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Register a new userDetection of Low Tension Cosmic Superstrings
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Cosmic superstrings of string theory differ from conventional cosmic strings of field theory. We review how the physical and cosmological properties of the macroscopic string loops influence experimental searches for these relics from the epoch of inflation. The universes average density of cosmic superstrings can easily exceed that of conventional cosmic strings having the same tension by two or more orders of magnitude. The cosmological behavior of the remnant superstring loops is qualitatively distinct because the string tension is exponentially smaller than the string scale in flux compactifications in string theory. Low tension superstring loops live longer, experience less recoil (rocket effect from the emission of gravitational radiation) and tend to cluster like dark matter in galaxies. Clustering enhances the string loop density with respect to the cosmological average in collapsed structures in the universe. The enhancement at the Suns position is $sim 10^5$. We develop a model encapsulating the leading order string theory effects, the current understanding of the string network loop production and the influence of cosmological structure formation suitable for forecasting the detection of superstring loops via optical microlensing, gravitational wave bursts and fast radio bursts. We evaluate the detection rate of bursts from cusps and kinks by LIGO- and LISA-like experiments. Clustering dominates rates for $G mu < 10^{-11.9}$ (LIGO cusp), $G mu<10^{-11.2}$ (LISA cusp), $G mu < 10^{-10.6}$ (LISA kink); we forecast experimentally accessible gravitational wave bursts for $G mu>10^{-14.2}$ (LIGO cusp), $G mu>10^{-15}$ (LISA cusp) and $G mu>10^{- 14.1}$ (LISA kink).
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We propose that the periodic fast radio bursts of FRB 180916.J0158+65 are sourced by axion emission (mass $m_{a} sim 10^{-14}$ eV) from cosmic superstrings. Some of the emitted axions are converted to photons by magnetic fields as they travel along the line of sight to Earth. An impulsive burst of axion emission generates a photon signal typically lasting for milliseconds and varying with frequency in the observed manner. We find a range of parameters in our cosmic string network model consistent with the properties of FRB 180916.J0158+65. We suggest followup gravitational wave observations to test our model.
We study the covariance in the angular power spectrum estimates of CMB fluctuations when the primordial fluctuations are non-Gaussian. The non-Gaussian covariance comes from a nonzero connected four-point correlation function -- or the trispectrum in Fourier space -- and can be large when long-wavelength (super-CMB) modes are strongly coupled to short-wavelength modes. The effect of such non-Gaussian covariance can be modeled through additional freedom in the theoretical CMB angular power spectrum and can lead to different inferred values of the standard cosmological parameters relative to those in $Lambda$CDM. Taking the collapsed limit of the primordial trispectrum in the quasi-single field inflation model as an example, we study how the six standard $Lambda$CDM parameters shift when two additional parameters describing the trispectrum are allowed. The reduced statistical significance of the Hubble tension in the extended model allows us to combine the {it Planck} temperature data and the type Ia supernovae data from Panstarrs with the distance-ladder measurement of the Hubble constant. This combination of data shows strong evidence for a primordial trispectrum-induced non-Gaussian covariance, with a likelihood improvement of $Delta chi^2 approx -15$ (with two additional parameters) relative to $Lambda$CDM.
Cosmic strings are predicted by many field-theory models, and may have been formed at a symmetry-breaking transition early in the history of the universe, such as that associated with grand unification. They could have important cosmological effects. Scenarios suggested by fundamental string theory or M-theory, in particular the popular idea of brane inflation, also strongly suggest the appearance of similar structures. Here we review the reasons for postulating the existence of cosmic strings or superstrings, the various possible ways in which they might be detected observationally, and the special features that might discriminate between ordinary cosmic strings and superstrings.
We perform a comprehensive study of cosmological constraints on non-standard neutrino self-interactions using cosmic microwave background (CMB) and baryon acoustic oscillation data. We consider different scenarios for neutrino self-interactions distinguished by the fraction of neutrino states allowed to participate in self-interactions and how the relativistic energy density, N$_{textrm{eff}}$, is allowed to vary. Specifically, we study cases in which: all neutrino states self-interact and N$_{textrm{eff}}$ varies; two species free-stream, which we show alleviates tension with laboratory constraints, while the energy in the additional interacting states varies; and a variable fraction of neutrinos self-interact with either the total N$_{textrm{eff}}$ fixed to the Standard Model value or allowed to vary. In no case do we find compelling evidence for new neutrino interactions or non-standard values of N$_{textrm{eff}}$. In several cases we find additional modes with neutrino decoupling occurring at lower redshifts $z_{textrm{dec}} sim 10^{3-4}$. We do a careful analysis to examine whether new neutrino self-interactions solve or alleviate the so-called $H_0$ tension and find that, when all Planck 2018 CMB temperature and polarization data is included, none of these examples ease the tension more than allowing a variable N$_{textrm{eff}}$ comprised of free-streaming particles. Although we focus on neutrino interactions, these constraints are applicable to any light relic particle.
We calculate the gravitational wave (GW) background spectra from kink propagation and kink-kink collisions on infinite cosmic superstrings. We take into account two characteristics of the cosmic superstring network: a small reconnection probability and Y-junctions. First, a small reconnection probability increases the number of infinite strings inside the horizon and enhances the kink production, which leads a larger amplitude of the GW background. Second, 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 superstrings. 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 above two effects, 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 there is a parameter space where the GW amplitude is slightly enhanced by the effect of a small reconnection probability.
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