The hype detector got a good workout with this one:
After an intergalactic search lasting more than two decades, an Australian-led team of scientists say they have finally found the universe’s “missing matter”, solving a mystery that has long stumped astronomers…
Astronomers had been looking at the universe using all sorts of different forms of light — from radio waves through to x-rays and visible light. None revealed the missing matter.
That was until they started to measure fast radio bursts — brief flashes of intense energy found racing across the universe — and discovered the missing matter hiding in the cold dispersed gas between galaxies…
Repeating the process with six different FRBs coming from different parts of the universe, the team was able to figure out how — and where — the missing matter was. The results were published today in the top scientific journal Nature.Michael Slezak and Penny Timms, “Astronomers find ‘missing matter’, solving decades-long mystery of outer space” at ABC Australia
Our physics color commentator Rob Sheldon suggests a timeout:
Well, like most astronomy press releases in the past decade, it is 3/4 hype, and 1/4 data, and it has nothing to do with dark matter. Since many people never read beyond the headline, the title is written to be as provocative as possible without outright lying.
The story starts in 2007 with the discovery of a narrow “whistle” in the archived Parkes Radio Telescope data, only a few milliseconds wide, that is, if our ears could hear radio waves.
In fact it would have been discarded as noise, except the grad student looking at the data had stacked the channels above each other, and watched how this narrow pulse went sliding down in frequency as it got later in time—like the back half of a “wolf whistle,” or an Irish slide-whistle.
At first it was thought to be contamination, like the 2010 “perytons” traced to the microwave oven at Parkes Observatory in Australia. But several more were found in archived data, and then in 2012, one repeated, and was linked to a distant galaxy.
They even got a name, “Fast Radio Bursts.” Theorists still had no idea what they were, with Harvard astronomer Abraham Loeb even publishing a paper to the effect that it was an alien microwave propulsion system.
Loeb likes to burnish his reputation as astronomy’s resident jester.
The Australian Parkes telescope, like the US Greenbank radio telescope, is a single steerable dish, which doesn’t have the resolution of the multiple radio telescopes like the VLBA in New Mexico. There didn’t seem to be a future for these dinosaurs, so the Fast Radio Burst was like fresh blood, or at least, fresh funding. In 2012 a new site in the central desert was dedicated for a “square kilometer array” like the VLBA called ASKAP. And U of Sidney was funded to upgrade an abandoned radio telescope MOST, and make it an FRB antenna just for this search, renamed UTMOST.
Australia really is perfect for radio telescopes, with miles of nothing but empty skies.
But by now it was a Gold Rush for radio telescopes, with new results coming in almost monthly. Just a month ago in April, CalTech astronomers made a claim that an FRB had been detected inside our own galaxy.
That would help the search for the source, but as of yet, no one knows what it is.
So that is the background for this paper on FRB studies. Without being able to crack the mystery of what an FRB is, the authors have decided to tackle the more mundane question, what does it do? FRB’s are thought to be created in unknown stellar upheavals, with highly polarized light that then passes through billions of lightyears on its way to us. This long journey permits the “shorter” or higher-pitched waves to travel faster than the longer, lower-pitched waves, which is why it sounds like a slide whistle when it arrives. And the reason the waves “disperse,” travelling different speeds, is that the ion and electron “plasma” that inhabits interstellar or intergalactic space, acts as a viscous “molasses” for radio waves.
So by locating the galaxy it came from, knowing how far away it is, and then calculating how far apart the high/low frequencies are, we get an estimate of the density or depth of the plasma between here and there. Then if you say, “Well everywhere in the universe has this plasma density,” then you can add it all up and weigh it to find out how much of the Big Bang was due to stars, and how much was due to plasma.
Now when people had done these calculations in the past, they knew how much matter “stuff” they had to put into the Big Bang simulation to get the right “Bang,” but they never actually saw more than 25% of that amount in the universe. (Mind you, this is ordinary matter, the stuff cosmologists call “baryonic matter,” not the mysterious “dark matter” you’ve heard so much about, which by definition, is non-baryonic.)
Well, when they finished their calculations, they got another 25% of the baryonic “stuff,” and were ready to call the press corps. For new investments like ASKAP, it is important that they get in the news early and often to justify their expense, and this is why the press release is both ambiguous and hyperbolic.
Is the hype justified?
Well, I would have to say not. Plasma is important, but it is by no means the only form of matter. Both dust and neutral hydrogen gas fill galaxies and to some extent, inter-galactic voids, and neither of these components can be measured with FRB dispersion. Further, the jump from 25% to 50% is big, but it still doesn’t account for the other half of the missing baryons.
First, we don’t know what a FRB really is. That means it could have been emitted by a giant slide whistle in a distant galaxy, (which is what Abraham Loeb is suggesting), so most of the dispersion was there from the very beginning. If so, then the entire calculation is wrong, and much less plasma exists between here and there. Most of the time, when we can’t solve a puzzle but desperately want to get something useful out of the data, we apply assumptions that are often later found to be wrong.
This paper relies on two or three unjustified assumptions: (1) FRBs start life as an instantaneous pulse over all wavelengths; (2) plasma is the most common form of matter in intergalactic space; and (3) The Big Bang model accurately predicts the baryonic density.
And second, I’ve mentioned before how the Hubble tension (discrepant H0 numbers) are causing people to revisit the BB model. There’s another half-dozen BB problems that have never been solved either, including 7Li abundances, D/H ratios, CMBR smoothness, early galactic formation, dark matter halos, Pop III halo stars, etc. So it is pretty clear that the Big Bang model is going to get a major overhaul in the near future, and who knows whether this 25% baryonic matter will turn into 75% or 5%, so hyping it as “solving the missing matter” is a bit premature.
But the unresolved problem is this: What process makes an FRB, and whether it is born with or without dispersion. That will be the judge of whether this paper is worthless or redeemable. And even Abraham Loeb doesn’t know the answer to that.