REVIEW
Gravitational radiation from cosmic (super)strings: bursts, stochastic background, and observational windows
High Energy Physics - Theory · Submitted 22 Oct 2004 · 43 references
Pith's verdict: A careful, honest extension of the Damour-Vilenkin string-GW program to two parameters (p, ε); the scaling laws are new and useful, but the p-exponents rest on a contested network-scaling assumption the abstract hedge doesn't cover.
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Referee report
The paper
The paper claims
The paper argues that cosmic-string gravitational-wave bursts and the stochastic background remain detectable when the reconnection probability is small and loop sizes are reduced, with simple scaling laws in those two parameters.
The reading
What carries the argument
The load-bearing machinery is a reparameterization of the standard string-network loop density. In the standard scenario the typical loop length is $alpha t$ with $alpha sim Gamma Gmu$, and the loop density is $n sim alpha^{-1}t^{-3}$. The paper replaces $alpha$ by $epsilonGamma Gmu$ and tracks how a low reconnection probability changes the number of long strings per Hubble volume, giving $n(t)sim (pGamma Gmu t^3)^{-1}$ for small loops and the same form in the matter era for large loops. This density change is absorbed into the cusp-event rate as $cto cepsilon/p$, or equivalently $dot Nto dot N p/epsilon$, which is what produces the scaling laws for bursts and the background. A Heaviside cutoff $Theta(1-theta_m)$ with $theta_msim(alpha f t_0)^{-1/3}(1+z)^{1/6}$ enforces that only Fourier modes with $|m|ge1$ contribute.
Extended reading notes
Core claim
The central claim is that the gravitational-wave predictions for cosmic string networks are not fragile to the two new parameters. For cusp bursts, the plot of expected amplitude versus $Gmu$ moves upward by $epsilon^{-3/8}p^{-5/8}$ and rightward by $epsilon^{-5/8}p^{-3/8}$ (Eqs. 3.13-3.14), so a smaller reconnection probability improves detectability and a smaller loop parameter mainly shifts the accessible tension range. For the stochastic background from overlapping loops, the low-frequency estimate is $Omega_g(f)propto p^{-1}epsilon^{-1/3}(Gmu)^{2/3}(f t_0)^{-1/3}$ in the regime where the loops are large enough to radiate into the observing band (Eqs. 4.9, 4.14). The important exception is a step-function cutoff when $alpha f t_0<1$, which can remove the signal from pulsar timing, and later from LIGO or LISA, if $epsilon$ is extremely small.
Load-bearing premise
The acknowledged load-bearing premise is the treatment of a low reconnection probability: the paper assumes the long-string coherence length still scales as $xi(t)sim t$ while the inter-string distance scales as $L(t)sim p^{1/2}t$, and every $p$-dependent result flows through that choice; the authors note conflicting claims in the literature and cite only preliminary simulations.
Editorial extensions
If this is right
- A smaller reconnection probability $p<1$ raises both the cusp-burst amplitude and the stochastic background at fixed $Gmu$, so superstring networks can be easier to detect than ordinary strings.
- For $epsilon<1$, burst detectability by LIGO and LISA is largely preserved until the mode cutoff becomes important, roughly when $epsilonlesssim10^{-11}$ for LIGO or $epsilonlesssim10^{-7}$ for LISA at $Gmusim10^{-10}$.
- The existing pulsar-timing limit $Omega_g h^2<6times10^{-8}$ implies $c^{3/2}Gmulesssim10^{-7}$, while a future pulsar-timing-array sensitivity of $Omega_g h^2sim2times10^{-9}$ could probe $Gmusim3.6times10^{-10}c^{-3/2}p^{3/2}epsilon^{1/2}$.
- Very small $epsilon$ can cut off the stochastic signal in the pulsar band when $alpha f t_0<1$, so pulsar timing may lose sensitivity to the smallest tensions even as higher-frequency detectors retain it.
Reading between the lines
- If brane inflation produces superstrings with $p$ and $epsilon$ far from one, the amplitude scalings and the low-frequency cutoff give a way to distinguish them from ordinary field-theory strings using combined LIGO, LISA, and pulsar-timing data.
- Measuring both a burst amplitude and a stochastic-background level could in principle break the degeneracy among $Gmu$, $p$, $epsilon$, and the cusp number $c$, because the two observables scale differently with these parameters.
