Infinite Cosmic String Research Articles

Gravitational wave spectrum from kinks on infinite cosmic strings with Y-junctions

We calculate the gravitational wave (GW) background spectrum from kink-kink collisions on infinite cosmic strings by taking into account the effect of Y-junctions. A kink going through a Y-junction transforms into three daughter kinks. In this way, the existence of Y-junctions also increases the number of kinks on cosmic strings with Y-junctions. However, at the same time, it smooths out the sharpness of kinks rapidly and reduces the number of sharp kinks, which are responsible for the emissions of strong GW bursts. We compute the number distribution of kinks as a function of the sharpness by taking into account the effect of Y-junctions and translate it to the amplitude of the GW background spectra. We first investigate the case of the string network with equal string tensions and find that the effect of Y-junctions to smooth out kink sharpness dominates that of the enhancement of the kink number by a small reconnection probability, and the GW amplitude turns out to be smaller than the ordinary cosmic string case. On the other hand, for non-equal string tensions, we find that the GW spectrum does not change significantly compared to the equal-tension case.

Impact of the small-scale structure on the Stochastic Background of Gravitational Waves from cosmic strings

Numerical simulations and analytical models suggest that infinite cosmic strings produce cosmic string loops of all sizes with a given power-law. Precise estimations of the power-law exponent are still matter of debate while numerical simulations do not incorporate all the radiation and back-reaction effects expected to affect the network at small scales. Previously it has been shown, using a Boltzmann approach, that depending on the steepness of the loop production function and the gravitational back-reaction scale, a so-called Extra Population of Small Loops (EPSL) can be generated in the loop number density. We propose a framework to study the influence of this extra population of small loops on the Stochastic Background of Gravitational Waves (SBGW). We show that this extra population can have a significant signature at frequencies higher than H0(Γ Gμ)−1 where Γ is of order 50 and H0 is the Hubble constant. We propose a complete classification of the Gravitational Wave (GW) power spectra expected from cosmic strings into four classes, including the model of Blanco-Pillado, Olum and Shlaer and the model of Lorenz, Ringeval and Sakellariadou. Finally we show that given the uncertainties on the Polchinski-Rocha exponents, two hybrid classes of GW power spectrum can be considered giving very different predictions for the SBGW.

Gravitational-wave background from kink-kink collisions on infinite cosmic strings

We calculate the power spectrum of the stochastic gravitational-wave (GW) background expected from kink-kink collisions on infinite cosmic strings. Intersections in the cosmic string network continuously generate kinks, which emit GW bursts by their propagation on curved strings as well as by their collisions. First, we show that the GW background from kink-kink collisions is much larger than the one from propagating kinks at high frequencies because of the higher event rate. We then propose a method to take into account the energy loss of the string network by GW emission as well as the decrease of the kink number due to the GW backreaction. We find that these effects reduce the amplitude of the GW background at high frequencies and produce a flat spectrum. Finally, we obtain a constraint on the string tension of $G\ensuremath{\mu}\ensuremath{\lesssim}{10}^{\ensuremath{-}5}$ using the current upper bound on the GW background by Advanced LIGO, and $G\ensuremath{\mu}\ensuremath{\lesssim}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ using pulsar timing arrays.

Instantons for particles joined by strings in three dimensional gravity

We study the instantons describing the production of particles at the ends of codimension-one objects (strings and struts) in (2 + 1)-dimensional Minkowski and de Sitter spaces. A Minkowskian background allows only for systems with vanishing total mass, so that either negative mass particles or negative tension struts are required. On a de Sitter background, on the other hand, we find processes describing the production of string/particle systems with no negative energies involved. We also compute the probabilities of creating and of breaking an infinite cosmic string in de Sitter space. We perform our analysis of the system in de Sitter space employing a generalization of the toroidal coordinate system to the three-sphere.

Backreaction on an infinite helical cosmic string

To understand the properties of a possible cosmic string network requires knowledge of the structures on long strings, which control the breaking off of smaller loops. These structures are influenced by backreaction due to gravitational wave emission. To gain an understanding of this process, we calculate the effect of gravitational backreaction on the "helical breather": an infinite cosmic string with a helical standing wave. We do the calculation analytically to first order in the amplitude of the oscillation. Our results for the rate of loss of length agree with the total power emitted from this system as calculated by Sakellariadou. We also find a rotation of the generators of the string that leads to an advancement of the phase of the oscillation in addition to that produced by the loss of length.

