Direct Imaging of the Cosmic Web Reveals Hidden Baryons and Tests Lambda-CDM

This Ashram video explains how the missing baryonic matter in the universe may be hidden in the cosmic web and how a 23 million light-year long filament has been directly imaged. The hot gas filament connects galaxy clusters in the Shapley supercluster, seen in X-ray emission at roughly 10 million degrees, suggesting the warm hot intergalactic medium hosts a substantial fraction of ordinary matter. The discovery, based on Suzaku and XMM Newton observations, aligns with large-scale cosmological simulations and tightens the baryon budget predicted by the cosmic microwave background. The video places this finding in the broader context of dark matter, dark energy, and future mapping with Euclid and other observatories to test our cosmological models.

Introduction

In this video, the host discusses the long standing question of missing baryons in the universe and how the cosmic web could hide ordinary matter in diffuse gas. The budget from the cosmic microwave background fixes the number of atoms that should exist today, while direct observations of stars and gas account for only about 60 percent, leaving a substantial reservoir to find.

Background: The cosmic web and baryons

The cosmic web, shaped by dark matter, consists of filaments and nodes where galaxies cluster. The warm hot intergalactic medium is predicted to fill these filaments and host much of the baryonic mass that remains unseen. The video explains how simulations and CMB measurements underpin this framework and why detecting such diffuse gas is challenging.

From indirect hints to direct imaging

The narrative traces milestones from indirect evidence to direct probing of the web. It covers the 2005 Chandra detection of intergalactic gas, the 2012 inference of a filament around MAX J0717 via gravitational lensing, the 2015 hydrogen filament imaging with quasar illumination, and the 2019 expansion to networks around protoclusters. The 2023 Keck based advances using Lyman alpha emission introduced new ways to image the web in darkness, and the 2025 MUSE observations began to map filaments on megaparsec scales in the local universe.

The 2025 Suzaku-XMM discovery in the Shapley supercluster

In June 2025 a European team led by Konstantinos Migas mapped a single filament across 23 million light years in the Shapley supercluster. Suzaku mapped the hot gas while XMM Newton identified and removed contamination from background sources such as supermassive black holes. The filament connects four galaxy clusters and contains roughly ten times the Milky Way's mass, with gas at about 10 million degrees Celsius. This represents a direct glimpse of the cosmic web and its role in linking clusters over vast distances.

Implications for cosmology and future missions

The observation supports the lambda cold dark matter model by locating the missing baryons within the cosmic web, but a broader map of filaments is needed. The Euclid mission will help by measuring dark matter through gravitational lensing and providing 3D galaxy positions to reveal the dark matter filaments and guide searches for gas in the local universe. By about 2030, Euclid could confirm whether the web observed in X-ray data and the CMB is a perfect match to our cosmological predictions, strengthening or challenging the current model.

Looking ahead: challenges and opportunities

While the discovery marks a milestone, observational challenges remain, including signal separation and extending mappings to many filaments. A multi-mission approach combining X-ray data, Lyman alpha measurements, and lensing will be essential for building a comprehensive 3D map of the cosmic web and refining our understanding of cosmic evolution.

Conclusion

The filament is described as a bone like strand in the cosmic skeleton that supports structure formation. The result tightens the link between theory and observation and opens paths toward locating more filaments and testing the standard cosmological model.