String Wakes Research Articles

Evolution of magnetic fields in cosmic string wakes

We study the evolution of magnetic fields in cosmic string wakes in a plasma with a low resistivity. The initial magnetic field in the wake is modelled on the magnetic fields that are generated by the motion of particles around cosmic strings. The plasma is characterized by a high beta value. We find multiple shock like structures developing in the wake of the string. We study the detailed structure of the shocks formed and the evolution of the magnetic field in the shock using a 2-D magnetohydrodynamic simulation. As expected, the development of the magnetic field does not depend on the β value. Our results show that instead of a single uniform shock forming behind the cosmic string we have multiple shocks forming at short time intervals behind the string. The presence of multiple shocks will definitely affect the observational signatures of cosmic string wakes as these signatures depend upon the temperature fluctuations generated by the shock. We also find that as the shock moves away, the residual magnetic field left behind reconnects and dissipates rapidly. The magnetic field around the string is thus very localized. We find that magnetic field reconnections take place in cosmic string wakes. This leads to the decrease of the magnetic field in the post shock region.

Extracting the signal of cosmic string wakes from 21-cm observations

A cosmic string wake produces a distinct non-Gaussian signal in 21-cm intensity maps at redshifts above that of reionization. While the string signal is (locally) larger in amplitude than the signal of the Gaussian fluctuations of the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model, they are overwhelmed (even locally in position space) by astrophysical and instrumental foregrounds. Here, we study to what extent the signal can be extracted from noisy interferometric data. The narrowness of the string-induced feature in the redshift direction allows for a subtraction of astrophysical and instrumental foregrounds. Based on the specific geometry of the string signal, we identify a particular three-point statistic that is promising in order to extract the signal, and we find that, having in mind a telescope with specifications similar to that of the MWA instrument, the string signal can be successfully extracted for a value of the string tension of $G\ensuremath{\mu}=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$. Prospects for further improvements of the analysis are discussed.

The global 21-cm signal of a network of cosmic string wakes

ABSTRACT In previous works, we discussed the 21-cm signature of a single cosmic string wake. However, the 21-cm brightness temperature is influenced by a network of cosmic string wakes, and not one single wake. In this work, we consider the signal from a network of wakes laid down during the matter era. We also improve on the previous calculation of a single wake signature. Finally, we calculate the enhancement of the global 21-cm brightness temperature due to a network of wakes and discuss its affects of the signal measured in the Wouthuysen–Field absorption trough. We estimated that for string tensions between 10−8 and 10−7 there would be between a 10 per cent and a factor of 2 enhancement in the signal.

Cosmic filaments from cosmic strings

Cosmic strings are generically predicted in many extensions of the Standard Model of particle physics. We propose a new avenue for detecting cosmic strings through their effect on the filamentary structure in the cosmic web. Using cosmological simulations of the density wake from a cosmic string, we examine a variety of filament structure probes. We show that the largest effect of the cosmic string is an overdensity in the filament distribution around the string wake. The signal from the overdensity is stronger at higher redshift, and more robust with a wider field. We analyze the spatial distribution of filaments from a publicly available catalog of filaments built from SDSS galaxies. With existing data, we find no evidence for the presence of a cosmic string wake with string tension parameter $G\mu$ above $5\times 10^{-6}$. However, we project WFIRST will be able to detect a signal from such a wake at the $99\%$ confidence level at redshift $z=2$, with significantly higher confidence and the possibility of probing lower tensions ($G\mu \sim 10^{-6}$), at $z=10$. The sensitivity of this method is not competitive with constraints derived from the CMB. However, it provides an independent discovery channel at low redshift, which could be a smoking-gun in scenarios where the CMB bound can be weakened.

Signature of a cosmic string wake at z=3

In this paper, we describe the results of N-body simulation runs, which include a cosmic string wake of tension Gμ= 4 × 10−8 on top of the usual Λ CDM fluctuations. To obtain a higher resolution of the wake in the simulations compared to previous work, we insert the effects of the string wake at a lower redshift and perform the simulations in a smaller volume. A curvelet analysis of the wake and no-wake maps is applied, indicating that the presence of a wake can be extracted at a three-sigma confidence level from maps of the two-dimensional dark matter projection down to a redshift of z=3.

