Cosmic String Loops Research Articles

Spectral energy distribution and generalized Wien's law for photons and cosmic string loops

Physical objects with energy , with l a characteristic length and w a dimensionless constant, lead to an equation of state , with p the pressure and ρ the energy density. Special entities with this property are, for instance, photons (, with l the wavelength) with , and some models of cosmic string loops (, with l the length of the loop and a a numerical constant), with . Here, we discuss some features of the spectral energy distribution of these systems and the corresponding generalization of Wien's law, which in terms of l has the form , being the most probable size of the mentioned entities.

The Stochastic Gravitational Wave Background Generated by Cosmic String Networks

AbstractCosmic string interactions often result in the formation of cosmic string loops that detach from the long string network and radiate their energy in the form of gravitational waves. Loop production occurs copiously throughout the cosmological evolution of a cosmic string network and the superimposition of their emissions gives rise to a stochastic gravitational In this essay, we briefly review our recent work on the stochastic gravitational wave background generated by cosmic string networks and introduce a set of numerical and analytical tools for the computation of this background.

Number of cosmic string loops

Using recent simulation results, we provide the mass and speed spectrum of cosmic string loops. This is the quantity of primary interest for many phenomenological signatures of cosmic strings, and it can be accurately predicted using recently acquired detailed knowledge of the loop production function. We emphasize that gravitational smoothing of long strings does not play any role in determining the total number of existing loops. We derive a bound on the string tension imposed by recent constraints on the stochastic gravitational wave background from pulsar timing arrays, finding $G\mu \leq 2.8\times 10^{-9}$. We also provide a derivation of the Boltzmann equation for cosmic string loops in the language of differential forms.

21 cm angular spectrum of cosmic string loops

The 21 cm signatures induced by moving cosmic string loops are investigated. Moving cosmic string loops seed filamentary nonlinear objects. We analytically evaluate the differential 21 cm brightness temperature from these objects. We show that the brightness temperature reaches 200 mK for a loop whose tension is about the current upper limit, $G\mu\sim10^{-7}$. We also calculate the angular power spectrum, assuming scaling in loop distribution. We find that the angular power spectrum for $G\mu>10^{-8}$ at $z=30$ or $G\mu>10^{-10}$ at $z=20$ can dominate the spectrum of the primordial density fluctuations. Finally we show that a future SKA-like observation has the potential to detect the power spectrum due to loops with $G\mu=10^{-8}$ at $z=20$.

A Boltzmann treatment for the vorton excess problem

We derive and solve a Boltzmann equation governing the cosmological evolution of the number density of current carrying cosmic string loops, whose centrifugally supported equilibrium configurations are also referred to as vortons. The phase space is three-dimensional and consists of the time variable, the loop size, and a conserved quantum number. Our approach includes gravitational wave emission, a possibly finite lifetime for the vortons and works with any initial loop distribution and for any loop production function. We then show how our results generalize previous approaches on the vorton excess problem by tracking down the time evolution of the various sub-populations of current-carrying loops in a string network.

Gravitational scattering of photons off cosmic strings

Photons can gravitationally scatter off a cosmic string loop and gain or lose energy. We consider the spectral distortion induced by cosmic string loops placed in an ambient thermal bath of photons. The fractional deviation from a thermal spectrum caused by cosmic strings is estimated to scale as (G_N \mu)^2 z^2, where G_N is Newton's constant, $\mu$ is the string tension, and z is the cosmological redshift after which spectral distortions can survive. This effect is large enough to potentially be of observational interest.

Bosonic (meta)stabilization of cosmic string loops

We consider the possibility of a bosonic (meta)stabilization of a cosmic gauge string loop due to the presence of a gas of low-mass bosonic particles which become trapped within the string core. This boson gas exerts a pressure which tends to counteract the string tension, allowing a circular string loop to find a finite equilibrium radius, provided that gas particles do not escape the string core. However, high-energy bosons do escape, and, consequently, the loop shrinks, and the temperature rises. Estimates indicate that the bosonic stabilization mechanism is ineffective, and the loop is unstable against decay.

