[Paper Review] Cosmology with Very-High-Energy Gamma Rays

E. Pueschel, J. Biteau|arXiv (Cornell University)|Dec 11, 2021

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper reviews the role of very-high-energy (VHE) gamma-ray astronomy in probing cosmological phenomena, including the extragalactic background light (EBL), intergalactic magnetic fields, and potential Lorentz invariance violation. By analyzing VHE gamma-ray absorption and propagation, it constrains EBL evolution, magnetic field strength and coherence, and sets stringent limits on new physics beyond the Planck scale, with future CTA expected to significantly advance these measurements.

ABSTRACT

In this chapter, we discuss the contributions of gamma-ray astronomy at TeV energies to our understanding of the visible content and structure of the universe. We start from the present epoch with the second most intense electromagnetic background field after the CMB: the extragalactic background light (EBL). The EBL is composed of all the light emitted by stars and galaxies since the beginning of reionization, including light absorbed and re-emitted by dust. As such, the EBL traces the history of radiating matter in the universe. We then further dive into the large voids of the universe to study the large-scale magnetic fields that should permeate them. These fields could originate from the onset of structure formation or early phase transitions, bringing us back to the infancy of the universe. We conclude by looking back to the elusive Planck time scale, where the standard models of cosmology and particle physics are no longer applicable. Observations with current-generation gamma-ray astronomy experiments have now started to scratch the surface of cosmology, as we will show in this chapter.

Motivation & Objective

• To assess the contribution of VHE gamma-ray observations to understanding the extragalactic background light (EBL) and its evolution across cosmic time.
• To evaluate the constraints on intergalactic magnetic fields (IGMF) derived from VHE gamma-ray propagation and source spectra.
• To investigate the potential for detecting Lorentz invariance violation (LIV) through energy-dependent time delays in VHE gamma-ray signals.
• To examine the implications of EBL and IGMF constraints for other astrophysical phenomena, such as the diffuse supernova neutrino background and ultra-high-energy cosmic ray propagation.
• To project the potential of the Cherenkov Telescope Array (CTA) in significantly improving current constraints on EBL, IGMF, and new physics.

Proposed method

• Modeling the EBL spectrum using empirical, phenomenological, and semi-analytic approaches, with inputs from deep-field UV-FIR observations (e.g., Hubble, Spitzer, Herschel).
• Using gamma-ray optical depth calculations based on pair production cross-sections and EBL intensity to infer EBL evolution and source redshifts.
• Applying the integral cross-section for pair production and the EBL spectrum to compute the optical depth τ for 1 TeV gamma rays as a function of redshift and distance.
• Employing the dispersion relation modification model with energy-dependent corrections to test for Lorentz invariance violation (LIV), using time delay and threshold energy constraints.
• Evaluating the inverse Compton scattering of electron-positron pairs produced in gamma-ray interactions with CMB photons to estimate secondary gamma-ray emission.
• Using the CTA sensitivity forecast to project improvements in angular, energy, and flux resolution for future EBL and IGMF measurements.

Figure 7.1: Spectrum of the EBL at $z=0$ , following the empirical model of Domínguez et al. ( 2011 ) , the phenomenological model of Finke et al. ( 2010 ) and the semi-analytical model of Gilmore et al. ( 2012 ) . The spectrum of the CMB, peaking around 1000 nW m -2 sr -1 , is shown as a dashed are

Experimental results

Research questions

• RQ1 What is the contribution of VHE gamma-ray observations to constraining the cosmic star formation history via the extragalactic background light (EBL)?
• RQ2 How do VHE gamma-ray observations constrain the strength and coherence length of intergalactic magnetic fields?
• RQ3 What limits can be placed on Lorentz invariance violation (LIV) using VHE gamma-ray time delays and threshold effects?
• RQ4 How do EBL and IGMF constraints affect the interpretation of the diffuse supernova neutrino background and ultra-high-energy cosmic ray propagation?
• RQ5 To what extent will the Cherenkov Telescope Array (CTA) improve current constraints on EBL, IGMF, and new physics beyond the Planck scale?

Key findings

• The EBL at z=0 has a bolometric intensity of ~30 nW m⁻² sr⁻¹ in both the cosmic optical background (COB) and cosmic infrared background (CIB), with a total energy budget of 6–7% that of the CMB.
• The optical depth for 1 TeV gamma rays is approximately τ ≈ 0.1 at z ≈ 0.1, consistent with the cosmic gamma-ray horizon being located at z ~ 0.1 for this energy.
• The EBL spectrum peaks in the near-infrared, with the most probable interaction energy for 1 TeV gamma rays being ~1 eV, corresponding to the COB peak.
• The total pair-production cross-section peaks at x ≈ 2.7, where x = Eγ × εEBL, indicating that 1 TeV gamma rays most efficiently interact with EBL photons of ~1 eV energy.
• The maximum of the pair-production cross-section occurs at x ≈ 2.7, and the most likely EBL photons interacting with 10 TeV gamma rays have a wavelength of ~0.46 μm (visible light).
• The inverse Compton scattering of electron-positron pairs produces secondary gamma rays with energies up to ~1.3 TeV for 20 TeV primary photons, under the Thomson regime assumption.

Figure 7.2: Attenuation factor, in percent, as a function of gamma-ray energy on Earth for sources located at $z=0.03$ , $z=0.1$ , and $z=1.0$ , following the empirical model of Domínguez et al. ( 2011 ) , the phenomenological model of Finke et al. ( 2010 ) , and the semi-analytical model of Gilmore