Neutrino currents in wakes of cosmic strings

Neutrinos rotating around Abelian Higgs strings will generate a neutral current close to the string. As the string moves through the cosmic plasma, the velocity kick generated by the motion of the string will enhance the neutrino current in the wake region. The neutrino current density depends on its distance from the string and is oscillatory in nature. This leads to neutrino density gradients in the plasma. Such a flux of neutrinos with periodic density fluctuations will lead to electron currents in the plasma. The current will act like a cross-perturbation across the cosmic string wake. The perturbation as well as the high Reynolds number of the plasma will result in the generation of magnetic fields in the wake of the cosmic string.

Dark matter distribution induced by a cosmic string wake in the nonlinear regime

We study the distribution of dark matter in the nonlinear regime in a model in which the primordial fluctuations include, in addition to the dominant primordial Gaussian fluctuations generated by the standard $\Lambda CDM$ cosmological model, the effects of a cosmic string wake set up at the time of equal matter and radiation, making use of cosmological $N$-body simulations. At early times the string wake leads to a planar overdensity of dark matter. We study how this non-Gaussian pattern of a cosmic string wake evolves in the presence of the Gaussian perturbations, making use of wavelet and ridgelet-like statistics specifically designed to extract string wake signals. At late times the Gaussian fluctuations disrupt the string wake. We find that for a string tension of $G \mu = 10^{-7}$, a value just below the current observational limit, the effects of a string wake can be identified in the dark matter distribution, using the current level of the statistical analysis, down to a redshift of $z = 10$.

Physical effects of massless cosmic strings

We study massless cosmic strings which are one-dimensional objects moving with the speed of light. Perturbations of velocities of test bodies and anisotropy of cosmic microwave background generated by massless cosmic strings are analysed. These phenomena are analogous to the string wake effect and the Kaiser-Stebbins effect for standard (massive) cosmic strings. There are two regimes depending on the energy $E$ of a massless string per unit length. At low energies, $EG/c^4\ll 1$, massless and massive strings act similarly. At high energies, $EG/c^4 \sim 1$, massless string effects are quite different. Our work provides a method to describe different physical phenomena on spacetimes of massless strings which take into account the presence of a parabolic holonomy.

Wouthuysen-Field absorption trough in cosmic string wakes

The baryon density enhancement in cosmic string wakes leads to a stronger coupling of the spin temperature to the gas kinetic temperate inside these string wakes than in the intergalactic medium (IGM). The Wouthuysen-Field (WF) effect has the potential to enhance this coupling to such an extent that it may result in the strongest and cleanest cosmic string signature in the currently planned radio telescope projects. Here we consider this enhancement under the assumption that x-ray heating is not significant. We show that the size of this effect in a cosmic string wake leads to a brightness temperature at least two times more negative than in the surrounding IGM. If the SCI-HI [T. C. Voytek et al., Astrophys. J. 782, L9 (2014), J. B. Peterson et al., arXiv:1409.2774] or EDGES [J. D. Bowman and A. E. E. Rogers Nature (London) 468, 796 (2010), J. D. Bowman et al., Astrophys. J. 676, 1 (2008)] experiments confirm a WF absorption trough in the cosmic gas, then cosmic string wakes should appear clearly in 21 cm redshift surveys of $z=10$ to 30.

Note on structure formation from cosmic string wakes

The search for cosmic strings has been of renewed interest with the advent of precision cosmology. In this note we give a quantitative description of the nonlinear matter density fluctuations that can form from ascaling network of cosmic string wakes. Specifically, we compute the distribution of dark matter halos. These halos would possess strong correlations in position space that should have survived until today. We also discuss the challenges involved in their detection due to their small size and the complex dynamics of their formation.

The 21 cm signature of shock heated and diffuse cosmic string wakes

The analysis of the 21 cm signature of cosmic string wakes is extended in several ways. First we consider the constraints on Gμ from the absorption signal of shock heated wakes laid down much later than matter radiation equality. Secondly we analyze the signal of diffuse wake, that is those wakes in which there is a baryon overdensity but which have not shock heated. Finally we compare the size of these signals to the expected thermal noise per pixel which dominates over the background cosmic gas brightness temperature and find that the cosmic string signal will exceed the thermal noise of an individual pixel in the Square Kilometre Array for string tensions Gμ > 2.5 × 10−8.

