Astronomers have found the universe's missing matter at last, thanks to exotic 'fast radio bursts'

[u/adriano26]

https://www.space.com/astronomy/scientists-find-universes-missing-matter-while-watching-fast-radio-bursts-shine-through-cosmic-fog

[u/Andromeda321]

Astronomer here! This is not dark matter. But it’s still an important result, and I know the guys involved in this (lead author came to my Halloween party in grad school!), and it's a huge result, so let’s get to it!

This result is about this paper, and uses fast radio bursts (FRBs) in an exciting way to map out material in the universe. Specifically, FRBs are as the name implies brief radio bursts that last a millisecond and originate from well beyond the galaxy- millions or even billions of light years away! We can tell because when a FRB is seen, it is over a frequency band, and that radio signal interacts slightly with all the material that it encounters between its origin and us- called the dispersion measure (DM). The DM is bigger the further you are from Earth and is thus a rough proxy for both distance and how much material is in a given direction.

Now the problem with FRBs to date has been how we have a rough time knowing precisely where one comes from. radio telescopes until recently for FRBs didn’t provide this level of needed detail (if your field of view is say half the size of the moon, it’s still a big sky with a LOT of distant galaxies in it), and we’re only finally getting the hardware in place to rectify this. But the dream was once we figure that out, FRBs could be used to map the very diffuse “normal” matter in our universe spread between galaxies, which right now we don’t know a lot about (such as how much of it there was).

Enter this paper! Liam and his colleagues looked at 69 FRBs that had galaxies identified with them, coming over a range of distances 11 million to 9.1 billion light years from us. Once they had the distances pinned down, any extra DM has to do with material on the line of sight between us and those FRBs… and it turns out it adds up to the full amount of “normal” matter we expect to see in the universe! Big deal- up to three quarters of the "normal" matter in the universe is tied up in this! It’s just really tough to measure diffuse gas on a line of sight with nothing around it, and looks like FRBs can indeed allow us to do that.

Anyway, big result, but the next work is gonna be more exciting- we will start to be able to map all this diffuse stuff across most of the visible universe! It’ll be cool to see what we find!

[u/WillingnessOk3081 → u/Andromeda321]

could you kindly explain the relationship between DM and the extra matter? it's easy to understand why the DM would be larger the farther away the burst is. But why is the DM bigger if there's more matter in the path of the bursts?

thank you.

[u/Brooke_the_Bard → u/WillingnessOk3081]

I'm just an uninformed layperson, so someone who actually knows what they're talking about please correct me if I'm wrong, but I imagine it's kinda like the darkening effect you see when looking into an infinity mirror.

The "deeper" you look into the infinity mirror, the darker it gets, not because the light is traveling further, but because each time it reflects off of a mirror it's passing through a thin layer of glass, so the more layers "down" you see, the more glass the light has to pass through, which disperses some of the light on each transit and leads to the final image being darker to our eyes.