What is Cosmic Strings?

Introduced by the British physicist T. W. B. Kibble in 1976 (1), cosmic strings are topological defects that would hypothetically formed during the first moments of the universe. The study of topological defects is a topic of interest to cosmologists, because it allows to better understand the physical conditions prevailing in the early universe.

Such objects should be major sources of gravitational waves. With the advent of "Astrophysics of gravitational waves", thanks to the recent detections made by LIGO and Virgo, physicists today have the tools to detect the potential presence of cosmic strings.

NB: Cosmic strings should not be confused with the strings of string theory, these are two very different objects.

Universe: spontaneous broken symmetries and topological defects

Topological defects are stable alleged hypothetical structures that would be formed in the first moments of the universe. Theories involving the formation of topological defects are predicting they would have appeared at the end of the period inflationary (3), at levels of energies of the order of the Grand unification theories (about 1015 GeV). These phenomena are part of the very high energy physics.

Specifically, the topological defects are known to have formed during the different transitions in the early universe phase. A phase transition is defined as the change of State or structure of a physical system produced by changing a setting outside. For example, the transition from the liquid state to solid state is a phase transition.

Graphic showing the different transitions of phases of matter from the beginnings of the universe. At the end of the Big Bang, the universe is extremely hot and energetic, it consists of a plasma of quarks and gluons. Then it cools down gradually with the expansion, its energy decreases correspondingly and the material goes through several States until today be arranged in atoms. Credits: gruppo3.ca.infn.it

About the universe, the transitions of phases occurred when the temperature, and thus the energy of the universe began to decline as a result of the expansion. In the Standard model, these phase transitions are accompanied of different spontaneous broken symmetries; that is, certain symmetries on which were based the physical laws were broken.

Of such broken symmetries so explain how four elementary interactions, which were then unified at the end of the Big Bang, gradually 'broke' from each other when the universe started to cool. The more well-known of spontaneous breaking of symmetry example is that of the "desunification" between the weak and electromagnetic interactions, caused by the coupling of W/Z bosons with the Higgs field.

In addition to basic interactions, the quantum vacuum underwent phase transitions changing its topology and impacting so structure in different areas of space-time. At the intersection of these altered space-time areas, atypical and stable material configurations have emerged (3) via the Kibble-Zurek mechanism (mechanism similar to the Higgs mechanism). These structures are called "topological defects", because they are derived from faults in the topology (structure) of the vacuum.

Computer simulation showing a network of walls of areas. The walls of areas are topological defects formed as a result of the spontaneous breaking of a discrete symmetry. They are "membranes" partitioning in several parts of the universe. Credits: Cambridge University / Carlos Martin

Depending on the symmetry is broken in the vacuum, different types of topological defects may appear. When a cylindrical symmetry is broken, it is a cosmic string. In the case of a spherical symmetry, it is a magnetic monopoly. For a discrete symmetry, it is a wall of field. For other types of symmetries, it can be of skyrmions, textures (unstable) or extra dimensions.

Cosmic strings: training, properties and predictions

Cosmic strings are therefore emerged when, at the end of the period inflationary, cylindrical and axial symmetries are broken. These are one-dimensional topological defects in linear form. The number of cosmic strings in the universe cannot be determined with certainty, however, Kibble calculations indicate that there will be approximately a cosmic string by Hubble volume (1), is a cosmic string every 1031 cubic light-years.

Theories in general recognize two types of cosmic strings depending on the magnitude of their effects and more specifically energy thresholds at which they formed. First, the local cosmic strings that have no fields (electric, magnetic...) at long range; These strings have an attractive effect of very short scale on the surrounding material. The energy density is therefore strongly localized at the periphery of the rope.

Secondly, global cosmic strings that have fields at long range. Indeed, the amplitude of the fields of a global cosmic string is given by the report E/Hc with 'E' energy of the string, 'H' the constant Hubble and 'c' the speed of light in a vacuum. This amplitude begins so to decrease than over distances of the order of the Hubble RADIUS (14 billion light years), it therefore extends to the entire observable universe.

