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Observational Constraints on Cosmic String Production During Brane Inflation

Levon Pogosian, S. -H. Henry Tye, Ira Wasserman, Mark Wyman

TL;DR

Brane-inflation scenarios naturally predict cosmic-string production at the end of inflation. The authors analyze a mixed model combining adiabatic curvature perturbations with active string perturbations, fixing the background cosmology to WMAP best-fit values and introducing a string-perturbation fraction parameter $B$ and a string-tension scale $G\mu$, to confront CMB and galaxy data. They use a scaling string network model and a modified CMBFAST pipeline to compute spectra and perform a $\chi^2$ analysis, finding that $B$ can be as large as $\sim0.1$ without being ruled out, though the best fits typically prefer $B=0$; this translates into a bound $G\mu \lesssim 3.5\times10^{-7}\left(\frac{\lambda}{0.25}\right)\sqrt{\frac{B}{0.1}}$, and implies that B-mode polarization and precise determinations of $n_s(k)$ could help discriminate brane-inflation models. Observationally, a smoking-gun signature would be a detectable small-scale $C_l^{BB}$ signal from string-induced vector modes, while non-Gaussianity tests may also reveal string contributions. Overall, the work delineates how current data tolerate a subdominant cosmic-string component and outlines concrete avenues for future confirmation or ruling out of brane-inflationary cosmic strings.

Abstract

Overall, brane inflation is compatible with the recent analysis of the WMAP data. Here we explore the constraints of WMAP and 2dFGRS data on the various brane inflationary scenarios. Brane inflation naturally ends with the production of cosmic strings, which may provide a way to distinguish these models observationally. We argue that currently available data cannot exclude a non-negligible contribution from cosmic strings definitively. We perform a partial statistical analysis of mixed models that include a sub-dominant contribution from cosmic strings. Although the data favor models without cosmic strings, we conclude that they cannot definitively rule out a cosmic-string-induced contribution of $\sim 10 %$ to the observed temperature, polarization and galaxy density fluctuations. These results imply that $Gμ\lesssim 3.5\times 10^{-7}(λ/0.25)\sqrt{B/0.1}$, where $λ$ is a dimensionless parameter related to the interstring distance, and $B$ measures the importance of perturbations induced by cosmic strings. We argue that, conservatively, the data available currently still permit $B\lesssim 0.1$. Precision measurements sensitive to the B-mode polarization produced by vector density perturbation modes driven by the string network could provide evidence for these models. Accurate determinations of $n_s(k)$, the scalar fluctuation index, could also distinguish among various brane inflation models.

Observational Constraints on Cosmic String Production During Brane Inflation

TL;DR

Brane-inflation scenarios naturally predict cosmic-string production at the end of inflation. The authors analyze a mixed model combining adiabatic curvature perturbations with active string perturbations, fixing the background cosmology to WMAP best-fit values and introducing a string-perturbation fraction parameter and a string-tension scale , to confront CMB and galaxy data. They use a scaling string network model and a modified CMBFAST pipeline to compute spectra and perform a analysis, finding that can be as large as without being ruled out, though the best fits typically prefer ; this translates into a bound , and implies that B-mode polarization and precise determinations of could help discriminate brane-inflation models. Observationally, a smoking-gun signature would be a detectable small-scale signal from string-induced vector modes, while non-Gaussianity tests may also reveal string contributions. Overall, the work delineates how current data tolerate a subdominant cosmic-string component and outlines concrete avenues for future confirmation or ruling out of brane-inflationary cosmic strings.

Abstract

Overall, brane inflation is compatible with the recent analysis of the WMAP data. Here we explore the constraints of WMAP and 2dFGRS data on the various brane inflationary scenarios. Brane inflation naturally ends with the production of cosmic strings, which may provide a way to distinguish these models observationally. We argue that currently available data cannot exclude a non-negligible contribution from cosmic strings definitively. We perform a partial statistical analysis of mixed models that include a sub-dominant contribution from cosmic strings. Although the data favor models without cosmic strings, we conclude that they cannot definitively rule out a cosmic-string-induced contribution of to the observed temperature, polarization and galaxy density fluctuations. These results imply that , where is a dimensionless parameter related to the interstring distance, and measures the importance of perturbations induced by cosmic strings. We argue that, conservatively, the data available currently still permit . Precision measurements sensitive to the B-mode polarization produced by vector density perturbation modes driven by the string network could provide evidence for these models. Accurate determinations of , the scalar fluctuation index, could also distinguish among various brane inflation models.

Paper Structure

This paper contains 10 sections, 56 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Brane Inflation and Cosmic String Properties
  3. Coulombic Inflation
  4. Power Law Inflation
  5. Cosmic String Properties
  6. Tensor modes
  7. Cosmic strings and the CMBR
  8. Methods
  9. Results
  10. Discussion

Figures (4)

  • Figure 1: (Left) Minimum reduced $\chi^2/\nu$ as a function of $B$ for the fit of the $b$-free model to the WMAP's TT (red triangles), TT+TE (red stars) and TT+TE+2dF (red hollow squares) with corresponding best fit values of $W-1$ (green squares) and $n_s-1$ (blue circles). We only show $W$ and $n_s$ from the fit to the TT data alone. The error bars correspond to the diagonal elements of the covariance matrix. Adding the remaining data sets changes the best fit values of $W$ and $n_s$ by only a small amount, well within the plotted error bar. (Right) Same as on the left but for the $b$-fixed model with all data sets included. In the left panel, $\nu=93$ and in the right $\nu=94$. The values of the reduced $\chi^2/\nu$ at $B=0$ are, in the left panel, 0.97 (TT), 1.1 (TT$+$TE) and 0.98 (TT$+$TE$+$2dF), and 1.1 in the right panel.
  • Figure 2: (Left) Plots of the best fit $C_l^{TT}$ and $C_l^{TE}$ computed using the $b$-free model for different values of $B$ together with the WMAP's binned data. (Right) Corresponding plots of the galaxy clustering power spectra together with the 2dFGRS data. On both plots, the solid line corresponds to $B=0$, dotted line - $B=0.05$, short dash line - $B=0.1$, long dash line - $B=0.15$ and the dash-dot line corresponds to the pure string contribution, i.e. $(W=0,B=1)$.
  • Figure 3: Same as in Fig. \ref{['fig:free']} but for the $b$-fixed model.
  • Figure 4: The $B$-type polarization spectrum $C_l^{BB}$ due to cosmic strings as predicted by our string model with the fit parameter $B$ set to $0.1$ is plotted as a solid red line. The expected $C_l^{BB}$ spectra for E to B lensing (dotted black line) and from primordial gravitational waves assuming a tensor-to-scalar ratio of $r=0.1$ (dash-dotted magenta line) are plotted for comparison.