Improved calculation of the gravitational wave spectrum from kinks on infinite cosmic strings

Gravitational wave observations provide unique opportunities to search for cosmic strings. One of the strongest sources of gravitational waves is discontinuities of cosmic strings, called kinks, which are generated at points of intersection. Kinks on infinite strings are known to generate a gravitational wave background over a wide range of frequencies. In this paper, we calculate the spectrum of the gravitational wave background by numerically solving the evolution equation for the distribution function of the kink sharpness. We find that the number of kinks for small sharpness is larger than the analytical estimate used in a previous work, which makes a difference in the spectral shape. Our numerical approach enables us to make a more precise prediction on the spectral amplitude for future gravitational wave experiments.

Radio jets and galaxies as cosmic string probes

The lensing effect of a cosmic string is studied, and some new methods are proposed to detect the cosmic string. The technique for using jets as extended gravitational lensing probes was firstly explored by Kronberg. We use the "alignment-breaking parameter" $ \eta_G $ as a sensitive indicator of gravitational distortion by a wiggly cosmic string. Then, we applied the non-constant deflection angle to jets, and $ \eta_G $ of a specific jet is just related to the projected slope of the jet. At least three jets in the sample of Square Kilometer Array (SKA) would have significant signals ($ \eta_G >10^\circ $) if the wiggly infinite cosmic string existed. The distortion of elliptical object is also studied and used to do a statistical research on directions of axes and ellipticities of galaxies. In the direction of the string, we find that galaxies appear to be more elliptical for an observer and the distribution of apparent ellipticity changes correspondingly. Ellipticity distribution of current SDSS spiral sample has the signal-to-noise ratio up to 8.48 which is large enough for astronomical observations. The future survey, such as Large Synoptic Survey Telescope (LSST) and Dark Energy Survey (DES), would weaken the requirement of special geometry in the data processing. As a result, all kinds of distributions, including ellipticity axis distribution, would serve as probes to detect wiggly strings in the near future. In brief, if a wiggly cosmic string existed, these signals would be convenient to be observed with the future weak lensing survey or other surveys in deep space. If there was no lensing signal in these distributions, it would give the upper limit of the abundance of infinite strings.

B-mode polarization induced by gravitational waves from kinks on infinite cosmic strings

We investigate the effect of the stochastic gravitational wave (GW) background produced by kinks on infinite cosmic strings, whose spectrum was derived in our previous work, on the B-mode power spectrum of the cosmic microwave background (CMB) anisotropy. We find that the B-mode polarization due to kinks is comparable to that induced by the motion of the string network and hence the contribution of GWs from kinks is important for estimating the B-mode power spectrum originating from cosmic strings. If the tension of cosmic strings \mu is large enough i.e., G\mu >~ 10^{-8}, B-mode polarization induced by cosmic strings can be detected by future CMB experiments.

Gravitational waves from kinks on infinite cosmic strings

Gravitational waves emitted by kinks on infinite strings are investigated using detailed estimations of the kink distribution on infinite strings. We find that gravitational waves from kinks can be detected by future pulsar timing experiments such as SKA for an appropriate value of the string tension, if the typical size of string loops is much smaller than the horizon at their formation. Moreover, the gravitational wave spectrum depends on the thermal history of the Universe and hence it can be used as a probe into the early evolution of the Universe.

Electrodynamic Casimir effect in a medium-filled wedge

We re-examine the electrodynamic Casimir effect in a wedge defined by two perfect conductors making dihedral angle alpha=pi/p. This system is analogous to the system defined by a cosmic string. We consider the wedge region as filled with an azimuthally symmetric material, with permittivity and permeability epsilon1, micro1 for distance from the axis r<a, and epsilon2, micro2 for r>a. The results are closely related to those for a circular-cylindrical geometry, but with noninteger azimuthal quantum number mp. Apart from a zero-mode divergence, which may be removed by choosing periodic boundary conditions on the wedge, and may be made finite if dispersion is included, we obtain finite results for the free energy corresponding to changes in a for the case when the speed of light is the same inside and outside the radius a , and for weak coupling, |epsilon1-epsilon2|<<1, for purely dielectric media. We also consider the radiation produced by the sudden appearance of an infinite cosmic string, situated along the cusp line of the pre-existing wedge.

Gravitational radiation and the small-scale structure of cosmic strings

We calculate the gravitational radiation emitted by an infinite cosmic string with two oppositely moving wave-trains, in the small amplitude approximation. After comparing our result to the previously studied cases we extend the results to a new regime where the wavelengths of the opposing wave-trains are very different. We show that in this case the amount of power radiated vanishes exponentially. This means that small excitations moving in only one direction may be very long lived, and so the size of the smallest scales in a string network might be much smaller than what one would expect from gravitational back reaction. This result allows for a potential host of interesting cosmological possibilities involving ultra-high energy cosmic rays, gamma ray bursts and gravitational wave bursts.

AN EXACT MODEL OF AN INFINITE COSMIC STRING

We present here an exact model of an infinite cosmic string. The linear mass density of the source is finite and the solution is regular everywhere. All the arbitrary constants of the solutions are expressed in terms of the known constants of the U(1) gauge theory.

