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NASA NTRS1989

Evolution of cosmic string networks

A discussion of the evolution and observable consequences of a network of cosmic strings is given. A simple model for the evolution of the string network is presented, and related to the statistical mechanics of string networks. The model predicts the long string density throughout the history of the universe from a single parameter, which researchers calculate in radiation era simulations. The statistical mechanics arguments indicate a particular thermal form for the spectrum of loops chopped off the network. Detailed numerical simulations of string networks in expanding backgrounds are performed to test the model. Consequences for large scale structure, the microwave and gravity wave backgrounds, nucleosynthesis and gravitational lensing are calculated.

Albrecht, Andreas↗

NASA NTRS1990

On the large-scale structures formed by wakes of open cosmic strings

Large-scale structures of the universe have been variously described as sheetlike, filamentary, cellular, bubbles or spongelike. Recently cosmic strings became one of viable candidates for a galaxy formation scenario, and some of the large-scale structures seem to be simply explained by the open cosmic strings. According to this scenario, sheets are wakes which are traces of moving open cosmic strings where dark matter and baryonic matter have accumulated. Filaments are intersections of such wakes and high density regions are places where three wakes intersect almost orthogonally. The wakes formed at t sub eq become the largest surface density among all wakes, where t sub eq is the epoch when matter density equals to radiation density. If we assume that there is one open cosmic string per each horizon, then it can be explained that the typical distances among wakes, filaments and clusters are also approx. 10(exp 2) Mpc. This model does not exclude a much more large scale structure. Open cosmic string may move even now and accumulate cold dark matter after its traces. However, the surface density is much smaller than the ones formed at t sub eq. From this model, it is expected that the typical high density region will have extended features such as six filaments and three sheets and be surrounded by eight empty regions (voids). Here, the authors are mainly concerned with such structures and have made numerical simulations for the formation of such large scale structures.

Hara, Tetsuya↗

NASA NTRS1991

Closed timelike curves produced by pairs of moving cosmic strings - Exact solutions

Exact solutions of Einstein's field equations are presented for the general case of two moving straight cosmic strings that do not intersect. The solutions for parallel cosmic strings moving in opposite directions show closed timelike curves (CTCs) that circle the two strings as they pass, allowing observers to visit their own past. Similar results occur for nonparallel strings, and for masses in (2+1)-dimensional spacetime. For finite string loops the possibility that black-hole formation may prevent the formation of CTCs is discussed.

Gott, J. Richard, III↗

NASA NTRS2004

Termination Proofs for String Rewriting Systems via Inverse Match-Bounds

Annotating a letter by a number, one can record information about its history during a reduction. A string rewriting system is called match-bounded if there is a global upper bound to these numbers. In earlier papers we established match-boundedness as a strong sufficient criterion for both termination and preservation of regular languages. We show now that the string rewriting system whose inverse (left and right hand sides exchanged) is match-bounded, also have exceptional properties, but slightly different ones. Inverse match-bounded systems effectively preserve context-free languages; their sets of normalized strings and their sets of immortal strings are effectively regular. These sets of strings can be used to decide the normalization, the termination and the uniform termination problems of inverse match-bounded systems. We also show that the termination problem is decidable in linear time, and that a certain strong reachability problem is deciable, thus solving two open problems of McNaughton's.

Butler, Ricky↗

NASA NTRS2006

The Strings of Eta Carina: The HST/STIS Spectra and [Ca II]

Long linear, filamentary ejecta, are found to move at very high velocity external to the Homunculus, the circumstellar hourglass-shaped ejecta surrounding Eta Carinae. The origin of the strings is a puzzle. As an example, the Weigelt Blobs have N at 10X solar and C, O at 0.01X solar abundance, along with He/H significantly enhanced. This abundance pattern is evidence for extreme CNO-processing. Similarly, the Strontium Filament has Ti/Ni at 100X solar, presumably due to the lack of oxygen to form Ti-oxide precipitates onto dust grains. We have obtained 2-D spectra with the HST/STIS of the Strontium Filament and a portion of a string. These deep spectral exposures, at moderate dispersion, span much of the near red spectral region from 5000 to 9000A. We have identified twelve emission lines in these spectra with proper velocities and spatial structure of this string and obtained line ratios for [Ca II] (7293/7325A) and [Fe Ill (7157/8619A) which are useful for determining physical conditions in this nebulosity. In an attempt to use the [Ca II] ratio to determine the physical parameters, and ultimately the abundances in the strings, we have constructed a statistical equilibrium model for Ca II , including radiative and collisional rates. These results incorporate our newly calculated atomic data for levels n = 3,4,5 and 6 configurations of Ca II. The aim is to compute the [Ca II] line ratios and use them as a diagnostic of the physical parameters. Using the [Fe II] ratio we find that for Te=10,000 K, the electron density is Ne approx.10(exp 6)/cu cm. We plan to use the [Ca II] ratio to confirm this result. Then, we will extend the use of this multilevel model Ca II atom to study the physical conditions of the Strontium filament where eight lines of Ca II, both allowed and forbidden, had been identified. With the physical conditions determined, we will be able to derive reliable estimates for the gas phase abundances in the strings.

