Intercommuting Probability Research Articles

Dark baryon from pure Yang-Mills theory and its GW signature from cosmic strings

We point out that SO(2N) pure Yang-Mills theory provides a candidate for dark matter (DM) without the explicit need to impose any additional symmetry. The DM candidate is a particular type of glueball, which we refer to as a baryonic glueball, that is naturally stable and produced by a novel production mechanism for a moderately large N. In this case, the intercommutation probability of cosmic strings (or macroscopic color flux tubes) is quite low, which offers characteristic gravitational wave signals to test our model. In particular, our model can simultaneously account for both abundance of DM and the recently reported gravitational wave signals detected in pulsar timing array experiments, including NANOGrav.

Probing the gravitational wave background from cosmic strings with LISA

Cosmic string networks offer one of the best prospects for detection of cosmological gravitational waves (GWs). The combined incoherent GW emission of a large number of string loops leads to a stochastic GW background (SGWB), which encodes the properties of the string network. In this paper we analyze the ability of the Laser Interferometer Space Antenna (LISA) to measure this background, considering leading models of the string networks. We find that LISA will be able to probe cosmic strings with tensions Gμ ≳ \U0001d4aa(10−17), improving by about 6 orders of magnitude current pulsar timing arrays (PTA) constraints, and potentially 3 orders of magnitude with respect to expected constraints from next generation PTA observatories. We include in our analysis possible modifications of the SGWB spectrum due to different hypotheses regarding cosmic history and the underlying physics of the string network. These include possible modifications in the SGWB spectrum due to changes in the number of relativistic degrees of freedom in the early Universe, the presence of a non-standard equation of state before the onset of radiation domination, or changes to the network dynamics due to a string inter-commutation probability less than unity. In the event of a detection, LISA's frequency band is well-positioned to probe such cosmic events. Our results constitute a thorough exploration of the cosmic string science that will be accessible to LISA.

Constraints on cosmic strings using data from the first Advanced LIGO observing run

Cosmic strings are topological defects which can be formed in GUT-scale phase transitions in the early universe. They are also predicted to form in the context of string theory. The main mechanism for a network of Nambu-Goto cosmic strings to lose energy is through the production of loops and the subsequent emission of gravitational waves, thus offering an experimental signature for the existence of cosmic strings. Here we report on the analysis conducted to specifically search for gravitational-wave bursts from cosmic string loops in the data of Advanced LIGO 2015-2016 observing run (O1). No evidence of such signals was found in the data, and as a result we set upper limits on the cosmic string parameters for three recent loop distribution models. In this paper, we initially derive constraints on the string tension $G\mu$ and the intercommutation probability, using not only the burst analysis performed on the O1 data set, but also results from the previously published LIGO stochastic O1 analysis, pulsar timing arrays, cosmic microwave background and Big-Bang nucleosynthesis experiments. We show that these data sets are complementary in that they probe gravitational waves produced by cosmic string loops during very different epochs. Finally, we show that the data sets exclude large parts of the parameter space of the three loop distribution models we consider.

Probing cosmic superstrings with gravitational waves

We compute the stochastic gravitational wave background generated by cosmic superstrings using a semi-analytical velocity-dependent model to describe their dynamics. We show that heavier string types may leave distinctive signatures on the stochastic gravitational wave background spectrum within the reach of present and upcoming gravitational wave detectors. We examine the physically motivated scenario in which the physical size of loops is determined by the gravitational backreaction scale and use NANOGRAV data to derive a conservative constraint of $G\mu_F<3.2 \times 10^{-9}$ on the tension of fundamental strings. We demonstrate that approximating the gravitational wave spectrum generated by cosmic superstring networks using the spectrum generated by ordinary cosmic strings with reduced intercommuting probability (which is often done in the literature) leads, in general, to weaker observational constraints on $G\mu_F$. We show that the inclusion of heavier string types is required for a more accurate characterization of the region of the $(g_s,G\mu_F)$ parameter space that may be probed using direct gravitational wave detectors. In particular, we consider the observational constraints that result from NANOGRAV data and show that heavier strings generate a secondary exclusion region of parameter space.