Bremsstrahlung and Pair Creation by a Photon in the Spacetime of a Cosmic String

We consider two tree-graph processes in the spacetime of an infinite, straight and static cosmic string, namely Bremsstrahlung and electron–positron pair production. Results based on a simplified model with scalar fields which were obtained by other authors are extended to the more realistic QED case. In empty Minkowski spacetime, energy and momentum conservation preclude these processes. In spite of the local flatness of the metric, it is a topological defect that gives rise to non-conservation of momentum in the plane perpendicular to the string. Analytic expressions for the total transition probabilities and, in the case of Bremsstrahlung, the total radiation energy are found and compared with the results for the scalar field model. In a sense, the cylindrical mode solutions of the Dirac equation allow a spatial localisation of the decay processes relative to the string axis.

Wormhole at the core of an infinite cosmic string

We study a solution of Einstein's equations that describes a straight cosmic string with a variable angular deficit, starting with a $2 \pi$ deficit at the core. We show that the coordinate singularity associated to this defect can be interpreted as a traversible wormhole lodging at the the core of the string. A negative energy density gradually decreases the angular deficit as the distance from the core increases, ending, at radial infinity, in a Minkowski spacetime. The negative energy density can be confined to a small transversal section of the string by gluing to it an exterior Gott's like solution, that freezes the angular deficit existing at the matching border. The equation of state of the string is such that any massive particle may stay at rest anywhere in this spacetime. In this sense this is 2+1 spacetime solution.

Average string dynamics.

The dynamics of an infinite cosmic string chopping off loops is modeled by passing over to a smearedout effective string which is obtained by a local average procedure. Using several approximations, the time-evolution equations for the effective two-point correlation functions are brought into a mathematically simple form in which all essential nonlinearities and nonlocalities have been absorbed into one single quantity, the loop production activity $K$. The evolution scheme may serve as a test for possible estimations of $K$. A toy model is presented and solved numerically.

Physical obstacles to time-travel

The author critically examines the properties of geometries generated by parallel infinite cosmic strings, or equivalently point particles in 2+1 gravity, that possess closed time-like curves (CTC). It is shown that CTC are physically excluded: their existence requires that the centre of mass of the sources be tachyonic, which entails unacceptable boundary conditions, in particular presence of CTC at spatial infinity. These conclusions also hold if one or both of the sources are lightlike.

Vacuum fluctuations of twisted fields in the spacetime of a cosmic string-thermal effects

In the case of quantum field theory at finite temperature the author investigates the thermal Euclidean Green function for massive twisted scalar fields in the spacetime of an infinite straight cosmic string. Then taking the limit, when mass goes to zero, the author obtains the integral form of the mean-square field as a function of temperature.

Stringy cosmic strings and axion cohomology.

The static stationary axially symmetric background ("infinite cosmic string") of the $D=4$ string theory provided with an axion charge is studied in the effective theory approach. The most general exact solution is constructed. It is found that the Kalb-Ramond axion charge, present in the string topology $R^{3} \times S^{1}$, produces nontrivial gravitational field configurations which feature horizons. The corresponding ``no-hair'' theorems are proved which stress uniqueness of black strings. Connection of the solutions with the gauged WZWN sigma model constructions on the world sheet is discussed since they are the only target spaces which hide their singularities behind horizons, and thus obey the cosmic censorship conjecture.

From wormhole to time machine: Remarks on Hawking's chronology protection conjecture.

The recent interest in ``time machines'' has been largely fueled by the apparent ease with which such systems may be formed in general relativity, given relatively benign initial conditions such as the existence of traversable wormholes or of infinite cosmic strings. This rather disturbing state of affairs has led Hawking to formulate his Chronology Protection Conjecture, whereby the formation of ``time machines'' is forbidden. This paper will use several simple examples to argue that the universe appears to exhibit a ``defense in depth'' strategy in this regard. For appropriate parameter regimes Casimir effects, wormhole disruption effects, and gravitational back reaction effects all contribute to the fight against time travel. Particular attention is paid to the role of the quantum gravity cutoff. For the class of model problems considered it is shown that the gravitational back reaction becomes large before the Planck scale quantum gravity cutoff is reached, thus supporting Hawking's conjecture.

Cosmic string evolution by exact methods (II). Gaussian model

A recently proposed framework to study loop production from a single infinite cosmic string in flat space-time by path-integral methods is worked out in detail by imposing a gaussian ansatz for the random string distribution. The constraints required by the general framework are satisfied approximately. The equations of motion for the two-point correlation functions are postulated to define the time evolution of the whole system. By an appropriate normalization, an improved version of the model is achieved. It is expected to mimic the essential features of the realistic case.