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Register a new userString length constraining from stochastic gravitational waves background
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String length is a fundamental parameter in string theory. A strategy on how to determine it through experiments is proposed. Our work focuses on the stochastic gravitational waves from string gas cosmology. With the help of the Lambert W function, we find the non-Hagedorn phase is ruled out by the B-mode polarization in the cosmic microwave background. The spectrum from the Hagedorn phase with a logarithmic term is found to be unique. We propose a strategy on how to constrain the string length through stochastic gravitational waves. Considering the sensitivities of the current and the upcoming detectors, the string length is found to be lower than 7 $sim$ orders of the Planck scale.
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Among all cosmological quantum-gravity or quantum-gravity-inspired scenarios, only very few predict a blue-tilted primordial tensor spectrum. We explore five of them and check whether they can generate a stochastic gravitational-wave background detectable by present and future interferometers: non-local quantum gravity, string-gas cosmology, new ekpyrotic scenario, Brandenberger-Ho non-commutative inflation and multi-fractional spacetimes. We show that non-local quantum gravity is unobservable, while all the other models can reach the strain sensitivity of DECIGO but not that of LIGO-Virgo-KAGRA, LISA or Einstein Telescope. Other quantum-gravity models with red-tilted spectra (most loop quantum cosmologies) or with exceptionally tiny quantum corrections (Wheeler-DeWitt quantum cosmology) are found to be non-detectable.
LIGO and Virgo have initiated the era of gravitational-wave (GW) astronomy; but in order to fully explore GW frequency spectrum, we must turn our attention to innovative techniques for GW detection. One such approach is to use binary systems as dynamical GW detectors by studying the subtle perturbations to their orbits caused by impinging GWs. We present a powerful new formalism for calculating the orbital evolution of a generic binary coupled to a stochastic background of GWs, deriving from first principles a secularly-averaged Fokker-Planck equation which fully characterises the statistical evolution of all six of the binarys orbital elements. We also develop practical tools for numerically integrating this equation, and derive the necessary statistical formalism to search for GWs in observational data from binary pulsars and laser-ranging experiments.
This is a summary of presentations delivered at the OC1 parallel session Primordial Gravitational Waves and the CMB of the 12th Marcel Grossmann meeting in Paris, July 2009. The reports and discussions demonstrated significant progress that was achieved in theory and observations. It appears that the existing data provide some indications of the presence of gravitational wave contribution to the CMB anisotropies, while ongoing and planned observational efforts are likely to convert these indications into more confident statements about the actual detection.
We point out that the observed time delay between the detection of the signal at the Hanford and Livingston LIGO sites from the gravitational wave event GW150914 places an upper bound on the speed of propagation of gravitational waves, $c_{gw}lesssim 1.7$ in the units of speed of light. Combined with the lower bound from the absence of gravitational Cherenkov losses by cosmic rays that rules out most of subluminal velocities, this gives a model-independent double-sided constraint $1lesssim c_{gw}lesssim 1.7$. We compare this result to model-specific constraints from pulsar timing and cosmology.
The formation of primordial black hole (PBH) dark matter and the generation of scalar induced secondary gravitational waves (SIGWs) have been studied in the generic no-scale supergravity inflationary models. By adding an exponential term to the Kahler potential, the inflaton experiences a period of ultra-slow-roll and the amplitude of primordial power spectrum is enhanced to $mathcal{O}(10^{-2})$. The enhanced power spectra of primordial curvature perturbations can have both sharp and broad peaks. A wide mass range of PBH is realized in our model, and the frequencies of the scalar induced gravitational waves are ranged form nHz to Hz. We show three benchmark points where the PBH mass generated during inflation is around $mathcal{O}(10^{-16}M_{odot})$, $mathcal{O}(10^{-12}M_{odot})$ and $mathcal{O}(M_{odot})$. The PBHs with masses around $mathcal{O}(10^{-16}M_{odot})$ and $ mathcal{O}(10^{-12}M_{odot})$ can make up almost all the dark matter, and the associated SIGWs can be probed by the upcoming space-based gravitational wave (GW) observatory. Also, the wide SIGWs associated with the formation of solar mass PBH can be used to interpret the stochastic GW background in the nHz band, detected by the North American Nanohertz Observatory for Gravitational Waves, and can be tested by future interferometer-type GW observations.
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