Scientists are finally figuring out how much dark matter – the almost imperceptible material said to tug on everything, yet emit no light – really weighs.
The new estimate helps pin down how heavy its particles could be – with implications for what the mysterious stuff actually is.
The research sharply narrows the potential mass of dark matter particles, from between an estimated 10^minus 24 electronvolts (eV) and 10^19 Gigaelectron volts (GeV) , to between 10^minus 3 eV and 10^7eV – a possible range of masses many trillions of trillions of times smaller than before.
The findings could help dark matter hunters focus their efforts on the indicated range of particle masses – or they might reveal a previously unknown force is at work in the universe, said Xavier Calmet, a professor of physics and astronomy at the University of Sussex in the United Kingdom.
Calmet, along with doctoral student Folkert Kuipers, also of the University of Sussex, described their efforts in a new study to be published in the March issue of Physical Letters B.
What is dark matter?
By some estimates, dark matter makes up about 83 percent of all the matter in the universe. It's thought only to interact with light and ordinary matter through gravity, which means it can only be seen by the way it curves light rays.
Astronomers found the first hints of dark matter when gazing at a galactic cluster in the 1930s, and theories that galaxies are threaded with and fringed by vast halos of dark matter became mainstream after the 1970s, when astronomers realized galaxies were whirling faster than they otherwise should, given how much visible matter they contained.
Possible candidates for dark matter particles include ghostly, tiny particles known as neutrinos, theoretical dark, cold particles known as axions, and proposed weakly-interacting massive particles, or WIMPs.
The new mass bounds could help eliminate some of these candidates, depending on the details of the specific dark matter model, Calmet said.
What scientists do know is that dark matter seems to interact with light and normal matter only through gravity, and not via any of the other fundamental forces; and so the researchers used gravitational theories to arrive at their estimated range for the masses of dark matter particles.
Importantly, they used concepts from theories of quantum gravity, which resulted in a much narrower range than the previous estimates, which used only Einstein's theory of general relativity.
"Our idea was a very simple one," Calmet told Live Science in an email. "It is amazing that people have not thought of this before."
Einstein's theory of general relativity is based on classical physics; it perfectly predicts how gravity works most of the time, but it breaks down in extreme circumstances where quantum mechanical effects become significant, such as at the center of a black hole.
Theories of quantum gravity, on the other hand, try to explain gravity through quantum mechanics, which can already describe the other three known fundamental forces – electromagnetic force, the strong force that holds most matter together, and the weak force that causes radioactive decay.
None of the quantum gravity theories, however, as yet have strong evidence to support them.
Calmet and Kuipers estimated the lower bound for the mass of a dark matter particle using values from general relativity, and estimated the upper bound from the lifetimes of dark matter particles predicted by quantum gravity theories.
The nature of the values from general relativity also defined the nature of the upper bound, so they were able to derive a prediction that was independent of any particular model of quantum gravity, Calmet said.
The study found that while quantum gravitational effects were generally almost insignificant, they became important when a hypothetical dark matter particle took an extremely long time to decay and when the universe was about as old as it is now (roughly 13.8 billion years), he said.
Physicists previously estimated that dark matter particles had to be lighter than the "Planck mass" – about 1.2 x 10^19 GeV, at least a 1,000 times heavier than the largest-known particles – yet heavier than 10^minus 24 eV to fit with observations of the smallest galaxies known to contain dark matter, he said.
But until now, few studies had attempted to narrow the range, even though great progress had been made in understanding quantum gravity over the last 30 years, he said. "People simply did not look at the effects of quantum gravity on dark matter before."
Calmet said the new bounds for the masses of dark matter particles, could also be used to test whether gravity alone interacts with dark matter, which is widely assumed, or if dark matter is influenced by an unknown force of nature.
"If we found a dark matter particle with a mass outside the range discussed our paper, we would not only have discovered dark matter, but also very strong evidence that … there is some new force beyond gravity acting on dark matter," he said.