The first to propose such a scenario for the evolution of the Universe was Yakov Borisovich Zeldovich. One can only be amazed at how many physical ideas belong to this scientist! In the theory of the Big Bang, in the field of elementary particles, in astrophysics and cosmology. Not all of them were confirmed by experiment, but they were always witty and provoked disputes and discussions.

They say that scientists are divided into classics and romantics. The former rarely write articles and books, but everything they write ends up in textbooks. Romantics, on the other hand, are very sociable; they discuss all new facts and phenomena. Sometimes they are called generators of ideas. They literally gush with them, and although not all of their hypotheses turn out to be one hundred percent correct, science without such people would be boring and grey. Yakov Zeldovich was one of them.

His idea of strings as the seeds of a “cosmic garden” (that’s also his term!) was unexpected and witty. Computer calculations (mostly performed by American scientists) showed that with its help, it is indeed possible to reconcile all the known astronomical data on the large-scale structure of the Universe. In particular, using only a few “fitting parameters” related to the properties of the “primordial vacuum,” it is possible to correctly explain both the shape of superclusters and their distribution by the number of galactic island-clusters they contain.

Interestingly, the distances between clusters, their spatial correlation, as astronomers say, also bear traces of their “string origin.” If you build a graph where the distance between two clusters is plotted on the “x-axis,” and the number of pairs of clusters with such distances is plotted on the “y-axis,” it is clear that for a random, uncorrelated birth of galaxies, this graph will be completely different from when they are born around cosmic strings. What astronomers observe corresponds to the second case.

However, if you consider not the clusters themselves, but individual galaxies within them—that is, if you go down one level—the traces of their string origin are less noticeable. This is probably due to the fact that—unlike clusters that are very far from each other and therefore weakly connected by gravitational fields—galaxies are more strongly affected by their mutual attraction, which over the multi-billion-year history of the Universe has managed to noticeably distort the picture of the original arrangement of galaxies in space.

Superconductivity in Space

They say that happiness is never complete. While scientists were rubbing their hands with delight, believing that they had finally managed to explain the structure of the world, nature presented them with a new riddle. An analysis of galaxy clusters, performed by astrophysicists at Harvard University, revealed numerous large-scale violations of the filamentary structure. In some places, the filaments are broken by gigantic “bubbles” hundreds of millions of light-years across, free of matter. Galaxy clusters are observed only on their surfaces, while inside, the visible, light-emitting matter is hundreds of times less dense. And there are quite a few such cases. They are what create the cellular “superstructure.”

String theory says that the filamentary chains of galaxies should be spaced from each other at an average distance of several tens of millions of light-years, while the bubble-cells are at least ten times larger. And the most important observation is that galaxies on the surfaces of the “bubbles” are flying away from their centers at speeds of thousands of kilometers per second.

The observed picture suggests that some mysterious, unusually powerful explosions occurred in many parts of the Universe, scattering galaxies far away and forming gigantic dark “bubbles.” It is impossible to explain all this with cosmic strings alone.

What happened there? What caused the monstrous explosions? There is no answer to these questions yet. Only hypotheses.

One of them suggests that the explosions were caused by accumulating electromagnetic radiation—a kind of gas of photon waves. The thing is, cosmic strings, as we already know, consist of homogeneous “primordial vacuum matter.” Their properties are completely identical throughout their length, so charged particles (and the hot young Universe was precisely such a plasma) accidentally captured by them from the surrounding plasma will move along the string without any resistance—like in a superconductor. These particles simply have nowhere to go.

If “clouds” of a magnetic field floated in the plasma of the young Universe, then when they were intersected by superconducting strings, powerful electric currents should have arisen in the latter—exactly as in Faraday’s famous experiment when a conductor is moved between the poles of a magnet. In addition to gravitational waves, such a string will also emit electromagnetic waves. If it also has the shape of a closed loop and is not too large in size (and there were many such strings “roaming” in the young Universe as a result of collisions and intersections), it would be an extremely powerful and very compact radio generator from a cosmic perspective. However, its radiation would not dissipate, since the surrounding plasma, like a metal screen, reflects and blocks electromagnetic waves in the nearby space.

The pressure of the radiation would begin to increase sharply and vigorously push away the surrounding electrically charged matter, forming a huge dark bubble. Light-emitting galaxies would “grow” (condense) on its walls, forming the cellular structure that American astronomers observed.

This is the qualitative picture of the formation of the cellular “honeycombs” of the Universe. It can also be justified with calculations. However, there is still no complete certainty that events developed in exactly this way and that the dark “bubbles” are just shells of electromagnetic bursts. Some scientists, criticizing this hypothesis, emphasize that, for example, it is completely unclear where magnetic field clouds come from in the Universe. “Indeed, it’s unclear,” supporters of the explosive hypothesis reply. “This is an additional assumption. But it still has to be introduced to explain the mechanism of star formation.” So, as they say, seven troubles—one answer! As we can see, there are still enough riddles here.

