The center of the Milky Way is mysteriously glowing.
Sure, there’s an entire bunch of stars there, along-with a 4 million times the mass of the Sun – but subtract the light from all that, and we’re still left with this mysterious excess gamma ray that suffuses the region.
It’s called the Galactic Center GeV Excess (GCE), and it’s puzzled scientists since its discovery by physicists Lisa Goodenough and Dan Hooper in 2009. In data from NASA’s Fermi telescope, they found excess gamma ray – a number of the foremost energetic-light in Universe – and that we haven’t been ready to directly detect whatever is causing it.
Now physicist Mattia Di Mauro of the National Institute for Nuclear physics in Italy has thrown his hat into the ring. His analysis, he said, points back at dark-matter because the GCE culprit (this was first floated as an evidence by Goodenough & Hooper).
We don’t know what dark matter’ is, just that there’s a mysterious mass out there liable for gravitational effects that can’t be explained by the things we might detect directly – the normal-matter like stars, dust, gas, & galaxies.
For example, galaxies rotate much faster than they ought to if they were just being gravitationally influenced by the normal-matter in them; gravitational lensing – the bending of spacetime around massive objects – is more stronger than it should be. Whatever is creating this extra gravity is beyond our ability to detect directly.
We know substance only by the gravitational effect it’s on other objects, and there’s more of it out there. Roughly 80 percent of all matter within the Universe is dark-matter, albeit we can’t see a scrap of it.
Goodenough & Hooper proposed that, if certain dark matter’ particles called WIMPS (Weakly Interacting Massive Particles) and their antiparticles were to collide, they might annihilate one another, exploding in shower of other particles, including gamma-ray photons. This explanation, they said, fit the info surprisingly well. Other physicists weren’t convinced, one even calling the-explanation “shaky”.
In 2018, another team of scientists proposed that very old, dead stars called pulsars that we haven’t yet seen might be causing the-excess. this is often plausible, because the galactic center is crowded, dusty, and really very-energetic – it might be pretty easy to miss a star or several.
Recent studies also found that the distribution of the GCE isn’t smooth – as you’d expect from dark-matter annihilation – but kind of clumpy & speckled, which the pulsar team interpreted as according to point sources, like stars.
Then another team came along and ruled that speckly gamma ray might be produced by dark-matter, putting it back on the table. Yet more researchers then generated a series of exhaustive models of the galactic center with dark-matter annihilation by-using a range of masses across the foremost commonly searched regimes. They found that WIMPs were unlikely to be the result of the GCE.
Back to Di Mauro. His study compares data from the Fermi telescope over the last 11 years against measurements of other astronomical anomalies recorded by the Pamela ionizing radiation detector aboard the Resurs-DK No.1 satellite and therefore the Alpha Magnetic Spectrometer experiment aboard the ISS.
In particular, his study uses the broadest set of information from Fermi collected over the last year, and minimizes the uncertainties introduced by background . This has provided, Di Mauro said, information about the spatial distribution of the GCE which will help rule out various explanations.
“If the excess was, for instance , caused by the interaction between cosmic rays & atoms, we might expect to-observe its greater spatial distribution at lower energies and its lower diffusion at higher energies thanks to the propagations of cosmic particles,” he explained.
“My study, on the other side, underlines how spatial distribution of the-excess doesn’t change as a function of energy.”
This, he said, had never been observed before, and might be explained by dark-matter, since we expect that dark matter’ particles should have similar energies.
“The analysis clearly shows that the-excess of gamma rays is concentrated within the galactic center, exactly what we might expect to seek out within the heart of the Milky Way if dark matter is actually a new type of particle,” he said.
As for what that particle is, it’s still an enormous mystery. In second preprint paper, Di Mauro and his colleague Martin Wolfgang Winkler of Stockholm University in Sweden have attempted to bring it out of the shadows by checking out a gamma-ray excess in nearby dwarf spheroidal galaxies. They didn’t find one, but that null detection has enabled constraints on the mass of the dark matter particle.
These constraints, they said, are compatible with the GCE.
So does this mean dark-matter is causing the GCE? No – but it means we can’t say dark matter’ isn’t causing it, either. Basically, the entire thing is as puzzling as it’s ever been, and we’re getting to need some pretty fascinating science (and tons more observations, probably) to even begin to untangle it.
We can feel high disturbance in force, like many physicists rubbing their hands together in anticipatory glee.
This research has been published in Physical Review D, and arXiv.
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