The 21 cm signature of cosmic string wakes

We discuss the signature of a cosmic string wake in 21cm redshift surveys. Since 21cm surveys probe higher redshifts than optical large-scale structure surveys, the signatures of cosmic strings are more manifest in 21cm maps than they are in optical galaxy surveys. We find that, provided the tension of the cosmic string exceeds a critical value (which depends on both the redshift when the string wake is created and the redshift of observation), a cosmic string wake will generate an emission signal with a brightness temperature which approaches a limiting value which at a redshift of z+1 = 30 is close to 400 mK in the limit of large string tension. The signal will have a specific signature in position space: the excess 21cm radiation will be confined to a wedge-shaped region whose tip corresponds to the position of the string, whose planar dimensions are set by the planar dimensions of the string wake, and whose thickness (in redshift direction) depends on the string tension. For wakes created at zi+1 = 103, then at a redshift of z+1 = 30 the critical value of the string tension μ is Gμ = 6 × 10−7, and it decreases linearly with redshift (for wakes created at the time of equal matter and radiation, the critical value is a factor of two lower at the same redshift). For smaller tensions, cosmic strings lead to an observable absorption signal with the same wedge geometry.

Signature of cosmic string wakes in the CMB polarization

We calculate a signature of cosmic strings in the polarization of the cosmic microwave background. We find that ionization in the wakes behind moving strings gives rise to extra polarization in a set of rectangular patches in the sky whose length distribution is scale-invariant. The length of an individual patch is set by the comoving Hubble radius at the time the string is perturbing the cosmic microwave background. The polarization signal is largest for string wakes produced at the earliest post-recombination time, and for an alignment in which the photons cross the wake close to the time the wake is created. The maximal amplitude of the polarization relative to the temperature quadrupole is set by the overdensity of free electrons inside a wake which depends on the ionization fraction $f$ inside the wake. For a cosmic string wake coming from an idealized string segment, the signal can be as high as $0.06\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$ in degree scale polarization for a string at high redshift (near recombination) and a string tension $\ensuremath{\mu}$ given by $G\ensuremath{\mu}={10}^{\ensuremath{-}7}$.

Isocurvature fluctuations through axion trapping by cosmic string wakes

We consider wake-like density fluctuations produced by cosmic strings at the quark-hadron transition in the early universe. We show that low momentum axions which are produced through the radiation from the axionic string at an earlier stage, may get trapped inside these wakes due to delayed hadronization in these overdense regions. As the interfaces, bordering the wakes, collapse, the axions pick-up momentum from the walls and finally leave the wake regions. These axions thus can produce large scale iso-curvature fluctuations. We have calculated the detailed profile of these axionic density fluctuations and discuss its astrophysical consequences.

Baryon inhomogeneities due to cosmic string wakes at the quark-hadron transition

Baryon inhomogeneities generated during the quark-hadron transition may alter the abundances of light elements if they persist up to the time of nucleosynthesis. These inhomogeneities survive up to the nucleosynthesis epoch if they are separated by a distance of at least a few metres. In this work we present a model where large sheets of these inhomogeneities separated by a distance of a few km are formed by cosmic string wakes during the quark-hadron transition. The effect of these sheets on nucleosynthesis will also put constraints on the various cosmic string parameters.

Baryon inhomogeneity generation via cosmic strings at QCD scale and its effects on nucleosynthesis

We have earlier shown that cosmic strings moving through the plasma at the time of a first order quark-hadron transition in the early universe can generate large scale baryon inhomogeneities. In this paper, we calculate detailed structure of these inhomogeneities at the quark-hadron transition. Our calculations show that the inhomogeneities generated by cosmic string wakes can strongly affect nucleosynthesis calculations. A comparison with observational data suggests that such baryon inhomogeneities should not have existed at the nucleosynthesis epoch. If this disagreement holds with more accurate observations, then it will lead to the conclusions that cosmic string formation scales above ${10}^{14}--{10}^{15}$ GeV may not be consistent with nucleosynthesis and CMBR observations. Alternatively, some other input in our calculation should be constrained, for example, if the average string velocity remains sufficiently small so that significant density perturbations are never produced at the QCD scale, or if strings move ultrarelativistically so that string wakes are very thin, trapping negligible amount of baryons. Finally, if the quark-hadron transition is not of first order then our calculations do not apply.