CMB ISW-lensing bispectrum from cosmic strings

We study the effect of weak lensing by cosmic (super-)strings on the higher-order statistics of the cosmic microwave background (CMB). A cosmic string segment is expected to cause weak lensing as well as an integrated Sachs-Wolfe (ISW) effect, the so-called Gott-Kaiser-Stebbins (GKS) effect, to the CMB temperature fluctuation, which are thus naturally cross-correlated. We point out that, in the presence of such a correlation, yet another kind of the post-recombination CMB temperature bispectra, the ISW-lensing bispectra, will arise in the form of products of the auto- and cross-power spectra. We first present an analytic method to calculate the autocorrelation of the temperature fluctuations induced by the strings, and the cross-correlation between the temperature fluctuation and the lensing potential both due to the string network. In our formulation, the evolution of the string network is assumed to be characterized by the simple analytic model, the velocity-dependent one scale model, and the intercommutation probability is properly incorporated in order to characterize the possible superstringy nature. Furthermore, the obtained power spectra are dominated by the Poisson-distributed string segments, whose correlations are assumed to satisfy the simple relations. We then estimate the signal-to-noise ratios of the string-induced ISW-lensing bispectra and discuss the detectability of such CMB signals from the cosmic string network. It is found that in the case of the smaller string tension, Gμ << 10-7, the ISW-lensing bispectrum induced by a cosmic string network can constrain the string-model parameters even more tightly than the purely GKS-induced bispectrum in the ongoing and future CMB observations on small scales.

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.

Weak lensing of CMB by cosmic (super-)strings

We study the effect of weak lensing by cosmic (super-)strings on the anisotropies of cosmic microwave background (CMB). In developing a method to calculate the lensing convergence field due to strings, and thereby temperature and polarization angular power spectra of CMB, we clarify how the nature of strings, characterized by the intercommuting probability, can influence these CMB anisotropies. Assuming that the power spectrum is dominated by Poisson-distributed string segments, we find that the convergence spectrum peaks at low multipoles and is mostly contributed from strings located at relatively low redshifts. As the intercommuting probability decreases, the spectra of the convergence and hence the lensed temperature and polarizations are gained because the number density of string segments becomes larger. An observationally important feature of the string-induced CMB polarizations is that the string-lensed spectra decay more slowly on small scales compared with primordial scalar perturbations from standard inflation.

Analytical model for CMB temperature angular power spectrum from cosmic (super-)strings

We present a new analytical method to calculate the small angle CMB temperature angular power spectrum due to cosmic (super-)string segments. In particular, using our method, we clarify the dependence on the intercommuting probability $P$. We find that the power spectrum is dominated by Poisson-distributed string segments. The power spectrum for a general value of $P$ has a plateau on large angular scales and shows a power-law decrease on small angular scales. The resulting spectrum in the case of conventional cosmic strings is in very good agreement with the numerical result obtained by Fraisse et al.. Then we estimate the upper bound on the dimensionless tension of the string for various values of $P$ by assuming that the fraction of the CMB power spectrum due to cosmic (super-)strings is less than ten percents at various angular scales up to $\ell=2000$. We find that the amplitude of the spectrum increases as the intercommuting probability. As a consequence, strings with smaller intercommuting probabilities are found to be more tightly constrained.

Skewness in CMB temperature fluctuations from curved cosmic (super-)strings

We compute the one-point probability distribution function of small-angle cosmic microwave background temperature fluctuations due to curved cosmic (super-)strings with a simple model of string network by performing Monte Carlo simulations. Taking into account of the correlation between the curvature and the velocity of string segments, there appear non-Gaussian features, specifically non-Gaussian tails and a skewness, in the one-point pdf. The obtained sample skewness for the conventional field-theoretic cosmic strings is g1 ≈ −0.14, which is consistent with the result reported by Fraisse et al. We also discuss the dependence of the pdf on the intercommuting probability. We find that the standard deviation of the Gaussian part increases and non-Gaussian features are suppressed as the intercommuting probability decreases. For sufficiently small intercommuting probability, the skewness is given by ≲ (a few) × 10−2.

Non-Gaussianity in the Cosmic Microwave Backgroundtemperature fluctuations from cosmic (super-)strings

We compute analytically the small-scale temperature fluctuationsof the cosmic microwave background from cosmic (super-)stringsand study the dependence on the string intercommuting probability P.We develop an analytical model which describes the evolutionof a string network and calculate the numbers ofstring segments and kinks in a horizon volume.Then we derive the probability distribution function (pdf)which takes account of finite angular resolution of observation. The resultant pdf consists of a Gaussian part due to frequent scatteringsby long string segments and a non-Gaussian tail due to closeencounters with kinks. The dispersion of the Gaussian part isreasonably consistent with that obtained by numerical simulationsby Fraisse et al.. On the other hand, the non-Gaussian tail containstwo phenomenological parameters which are determined by comparisonwith the numerical results for P = 1. Extrapolating the pdfto the cases with P < 1, we predict that the non-Gaussianfeature is suppressed for small P.

Cosmic string loops: Large and small, but not tiny

We develop an analytical model to study the production spectrum of loops in the cosmic string network. In the scaling regime, we find two different scales corresponding to large (one order below horizon) and small (few orders below horizon) loops. The very small (tiny) loops at the gravitational backreaction scale are absent, and thus, our model has no ultraviolet divergences. We calculate the spectrum of loops and derive analytical expressions for the positions and magnitudes of the small and large scale peaks. The small loops are produced by large bursts of similar loops moving with very high velocities in the same direction. We describe the shape of the large loops, which would usually consist of few kinks and few cusps per oscillation cycle. We also argue that the typical size of large loops is set by the correlation length, which does not depend on the intercommutation probability $p$, while the interstring distance scales as ${p}^{1/3}$.

