[Paper Review] Inflation, superheavy metastable strings and gravitational waves in non-supersymmetric flipped SU(5)

G. Lazarides, Rinku Maji+2 more|arXiv (Cornell University)|Aug 14, 2023

Cosmology and Gravitation TheoriesPhysics and Astronomy78 references3 citations

TL;DR

This paper proposes a non-supersymmetric flipped SU(5) hybrid inflation model in which superheavy metastable cosmic strings survive inflation and produce a stochastic gravitational wave background compatible with NANOGrav 15-year data and advanced LIGO-Virgo third-run constraints. The model achieves $G\mu \sim 10^{-6}$ via quantum tunneling-mediated monopole-antimonopole pair creation on strings, with inflation suppressing high-frequency power, while predicting a proton lifetime of $10^{36}-10^{37}$ years.

ABSTRACT

Motivated by the NANOGrav 15 year data and other recent investigations of stochastic gravitational background radiation based on pulsar timing arrays, we show how superheavy strings survive inflation but the slightly heavier monopoles do not in a non-supersymmetric hybrid inflation model based on flipped $SU(5)$. With the dimensionless string tension parameter $G μ\sim 10^{-6}$, the gravitational wave spectrum emitted by the strings, which are metastable due to breaking caused by monopole-antimonopole quantum mechanical tunneling, is compatible with the latest NANOGrav measurement as well as the advanced LIGO-VIRGO third run data. The string network undergoes about 30 $e$-foldings of inflation which suppresses the spectrum in the LIGO-VIRGO frequency range. With the symmetry breaking chain $SU(5) imes U(1)_X o SU(3)_c imes SU(2)_L imes U(1)_Z imes U(1)_X o SU(3)_c imes SU(2)_L imes U(1)_ Y$, the estimated proton lifetime is of order $10^{36}-10^{37}$ yrs.

Motivation & Objective

• To reconcile the observed stochastic gravitational wave background from pulsar timing arrays (NANOGrav) with advanced LIGO-Virgo data using a viable particle physics model.
• To explain the survival of superheavy metastable cosmic strings after inflation, despite the inflationary suppression of heavier monopoles.
• To construct a non-supersymmetric flipped SU(5) model that simultaneously accommodates inflation, metastable strings, and a proton lifetime consistent with experimental bounds.
• To ensure the scalar spectral index and tensor-to-scalar ratio are in agreement with current CMB observations.
• To compute the gravitational wave spectrum from metastable strings and verify compatibility with both low- and high-frequency observational constraints.

Proposed method

• Utilizes a non-supersymmetric flipped SU(5) gauge group with $SU(5) \times U(1)_X$ as the unbroken gauge symmetry at high energies.
• Employs a two-stage symmetry breaking chain: $SU(5) \times U(1)_X \to SU(3)_c \times SU(2)_L \times U(1)_Z \times U(1)_X \to SU(3)_c \times SU(2)_L \times U(1)_Y$, generating monopoles and strings via Higgs fields $\Phi$ and $\Psi$.
• Introduces a real singlet scalar $S$ as the inflaton field, driving hybrid inflation with a Coleman-Weinberg one-loop corrected potential.
• Applies quantum tunneling of monopole-antimonopole pairs to stabilize strings, making them metastable despite monopole inflationary suppression.
• Computes the gravitational wave spectrum from the string network using $f^2$ low-frequency, scale-invariant plateau, and $f^{-1/3}$ high-frequency behavior, dependent on $t_F$.
• Uses $e$-folding number $\sim 30$ to suppress the high-frequency spectrum, reconciling $G\mu \sim 10^{-6}$ with LIGO-Virgo third-run data.

Experimental results

Research questions

• RQ1 Can a non-supersymmetric flipped SU(5) model produce metastable superheavy cosmic strings that survive inflation and generate a gravitational wave spectrum consistent with NANOGrav 15-year data?
• RQ2 How does inflation suppress the gravitational wave spectrum in the LIGO-Virgo frequency band while preserving a detectable signal at lower frequencies?
• RQ3 What is the predicted proton lifetime in this model, and does it satisfy current experimental bounds from Super-Kamiokande and Hyper-Kamiokande?
• RQ4 How do Coleman-Weinberg one-loop corrections affect the inflationary potential and observables like $n_s$ and $r$?
• RQ5 What is the role of quantum tunneling in enabling metastable strings despite the inflationary suppression of monopoles?

Key findings

• The model predicts a gravitational wave spectrum with $G\mu \sim 10^{-6}$, consistent with the NANOGrav 15-year data and the advanced LIGO-Virgo third-run constraints.
• The string network undergoes approximately 30 $e$-foldings of inflation, which suppresses the high-frequency gravitational wave power, enabling compatibility with LIGO-Virgo observations.
• The scalar spectral index is predicted to be $n_s \simeq 0.963$, in good agreement with Planck and other CMB measurements.
• The tensor-to-scalar ratio is estimated in the range $r \sim 10^{-4} - 10^{-3}$, consistent with current observational limits.
• The proton lifetime is estimated at $\tau_p \approx 2.1 \times 10^{36}$ years (BP1) and $1.3 \times 10^{37}$ years (BP2), both above the Super-Kamiokande lower bound.
• The one-loop Coleman-Weinberg correction to the inflation potential slightly improves the agreement with inflationary observables, though it remains small in magnitude.