🔭 astrophysics

Simulating the LOcal Web (SLOW) -- VI: Gamma-ray Emission in the Local Universe

Using the first cosmological magnetohydrodynamic simulation with an on-the-fly spectral cosmic-ray model, this study estimates that diffuse gamma-ray emission from cosmic-ray protons in the local Universe's galaxy clusters and filaments is several orders of magnitude below current Fermi-LAT detection limits, requiring significantly improved sensitivity for future observation.

Original authors: Ludwig M. Böss, Ildar Khabibullin, Daniel Karner, Klaus Dolag, Ulrich P. Steinwandel, Elena Hernandez-Martinez, Jenny G. Sorce

Published 2026-07-21

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Ludwig M. Böss, Ildar Khabibullin, Daniel Karner, Klaus Dolag, Ulrich P. Steinwandel, Elena Hernandez-Martinez, Jenny G. Sorce

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Invisible Ghosts of the Cosmos

Imagine the universe not just as a collection of stars and galaxies, but as a vast, invisible ocean filled with high-speed particles. These aren't the gentle waves of water, but "cosmic rays"—protons and electrons zooming through space at nearly the speed of light. When these speedy particles crash into the gas floating between galaxies, they don't just bounce off; they create a flash of light called gamma rays. Think of it like a cosmic pinball machine: the faster the ball (the proton) and the more crowded the table (the gas), the brighter the flash.

Astronomers have been trying to catch these faint flashes for years using powerful telescopes like Fermi-LAT. Why does it matter? Because these gamma rays are the only way we can "see" the invisible pressure these cosmic rays exert on the universe. If we can measure them, we can understand how much energy is hidden in the dark, empty spaces between galaxies and how the universe builds its giant structures, like clusters of galaxies. However, so far, the telescope has mostly seen nothing but darkness, leaving scientists to wonder: Are the cosmic rays weaker than we thought, or are we just looking with the wrong eyes?

The Great Cosmic Hunt: Simulating the Invisible

In this new study, a team of scientists decided to stop guessing and start building a virtual universe to find the answer. They created a massive computer simulation called "SLOW" (Simulating the LOcal Web), which is essentially a digital sandbox of our entire local neighborhood in the cosmos. Unlike previous attempts that just guessed how many cosmic rays might be hiding, this simulation actually tracks them particle by particle as they are born, travel, and crash.

The researchers set up their digital universe to look exactly like our real one, complete with the Milky Way at the center and the giant clusters of galaxies surrounding it. They programmed their simulation to watch for "shocks"—violent collisions that happen when galaxies crash into each other or when gas falls into the cosmic web. In the real world, these shocks act like giant particle accelerators, smashing protons to high speeds. The team's computer code followed these protons as they got accelerated, bounced around, and eventually smashed into gas atoms to create gamma rays.

What they found was a bit of a bummer, but a very important one.

The simulation showed that while cosmic rays are being created in these cosmic collisions, they are far less abundant than some theories predicted. When the team calculated how much gamma-ray light these virtual protons should produce, the result was a whisper compared to the roar we expected. The simulated gamma-ray glow is roughly 1,000 to 100,000 times fainter than the current upper limits set by the Fermi-LAT telescope. In fact, to actually see this faint glow in a famous cluster like Coma, our telescopes would need to be about 100 to 1,000 times more sensitive than they are right now (requiring a sensitivity of Fγ < 10−11 γ s−1 cm−2).

The paper suggests that the reason we haven't seen this glow yet isn't because the cosmic rays don't exist, but because the "accelerators" in space aren't as efficient as we hoped. The simulation revealed that the magnetic fields in space act like a filter. They only let protons get accelerated if they hit the shock waves at a very specific angle. Since most of the shocks in the universe seem to hit at the "wrong" angles for protons, very few get the speed boost they need to make bright gamma rays.

The authors are careful to note that this is a result of their specific simulation rules. They didn't prove that cosmic rays are weak in reality; they proved that if the universe works the way their model says it does, the gamma rays should be incredibly faint. They also pointed out that their computer model might be missing some tiny, weak shocks that could be hiding more protons, which means the real universe might be slightly brighter than their simulation suggests. However, even with those missing pieces, the glow is likely still too dim for our current telescopes to catch.

In short, this paper is a sophisticated "what-if" story. It tells us that if our current understanding of how cosmic rays are born is correct, then the universe is much quieter in gamma rays than we thought. It's a reminder that sometimes, the most exciting discovery is realizing that the universe is hiding its secrets even better than we imagined, and that we'll need to build even better telescopes to hear its faintest whispers.

Technical Summary: Simulating the LOcal Web (SLOW) VI. Gamma-ray emission in the local Universe

Problem Statement
Diffuse gamma-ray ( $\gamma$ -ray) emission resulting from cosmic-ray (CR) protons scattering off thermal gas in the intracluster medium (ICM) and intergalactic medium (IGM) remains undetected by current instruments, specifically the Fermi Large Area Telescope (Fermi-LAT). While non-detections place upper limits on the relativistic proton energy density (roughly 1% of thermal energy density), theoretical models often struggle to reconcile efficient proton acceleration at merger shocks with these observational constraints. Previous simulations frequently modeled CRs via total energy budgets without explicit spectral evolution or applied post-processing spectral assumptions, potentially overlooking the complex interplay between shock acceleration, advection, and spectral steepening in realistic cosmological environments.