- A targeted time-domain search for transient near-cusp events in existing pulsar residuals is a low-cost test of the authors' suggestion that the Green Bank scatter is a burst rather than a steady red-noise background.
- Future network simulations that map $alpha$ and $c$ as functions of $Gmu$ and $p$ would turn the paper's scaling laws into a predictive parameter space rather than a family of allowed extrapolations.
Original abstract
The gravitational wave (GW) signals emitted by a network of cosmic strings are reexamined in view of the possible formation of a network of cosmic superstrings at the end of brane inflation. The reconnection probability $p$ of intersecting fundamental or Dirichlet strings might be much smaller than 1, and the properties of the resulting string network may differ significantly from those of ordinary strings (which have $p=1$). In addition, it has been recently suggested that the typical length of newly formed loops may differ by a factor $\epsilon \ll 1$ from its standard estimate. Here, we analyze the effects of the two parameters $p$ and $\epsilon$ on the GW signatures of strings. We consider both the GW bursts emitted from cusps of oscillating string loops, which have been suggested as candidate sources for the LIGO/VIRGO and LISA interferometers, and the stochastic GW background, which may be detectable by pulsar timing observations. In both cases we find that previously obtained results are \textit{quite robust}, at least when the loop sizes are not suppressed by many orders of magnitude relative to the standard scenario. We urge pulsar observers to reanalyze a recently obtained 17-year combined data set to see whether the large scatter exhibited by a fraction of the data might be due to a transient GW burst activity of some sort, e.g. to a near cusp event.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
The gravitational wave (GW) signals emitted by a network of cosmic strings are reexamined in view of the possible formation of a network of cosmic superstrings at the end of brane inflation. The reconnection probability $p$ of intersecting fundamental or Dirichlet strings might be much smaller than 1, and the properties of the resulting string network may differ significantly from those of ordinary strings (which have $p=1$). In addition, it has been recently suggested that the typical length of newly formed loops may differ by a factor $\epsilon \ll 1$ from its standard estimate. Here, we analyze the effects of the two parameters $p$ and $\epsilon$ on the GW signatures of strings. We consider both the GW bursts emitted from cusps of oscillating string loops, which have been suggested as candidate sources for the LIGO/VIRGO and LISA interferometers, and the stochastic GW background, which may be detectable by pulsar timing observations. In both cases we find that previously obtained results are \textit{quite robust}, at least when the loop sizes are not suppressed by many orders of magnitude relative to the standard scenario. We urge pulsar observers to reanalyze a recently obtained 17-year combined data set to see whether the large scatter exhibited by a fraction of the data might be due to a transient GW burst activity of some sort, e.g. to a near cusp event.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The gravitational wave (GW) signals emitted by a network of cosmic strings are reexamined in view of the possible formation of a network of cosmic superstrings at the end of brane inflation. | background 0.57 | 0.38 | 0.04 | hedged possibility 0.52 |
| s1 | The reconnection probability $p$ of intersecting fundamental or Dirichlet strings might be much smaller than 1, and the properties of the resulting string network may differ significantly from those of ordinary strings (which have $p=1$). | background 0.99 | 0.05 | 0.03 | hedged possibility 0.55 |
| s2 | In addition, it has been recently suggested that the typical length of newly formed loops may differ by a factor $\epsilon \ll 1$ from its standard estimate. | background 0.98 | 0.04 | 0.02 | hedged possibility 0.94 |
| s3 | Here, we analyze the effects of the two parameters $p$ and $\epsilon$ on the GW signatures of strings. | contribution 0.99 | 0.73 | 0.04 | asserted 0.90 |
| s4 | We consider both the GW bursts emitted from cusps of oscillating string loops, which have been suggested as candidate sources for the LIGO/VIRGO and LISA interferometers, and the stochastic GW background, which may be detectable by pulsar timing observations. | contribution 0.69 | 0.24 | 0.04 | hedged possibility 0.82 |
| s5 | In both cases we find that previously obtained results are \textit{quite robust}, at least when the loop sizes are not suppressed by many orders of magnitude relative to the standard scenario. | result 0.97 | 0.94 | 0.81 | asserted 0.98 |
| s6 | We urge pulsar observers to reanalyze a recently obtained 17-year combined data set to see whether the large scatter exhibited by a fraction of the data might be due to a transient GW burst activity of some sort, e.g. to a near cusp event. | future work 1.00 | 0.15 | 0.07 | hedged possibility 0.64 |
What was examined, and what was not
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- P abstract
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References
How the literature uses it
121 arXiv papers cite this in 187 sentences
Papers citing it · 97 sentences without a confident label
Builds on it 3 papers
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In this section we consider the GW emission from a cosmic superstring loop, and generalize the results of Damour and Vilenkin [ref] on the emission of gravitational bursts from standard loops without junctions.
