From The University of Melbourne (AU): “Using neutron stars to detect dark matter” 


From The University of Melbourne (AU)

Nicole Bell

Illustration showing gamma-rays from a neutron star. Credit: The National Aeronautics and Space Agency (US).

The search for dark matter may need a detector larger than we can build on Earth, but it could be that a neutron star can do the job.

The quest to uncover the nature of dark matter is one of the greatest challenges in science today, but the key to finally understanding this mysterious substance may well lie in the stars.

Or to be precise, one particular type of star – the neutron star.

Neutron stars are dense enough to capture dark matter. Picture: Illustration of a neutron star/NASA.

So far, scientists have been able to infer the existence of dark matter, but not directly observe it. Actually detecting dark matter particles in experiments on Earth is a formidable task, because the interactions of dark matter particles with regular matter are exceedingly rare.

To search for these incredibly rare signals, we need a very large detector – perhaps so big that it is impracticable to build a detector large enough on Earth. However, Nature provides an alternative option in the form of neutron stars – an entire neutron star can act as the ultimate dark matter detector.

In research published in Physical Review Letters, we have determined how to much more accurately use information gained from these unique natural dark matter detectors.

Neutron stars are the densest stars known to exist and form when giant stars die in supernovae explosions. Left behind is a collapsed core, in which gravity presses matter together so tightly that protons and electrons combine to make neutrons. With a mass comparable to that of the Sun – compressed into a 10km radius – one teaspoon of neutron star material has a mass of about a billion tons!

These stars are ‘cosmic laboratories’, enabling us to study how dark matter behaves under extreme conditions that cannot be replicated on Earth.

Dark matter interacts only very weakly with ordinary matter. For example, it can pass through a light-year of lead (about 10 trillion kilometres) without being stopped. Incredibly, however, neutron stars are so dense that they may be able to trap all dark matter particles that pass through them.

Theoretically, the dark matter particles would collide with neutrons in the star, lose energy, and become gravitationally trapped. Over time, dark matter particles would accumulate in the core of the star. This is expected to heat up old, cold, neutron stars to a level that may be in reach of future observations. In extreme cases, the accumulation of dark matter may trigger the collapse of the star to a black hole.

This means that neutron stars may allow us to probe certain types of dark matter that would be difficult or impossible to observe in experiments on Earth.

On Earth, dark matter experiments look for tiny nuclear-recoil signals, caused by incredibly rare collisions of slow-moving dark matter particles. In comparison, the strong gravitational field of a neutron star accelerates dark matter to quasi-relativistic speeds, resulting in much higher energy collisions.

Another problem for Earth-based detection is that nuclear-recoil experiments are most sensitive to dark matter particles that have a similar mass to atomic nuclei, making it harder to detect dark matter that might be much lighter or heavier.

However, dark matter particles can theoretically be trapped in stars and planets in considerable amounts, regardless of how light or heavy they are.

A critical challenge in using neutron stars to detect dark matter is ensuring that the calculations scientists use, fully account for the unique environment of the star. Although the capture of dark matter in neutron stars had been studied for decades, existing calculations have missed important physical effects.

The calculations used to detect dark matter in neutron stars need to fully account for the star’s unique environment. Picture: Getty Images.

So, our team set about making key improvements to the calculation of the dark matter capture rate – i.e., how fast the dark matter accumulates in neutron stars – which changed the answers considerably.

Our research correctly accounts for nucleon structure, rather than treating the neutrons as point particles, and includes the effects of strong forces between nucleons, rather than modelling the neutrons as a free gas of particles. This built upon our earlier work [Journal of Cosmology and Astroparticle Physics] in which we incorporated the composition of the star, relativistic effects, quantum statistics and gravitational focusing.

Put simply, we showed how to correctly think about dark matter collisions in the extreme neutron star environment, which is so very different to dark matter detectors on Earth.

This new research greatly increases the accuracy and robustness of our estimates of the dark matter capture rate. This paves the way for us to better determine the strength of dark matter interactions with ordinary matter.

Ultimately, evidence (or lack of evidence) of dark matter accumulation in stars would provide valuable clues about where to target experimental efforts on Earth, helping to unlock the mystery of dark matter.

The research team involved scientists from the ARC Centre of Excellence for Dark Matter Particle Physics (AU), including Dr Sandra Robles, Michael Virgato and Professor Nicole Bell from the University of Melbourne, Dr Giorgio Busoni from The MPG Institute for Nuclear Physics [MPG Institut für Kernphysik](DE), and Theo Motta and Professor Anthony Thomas AC from The University of Adelaide(AU).

Dark Matter Background
Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM, denied the Nobel, some 30 years later, did most of the work on Dark Matter.

Fritz Zwicky from http://

Coma cluster via NASA/ESA Hubble.

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.
Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science).

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL).

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970

Dark Matter Research

LBNL LZ Dark Matter Experiment (US) xenon detector at Sanford Underground Research Facility(US) Credit: Matt Kapust.

Lamda Cold Dark Matter Accerated Expansion of The universe http the-cosmic-inflation-suggests-the-existence-of-parallel-universes. Credit: Alex Mittelmann.

DAMA at Gran Sasso uses sodium iodide housed in copper to hunt for dark matter LNGS-INFN.

Yale HAYSTAC axion dark matter experiment at Yale’s Wright Lab.

DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB (CA) deep in Sudbury’s Creighton Mine

The LBNL LZ Dark Matter Experiment (US) Dark Matter project at SURF, Lead, SD, USA.

DAMA-LIBRA Dark Matter experiment at the Italian National Institute for Nuclear Physics’ (INFN’s) Gran Sasso National Laboratories (LNGS) located in the Abruzzo region of central Italy.

DARWIN Dark Matter experiment. A design study for a next-generation, multi-ton dark matter detector in Europe at the University of Zurich.

PandaX II Dark Matter experiment at Jin-ping Underground Laboratory (CJPL) in Sichuan, China.

Inside the Axion Dark Matter eXperiment U Washington (US) Credit : Mark Stone U. of Washington. Axion Dark Matter Experiment.

See the full article here .

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The University of Melbourne (AU) is an Australian public research university located in Melbourne, Victoria. Founded in 1853, it is Australia’s second oldest university and the oldest in Victoria. Times Higher Education ranks Melbourne as 33rd in the world, while the Academic Ranking of World Universities places Melbourne 44th in the world (both first in Australia).

Melbourne’s main campus is located in Parkville, an inner suburb north of the Melbourne central business district, with several other campuses located across Victoria. Melbourne is a sandstone university and a member of the Group of Eight, Universitas 21 and the Association of Pacific Rim Universities. Since 1872 various residential colleges have become affiliated with the university. There are 12 colleges located on the main campus and in nearby suburbs offering academic, sporting and cultural programs alongside accommodation for Melbourne students and faculty.

Melbourne comprises 11 separate academic units and is associated with numerous institutes and research centres, including the Walter and Eliza Hall Institute of Medical Research, Florey Institute of Neuroscience and Mental Health, the Melbourne Institute of Applied Economic and Social Research and the Grattan Institute. Amongst Melbourne’s 15 graduate schools the Melbourne Business School, the Melbourne Law School and the Melbourne Medical School are particularly well regarded.

Four Australian prime ministers and five governors-general have graduated from Melbourne. Nine Nobel laureates have been students or faculty, the most of any Australian university.