If some of the dark matter in the universe is decaying into undetectable radiation, it would solve a niggling mystery about the rate at which our universe is expanding. And if dark matter is decaying, it has major implications for the experiments that are looking for it.
Astronomers infer the presence of dark matter because of its gravitational influence on stars and galaxies. The standard model of cosmology assumes that the amount of dark matter has remained constant from when matter began dominating over radiation in our universe鈥檚 history, about 50,000 years after the big bang.
But, 鈥渋t鈥檚 by no means clear that this is true,鈥� says at University of Oslo in Norway.
Advertisement
So, what if it weren鈥檛? Bringmann and colleagues decided to test whether the observational data sit well with another model of dark matter, in which it is slowly decaying into radiation. This radiation would have to be something 鈥渄ark鈥� and thus invisible to our instruments, otherwise we鈥檇 have seen it by now.
Clashing calculations
One of the strongest constraints on any change in the amount of dark matter over the course of the universe鈥檚 evolution comes from precision measurements of the cosmic microwave background (CMB), the radiation left over from the big bang.
The variations in the temperature of the CMB from place to place in the sky can be linked to fluctuations in the density of dark matter when CMB was formed, about 380,000 years after the big bang.
Bringmann鈥檚 team found that the CMB data only allows for a few per cent of the dark matter to turn into radiation in the matter-dominated era of the universe. But even this small amount is enough to explain a puzzling but persistent tension or discrepancy in our measurements of the rate at which the universe is expanding, the so-called Hubble rate.
When cosmologists calculate the Hubble rate of today鈥檚 universe by extrapolating the CMB data and compare it to direct measurements of the current Hubble rate based on counts of galaxies and galaxy clusters in the nearby universe, they find a mismatch. The results are off by about 10 per cent.
鈥淚t鈥檚 not a strong enough discrepancy to call it a crisis, but something may be going on,鈥� says Bringmann. 鈥淎nd it is not going away. It鈥檚 stuck with us for the last 2 or 3 generation of experiments.鈥�
Possible new physics
The researchers found that even if a few per cent of the dark matter in the universe has decayed into dark radiation, this would resolve the tension, increasing the CMB-derived Hubble rate and bringing it in line with the observational calculations from the local universe. The next step, says Bringmann, is to improve the accuracy of our measurements of today鈥檚 Hubble rate.
, a cosmologist at the University of California at Berkeley, finds the study interesting enough to warrant further study. 鈥淚t is important to explore the alternatives,鈥� he says. 鈥淚f the tensions persist we might not only rule out [the standard model of cosmology], but also get a handle of whatever new physics is needed to supersede it.鈥�
If dark matter is indeed decaying, it鈥檇 have far reaching implications for all the experiments that are searching for so-called weakly interacting massive particles (WIMPs), the current favoured type of dark matter. In most models of WIMPs, the particles don鈥檛 decay. 鈥淚t鈥檇 immediately imply that a whole class of vanilla WIMP dark matter scenarios would be ruled out,鈥� says Bringmann.
Reference:
Read more: The universe still seems to be expanding faster than it ought to