Cosmic strings as emitters of extremely high energy neutrinos

We study massive particle radiation from cosmic string kinks, and its observability in extremely high energy neutrinos. In particular, we consider the emission of moduli---weakly coupled scalar particles predicted in supersymmetric theories---from the kinks of cosmic string loops. Since kinks move at the speed of light on strings, moduli are emitted with large Lorentz factors, and eventually decay into many pions and neutrinos via hadronic cascades. The produced neutrino flux has energy $E\ensuremath{\gtrsim}{10}^{11}\text{ }\text{ }\mathrm{GeV}$, and is affected by oscillations and absorption (resonant and nonresonant). It is observable at upcoming neutrino telescopes such as JEM-EUSO, and the radio telescopes LOFAR and SKA, for a range of values of the string tension, and of the mass and coupling constant of the moduli.

Anisotropies in the gravitational-wave stochastic background

We consider anisotropies in the stochastic background of gravitational-waves (SBGW) arising from random fluctuations in the number of gravitational-wave sources. We first develop the general formalism which can be applied to different cosmological or astrophysical scenarios. We then apply this formalism to calculate the anisotropies of SBGW associated with the fluctuations in the number of cosmic string loops, considering both cosmic string cusps and kinks. We calculate the anisotropies as a function of angle and frequency.

Constraints on superconducting cosmic strings from early reionization

Electromagnetic radiation from superconducting cosmic string loops can reionize neutral hydrogen in the universe at very early epochs, and affect the cosmic microwave background (CMB) temperature and polarization correlation functions at large angular scales. We constrain the string tension and current using WMAP7 data, and compare with earlier constraints that employed CMB spectral distortions. Over a wide range of string tensions, the current on the string has to be less than 10^7 GeV.

Constraints on cosmic string tension imposed by the limit on the stochastic gravitational wave background from the European Pulsar Timing Array

We investigate the constraints that can be placed on the cosmic string tension by using the current Pulsar Timing Array limits on the stochastic gravitational wave background (SGWB). We have developed a code to compute the spectrum of gravitational waves (GWs) based on the widely accepted one-scale model. In its simplest form the one-scale model allows one to vary: (i) the string tension, G\mu/c^2; (ii) the size of cosmic string loops relative to the horizon at birth, \alpha; (iii) the spectral index of the emission spectrum, q; (iv) the cut-off in the emission spectrum, n_*; and (v) the intercommutation probability, p. The amplitude and slope of the spectrum in the nHz frequency range is very sensitive to these unknown parameters. We have also investigated the impact of more complicated scenarios with multiple initial loop sizes, in particular the 2-\alpha models proposed in the literature and a log-normal distribution for \alpha. We have computed the constraint on G\mu/c^2 due to the limit on a SGWB imposed by data from the European Pulsar Timing Array. Taking into account all the possible uncertainties in the parameters we find a conservative upper limit of G\mu/c^2<5.3x 10^{-7} which typically occurs when the loop production scale is close to the gravitational backreaction scale, \alpha\approx\Gamma G\mu/c^2. Stronger limits are possible for specific values of the parameters which typically correspond to the extremal cases \alpha\ll \Gamma G\mu/c^2 and \alpha\gg \Gamma G\mu/c^2. This limit is less stringent than the previously published limits which are based on cusp emission, an approach which does not necessarily model all the possible uncertainties. We discuss the prospects for lowering this limit by two orders of magnitude, or even a detection of the SGWB, in the very near future in the context of the Large European Array for Pulsars and the Square Kilometre Array.

The evolution of circular loops of a cosmic string with periodic tension

In this Letter the equation of circular loops of cosmic string with periodic tension is investigated in the Minkowski spacetime and Robertson–Walker universe respectively. We find that the cosmic string loops possessing this kind of time-varying tension will evolve to oscillate instead of collapsing to form a black hole if their initial radii are not small enough.

Early structure formation from cosmic string loops

We examine the effects of cosmic strings on structure formation and on the ionization history of the universe. While Gaussian perturbations from inflation are known to provide the dominant contribution to the large scale structure of the universe, density perturbations due to strings are highly non-Gaussian and can produce nonlinear structures at very early times. This could lead to early star formation and reionization of the universe. We improve on earlier studies of these effects by accounting for high loop velocities and for the filamentary shape of the resulting halos. We find that for string energy scales Gμ≳10−7, the effect of strings on the CMB temperature and polarization power spectra can be significant and is likely to be detectable by the Planck satellite. We mention shortcomings of the standard cosmological model of galaxy formation which may be remedied with the addition of cosmic strings, and comment on other possible observational implications of early structure formation by strings.

