Scientists believe they have found a way to explain why there are not as many galaxies orbiting the Milky Way
as expected. Computer simulations of the formation of our galaxy
suggest that there should be many more small galaxies around the Milky
Way than are observed through telescopes.
This has thrown doubt on the generally accepted theory of cold dark
matter, an invisible and mysterious substance that scientists predict
should allow for more galaxy formation around the Milky Way than is
seen.
Now cosmologists and particle physicists at the Institute for Computational Cosmology and the Institute for Particle Physics Phenomenology, at Durham University, working with colleagues at LAPTh College & University in France, think they have found a potential solution to the problem.
Writing in the journal Monthly Notices of the Royal Astronomical Society,
the scientists suggest that dark matter particles, as well as feeling
the force of gravity, could have interacted with photons and neutrinos
in the young Universe, causing the dark matter to scatter.
Scientists think clumps of dark matter – or haloes – that emerged
from the early Universe, trapped the intergalactic gas needed to form
stars and galaxies. Scattering the dark matter particles wipes out the
structures that can trap gas, stopping more galaxies from forming around
the Milky Way and reducing the number that should exist.
Lead author Dr Celine Boehm, in the Institute for Particle Physics
Phenomenology at Durham University, said: "We don’t know how strong
these interactions should be, so this is where our simulations come in."
"By tuning the strength of the scattering of particles, we change the
number of small galaxies, which lets us learn more about the physics of
dark matter and how it might interact with other particles in the
Universe."
"This is an example of how a cosmological measurement, in this case
the number of galaxies orbiting the Milky Way, is affected by the
microscopic scales of particle physics."
There are several theories about why there are not more galaxies
orbiting the Milky Way, which include the idea that heat from the
Universe’s first stars sterilised the gas needed to form stars. The
researchers say their current findings offer an alternative theory and
could provide a novel technique to probe interactions between other
particles and cold dark matter.
Co-author Professor Carlton Baugh said: "Astronomers have long since
reached the conclusion that most of the matter in the Universe consists
of elementary particles known as dark matter."
"The model predicts that there should be many more small satellite galaxies around our Milky Way than we can observe."
"However, by using computer simulations to allow the dark matter to
become a little more interactive with the rest of the material in the
Universe, such as photons, we can give our cosmic neighbourhood a
makeover and we see a remarkable reduction in the number of galaxies
around us compared with what we originally thought."
The calculations were carried out using the COSMA supercomputer at Durham University, which is part of the UK-wide DiRAC super-computing framework.
The work was funded by the Science and Technology Facilities Council and the European Union.
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All the contacts are available for interview on Monday 8 and Tuesday 9 September.
Professor Carlton Baugh
Institute for Computational Cosmology
Durham University
Tel: +44 (0)191 33 43542
c.m.baugh@durham.ac.uk
Ryan Wilkinson
Institute for Computational Cosmology
Durham University
Tel: +44 (0)191 33 45753
ryan.wilkinson@durham.ac.uk
Jascha Schewtschenko
Institute for Computational Cosmology
Durham University
Tel: +44 (0)191 33 43710
j.a.schewtschenko@durham.ac.uk
Images and captions
Images are available for download from the RAS website and from the Durham media relations team.
The simulated distribution of dark matter in a Milky Way-like galaxy
for standard, non-interacting dark matter (top left), warm dark matter
(top right) and the new dark matter model that interacts with the photon
background (bottom). Smaller structures are erased up to the point
where, in the most extreme model (bottom right), the galaxy is
completely sterilised. Credit: Durham University.
Two models of the dark matter distribution in the halo of a galaxy like the Milky Way,
separated by the white line. The colours represent the density of dark
matter, with red indicating high-density and blue indicating
low-density. On the left is a simulation of how non-interacting cold
dark matter produces an abundance of smaller satellite galaxies. On the
right the simulation shows the situation when the interaction of dark
matter with other particles reduces the number of satellite galaxies we
expect to observe around the Milky Way. Credit: Durham University
Further information
This research has been published in Boehm C. el al., 2014, "Using the Milky Way satellites to study interactions between cold dark matter and radiation", Monthly Notices of the Royal Astronomical Society, vol. 445, p. L31-L35, published by Oxford University Press. A preprint version is available on the arXiv.
Useful Web Links
- Institute for Particle Physics PhenomenologyInstitute for Computational Cosmology
- Department of Physics at Durham University
- STFC
- Monthly Notices of the Royal Astronomical Society, Oxford University Press
Notes for editors
The DiRAC Data Centric system at Durham University is operated by the
Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility.
The DiRAC system is funded by BIS National E-infrastructure capital
grant ST/K00042X/1, STFC capital grant ST/H008519/1, STFC DiRAC
Operations grant ST/K003267/1, and Durham University. DiRAC is part of
the National E-Infrastructure.
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