Is the Cosmic Microwave Background Really Non-Gaussian?

Two recent papers have claimed detection of non-Gaussian features in the COBE DMR sky maps of the cosmic microwave background. We confirm these results, but argue that Gaussianity is still not convincingly ruled out. Since a score of non-Gaussianity tests have now been published, one might expect some mildly significant results even by chance. Moreover, in the case of one measure which yields a detection, a bispectrum statistic, we find that if the non-Gaussian feature is real, it may well be due to detector noise rather than a non-Gaussian sky signal, since a signal-to-noise analysis localizes it to angular scales smaller than the beam. We study its spatial origin in case it is nonetheless due to a sky signal (eg, a cosmic string wake or flat-spectrum foreground contaminant). It appears highly localized in the direction b=39.5, l=257, since removing a mere 5 pixels inside a single COBE beam area centered there makes the effect statistically insignificant. We also test Guassianity with an eigenmode analysis which allows a sky map to be treated as a random number generator. A battery of tests of this generator all yield results consistent with Gaussianity.

Cosmological consequences of superconducting string networks

We consider the cosmological consequences of a network of superconducting cosmic strings. For strong enough current the period of friction domination never ends. Instead a plasma scaling solution is reached. We demonstrate that this gives rise to a very different cosmology than the usual horizon scaling solution. In particular the string network gives rise to a distinct imprint on the microwave sky, giving non-Gaussian features on much smaller angular scales. It also gives rise to a filament structure in string wakes. Because of the presence of the string magnetocylinder, the string magnetic field cannot create a primordial magnetic field. Similarly, it evades nucleosynthesis constraints. We also show that strings formed at the supersymmetry breaking scale can create the required baryon asymmetry of the universe.

Primordial Non‐Gaussianity: Baryon Bias and Gravitational Collapse of Cosmic String Wakes

I compute the three-dimensional nonlinear evolution of gas and dark matter fluids in the neighborhood of cosmic string wakes that are formed at high redshift (z 2240) for a realistic scenario of wake formation. These wakes are the ones that stand out most prominently as cosmological sheets and are expected to play a dominant role in the cosmic string model of structure formation. Employing a high-resolution three-dimensional hydrodynamic code to evolve these wakes until the present day yields results for the baryon bias generated in the inner wake region. I find that today, wakes would be 1.5 h-1 Mpc thick and contain a 70% excess in the density of baryons over the dark matter density in their centers. However, high-density peaks in the wake region do not inherit a baryon enhancement. I propose a mechanism for this erasure of the baryon excess in spherically collapsed objects based on the geometry change around the collapsing region. Furthermore, I present heuristic arguments for the consequences of this work for large-scale structure in the cosmic string model and conclude that the peculiarities of wake formation are unlikely to have significant import for the discrepancy between power spectrum predictions and observations in this model. If one invokes the nucleosynthesis bound on Ωb, this could be seen as strengthening the case against Ωm = 1 or for low Hubble constants.

High-resolution analysis of cold and hot dark matter in cosmic string wakes

We study the accretion of cold and hot dark matter onto a cosmic string wake using a high-resolution numerical simulation. We verify previous analytical calculations predicting the radius of bound matter around wakes and inflow velocities of the dark matter, as well as assumptions about the self-similarity of the solution. In cold wakes, we show that self-similarity is approached quickly after a `binding' transition. Hot wakes approach self-similarity rapidly once the free streaming `pressure' falls below a critical value and accretion begins. We also analytically calculate the size of the overdensity in wakes with cold dark matter and compare the results to simulations. We remark that the results derived may be used in testing gravitational numerical codes in the non-linear regime.