Method to weigh galaxy clusters could help astronomers understand mysterious 'dark matter' structures | 10/17/2017 | Staff
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The furthest galaxy ever observed is so far away that the starlight we now detect was emitted less than 500m years after the Big Bang. It has taken about 13 billion years to reach us. But there's a lot of things about a galaxy that we can't see. For example, we think galaxies are immersed within gigantic "halos" of an invisible substance dubbed dark matter. Scientists don't actually know what dark matter is, but they know it exists because it has a gravitational pull on surrounding matter.

Now our new research, published in Nature Astronomy, presents a way in which we could learn just how galaxies have evolved within this strange, dark matter across most of cosmic time.

Years - Galaxies - Millennia - Formation - Universe

That we can see light emitted 13 billion years ago may sound amazing. But we can actually see light emitted even earlier – before galaxies formed. For a few hundred millennia after its formation, the universe was a hot mess of light particles (photons), electrically charged protons and electrons (plasma), as well as dark matter. The photons were trapped among the plasma: continuously "scattered" in random directions by near-constant interactions with the free electrons.

Like trying to traverse a crowded, bustling room, the average path length of each photon was very short before its next interaction. This made the universe opaque – if you were trying to look through this medium it would be like looking into a bank of fog.

Years - Big - Bang - Universe - Point

But 380,000 years after the Big Bang, the universe had expanded and cooled to a point when the free electrons could bind with the protons to form atoms of hydrogen. The scattering quickly ceased, allowing the photons to stream freely across the universe with no free electrons in the way.

As this transition happened everywhere in the universe quite quickly, from our point of view it's...
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