Possible first signal of dark matter detected

A simulated map of the universe's dark matter (in blue) compiled after extensive observations with the Hubble space telescope.
A simulated map of the universe’s dark matter (in blue) compiled after extensive observations with the Hubble space telescope. Credit: J.-P. Kneib (Observatoire Midi-Pyrenees, Caltech) et al., ESA, NASA

Dark matter is thought to make up more than 80% of the matter in our universe. However, it is relatively difficult to detect for various reasons. The two most important are because

  1. Scientists don’t know what the constituent particles of dark matter are, or how much they could weigh. There are various theories – each of them describes a different particle with distinct properties. Various observatories around the world and in space have been looking for them, with little success.
  2. Dark matter interacts with normal matter only through the force of gravity. And of the four known fundamental forces in nature, gravity is the weakest even though it acts across the largest distances. Moreover, there are enough objects in the universe that exert the gravitational forces. Filtering out a gravitational signal coming solely from dark matter is difficult in this sense.

Thankfully, many of these theories postulate other ways to find dark matter. One of them predicts that the particles of dark matter are sterile neutrinos. Neutrinos are a class of fundamental particles that have the lowest mass in nature – aside from the massless photon – and interact excruciatingly rarely with normal matter.

These interactions are confined to happen through the gravitational force and the weak force. However, sterile neutrinos are unique because they interact only through the gravitational force.

The sterile neutrino

When a sterile neutrino decays, it yields one massless photon and one normal neutrino, according to the sterile neutrino model. Because of the configuration of masses, the photon is detectable as an X-ray emission. Moreover, if the dark matter particle has mass in the keV region, the X-ray photon should have an energy of a few keVs.

Precisely this emission line was detected by two groups of astrophysicists who were studying the Perseus cluster of galaxies, located in the constellation of Perseus, in 2012. It is one of the most massive objects in the universe, and is thought to contain 190 million trillion trillion trillion kilograms of dark matter. This vast quantity means that even if the dark matter decay rate is slow – with lifetimes of 1021 years – there are still about 1077 sterile neutrinos of keV mass decaying into X-ray photons and neutrinos in the Perseus cluster.

One group, lead from the Institute for Theoretical Physics in the University of Leiden, Germany, used the ESA XMM-Newton X-ray observatory to measure an X-ray emission of 3.5 keV coming from the Perseus cluster. Another group, lead from the Harvard-Smithsonian Center for Astrophysics (CfA), USA, used the NASA Chandra X-ray observatory to observe the same emissions. XMM-Newton and Chandra are space-based observatories.

Both groups published their papers in December 2014, and both groups were able to measure the emission with confidence levels more than 99.999%. However, their observations need further confirmation before they can graduate from being scientific evidence to knocking on the doors of scientific fact.

The nature of these confirmations will be twofold. On the one hand, scientists will have to gather more evidence to assert that the X-ray emission is indeed from dark matter decays. To this end, they will need to see if the emission intensity varies according to how the density of dark matter varies through space. They will also look for a Doppler effect, which would make the emission look like a smear in a spectrograph.

On the other hand, to deny that the X-ray emission could be from other sources will require a thorough knowledge of other sources in the same volume of space that could emit X-ray photons and their behavior over time. Fortunately, this knowledge already exists – it was on its strength that the two groups that made the observation were able to postulate that the emission was from dark matter decays. Nevertheless, more detailed descriptions of the gases, elements, compounds and other objects in the area will be sought.

The Astro-H telescope

Even so, there’s one more problem: if the observation was made with high confidence, why was the signal weak? If this is indeed the dark matter signature that scientists have been looking for for over six decades, why isn’t the X-ray emission line more pronounced?

Alexey Boyarsky from the University of Leiden notes,

The [dark matter] decay line is much narrower than the spectral resolution of the present day X-ray telescopes and, as previous searches have shown, should be rather weak.

As if by coincidence, Esra Bulbul from the CfA highlights a solution that’s on the horizon: the planned 14-meter-long Astro-H X-ray telescope to be launched in 2015 by the Japanese Aerospace Exploration Agency. As Bulbul writes,

The future high-resolution Astro-H observations will be able to measure the broadening of the line, which will allow us to measure its velocity dispersion. To detect a dark matter decay line [that is weaker than other lines] will require a significantly long exposure.

The excitement these discoveries have set off is palpable, and deserves to be. Bulbul had told NASA in 2012, “After we submitted the paper, theoreticians came up with about 60 different dark matter types which could explain this line. Some particle physicists have jokingly called this particle a ‘bulbulon’.”

Apart from trying to assert the findings and invalidate competing theories, scientists will – rather could – also look for sterile neutrinos through neutrino experiments on Earth. Although some particles similar to them have been detected in the past, experiments looking for sterile neutrinos henceforth will also have to focus on the 3.5 keV mass scale.