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B polarization of cosmic microwave background as a tracer of strings

Uros Seljak, Anze Slosar

TL;DR

This work investigates whether cosmic strings from string-inspired brane inflation can imprint a detectable $B$-mode polarization signal in the CMB. Using a generalized KKLMMT potential and a calibrated cosmic string network with wiggliness, the authors compute unequal-time correlators of the string energy–momentum tensor and propagate them through a Boltzmann solver to obtain CMB spectra, showing that vector modes dominate the $B$-mode signal. They find two distinct peaks in the $B$-mode spectrum: a reionization peak at low multipoles and a recombination peak near $ℓ∼1000$, with the latter offering the strongest detectability prospects. With delensing and next-generation polarization experiments, they forecast sensitivity to string tension as low as $Gμ∼(1.4$–$2.7)×10^{-9}$, though realistic limits including lensing and foregrounds are closer to $∼10^{-8}$, implying that high-resolution $B$-mode measurements can test string-inspired inflation scenarios and constrain $Gμ$.

Abstract

String models can produce successful inflationary scenarios in the context of brane collisions and in many of these models cosmic strings may also be produced. In scenarios such as KKLMMT the string contribution is naturally predicted to be well below the inflationary signal for cosmic microwave background (CMB) temperature anisotropies, in agreement with the existing limits. We find that for $B$ type polarization of CMB the situation is reversed and the dominant signal comes from vector modes generated by cosmic strings, which exceeds the gravity wave signal from both inflation and strings. The signal can be detected for a broad range of parameter space: future polarization experiments may be able to detect the string signal down to the string tension $Gμ=10^{-9}$, although foregrounds and lensing are likely to worsen these limits. We argue that the optimal scale to search for the string signature is at $\ell\sim 1000$, but in models with high optical depth the signal from reionization peak at large scales is also significant. The shape of the power spectrum allows one to distinguish the string signature from the gravity waves from inflation, but only with a sufficiently high angular resolution experiment

B polarization of cosmic microwave background as a tracer of strings

TL;DR

This work investigates whether cosmic strings from string-inspired brane inflation can imprint a detectable -mode polarization signal in the CMB. Using a generalized KKLMMT potential and a calibrated cosmic string network with wiggliness, the authors compute unequal-time correlators of the string energy–momentum tensor and propagate them through a Boltzmann solver to obtain CMB spectra, showing that vector modes dominate the -mode signal. They find two distinct peaks in the -mode spectrum: a reionization peak at low multipoles and a recombination peak near , with the latter offering the strongest detectability prospects. With delensing and next-generation polarization experiments, they forecast sensitivity to string tension as low as –, though realistic limits including lensing and foregrounds are closer to , implying that high-resolution -mode measurements can test string-inspired inflation scenarios and constrain .

Abstract

String models can produce successful inflationary scenarios in the context of brane collisions and in many of these models cosmic strings may also be produced. In scenarios such as KKLMMT the string contribution is naturally predicted to be well below the inflationary signal for cosmic microwave background (CMB) temperature anisotropies, in agreement with the existing limits. We find that for type polarization of CMB the situation is reversed and the dominant signal comes from vector modes generated by cosmic strings, which exceeds the gravity wave signal from both inflation and strings. The signal can be detected for a broad range of parameter space: future polarization experiments may be able to detect the string signal down to the string tension , although foregrounds and lensing are likely to worsen these limits. We argue that the optimal scale to search for the string signature is at , but in models with high optical depth the signal from reionization peak at large scales is also significant. The shape of the power spectrum allows one to distinguish the string signature from the gravity waves from inflation, but only with a sufficiently high angular resolution experiment

Paper Structure

This paper contains 4 sections, 2 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Model and Analysis Method
  3. Results
  4. Discussion

Figures (1)

  • Figure 1: This figure shows various CMB temperature and polarization power spectra. The left panel shows $TT$ (top), $EE$ (middle) and the absolute value of the $TE$ (bottom) power spectra, while the larger right-hand side plot shows the same for the $BB$ power spectrum. The thick dashed and dot-dashed lines (orange) correspond to the inflationary contribution for the scalar and tensor modes, respectively; in the $BB$ power spectrum the scalar contribution comes exclusively from lensing of the $EE$ polarisation modes by the intervening structure between us and the surface of the last scattering. The thin dashed (red), dot-dashed (green) and dotted (blue) lines show the scalar, vector and tensor contributions to the total string contribution plotted as a thick solid line (black), assuming $G\mu = 10^{-8}$. The red thin straight lines in the BB power spectrum correspond to rough limits on residual noise obtained by cleaning the lensing contamination from $E$ polarization by the quadratic estimator (top) and iterative method (middle), while the bottom is the instrumental noise for a hypothetical future instrument with polarisation sensitivity of $\sim 0.25 \mu$K arcmin and beam size of $2'$.