The 21 cm signature of a cosmic string loop

Cosmic string loops lead to nonlinear baryon overdensities at early times, even before the timewhich in the standard LCDM model corresponds to the time of reionization. These overdensestructures lead to signals in 21 cm redshift surveys at large redshifts. In this paper, we calculate the amplitude and shape of the string loop-induced 21 cm brightness temperature.We find that a string loop leads to a roughly elliptical region in redshift space withextra 21 cm emission. The excess brightness temperature for strings with a tension close to the current upper bound can be as high as 1°Kfor string loops generated at early cosmological times (times comparable to the timeof equal matter and radiation) and observed at a redshift of z+1 = 30.The angular extent of these predicted “bright spots” is x′. These signalsshould be detectable in upcoming high redshift 21 cm surveys. We also discussthe application of our results to global monopoles and primordial black holes.

Layzer-Irvine equation: New perspectives and the role of interacting dark energy

We derive the Layzer-Irvine equation in the presence of a homogeneous (or quasi-homogeneous) dark energy component with an arbitrary equation of state. We extend the Layzer-Irvine equation to homogeneous and isotropic universes with an arbitrary number of dimensions and obtain the corresponding virial relation for sufficiently relaxed objects. We find analogous equations describing the dynamics of cosmic string loops and other p-branes of arbitrary dimensionality, discussing the corresponding relativistic and non-relativistic limits. Finally, we generalize the Layzer-Irvine equation to account for a non-minimal interaction between dark matter and dark energy, discussing its practical use as a signature of such an interaction.

Formation of superdense dark matter lumps at the radiation-dominated cosmological stage

We study the process of superdense dark matter clump (DMC) formation at the radiation-dominated stage of the Universe. High-magnitude density perturbations that lead to DMC formation can be peaks in the inflationary perturbation spectrum or closed loops of cosmic strings. A model with superdense DMCs provides new opportunities for annihilation of particles having small annihilation cross-sections. Superdense DMCs can be detected by their tidal effect on gravitational wave detectors.

Dense DM clumps seeded by cosmic string loops and DM annihilation

We develop a model of production of the very dense clumps of DM in RD epochdue to the accretion of DM on the loops of cosmic strings as the seeds.At some time the loops disappear, for example due to the gravitationalradiation, and the remaining dense clumps produce the enhancement of the annihilation signal. We take into account the velocity distributionof the strings, and consider the two extreme regimes of DM annihilation: fastdecay and continuous evaporation. The produced annihilationflux of gamma radiation is detectable, and for some parameters of DMparticles and the strings can exceed the extragalactic flux of the gamma-radiation observed by Fermi. For the fixed parameters of DMparticles (e.g. neutralino with fixed masses and cross-section of annihilation)one can obtain the limits on the basic string parameter, tension μ,which is stronger than (more general) limits obtained from WMAPobservations, cosmological nucleosynthesis and gravitational lensing. Inparticular for the neutralino with 100 GeV mass we exclude theinterval 5 × 10−10 < Gμ/c2 < 5.1 × 10−9.

Gravitational power from cosmic string loops with many kinks

We investigate the effect of a large number of kinks on the gravitational power radiated by cosmic string loops. We show that the total power radiated by a loop with N left-moving and right-moving kinks is proportional to N and increases with the typical kink angle. We then apply these results to loops containing junctions which give rise to a proliferation of the number of sharp kinks. We show that the time of gravitational decay of these loops is smaller than previously assumed. In light of this we revisit the gravitational wave burst predictions from a network containing such loops. We find there is no parameter regime in which the rate of individual kink bursts is enhanced with respect to standard networks. By contrast, there remains a region of parameter space for which the kink-kink bursts dominate the stochastic background. Finally, we discuss the order of magnitude of the typical number of sharp kinks resulting from kink proliferation on loops with junctions.

Effects of anisotropic dynamics on cosmic strings

The dynamics of cosmic strings is considered in anisotropicbackgrounds. In particular, the behaviour of infinitely long straightcosmic strings and of cosmic string loops is determined. Smallperturbations of a straight cosmic string are calculated. Therelevance of these results is discussed with respect to the possibleobservational imprints of an anisotropic phase on the behaviour of acosmic string network.

Cosmic string in the BTZ black hole background with time-dependent tension

In this Letter, we study the equation of circular loops with time-dependent tension in the BTZ black hole background. We obtain various cases where cosmic string loops finally collapse to form black holes. Also, we study effect of the BTZ black hole mass and angular momentum on the evolution of cosmic string loops. We find the critical values of initial radii as a limit for the cosmic string loops collapsing to form black holes.