Dispersion Measures of Fast Radio Bursts as a Probe of the Epoch of Reionization: Predictions from the CoDa II Simulation

Core Concepts

This paper presents the first predictions for the mean and standard deviation of fast radio burst (FRB) dispersion measures (DM) as functions of redshift, using the Cosmic Dawn (CoDa) II simulation to model the density and ionization fields of the universe through the epoch of reionization (EOR).

Abstract

• Bibliographic Information: Ziegler, J. J., Shapiro, P. R., Dawoodbhoy, T., Beniamini, P., Kumar, P., Freese, K., ... & Gottlöber, S. (2024). Dispersion Measures of Fast Radio Bursts through the Epoch of Reionization. Monthly Notices of the Royal Astronomical Society, 000, 1–14.

• Research Objective: This study aims to predict the mean and standard deviation of FRB dispersion measures as functions of redshift, utilizing the CoDa II simulation to model the density and ionization fields of the universe during and after the EOR. The authors focus on the potential of FRB DMs to probe the patchiness of reionization.

• Methodology: The researchers employ the CoDa II simulation, a large-scale, radiation-hydrodynamical simulation of galaxy formation and reionization, to model the density and ionization fields of the universe down to redshift z ≈ 6.1, encompassing the end of the EOR. They combine this with an N-body simulation, CoDa II–Dark Matter, which simulates the fully-ionized epoch from the end of the EOR to the present day (z = 0). Using these simulations, they calculate the mean and standard deviation of FRB DMs as functions of redshift.

• Key Findings: The study finds that both the mean and standard deviation of FRB DMs increase with redshift, reaching a plateau by z(xHII ≲ 0.25) ≳ 8, well above the redshift at which reionization ends. The mean DM asymptote, DMmax ≈ 5900 pc cm−3, reflects the end of the EOR and its duration. The standard deviation at this plateau is σDM,max ≈ 497 pc cm−3, reflecting inhomogeneities in both patchy reionization and density. Importantly, the study finds that inhomogeneities in ionization during the EOR contribute approximately 1% of the total standard deviation (σDM,max) for FRBs at redshifts z ≳ 8.

• Main Conclusions: The authors conclude that the patchiness of reionization during the EOR leaves a potentially detectable imprint on the standard deviation of FRB dispersion measures. They suggest that this effect might be observable within a few years, given current estimates of FRB rates and advancements in observational capabilities.

• Significance: This research significantly contributes to the field of cosmology by demonstrating the potential of FRB dispersion measures as a novel probe for studying the EOR and its patchiness. The findings provide valuable theoretical predictions that can guide future observations and analyses of FRB data to constrain models of reionization and the early universe.

• Limitations and Future Research: The study acknowledges the dependence of the results on the specific ionization history predicted by the CoDa II simulation. Future research could explore the effects of different reionization scenarios and incorporate more sophisticated models of FRB sources and their environments. Additionally, further investigation into the observational challenges and strategies for detecting the predicted signal is warranted.

Stats

The mean dispersion measure at the plateau is approximately DMmax = 5900 pc cm−3.
The standard deviation at the plateau is approximately σDM,max ≈ 497 pc cm−3.
Inhomogeneities in ionization during the EOR contribute approximately 1% of the total standard deviation (σDM,max) for FRBs at redshifts z ≳ 8.
An estimated 10^4 FRBs occur per day.
Approximately 0.1% of detectable FRBs are produced prior to the end of reionization.

Quotes

"While previous study focused on low-redshift, FRBs are potentially detectable out to high redshift, where their DMs can, in principle, probe the epoch of reionization (EOR) and its patchiness."
"The mean and standard deviation of DM increase with redshift, reaching a plateau by z(xHII ≲0.25) ≳8, i.e. well above zre."
"Inhomogeneities in ionization during the EOR contribute O(1 per cent) of this value of σDM,max from FRBs at redshifts z ≳8."

Key Insights Distilled From

Dispersion Measures of Fast Radio Bursts through the Epoch of Reionization

by Joshua J. Zi... at arxiv.org 11-06-2024

https://arxiv.org/pdf/2411.02699.pdf

Deeper Inquiries

How might the presence of primordial magnetic fields in the early universe affect the propagation of FRB signals and their observed dispersion measures?

Answer:
The presence of primordial magnetic fields (PMFs) in the early universe could indeed have intriguing effects on the propagation of FRB signals and their observed dispersion measures (DM). Here's how:

Faraday Rotation: PMFs would introduce Faraday rotation, a phenomenon where the polarization plane of linearly polarized light rotates as it traverses a magnetized plasma. The degree of rotation is proportional to the magnetic field strength, electron density, and path length. By measuring the Faraday rotation measure (RM) of FRBs at different frequencies, we could potentially probe the strength and structure of PMFs along the line of sight.

