Deep Global 21-cm Trough (EDGES)

• Deep Global 21-cm Absorption Trough is a prominent cosmic dawn signal defined by a ~-500 mK absorption at z~17, challenging standard astrophysical predictions.
• The signal’s depth and timing imply either enhanced star formation efficiency in low-mass halos or the presence of exotic cooling and radio background mechanisms.
• Rigorous Bayesian analyses and upcoming JWST observations aim to resolve uncertainties by testing high-redshift galaxy formation models and alternative physics scenarios.

The Deep Global 21-cm Absorption Trough, as reported by the EDGES (Experiment to Detect the Global Epoch of Reionization Signature) collaboration, represents a major observational milestone in cosmic dawn studies. The detection consists of a prominent, sky-averaged absorption feature centered at approximately $z\sim 17$ (corresponding to $\nu\sim 78$ MHz), with a reported amplitude of roughly $-500$ mK—significantly deeper than standard predictions. This result has prompted extensive inquiries into both early galaxy formation models and the possibility of exotic physical mechanisms in the high-redshift universe.

1. Observational Signal and Cosmological Context

The EDGES trough is characterized by its redshift location $z\sim 17$, depth $\sim -500$ mK, and distinctive flattened profile, which standard $\Lambda$CDM-based astrophysical models struggle to reproduce within established parameter ranges (Mirocha et al., 2018). In standard theory, the sky-averaged differential brightness temperature of the 21-cm line is given by: $$\Delta T_{21}(z) \simeq 27\,{\bar{x}}_{\rm HI}(z) \left[1-\frac{T_\gamma(z)}{T_S(z)}\right]\left(\frac{\Omega_b h^2}{0.02}\right)\left(\frac{0.15}{\Omega_m h^2}\right)^{1/2}\left(\frac{1+z}{10}\right)^{1/2}\ {\rm mK},$$ where $T_S$ is the spin temperature of neutral hydrogen and $T_\gamma$ is the radio background temperature, which is usually dominated by the CMB. Any absorption stronger than $\sim -200$ mK requires $T_S \ll T_\gamma$ or $T_\gamma$ substantially exceeding the CMB.

The timing of the absorption places critical constraints on the astrophysical processes responsible for Ly$\alpha$ coupling and X-ray heating. Typical galaxy formation models—where the star-formation rate tracks assembly of dark matter halos—predict the absorption feature to occur at higher frequencies (lower redshift) than observed. The early formation of the EDGES trough thus demands either an accelerated or more efficient build-up of the radiative backgrounds necessary for 21-cm absorption (Mirocha et al., 2018, Witte et al., 2018).

2. Galaxy Formation and Star Formation Efficiency at Cosmic Dawn

To reproduce the timing and depth of the EDGES feature within standard astrophysics, a revision of galaxy formation scenarios at $z \gtrsim 10$ is required (Mirocha et al., 2018). Specifically, the star formation efficiency (SFE) in low-mass halos ($M_{\rm halo} \sim 10^8$–$10^{10}\,M_{\odot}$) must be elevated relative to canonical extrapolations from lower-redshift UV luminosity functions (UVLFs).

In standard UVLF-based models, SFE typically declines sharply with decreasing halo mass. However, matching the EDGES timing necessitates a nearly constant (or weakly declining) SFE to increase the abundance of faint galaxies—thus steepening the faint-end slope of the UVLF at high redshifts. Concretely, this would imply more galaxies with $M_{\rm UV} \lesssim -12$ at $z \gtrsim 12$, a prediction directly testable by deep JWST and lensed field surveys.

Table 1: Required Modifications for Standard Models to Fit the EDGES Trough

Physical Parameter	Standard Model	Needed for EDGES Trough
SFE in $10^8$–$10^{10} M_\odot$ halos	declining	nearly flat / high
UVLF faint-end slope	$\alpha \sim -2$	steeper at $z \gtrsim 10$
Ly$\alpha$ and X-ray backgrounds	gradual buildup	rapid increase by $z\sim 18$

The empirical constraint is that without invoking sources in halos below the atomic cooling threshold, only enhanced SFE in canonical halos can reconcile the early timing of the observed absorption trough (Mirocha et al., 2018, Madau, 2018). In this scenario, the high-redshift galaxy population is dominated by faint, metal-poor systems that generate the required Ly$\alpha$ photons for Wouthuysen–Field coupling.

3. Explanations Beyond Standard Astrophysics

Given the exceptional amplitude of the EDGES trough, several models invoke physics beyond simple star-formation-driven scenarios.

a) Enhanced Cooling Mechanisms

A plausible resolution is to introduce additional gas cooling channels, such as baryon–dark matter interactions. These processes—e.g., millicharged dark matter scattering—can enhance energy loss from the baryons, driving $T_K$ below the adiabatic limit and thus increasing absorption (Mirocha et al., 2018, Burns et al., 2019). The gas temperature evolution in such models is parameterized as: $$d\log(T)/d\log(t) = (\alpha/3) - [(2+\alpha)/3] \left\{1+\exp{\left[-(z/z_0)^\beta\right]}\right\},$$ where setting $\alpha = -4$ recovers standard evolution, and more negative $\alpha$ captures additional cooling.

b) Excess Radio Background

An alternative is to posit an extra radio background (beyond the CMB) during Cosmic Dawn. If present, this would raise $T_\gamma$ in the 21-cm formula, deepening the absorption trough even if $T_S$ remains unchanged. The required radio luminosity density must scale as: $$L_R = 10^{22} f_R \left(\frac{\rm SFR}{M_\odot\,{\rm yr}^{-1}}\right) \; {\rm W\,Hz}^{-1},$$ with $f_R \sim 10^3$ compared to the local universe. Successful models necessitate both a very large efficiency of low-frequency photon production and an abrupt truncation by $z\lesssim 15$ to avoid violating constraints from ARCADE2 at $z=0$ (Mirocha et al., 2018, Cang et al., 12 Nov 2024).