Computer simulation showing the distribution of cosmic strings in a bounded area of the universe. Credits: Cambridge University

Although they can reach lengths of the order of the observable universe, cosmic strings are Superfine objects which diameter approach that of the proton, i.e. 1 femtometer. In fact, to simplify calculations, physicists consider cosmic strings objects of thickness zero. This approximation allows them to use a particular mathematical tool, 'Nambu-Goto action', taken from the theory of the strings and allowing to study any object one-dimensional shaped string of energy non-zero.

When the gravitational field in general relativity equations are applied to the cosmic strings, they reveal that they are necessarily in tension objects in space-time, and thus have a mass proportional to this voltage (4). Thus, despite their thin diameter, cosmic strings are extremely dense. For example, a kilometre-long cosmic string has a mass similar to that of the Earth (4).

Two theories postulate the existence of cosmic strings. First, as part of the Standard model, the quantum field theory predicts the formation of cosmic strings through the Abelian Higgs mechanism. The latter shows that in the early universe, when some gauge symmetries were broken (so that some quantum gauge field acquires a mass), cosmic strings appeared in the place of these broken symmetries.

Then, cosmic strings are also predicted by the superstring theory. The superstring theory contains several types of strings, including fundamental strings rated "F-strings". The rope access physicist Joseph Polchinski, relying on the work of the physicist Henry Tye (5), showed that during the first moments of the expansion of the universe, the F strings were stretched up to reach the Galactic sizes, thus constituting a "cosmic superstring.

Observe cosmic strings?

Cosmic strings are important sources of gravitational waves. Indeed, they fit in networks within which strings, because of chaotic oscillations, can sometimes become generally accepted, leading to the formation of local configurations specific and very unstable at the origin of bursts of waves gravitational. These events are predicted as being singularly violent gravitational phenomena, and therefore detectable.

In addition, cosmic strings also form loops, also unstable, that decay by emitting gravitational waves. However, unlike the gravitational bursts, these waves are much less powerful and therefore more difficult to detect, since they eventually "drown" in the background of gravitational waves.

Computer simulation showing the configuration in networks of cosmic strings, to two different eras of the universe. During the era of the radiation, networks of cosmic strings have a high density of loops and knots, sources of gravitational bursts. During the era of matter, networks are looser and loops density decreases, the gravitational waves emitted are much lower intensity. Credits: Cambridge University

Among the observational predictions related to cosmic strings, these, in view of their extreme density, should be the source of phenomena of gravitational lenses, leading to optical duplication of the image of a same galaxy far away. However, until today, no phenomenon of this type were observed (5). In the same way, an optical duplication of fluctuations in the cosmic microwave background should be kept, but nothing 2013 Planck mission results have revealed a conclusive (6).

In 1979, astronomers Dennis Walsh, Bob Carswell, and Ray Weymann discover the quasar Q0957 + 561 A, B. The letters "A, B" refer to the fact that when they are discovered, astronomers have observed a double image of the same quasar, which they then called "double quasar. Then, this optical duplication was explained by the presence of a Galaxy between Earth and the quasar, creating a gravitational lens effect and leading to a shift of observation about 415 days between the two images of the same quasar.

However, between September 1994 and July 1995, astrophysicists of the Harvard-Smithsonian Center for Astrophysics again observe the quasar and, in addition to observe variations in brightness of the two images, only detect this time none difference between reception of the latter. The explanation given by astrophysicists in a March 25, 2004 publication in the journal Astronomy & Astrophysics is that during this period, a cosmic string, with a period of oscillation of 100 days, would be passed between the Earth and quasar (7).

Although currently there is no observational evidence to confirm or deny the existence of cosmic strings, many experiences have allowed to ask quite specific constraints on their conditions of existence and observation) (6) with data from LIGO and Virgo collaborations, physicists should finally be able to shed light on these cosmic objects.