Around 500 astronomers and space scientists will gather at Venue Cymru in Llandudno, Wales, from 5-9 July, for the Royal Astronomical Society National Astronomy Meeting 2015 (NAM2015, Cyfarfod Seryddiaeth Cenedlaethol 2015). The conference is the largest regular professional astronomy event in the UK and will see leading researchers from around the world presenting the latest work in a variety of fields. Science writer and editor Kulvinder Singh Chadha presents his second report from the event:
Sterile neutrino detections: dark matter candidate?
What is dark matter made of? A postgraduate student giving a talk at NAM2015 today may have an answer in the form of a mysterious spectral line in the X-ray region.
Normal, ‘everyday’ matter that makes up stars, planets, gas and dust is something that astronomers can easily detect. It either gives off, reflects, or absorbs light, electromagnetism or radiation. Dark matter’s much more shifty. It’s completely invisible, yet comprises 84.5 percent of all the mass in the Universe. Scientists only know of its existence because of the gravitational effect it has on its environment — even lensing light from distant galaxies by bending the spacetime around it.
Just as the normal matter around us is made of subatomic particles (protons, neutrons, electrons, etc), so scientists surmise, must dark matter be also. The current model favoured by many scientists for dark matter involves ‘Cold Dark Matter’ (CDM) particles, which are slow-moving compared to the speed of light. But no dark matter particle has ever been detected. However, in a talk at NAM2015, Sownak Bose from Durham University’s Institute for Computational Cosmology (ICC) presented new predictions for a different sort of candidate particle: the sterile neutrino. And it may have been detected in an X-ray spectral line.Neutrinos (often called ‘ghost particles’) almost never interact with ordinary matter. This makes them extremely difficult to detect, even though trillions of them pass through every square inch of Earth (and everything on it) every second. So-called ‘sterile neutrinos’ are thought to be even less willing to interact with ordinary matter. Unlike standard neutrinos, however, which have been detected many times despite their elusive nature, sterile neutrinos are only theoretical. “The neutrinos are sterile in that they interact even more weakly than ordinary neutrinos. Their predominant interaction is via gravity,” says Bose. “The key difference with CDM is that just after the Big Bang, sterile neutrinos would have had comparatively large velocities and would thus have been able to move away in random directions.”
In 2014, two independent groups detected an unexplained emission line at X-ray wavelengths in clusters of galaxies, using the Chandra and XMM-Newton X-ray telescopes. The energy of the line fits with predictions for the energies at which sterile neutrinos would decay over the lifetime of the Universe. Bose and colleagues from the ICC in Durham are using sophisticated models of galaxy formation to investigate whether sterile neutrinos corresponding to such a signal could help find the true identity of dark matter. “By modelling how the Universe has evolved from its starting point and looking at the distribution of present-day structures, such as dwarf galaxies, we can test whether sterile neutrinos or CDM best fit with observations,” says Bose. “We may have seen the first evidence for sterile neutrinos and this would be hugely exciting.”
The session chair, Professor Peter Coles says, “Although cosmology has made great progress in recent years, many questions remain unanswered. And indeed, many are unasked. This meeting is a timely opportunity to look at some of the gaps in our current understanding and some of the ideas that are being put forward for how they might be filled.”
Do cosmic jets live in dark matter ‘ponds’?
Cosmic jets — jets of material from active galaxies travelling close to the speed of light, may correlate with dense regions of dark matter in the Universe. A team of PhD students presented results at NAM2015 today that show how the Cosmic Microwave Background Radiation (CMB) can be used as an indicator to test for such a correlation.Cosmic jets exist in active galaxies, many of which have supermassive black holes in their cores that are millions to billions of times the mass of our Sun. The infalling material spiralling towards a galaxy’s black hole forms an accretion disc that gets hotter as it gets closer to the black hole. Eventually the material becomes a super-hot plasma that radiates brightly in X-rays, ultraviolet, visible, infrared and radio wavelengths. Some of this charged plasma can is then channelled along powerful magntic fields at enormous speeds out of the galactic disc. These cosmic jets are easily spotted by astronomers.
Dark matter, however, is completely invisible. It doesn’t emit, reflect or absorb electromagnetic radiation of any kind and is only detectable by its gravitational effects (see also the previous story for today). This gravitational property is what enables dark matter to attract normal matter into ‘ponds’ that may eventually become galaxies. Indeed, many scientists think that this is how major galaxies came into existence — though this is still an intense area of research. And as of yet the nature of dark matter is completely unknown.
The CMB is the oldest, most distant radiation detectable. It originates from a time, just 300,000 years after the Big Bang, when the Universe changed from being hot, dense and opaque, to being transparent. As such, CMB photons traversing the Universe should be affected by certain things lying in their path. Speaking to Astronomy Now, lead author Rupert Allison of the University of Oxford says, “We think that the really dense regions of dark matter are where we should see more of these cosmic jets.” The reason for this is that cosmic jets are indicative of supermassive black holes and thus, major galaxies. Such galaxies are expected to form inside dark matter halos.
Along with Dr Sam Lindsay of Oxford University and Dr Blake Sherwin of UC Berkley, Allison looked for distortions in a map of the CMB using data from the Atacama Cosmology Telescope and the Very Large Array’s First Images of the Radio Sky at Twenty centimetres. Such distortions are giveaways of high mass regions, where large scale gravitation is strong. Einstein’s Theory of General Relativity predicts that strong gravity will distort spacetime, and thus the path of light, or other radiation — almost as if there was a lens in the way. The team saw just such distortions. When compared with a map showing high-than average concentrations of cosmic jets, there was a correlation.
“We can estimate the size of these dark matter halos and they’re around 10^13.6 solar masses in size,” says Allison, who is excited by the prospects of more robust data with the Square Kilometre Array; the largest radio telescope array ever-made and currently under development. As Allison says to Astronomy Now, “What we’re expecting to see is how these jets and their galaxies change over the evolutionary history of the Universe.” The existence of a correlation is just the beginning.