Gravitational Wave Bursts from Cosmic Superstrings with Y-junctions · GW emission from cosmic superstrings with junctions · 1.00
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In particular, the mechanism for the emission of gravitational and particle radiation from cusps on loops of cosmic string [ref] also applies to cusps on strings between junctions.
Cusps on cosmic superstrings with junctions · Phenomenology · 0.86
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In a follow up paper [ref] , they have considered GW bounds on networks of cosmic superstrings allowing for smaller reconnection probabilities, as well as allowing for the length of newly formed loops to be a free parameter.
Seeking String Theory in the Cosmos · Gravitational waves · 0.84
Uses its method or tool 5 papers
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This was pointed out in Refs. [ref] and we follow the formalism of Ref. [ref] .
Cosmological effects of decaying cosmic string loops with TeV scale width · Gravitational waves · 0.90
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We should calculate the stochastic GW background spectrum using the following formulae [ref]
Gravitational waves from kinks on infinite cosmic strings · THE STOCHASTIC BACKGROUND OF GRAVITATIONAL WAVES FROM KINKS · 0.98
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In this section we calculate the GW burst signal following the methods of Refs. [ref] together with the refinements of Refs. [ref] .
Gravitational Wave Bursts as Harbingers of Cosmic Strings Diluted by Inflation · Gravitational Wave Signals from String Loops · 0.99
Uses its data or values 2 papers
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Equation 4.8 in [ref] gives an analytic approximation to $h_c^cusp$ valid for small frequencies.
Gravitational wave signatures from kink proliferation on cosmic (super-) strings · Characteristic amplitude of stochastic background · 0.83
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Pulsar timing, specifically, places a bound $Gμ<10^-7-10^-8$ on the cosmic string tension [ref] .
Edge Detection, Cosmic Strings and the South Pole Telescope · Introduction · 0.88
Compares with it 3 papers
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The lines (solid or dotted/dashed) represent the graphs obtained using the DV interpolating functions, allowing for a direct comparison with [ref] .
The effect of extra dimensions on gravity wave bursts from cosmic string cusps · The gravitational waveform · 0.87
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As expected, away from the junctions we recover the same GW signal as Damour and Vilenkin [ref] , both for a kink propagating on a string - for which $α=2/3$ and the emission occurs in a fan-like set of directions - as well as for a cusp, where $α=1/3$ and the emission occurs in a small cone around a particular direction.
Gravitational Wave Bursts from Cosmic Superstrings with Y-junctions · Conclusion · 0.89
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This is in contrast with [ref] which focus on the burst of GW produced by the cusps and kinks.
Analytic Approaches to the Study of Small Scale Structure on Cosmic String Networks · Summary and discussion · 1.00
Cites it as support 1 paper
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Furthermore, it has been shown that the background from cosmic strings could be dominated by a non-Gaussian contribution arising from the closest sources [ref] .
Detecting non-Gaussian gravitational wave backgrounds: a unified framework · Introduction · 0.97
Cites it as background 70 papers
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Although the SGWB from SMBHBs is supposed to be the most promising source for the signal detected by the PTAs, the signal at nanoHertz can also originate from some cosmological processes [ref] , such as cosmological first-order phase transition [ref] , cosmic strings [ref] , domain walls [ref] , and scalar-induced GW [ref] accompanying the formation of primordial black holes [ref] .
Cosmological Interpretation for the Stochastic Signal in Pulsar Timing Arrays · Introduction · 0.95
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The gravitational emission by cusps and kinks has been well-studied [ref] .
Periodic Fast Radio Bursts from Axion Emission by Cosmic Superstrings · Power of Axion Emission · 0.95
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In addition to the SGWB, more recent bursts of GWs from cusps or kinks on string loops can potentially be resolved as individual events [ref] , but for standard scaling strings they are harder to find than the unresolved SGWB [ref] .
Gravitational Wave Bursts as Harbingers of Cosmic Strings Diluted by Inflation · Introduction · 0.84
Discussion
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