Comparison of Cold Dark Matter (CDM) and sterile neutrino simulations of Milky Way-like dark matter haloes (the invisible “skeleton" within which the galaxy will actually form). Credit: M Lovell/ICC Durham. Click here for an enlarged image
What are the mysterious dark matter and dark energy that seem to account for so much of our Universe? Why is the Universe expanding? For the past 30 years, most cosmologists have looked to the ‘standard model’ to answer these questions, and have had wide-ranging success in simulating formation in the universe and matching observational data. But not everything quite fits the predictions. Are these discrepancies down to the interpretation of observations, or is a more fundamental rethink required? On Tuesday 7th July, a special session at the National Astronomy Meeting (NAM) 2015 has been convened for astronomers to take stock of the evidence and stimulate further investigation of cosmology beyond the standard model.
The most popular candidate for the
elusive particles that give the Universe extra mass is Cold Dark Matter
(CDM). CDM particles are thought to move slowly compared to the speed
of light and interact very weakly with electromagnetic radiation.
However, no one has managed to detect CDM to date. Sownak Bose from
Durham University’s Institute for Computational Cosmology (ICC) will
present new predictions at NAM 2015 for a different candidate for dark
matter, the sterile neutrino, which may have been detected recently.
“The neutrinos are sterile in that they interact even more weakly
than ordinary neutrinos; their predominant interaction is via gravity,”
explained Bose. “The key difference with CDM is that just after the Big
Bang, sterile neutrinos would have had comparatively larger velocities
than CDM and would thus have been able to move in random directions away
from where they were born. Structures in the sterile neutrino model are
smeared out, compared to CDM, and the abundance of structures on small
scales is reduced. By modelling how the Universe has evolved from that
starting point and looking at the distribution of present-day
structures, such as dwarf-mass galaxies, we can test which model --
sterile neutrinos or CDM -- fits best with observations.”
Last year, 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 neutrino corresponding
to such a signal could help zero-in on the true identity of dark matter.
“Our models show that a sterile neutrino with a mass corresponding to
the signal detected would also be able to pass many current
astrophysical tests of dark matter," said Bose. “We may have seen the
first evidence for sterile neutrinos and this would be hugely exciting."
However, not everyone believes that extra mass from dark matter is
needed to explain observations. Indranil Banik and colleagues at the
University of St Andrews believe that a modified theory of gravity may
be the answer. Banik and colleagues have constructed a detailed model
predicting velocities of galaxies in the local group, which is dominated
by the mass of our own Milky Way and the neighbouring Andromeda
galaxy.
“On large scales, our Universe is expanding – galaxies further away
are going away from us faster. But on local scales, the picture is more
confusing,” said Banik. “We found that running our model in the context
of Newtonian gravity did not match the observations very well. Some
local group galaxies are travelling outwards so fast that it’s as if the
Milky Way and Andromeda are exerting no gravitational pull at all!”
The St Andrews group suggests that these fast-moving outliers could
be explained by a gravitational boost from a close encounter between the
Milky Way and Andromeda about 9 billion years ago. The very fast
motions of the two galaxies as they flew past each other, at around 600
kilometres per second, would have caused gravitational slingshot effects
on other galaxies in the local group.
“This is like the trick spacecraft use to build up speed to reach the
outer planets in our Solar System. Essentially, the big object – in
this case the Milky Way or Andromeda – is slowed down slightly by the
gravity from a passing object – the dwarf galaxy – which greatly speeds
up as it's much lighter. This fits our observations – but not
predictions with Newtonian gravity. This is just not strong enough to be
compatible with such a close encounter between the Milky Way and
Andromeda. Thus, we believe that our work favours a modified gravity
theory and adds to a growing body of evidence from observations of
galaxies,” said Banik.
The amount of dark energy in the Universe is also a matter of debate.
