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Cosmology

In 2008, JILA scientist Gayler Harford and Fellow Andrew Hamilton began investigating the structures formed by ordinary matter (baryons) and dark matter by the time the early Universe became transparent to light, which occurred 12.7 billion years ago—or about a billion years after the Big Bang. Harford analyzed a high-resolution cosmological simulation developed by Nickolay Gnedin of Fermilab. She has made some surprising discoveries.

Harford discovered that ordinary matter formed filaments connecting the largest galaxies. These filaments functioned as superhighways that not only routed gas to the monster black holes growing inside each galaxy, but also supplied the raw materials for billions and billions of stars. The long filaments often linked together strings of galaxies that were already decoupled from dark matter. Dark matter was found along the filaments, but concentrated somewhat irregularly into globular shapes in distinctly different locations from nascent galaxies.

Harford’s analyses suggested that the properties of the gas entrained in the filaments were far more important in determining the structure of the filamentous superhighways than any nearby dark matter. The gas hitchhiking along the filaments appeared to be far more important than dark matter in determining whether an emerging galaxy could pull in enough gas to form stars.

In fact, simulated galaxies that were not connected to filaments may not have been able to accumulate enough gas to form stars. These galaxies may not have been detected because they would not be luminous. It is possible that some of these dark galaxies may be found near our own Milky Way galaxy, which is predicted to have more satellite galaxies than have ever been found.

To learn more about dark galaxies and the gaseous filaments linking luminous galaxies, Harford is now studying the development of filaments and galaxies during the several hundred years before the Universe became transparent to light. She wonders whether the dark galaxies could be detected in observations of Lyman-alpha radiation in the early Universe. She is also working on a theory to explain how ordinary matter and dark matter separated from one another so early in the history of galaxy formation.