Cosmic D-strings and vortons in supergravity

Recent developments in string inspired models of inflation suggest that D-strings are formed at the end of inflation. Within the supergravity model of D-strings there are 2 ( n − 1 ) chiral fermion zero modes for a D-string of winding n . Using the bounds on the relic vorton density, we show that D-strings with winding number n > 1 are more strongly constrained than cosmic strings arising in cosmological phase transitions. The D-string tension of such vortons, if they survive until the present, has to satisfy 8 π G N μ ≲ p × 10 −26 where p is the intercommutation probability. Similarly, D-strings coupled with spectator fermions carry currents and also need to respect the above bound. D-strings with n = 1 do not carry currents and evade the bound. We discuss the coupling of D-strings to supersymmetry breaking. When a single U ( 1 ) gauge group is present, we show that there is an incompatibility between spontaneous supersymmetry breaking and cosmic D-strings. We propose an alternative mechanism for supersymmetry breaking, which includes an additional U ( 1 ) , and might alleviate the problem. We conjecture what effect this would have on the fermion zero modes.

Effect of reconnection probability on cosmic (super)string network density

We perform numerical simulations of cosmic string evolution with intercommuting probability $P$ in the range $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ensuremath{\le}P\ensuremath{\le}1$, both in the matter and radiation eras. We find that the dependence of the scaling density on $P$ is significantly different than the suggested $\ensuremath{\rho}\ensuremath{\propto}{P}^{\ensuremath{-}1}$ form. In particular, for probabilities greater than $P\ensuremath{\simeq}0.1$, $\ensuremath{\rho}(1/P)$ is approximately flat, but for $P$ less than this value it is well-fitted by a power-law with exponent ${0.6}_{\ensuremath{-}0.12}^{+0.15}$. This shows that the enhancement of string densities due to a small intercommuting probability is much less prominent than initially anticipated. We interpret the flat part of $\ensuremath{\rho}(1/P)$ in terms of multiple opportunities for string reconnections during one crossing time, due to small-scale wiggles. We also propose a two-scale model, which satisfactorily fits our results over the whole range of $P$ covered by the simulations.

Observational constraints on cosmic string production during brane inflation

Overall, brane inflation is compatible with the recent analysis of the Wilkinson microwave anisotropy probe (WMAP) data. Here we explore the constraints of WMAP and the 2dF Galaxy Redshift Survey (2dFGRS) data on the various brane inflationary scenarios. Brane inflation naturally ends with the production of cosmic strings, which may provide a way to distinguish these models observationally. We argue that currently available data cannot exclude a non-negligible contribution from cosmic strings definitively. We perform a partial statistical analysis of mixed models that include a subdominant contribution from cosmic strings. Although the data favor models without cosmic strings, we conclude that they cannot definitively rule out a cosmic-string-induced contribution of $\ensuremath{\sim}10%$ to the observed temperature, polarization and galaxy density fluctuations. These results imply that $G\ensuremath{\mu}\ensuremath{\lesssim}1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\sqrt{B\ensuremath{\lambda}/0.1},$ where $\ensuremath{\lambda}l~1$ is a measure of the intercommutation probability of the cosmic string networks and B measures the importance of perturbations induced by cosmic strings. We argue that, conservatively, the data available currently still permit $B\ensuremath{\lesssim}0.1.$ Precision measurements sensitive to the B-mode polarization produced by vector density perturbation modes driven by the string network could provide evidence for these models. Accurate determinations of ${n}_{s}(k),$ the scalar fluctuation index, could also distinguish among various brane inflation models.

Cosmic string interactions

The cosmic string scenario has attracted much interest recently because of the alluring model of galaxy formation it produces. The viability of the scenario, however, rest on the untested assumption that colliding strings will intercommute or “exchange partners”. This paper examines the detailed interaction properties of global U(1)-strings, both numerically and analytically, to determine the validity of this assumption. Numerical simulations demonstrated that these strings intercommute under most circumstances. A threshold collision energy was observed, above which the strings simply passed through one another ( υ ≈ 0.9 c in the centre of mass frame). Generically, corners of “kinds” formed and persisted after intercommuting. Such kinks could constitute a major energy loss mechanism. Neither this behaviour nor the threshold phenomenon, however, altered the central conclusion that the intercommuting probability is of order unity. It is argued that this conclusion will also apply to other varieties of strings. These results provide a much firmer foundation for the cosmic string scenario.