Both classes of models (enhanced cooling and radio excess) are highly nontrivial to realize in early-universe environments. Exotic cooling requires fine-tuning of dark matter microphysics, while a strong, rapidly vanishing high-redshift radio background is difficult to engineer with known astrophysical sources (Mirocha et al., 2018, Cang et al., 12 Nov 2024).

4. Model Challenges, Systematics, and Bayesian Data Analysis

Explanations invoking a strong radio background confront two major obstacles (Mirocha et al., 2018, Cang et al., 12 Nov 2024):

• The need for extremely efficient low-frequency radio photon production from $z>15$ star-forming galaxies (up to $\sim 10^3$ times their local efficiency).
• The requirement for this radio luminosity to sharply cease by $z\lesssim 15$ so as not to overproduce the present-day cosmic radio background measured by ARCADE2.

Recent Bayesian analyses of the EDGES dataset using physically motivated models for the radio background, joint foreground/systematic modeling (via log-polynomials and calibration residuals), and full data–space forward modeling decisively disfavor a cosmic signal with excess radio background-induced depth. The preferred solution—when sufficient freedom is allowed for foreground and systematics—is a 21-cm absorption consistent with standard physics (i.e., an amplitude not exceeding about $-210$ mK) (Cang et al., 12 Nov 2024). These results highlight the dangers of pseudo-likelihood approaches that fit phenomenological "flattened Gaussian" signals divorced from physically motivated templates.

5. Astrophysical and Cosmological Implications

The early timing and shape of the EDGES trough remain robust inferences, indicating:

• Either an increased SFE in low-mass halos, supporting a scenario with large populations of faint galaxies at $z \gtrsim 10$, which can be directly tested by JWST.
• Or a need for truly exotic cooling mechanisms or enhanced radio backgrounds, both of which face stringent theoretical and empirical challenges (Mirocha et al., 2018, Cang et al., 12 Nov 2024).

Key predictions of the high-$z$ SFE scenario include a steepening of the faint-end slope of the UVLF at $M_{\rm UV} \lesssim -12$ and a higher early SFRD, accessible to deep lensed and unlensed field observations. The confirmation or falsification of abundant faint galaxies at $z \gtrsim 12$ would thus directly constrain the physical origin of the EDGES feature (Mirocha et al., 2018).

Table 2: Summary of Scenarios and Observational Consequences

Explanation	Observational Test	Theoretical Challenge
High $z$ SFE (steep UVLF)	Detect faint galaxies with JWST at $z > 12$	No fundamental conflict, but requires rapid evolution of galaxy formation physics
Exotic cooling (e.g., dark matter–baryon coupling)	Test for extra cooling with future 21-cm and CMB experiments	Must tune dark matter properties to fit amplitude and not violate other constraints
Radio background excess	Present-day ARCADE2 limits, forward modeling of EDGES sky	Impractical efficiency and fine-tuned redshift cut-off, disfavored by joint Bayesian analysis

6. Future Observations and Prospects

Ongoing and planned surveys with JWST (and WFIRST/ROMS) targeting ultra-deep fields are expected to probe the abundance of faint galaxies at $z \sim 12$–15. The observation of a steep faint-end UVLF, with a high density of galaxies at $M_{\rm UV} \lesssim -12$, would support the scenario of enhanced SFE driving the signal (Mirocha et al., 2018). Alternatively, confirmation of the absence of such galaxies would bolster the case for new physics.

In the radio, higher-fidelity measurements of the global 21-cm signal and its fluctuations—accompanied by advanced foreground and systematic modeling—will solidify the interpretation of EDGES-like features. Forward-modeling approaches that consistently treat foregrounds, sky calibration, and cosmic signal are essential for robust inference (Cang et al., 12 Nov 2024, Sims et al., 2019).

7. Conclusions

The EDGES deep global 21-cm absorption trough at $z\sim 17$ places stringent constraints on both astrophysical and exotic explanations for the onset of cosmic dawn. Only two classes of scenarios can explain its timing and depth: (a) increased star formation efficiency in low-mass halos (driving a steepening of the UVLF) or (b) new physics that either cools the gas further or raises the effective radio background. Bayesian analyses using physical models and rigorous treatment of systematic uncertainties currently disfavor an excess radio background scenario; the amplitude and timing of the absorption feature remain critical discriminants between astrophysical and exotic origins, with imminent tests available from JWST and next-generation low-frequency radio experiments. The EDGES result thus continues to motivate the convergence of 21-cm cosmology, deep galaxy surveys, and the search for new physics in the early universe.