We still have too few reliable facts to rely on when building a theory. For example, if we had gravitational wave detectors in addition to optical and radio telescopes, we could look inside such bubbles and check whether the filaments of galaxy clusters, consisting of dark, electrically neutral matter, have survived there. This would be a convincing experimental verification of the hypothesis of electromagnetic explosions of superconducting strings. Unfortunately, such observations are a matter of the distant future; gravitational astronomy is only just being created.

To Space for Strings

As mentioned above, cosmic strings are extremely heavy objects. Although over billions of years they have “thinned out” noticeably, decreasing in size and weight, they still have a huge gravitational field today, and their presence in space cannot go unnoticed.

Estimates show that the strings that have survived to this day have a size of about a thousand light-years. In cosmic terms, this is not much—a string is a hundred times smaller than the diameter of the Galaxy—but by our earthly standards, it is an enormous value, especially if you remember that the nearest star to us is ten light-years away, which is a hundred times smaller than a cosmic string.

It must be said that only those strings around which galaxy clusters formed have survived to this day; the “embryos” around which the condensation of scattered galaxies occurred have already melted away.

The average distances between strings are also very large—tens of millions of light-years. Each of them is like a tiny millimeter-long speck in a ten-meter ball. And yet, despite all these cyclopean “mores” and “lesses,” we can still try to notice their presence in space. The task is, of course, extremely difficult. Direct observations are impossible, since strings do not emit light or other electromagnetic rays. All hope lies in their powerful gravitational field.

With their gravitational pull, strings should deflect the light rays emitted by objects behind them. Light that bypasses a string from two sides will be deflected differently. For example, luminous points—like the very distant quasars located at the edge of the visible part of the Universe that we observe in telescopes—will split and turn into two pale, closely located stars. Therefore, if a chain of paired light spots can be detected, it will indicate that a cosmic string is located in the path of the light rays.

Recently, American astronomers recorded something similar, but their observations still require verification.

If a cosmic string is located in the path of the rays emitted by a galaxy, its tiny visible disk will be sharply bisected. So far, no one has observed such objects, although searches have been underway at many observatories for several years.

Finally, the gravitational background of the Universe, created by the radiation of “melted” and “thinning” strings, causes weak disturbances in the very precisely repeating periodic radio emission of some pulsars. If this radiation is observed for about ten years, such disturbances can be identified.

Thus, the “hunt” for cosmic strings, although extremely difficult, is not a hopeless task. Especially since these objects, especially their superconducting variety, are so unusual in their properties that other, more vivid manifestations of them will undoubtedly be found. There’s plenty to rack your brains over!

Infinite and Inexhaustible

The reader may be somewhat discouraged by how quickly scientists replace one model of the evolution of the Universe with the next, and how all of them are based on hard-to-verify, fantastic hypotheses about the properties and behavior of the Universe in the first moments of its life.

Observing how quickly hypotheses and theoretical schemes replace each other, a person far from science feels a certain uncertainty: how can one believe the predictions of scientists if they themselves give up on them so quickly? The hadronic model of the hot Universe was replaced by the neutrino model, which was in turn replaced by cosmic strings, and soon, something else will probably appear. As one humorist noted, astronomers see the Universe but cannot explain it, while cosmologists see nothing but can explain everything. Can their “fantastic” pictures be trusted?

First of all, it should be kept in mind that the epithet “fantastic,” so often exploited by popularizers of science, is not entirely appropriate when it comes to the first seconds of our world’s life. Amazing, astonishing—that’s true—but not fantastic, because unlike science fiction writers, a cosmologist deals not with arbitrary assumptions, but with the logical consequences of astronomical observations and experimental facts from elementary particle physics.

True, these consequences cannot be called “ironclad”—they are ambiguous, especially in quantitative terms, due to the inaccuracy and insufficient number of experimental data. However, as we saw above, new observations strictly control them, which is why there is a rapid change in cosmological models. Or, rather, an improvement in the scientific picture of the world, since each subsequent model grasps some new element of the truth.

In previous centuries, major astronomical discoveries were extremely rare. Today, thanks to the rapid progress of technology, they are separated by time intervals of five to ten years. Cosmology is being updated at the same pace. And this process of deepening our knowledge is endless. It is impossible to create a final, completely finished theory—both the microworld and the cosmos are inexhaustible in their properties.

To back this up, we note that today, cosmology is moving to the next level of “superstructures,” the elements of which are now entire universes. But that is a subject for a separate conversation.