Melendez, M. B.↗

NASA NTRS2021

Constraints on Cosmic Strings Using Data from the Third Advanced LIGO-Virgo Observing Run

We search for gravitational-wave signals produced by cosmic strings in the Advanced LIGO and Virgo full O3 dataset. Search results are presented for gravitational waves produced by cosmic string loop features such as cusps, kinks, and, for the first time, kink-kink collisions. A template-based search for short-duration transient signals does not yield a detection. We also use the stochastic gravitational-wave background energy density upper limits derived from the O3 data to constrain the cosmic string tension Gμ as a function of the number of kinks, or the number of cusps, for two cosmic string loop distribution models. Additionally, we develop and test a third model that interpolates between these two models. Our results improve upon the previous LIGO–Virgo constraints on Gμ by 1 to 2 orders of magnitude depending on the model that is tested. In particular, for the one-loop distribution model, we set the most competitive constraints to date: Gμ ≲ 4×10−15. In the case of cosmic strings formed at the end of inflation in the context of grand unified theories, these results challenge simple inflationary models.

J B Camp↗

NASA NTRSDate not supplied

PPE Electric Propulsion Advancing to CDR: 12kW String

String level testing is a vital part of electric propulsion development. With the Power and Propulsion Element (PPE) spacecraft flying two new models of Hall Thrusters, this work is even more critical. PPE will have both 6kW and 12kW thruster strings onboard, all powered by Maxar power processing units (PPU). Both strings have recently undergone end-to-end string level hot fire testing to verify the ability of all components to work together. This entails operating the thruster with the Xenon Flow Controller (XFC) and PPUs under both nominal and off-nominal conditions to stress the system and determine weak points. Such testing has long been a standard Maxar practice since our first 1.5kW electric propulsion system in 2004. This paper will focus on Phase 1 of the 12kW testing recently completed at NASA Glenn. The primary conclusion of the test effort was that the 12kW string operated nominally and that the PPU and XFC could operate the thruster within specifications.

Electric propulsion↗

DOE OSTI2020

Will LISA Detect Harmonic Gravitational Waves from Galactic Cosmic String Loops?

Rapid advancement in the observation of cosmic strings has been made in recent years placing increasingly stringent constraints on their properties, with $G\mu\lesssim 10^{-11}$ from Pulsar Timing Array (PTA). Cosmic string loops with low string tension clump in the Galaxy due to slow loop decay and low gravitational recoil, resulting in great enhancement to loop abundance in the Galaxy. With an average separation of down to just a fraction of a kpc, and the total power of gravitational wave (GW) emission dominated by harmonic modes spanning a wide angular scale, resolved loops located in proximity are powerful, persistent, and highly monochromatic sources of GW with a harmonic signature not replicated by any other sources, making them prime targets for direct detection by the upcoming Laser Interferometer Space Antenna (LISA), whose frequency range is well-matched. Unlike detection of bursts where the detection rate scales with loop abundance, the detection rate for harmonic signal is the result of a complex interplay between the strength of GW emission, loop abundance, and other sources of noise, and is most suitably studied through numerical simulations. We develop a robust and flexible framework for simulating loops in the Galaxy for predicting direct detection of harmonic signal from resolved loops by LISA. Our simulation reveals that the most accessible region in the parameter space consists of large loops $\alpha=0.1$ with low tension $10^{-21}\lesssim G\mu\lesssim 10^{-19}$. Direct detection of field theory cosmic strings is unlikely, with the detection probability $p_{\mathrm{det}}\lesssim 2\%$ for a 1-year mission. An extension suggests that direct detection of cosmic superstrings with a low intercommutation probability is very promising. Searching for harmonic GW signal from resolved loops through LISA observations will potentially place physical constraints on string theory.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

On unitarity of tree-level string amplitudes

Four-particle tree-level scattering amplitudes in string theory are magically consistent with unitarity, reflected in the non-trivial fact that beneath the critical dimension, the residues of the amplitudes on massive poles can be expanded in partial waves with all positive coefficients. While this follows (rather indirectly) from the no-ghost theorem, the simplicity of the statement and its fundamental importance for the physical consistency of string theory begs for a more direct and elementary understanding. In this note we take a step in this direction by presenting a new expression for the partial wave coefficients of string amplitudes, given by surprisingly simple double/triple contour integrals for open/closed strings. This representation allows us to directly prove unitarity of all superstring theories in D ≤ 6 spacetime dimensions, and can also be used to determine various asymptotics of the partial waves at large mass levels.