Methodology
This work presents results from the first constrained cosmological magnetohydrodynamic (MHD) simulation of a $500 h^{-1}$ cMpc volume (centered on the Milky Way) that employs an on-the-fly spectral CR model. The simulation, named SLOW-CR30723, utilizes the OpenGadget3 code and the Crescendo Fokker-Planck solver to evolve the distribution functions of CR protons and electrons.

Key methodological components include:

Initial Conditions: Constrained using the CosmicFlows-2 survey to reproduce the local Universe's large-scale structure.
CR Physics: The model tracks CR protons as piecewise power laws in momentum space, evolving them via a two-moment diffusion-advection approach. It explicitly includes:
- Injection at shocks via Diffusive Shock Acceleration (DSA), dependent on the sonic Mach number ( $M_s$ ) and shock obliquity ( $\theta_B$ ).
- Adiabatic energy changes due to gas expansion/contraction.
- Advection of CRs with the thermal gas (diffusion and streaming are neglected due to computational cost and recent findings suggesting efficient confinement).
- Energy losses for electrons (synchrotron and Inverse Compton), though protons are treated as long-lived in these environments.
Gamma-ray Calculation: $\gamma$ -ray emissivity is derived directly from the simulated CR proton spectra and the local thermal gas density. The calculation assumes $\pi^0$ -decay as the dominant channel for the Fermi-LAT band (100 MeV – 100 GeV), utilizing cross-section parameterizations from Kafexhiu et al. (2014).
Comparison: Results are compared against Fermi-LAT upper limits (Ackermann et al. 2014) and analytic models (Pfrommer & Enßlin 2004). The study also performs resolution tests using zoom-in simulations (Dianoga set) to assess shock-finder accuracy.

Key Results

CR Proton Distribution: CR protons are accelerated at all structure formation and accretion shocks within galaxy clusters and cosmic web filaments. These protons are advected throughout the cluster volume, forming halos that extend to and beyond the virial radius, closely tracing the cosmic web.
Gamma-ray Flux and Luminosity: The absolute value of the diffuse $\gamma$ $γ$ -ray emission in the simulation is three to five orders of magnitude below current Fermi-LAT upper limits for massive clusters (e.g., Coma, Virgo, Perseus).
- The volume-averaged ratio of CR proton pressure to thermal pressure ( $X_{CR}$ ) is found to be significantly lower than the $\sim 1\%$ limit often assumed in analytic models.
- When considering only gas particles that actually contain a CR population, the pressure ratio increases but remains one to two orders of magnitude below observational limits.
Spectral Characteristics: The simulated $\gamma$ -ray spectra exhibit a slope of $\alpha_\gamma \approx -1$ (implying a proton spectral index $\alpha_p \approx 2.0$ ) for energies above 1 GeV. This is flatter than some previous predictions but is attributed to the simulation's bias toward detecting stronger shocks due to resolution limitations.
Surface Brightness: Central surface brightness values for clusters range from $10^{-9}$ to $10^{-8}$ erg s $^{-1}$ cm $^{-2}$ , decreasing by 2–3 orders of magnitude toward the virial radius. Diffuse emission in filaments is 4–5 orders of magnitude fainter than cluster centers.

Significance and Limitations
The paper claims that its primary significance lies in providing a lower limit for diffuse $\gamma$ -ray emission based on a self-consistent, on-the-fly treatment of CR acceleration and transport in a cosmological context. The authors emphasize that their results are driven by specific physical and numerical choices:

Shock Obliquity: The inclusion of an obliquity-dependent acceleration efficiency (based on Caprioli & Spitkovsky 2014; Pais et al. 2018) suppresses proton acceleration at quasi-perpendicular shocks, which are common in merger shocks.
Resolution and Shock Finding: The current resolution ( $3072^3$ particles) limits the detection of low-Mach number shocks ( $M_s < 2.25$ ) where acceleration is inefficient in the adopted model. Resolution tests suggest that increasing resolution by a factor of 10 could increase CR energy by a factor of 2–3.
Injection Parameters: The choice of injection momentum ( $\hat{p}_{inj} = 0.1$ ) and the specific DSA parameterization (Ryu et al. 2019) significantly impact the resulting emissivity.

The authors conclude that under their specific model assumptions, detecting diffuse $\gamma$ -ray emission from the Coma cluster would require a sensitivity of $F_\gamma < 10^{-11}$ $\gamma$ s $^{-1}$ cm $^{-2}$ . They note that the discrepancy between their low predictions and previous higher estimates (or potential hints of emission) likely stems from the combination of shock-obliquity effects, resolution limits on shock detection, and the specific efficiency parameterizations used. Future work is required to vary these CR acceleration parameters to provide more conclusive predictions.

The Invisible Ghosts of the Cosmos

The Great Cosmic Hunt: Simulating the Invisible

More like this