It’s far from a perfect image. [Note added: if you need an introduction to what images like this actually represent (they aren’t photographs of black holes, which are, after all, black…), start with this.]
It’s blurred out in space by imperfections in the telescopic array that is the “Event Horizon Telescope” (EHT) and by dust between us and our galaxy’s center. It’s blurred out in time by the fact that the glowing material around the black hole changes appreciably by the hour, while the EHT’s effective exposure time is a day. There are bright spots in the image that may just be artifacts of exactly where the telescopes are located that are combined together to make up the EHT. The details of the reconstructed image depend on exactly what assumptions are made.
At best, it shows us just a thick ring of radio waves emitted over a day by an ever-changing thick disk of matter around a black hole.
But it’s our galaxy’s black hole. And it’s just the first image. There will be many more to come, sharper and more detailed. Movies will follow. A decade or two from now, what we have been shown today will look quaint.
We already knew the mass of this black hole from other measurements, so there was a prediction for the size of the ring to within twenty percent or so. The prediction was verified today, a basic test of Einstein’s gravity equations. Moreover, EHT’s results now provide some indications that the black hole spins (as expected). And (by pure luck) its spin axis points, very roughly, toward Earth (much like M87’s black hole, whose image was provided by EHT in 2019.)
We can explore these and other details in coming days, and there’s much more to learn in the coming years. But for now, let’s appreciate the picture for what it is. It is an achievement that history will always remember.
The reason we are almost entirely convinced that the universe has lots of matter that doesn’t shine is that we can see many signs of its gravitational effects — for instance, its effect on the motions of stars within galaxies, its ability to bend light a la Einstein, etc. It’s almost certain that most of a galaxy is dark matter. And over the years we’ve convinced ourselves this dark matter almost certainly can’t be made from any type of particle that we already know about.
But to learn more about what it is, we need to find signs of some of its non-gravitational effects, if it has any. One possibility is that dark matter particles, if and when they collide, might annihilate into ordinary known particles. If those known particles are photons, we might be able to detect them. A good way to look for them would be to point a suitable telescope toward the center of the Milky Way, our galaxy, which is one place where we expect dark matter particles to be especially numerous, and collisions among them to be especially common.
And the reason I’m doing this now is that there is a new paper claiming that a signal of this type may have been seen (with a claimed significance of 3.3 standard deviations, after including the look-elsewhere effect.) This is a paper by a theorist, analyzing publicly available data taken by the experimental group that operates the Fermi Large Area Telescope satellite. One should note that the record of theorists making discoveries using experimentalists’ data is very poor. Typically there are either detector-related or statistics-related issues that theorists screw up. And there are risks of bias — I am not yet sure whether the rather sophisticated analysis method used by this theorist was chosen in a blinded fashion. [For instance, did he choose his method first and then look at the data, or did he already know there was a hint of a peak in the data before he started designing his method?] So I would be skeptical of this claim for now. (And the theorist, knowing he’s out on a limb, was careful [and wise] to put the word “Tentative” in his title.) However, stranger things have happened, so I wouldn’t dismiss this claim out of hand either, at least not until the Fermi experimentalists tell us that in their opinion the theorist over-estimated the statistical significance of this particular bump. We’ll be looking forward to what they have to say.
I’ll have a few more details about this for you soon.
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From the CMS experiment at the Large Hadron collider, a proton-proton collision that created a Higgs boson, which subsequently decayed to two particles of light (shown as green rods.)