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Message:

> Just discovered:
> 
> https://mail.google.com/mail/u/0/#inbox/FMfcgzGwHLfLNTfSMvhDkRvdKVlwKCxD
> 
> "Scientists from the United States and Australia have for the first time seen intergalactic gas filaments. These gas filaments are mostly made of hydrogen. Because hydrogen is the simplest element, a lot of it was created in the early universe. If it clumps enough, it forms stars and solar systems and galaxies. But where it doesn’t clump it just lingers around and it’s hard to see.[b] And yet, seeing this stuff is important to confirm that our model of the universe is correct. [/b] ( my bold)
> 
> "Measuring this hydrogen is really difficult. They did it by looking for a particular emission line of hydrogen, known as the Lyman alpha line. If hydrogen atoms wiggle, this is one of the wavelengths they emit. The Lyman alpha line is in the ultraviolet when emitted, so we can’t see it.  But the universe expands while the light travels towards us, so the wavelength stretches, and it’s shifted into the visible range. These emission lines are faint and difficult to tease out of the data from the rest of the universe’s light.
> 
> "They did it with instrument called the Cosmic Web Imager at the Keck Observatory in Hawaii using a sophisticated background removal technique called nod-and-shuffle. This entails shifting the focus of the instrument from the source you want to image to its background and tracking how the combination of both changes. Then you can identify and subtract much of the background.
> 
> "The sources they looked at were at redshift around 2 point 5, so about 10 billion light-years away. The volume that their observations covered is a slice of roughly 3 million light-years wide and 600 million light-years long. So it ain’t small.
> 
> "If dark matter exists, which it may not, then it should fit with the structure of these filaments, so measuring them is another way to probe dark matter. It’s an important test because most of the *normal matter in our universe is actually not in stars, but floats around as such barely visible gas, either inside of galaxies or between them.
> 
> "Space is really a bit like society, the stars attract all the attention, but the real power is in the dark web."
> 
> Comment: Thanks to Sabine Hossenfelder for this coverage and her analysis.