how scientists accidentally solved the universe’s weight problem

While studying mysterious fast radio bursts, astronomers figured out why the universe weighs as much as it does and where that mass went.

pondering the universe

No one seems to be exempt from having some sort of an issue with weight nowadays, even all the matter in the universe. You see, by measuring the gravitational pull of all the galaxies, we’re able to see about how much the cosmos weighs. Of course, since the measurement is indirect, our observations don’t necessarily line up with each other and leave us with something a lot like placing a person on a scale only to see a weight roughly twice what would make sense for any human this size. For a long time, astronomers looked for any trace of missing matter and found that sometimes, it’s hiding in plain sight behind clouds of gas and dust. Still, just because you’re now able to see stars and galaxies you couldn’t see before in a region of space, doesn’t mean you can call the whole matter resolve and retire for drinks at the local pub. You still have to get data showing that the same kind of phenomena is hiding stars and galaxies everywhere, which is no trivial task. You have to keep scouring the cosmos for any sign of them to be sure.

Sometimes, though, the universe gives you a break from an unexpected source. In this case, a stray signal from an FRB, which, despite media reports to the contrary are not aliens or just an open microwave door in the telescope’s facility, but real, violent cosmic phenomena, traveled a mind-boggling 6 billion light years to get to Earth. Not only was it the first time that an FRB was pinned down to a particular galaxy, but the radio afterglow studied in unprecedented detail by a significant team of researchers all over the world showed that the missing matter we’ve inferred to exist is in fact there and the radio waves have been bouncing off of it in ways our models say they should. How can we tell? Well, we can see it in the dispersion patterns of the FRB’s signal, which were affected by traveling through the interstellar and intergalactic medium. The more it hit on its way to Earth, the more pronounced the effects which can be generalized, since at the scales involved, the universe is more or less homogeneous in density. Averaging out the matter we think is there at the density the equations say it should be based on its mass gives us a very straightforward benchmark for expected dispersion patterns, which this FRB matched.

Now, newspapers and blogs less familiar with science or unable to read scientific papers will be trumpeting that we’ve solved the mystery of dark matter, which is boring galaxies astronomers couldn’t see before. But that’s not true. That missing matter is actually just part of the 5%, or so of standard, baryonic matter the observable universe is made of. Dark matter and dark energy still make up over 95% of the universe’s mass and their existence was inferred using the same exact methods showing that the missing matter found in the FRB’s dispersion patterns needed to be there. Still, the fact that we now know that we’ve weighed the universe correctly is a huge boon to further research in astronomy and cosmology. Science is very exciting when new data overturns something we’ve long held to be true. But at the same time, we do need at least core principles of how we think the universe works to hold as firm foundations so we could capitalize on breakthroughs and have a context for them. This discovery of missing matter alongside the recent detection of gravitational waves is exactly what we needed: nature’s confirmation that as science moves forward, we’re starting to get key things about how the universe works right.

# space // astronomy / cosmology / frb / physics


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