Plasma Instabilities and Scattering: PMFs can trigger plasma instabilities, leading to density fluctuations in the intergalactic medium (IGM). These fluctuations can scatter FRB signals, causing both temporal broadening of the pulse and angular broadening of the source. The scattering properties would carry information about the turbulence and magnetization of the IGM.

Magnetic Field Contribution to DM: While the dominant contribution to DM comes from the electron density, strong magnetic fields could, in principle, also contribute to the observed DM. This effect is generally expected to be small but might be non-negligible for extremely strong PMFs or in specific astrophysical environments.
Observational Challenges and Opportunities:
Measuring the subtle effects of PMFs on FRB signals is challenging. It requires:

High-frequency Observations: Faraday rotation is more pronounced at lower frequencies. Observing FRBs at very low frequencies (e.g., using future radio telescopes like the Square Kilometre Array) would enhance our sensitivity to PMFs.
Precision Polarimetry: Detecting Faraday rotation requires precise measurements of the polarization properties of FRB signals across a wide range of frequencies.
If we can overcome these challenges, FRBs could become valuable tools for probing the elusive PMFs, providing insights into the magnetic universe's evolution from the early epochs to the present day.

Could alternative theories of gravity, which propose modifications to general relativity, potentially explain the observed distribution of FRB dispersion measures without invoking patchy reionization?

Answer:
It's possible, though not definitively proven, that alternative theories of gravity could offer explanations for the observed distribution of FRB dispersion measures (DM) without requiring patchy reionization. Here's a breakdown of the concept:

Modified Gravity and Cosmic Structure: Many modified gravity theories, often proposed as alternatives or extensions to general relativity, predict differences in how cosmic structures like galaxies and galaxy clusters form and evolve. These differences could lead to variations in the distribution of matter, including ionized baryons, in the universe.

Impact on DM: Since DM is directly proportional to the integrated electron density along the line of sight, modifications to the matter distribution due to alternative gravity could potentially alter the expected DM-redshift relation.

Distinguishing from Patchy Reionization: The key challenge lies in disentangling the effects of modified gravity from those of patchy reionization. Both can introduce fluctuations in the DM distribution. To differentiate them, we need:

Precise DM Measurements: A large sample of FRBs with accurately measured DMs and, ideally, known redshifts is crucial.
Complementary Cosmological Probes: Combining FRB data with other cosmological observations sensitive to the matter distribution and expansion history (e.g., galaxy surveys, cosmic microwave background anisotropies) is essential to break degeneracies between different models.
Current Status:
Currently, there's no compelling evidence that definitively favors modified gravity as the primary explanation for the observed FRB DM distribution. Patchy reionization remains a more widely accepted and well-supported model. However, as FRB observations improve in quantity and quality, they have the potential to provide valuable constraints on alternative gravity theories and contribute to our understanding of fundamental physics.

If we could develop technology to precisely measure the polarization of FRB signals across a wide range of frequencies, what additional insights might we gain about the properties of the intergalactic medium and the nature of the EOR?

Answer:
The ability to precisely measure the polarization of FRB signals across a wide range of frequencies would be revolutionary, opening up a treasure trove of information about the intergalactic medium (IGM) and the epoch of reionization (EOR). Here are some key insights we could gain:

Mapping Magnetic Fields: As mentioned earlier, Faraday rotation of the polarization plane provides a direct probe of magnetic fields along the line of sight. With precise polarimetry, we could create detailed maps of the magnetic field structure in the IGM, tracing its evolution from the early universe to the present. This would be invaluable for understanding the role of magnetic fields in cosmic structure formation and galaxy evolution.

Probing the Ionization State of the IGM: The degree of Faraday rotation is also sensitive to the electron density, which is directly related to the ionization state of the IGM. By analyzing the frequency dependence of polarization, we could map out the distribution of ionized and neutral gas during the EOR, providing a 3D view of this crucial epoch.

Characterizing Reionization Sources: Different reionization scenarios (e.g., dominated by early stars or quasars) predict different morphologies for the ionized bubbles during the EOR. Polarization measurements of FRBs could help distinguish between these scenarios by revealing the size and distribution of ionized regions.

Studying IGM Turbulence: Fluctuations in the electron density due to turbulence in the IGM can cause depolarization of FRB signals. By analyzing the depolarization properties, we could characterize the strength and scale of turbulence, providing insights into the dynamic processes at play in the early universe.

Testing Fundamental Physics: Precise polarization measurements could also be used to test fundamental physics, such as searching for potential variations in fundamental constants or testing the validity of Einstein's equivalence principle.
In essence, high-precision FRB polarimetry would transform our understanding of the IGM, the EOR, and the evolution of the universe as a whole. It would be akin to having a powerful new microscope to study the cosmos in unprecedented detail.