Introduced by the British physicist T. W. B. Kibble in 1976 (1), cosmic strings are topological defects that would have hypothetically formed during the first moments of the universe. The study of topological defects is a subject of interest for cosmologists because it allows a better understanding of the physical conditions that prevailed in the early universe.
Such objects should be important sources of gravitational waves. With the advent of “gravitational wave astrophysics”, in particular thanks to recent detections made by LIGO and Virgo, physicists now have the necessary tools to detect the potential presence of cosmic strings.
NB:cosmic strings should not be confused with string theory strings, they are two very different objects.
Topological faults are hypothetical structures presumed to be stable that would have formed in the first moments of the universe. Theories involving the formation of topological defects predict that they would have appeared at the end of the inflationary period (3), i.e. at energy levels of the order of grand unification theories (about 10 15 GeV). These phenomena are therefore part of very high energy physics.
More specifically, topological faults are said to have formed during the various phase transitions of the early universe. A phase transition is defined as the change of state or structure of a physical system produced by the modification of an external parameter. For example, the passage from the liquid state to the solid state is a phase transition.
Regarding the universe, phase transitions occurred when the temperature, and therefore the energy, of the universe began to decrease as a result of expansion. In the Standard Model, these phase transitions are accompanied by various spontaneous symmetry breakings; that is to say that certain symmetries on which the physical laws were based have been broken.
Such broken symmetries thus explain how the four elementary interactions, which were then unified at the end of the Big Bang, gradually "detached" from each other when the universe began to cool. The best-known example of spontaneous symmetry breaking is that of “disunification” between weak and electromagnetic interactions, caused by the coupling of W/Z bosons with the Higgs field.
Alongside elementary interactions, the quantum vacuum has also undergone phase transitions modifying its topology and thus impacting the structure of different areas of space-time. At the intersection of these altered spatio-temporal zones, atypical and stable configurations of matter appeared (3) via the Kibble-Zurek mechanism (mechanism similar to the Higgs mechanism). These structures are called "topological defects", because they arise from defects in the topology (structure) of the void.
Depending on the broken symmetry in the vacuum, different types of topological defects can appear. When a cylindrical symmetry is broken, it is a cosmic string. In the case of spherical symmetry, it is a magnetic monopole. For an unobtrusive symmetry, it is a domain wall. For other types of symmetries, it can be skyrmions, textures (unstable) or even additional dimensions.
The cosmic strings therefore appeared when, at the end of the inflationary period, cylindrical and axial symmetries were broken. They are one-dimensional topological defects of linear form. The number of cosmic strings in the universe cannot be determined with certainty, however, Kibble's calculations indicate that there would be approximately one cosmic string per Hubble volume (1), i.e. one cosmic string every 10 31 cubic light years.
Theories generally admit two types of cosmic strings according to the amplitude of their effects and more precisely according to the energy thresholds at which they are formed. First, the local cosmic strings which do not have long-range fields (electric, magnetic, etc.); these strings therefore have a very short-scale attractive effect on the surrounding matter. The energy density is therefore strongly localized at the periphery of the string.
Second, the global cosmic strings which possess long-range fields. Indeed, the amplitude of the fields of a global cosmic string is given by the ratio E/Hc with "E" the energy of the string, "H" the Hubble constant and "c" the speed of light in the void. This amplitude therefore begins to decrease only over distances of the order of the Hubble radius (i.e. 14 billion light-years), it therefore extends to the entire observable universe.
Although they can reach lengths of the order of the observable universe, cosmic strings are ultra-thin objects whose diameter approaches that of the proton, that is to say 1 femtometer. Moreover, to simplify the calculations, physicists consider cosmic strings as objects of zero thickness. This approximation allows them to use a special mathematical tool, "the Nambu-Goto action", taken from string theory and allowing them to study any one-dimensional object in the shape of a string, with non-zero energy.
When the gravitational field equations of general relativity are applied to cosmic strings, they reveal that the latter are necessarily objects in tension in space-time, and therefore have a mass proportional to this tension (4). Thus, despite their ultra-thin diameter, cosmic strings are extremely dense. For example, a cosmic string one kilometer long has the same mass as the Earth (4).
Two theories postulate the existence of cosmic strings. First, within the framework of the Standard Model, quantum field theory predicts the formation of cosmic strings through the abelian Higgs mechanism. The latter shows that in the early universe, when certain gauge symmetries were broken (so that certain gauge quantum fields would acquire mass), cosmic strings appeared at the location of these symmetry breaks.
Then cosmic strings are also predicted by superstring theory. Superstring theory contains several types of strings, among them the fundamental strings denoted "F-strings". String physicist Joseph Polchinski, drawing on the work of physicist Henry Tye (5), showed that during the first moments of the expansion of the universe, the F-strings were stretched to sizes galactic, thus constituting "cosmic superstrings".
Cosmic strings are important sources of gravitational waves. Indeed, they are arranged in networks inside which the cords, because of chaotic oscillations, can sometimes intertwine, leading to the formation of specific and very unstable local configurations at the origin of wave bursts. gravitational. These events are predicted to be singularly violent gravitational phenomena, and therefore detectable.
In addition, the cosmic strings also form loops, also unstable, which disintegrate by emitting gravitational waves. However, unlike gravitational bursts, these waves are much less powerful and are therefore more difficult to detect, because they end up "drowning" in the background of gravitational waves.
Among the observational predictions attached to the cosmic strings, these, in view of their extreme density, should be the source of gravitational lensing phenomena, leading to the optical duplication of the image of the same distant galaxy. However, until today, no such phenomenon has been observed (5). In the same way, an optical duplication of the fluctuations of the cosmic microwave background should be observed, but the results of the Planck mission of 2013 did not reveal anything conclusive (6).
In 1979, astronomers Bob Carswell, Dennis Walsh and Ray Weymann discovered the quasar Q0957+561A,B. The letters "A,B" refer to the fact that when they discovered the astronomers observed a double image of the same quasar, which they then called "double quasar". This optical duplication was then explained by the presence of a galaxy between the Earth and the quasar, creating a gravitational lensing effect and leading to an observation lag of about 415 days between the two images of the same quasar.
However, between the months of September 1994 and July 1995, astrophysicists from the Harvard-Smithsonian Center for Astrophysics observed the quasar again and, in addition to observing variations in luminosity of the two images, did not detect this time any shift between receipt of these. The explanation given by astrophysicists in a publication published on March 25, 2004 in the journal Astronomy &Astrophysics is that during this period, a cosmic string, with an oscillation period of 100 days, would have passed between the Earth and the quasar ( 7).
Even if currently no observational data can confirm or invalidate the existence of cosmic strings, many experiments have made it possible to pose relatively precise constraints on their conditions of existence and observation (6). With the data collected by the LIGO and Virgo collaborations, physicists should finally be able to shed light on these cosmic objects.
