E7 Galaxy Clusters

Topic

The central subject of this model is the galaxy cluster—the most massive virialized object in the entire universe.

The mass of such a cluster is distributed—very roughly—among a small fraction of visible galaxies, a similar fraction of hot gas (detectable via X-rays due to its own thermal radiation), and a clearly dominant fraction of dark matter.

The Sunyaev-Zel'dovich effect consists of the energy gain experienced by cosmic microwave background photons as they scatter off the hot electrons within the cluster; this effect results in a decrease in observed temperature below a specific frequency and an increase above that same frequency.

The mass fraction accounted for by the cluster's gas, when compared to its total mass, provided one of the first historical pieces of evidence for the existence of dark matter. Thus, from the galaxy cluster that opens this discussion to the historical evidence that concludes it—encompassing the distribution of mass and the Sunyaev-Zel'dovich effect—galaxy clusters demonstrate how a single object (the most massive in the universe) reveals, through the simple distribution of its own mass, the first historical clue that the vast majority of the universe is invisible.

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