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Register a new userSneutrino Tribrid Inflation, Metastable Cosmic Strings and Gravitational Waves
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We present a successful realization of sneutrino tribrid inflation model based on a gauged $U(1)_{B-L}$ extension of Minimal Supersymmetric Standard Model (MSSM). A single interaction term involving the $B-L$ Higgs field and the right-handed neutrinos serves multiple purposes. These include the generation of heavy Majorana masses for the right-handed neutrinos to provide an explanation for the tiny neutrino masses via the seesaw mechanism, a realistic scenario for reheating and non-thermal leptogenesis with a reheat temperature as low as $10^6$ GeV, and a successful realization of inflation with right-handed sneutrino as the inflaton. The matter parity which helps avoid rapid proton decay survives as a $Z_{2}$ subgroup of a $U(1)$ $R$-symmetry. Depending on the choice of model parameters yields the following predicted range of the tensor to scalar ratio, $3 times 10^{-11}lesssim rlesssim 7times 10^{-4}$ ($ 6 times 10^{-7} lesssim r lesssim 0.01 $), and the running of the scalar spectral index, $-0.00022 lesssim dn_s/dln k lesssim -0.0026$ ($-0.00014 lesssim dn_s/dln k lesssim 0.005$), along with the $B-L$ breaking scale, $ 3 times 10^{14}lesssim M/ text{GeV}lesssim 5 times 10^{15}$ ($ 6 times 10^{15}lesssim M/ text{GeV}lesssim 2 times 10^{16}$), calculated at the central value of the scalar spectral index, $n_s =0.966$, reported by Planck 2018. The possibility of realizing metastable cosmic strings in a grand unified theory setup is briefly discussed. The metastable cosmic string network admits string tension values in the range $10^{-8} lesssim Gmu_s lesssim 10^{-6}$, and predicts a stochastic gravitational wave background lying within the 2-$sigma$ bounds of the recent NANOGrav 12.5-yr data.
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A metastable cosmic-string network is a generic consequence of many grand unified theories (GUTs) when combined with cosmic inflation. Metastable cosmic strings are not topologically stable, but decay on cosmic time scales due to pair production of GUT monopoles. This leads to a network consisting of metastable long strings on superhorizon scales as well as of string loops and segments on subhorizon scales. We compute for the first time the complete stochastic gravitational-wave background (SGWB) arising from all these network constituents, including several technical improvements to both the derivation of the loop and segment contributions. We find that the gravitational waves emitted by string loops provide the main contribution to the gravitational-wave spectrum in the relevant parameter space. The resulting spectrum is consistent with the tentative signal observed by the NANOGrav and Parkes pulsar timing collaborations for a string tension of Gmu ~ 10^-11...-7 and has ample discovery space for ground- and space-based detectors. For GUT-scale string tensions, Gmu ~ 10^-8...-7, metastable strings predict a SGWB in the LIGO-Virgo-KAGRA band that could be discovered in the near future.
The NANOGrav collaboration has recently presented its pulsar timing array data which seem compatible with the presence of a stochastic gravity wave background emitted by cosmic strings with a dimensionless string tension $Gmusimeq 2times 10^{-11}-3times 10^{-10}$ at $95%$ confidence level ($G$ is Newtons constant and $mu$ denotes the string tension). However, there is some tension between these results and the previous pulsar timing array bound $Gmulesssim 4times 10^{-11}$ from the PPTA experiment. We propose a relaxation of this tension by invoking primordial inflation which partially inflates the string network. The latter re-enters the horizon at later times after the end of inflation, and thus the short string loops are not produced. This leads to a reduction of the gravity wave spectrum which is more pronounced at higher frequencies. The reconciliation of the NANOGrav results with the PPTA bound is possible provided that the strings re-enter the horizon at adequately late times. We consider an example of a realistic $SO(10)$ model incorporating successful inflation driven by a gauge singlet real scalar field with a Coleman-Weinberg potential. This model leads to the production of intermediate scale topologically stable cosmic strings that survive inflation. We show regions of the parameter space where the tension between NANOGrav and PPTA is alleviated. Finally, we present an example in which both monopoles and strings survive inflation with the above tension resolved.
We consider strings with the Nambu action as extremal surfaces in a given space-time, thus, we ignore their back reaction. Especially, we look for strings sharing one symmetry with the underlying space-time. If this is a non-null symmetry, the problem of determining the motion of the string can be dimensionally reduced. We get exact solutions for the following cases: straight and circle-like strings in a Friedmann background, straight strings in an anisotropic Kasner background, different types of strings in the metric of a gravitational wave. The solutions will be discussed.
We investigate the stability of the electroweak Z-string at high temperatures. Our results show that while finite temperature corrections can improve the stability of the Z-string, their effect is not strong enough to stabilize the Z-string in the standard electroweak model. Consequently, the Z-string will be unstable even under the conditions present during the electroweak phase transition. We then consider phenomenologically viable models based on the gauge group $SU(2)_L times SU(2)_R times U(1)_{B-L}$ and show that metastable strings exist and are stable to small perturbations for a large region of the parameter space for these models. We also show that these strings are superconducting with bosonic charge carriers. The string superconductivity may be able to stabilize segments and loops against dynamical contraction. Possible implications of these strings for cosmology are discussed.
A standard expectation of primordial cosmological inflation is that it dilutes all relics created before its onset to unobservable levels. We present a counterexample to this expectation by demonstrating that a network of cosmic strings diluted by inflation can regrow to a level that is potentially observable today in gravitational waves~(GWs). In contrast to undiluted cosmic strings, whose primary GW signals are typically in the form of a stochastic GW background, the leading signal from a diluted cosmic string network can be distinctive bursts of GWs within the sensitivity reach of current and future GW observatories.
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