The first evidence for dark energy – an energy field causing the
expansion of the Universe to accelerate – came through measurements of
Type Ia supernovae, which are used by astronomers as cosmic lighthouses
to determine distances. However, there is now increasing evidence that
Type Ia supernovae are not ‘standard candles’ and the precise brightness
reached by these exploding white dwarf stars depends on the environment
in the host galaxy. Now, Dr Heather Campbell and colleagues at the
University of Cambridge have used the largest sample of supernovae and
host galaxies to date to study the relation between host galaxy and
supernova luminosity.
“Understanding the effect of the properties of the host is critical
if astronomers are to make the most precise measurements possible of
dark energy,” said Campbell. “More massive galaxies tend to have
fainter supernovae. If the galaxy properties are not accounted for
properly, then the amount of dark energy in the Universe is
underestimated. This work is crucial for future telescopes and space
missions such as LSST and Euclid, which will attempt to make precision
measurements of the expansion of the Universe.”
The session convener, Prof Peter Coles said, “Although cosmology has
made great progress in recent years, many questions remain unanswered
and indeed many questions 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 those gaps might be filled.”
Images and captions
Comparison of Cold Dark Matter (CDM) and sterile neutrino simulations
of Milky Way-like dark matter haloes (the invisible “skeleton" within
which the galaxy will actually form). The "Milky Way" would form
somewhere near the centre (the yellowish bit), with its satellite
galaxies distributed among the many of smaller haloes around it. On the
left is a visualisation of the Milky Way environment in a Universe
dominated by CDM; on the right is the same object seen in a sterile
neutrino dark matter Universe. While there are thousands of satellite
galaxies in the CDM model, their abundance is greatly reduced in the
sterile neutrino case. The net result is a “smoother” halo in the
sterile neutrino case, compared to the “lumpy” CDM one. The simulations
were created at the Institute for Computational Cosmology in Durham as
part of the Aquarius supercomputing project undertaken by the Virgo
consortium.
Is this what the night sky looked like billions of years ago?
Cosmologists from St Andrews think that the motion of outlying galaxies
in the Local Group could be explained by a close encounter between the
Milky Way and Andromeda 9 billion years ago.Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger
Type Ia supernovae, such as supernova 1994D in galaxy NGC 4526
(imaged here by the Hubble Space Telescope), are used as cosmic
lighthouses by astronomers to measure distance in the Universe. A team
from the University of Cambridge has used the largest sample of
supernovae and host galaxies to date to study the relation between host
galaxy and the precise brightness of the supernova. Credit: NASA/ESA,
The Hubble Key Project Team and The High-Z Supernova Search Team
Media contacts
Dr Robert Massey
Royal Astronomical Society
Mob: +44 (0)794 124 8035
rm@ras.org.uk
Ms Anita Heward
Royal Astronomical Society
Mob: +44 (0)7756 034 243
anitaheward@btinternet.com
Dr Sam Lindsay
Royal Astronomical Society
Mob: +44 (0) 7957 566 861
sl@ras.org.uk
Science contacts
Mr Sownak Bose
Institute for Computational Cosmology
Durham University
sownak.bose@durham.ac.uk
Dr Heather Campbell
Institute of Astronomy
University of Cambridge
hcc@ast.cam.ac.uk
Mr Indranil Banik
School of Physics and Astronomy
University of St Andrews
ib45@st-andrews.ac.uk
Prof Peter Coles
Head of School of Mathematical and Physical Sciences, Astronomy Centre
University of Sussex
P.Coles@sussex.ac.uk
Futher information
‘Dynamical History of the Local Group in LCDM’, Indranil Banik and Hongsheng Zhao. Submitted to Monthly Notices of the Royal Astronomical Society, June 2015.
http://arxiv.org/abs/1506.07569
Did Andromeda crash into the Milky Way 10 billion years ago?
http://www.ras.org.uk/news-and-press/224-news-2013/2303-did-andromeda-crash-into-the-milky-way-10-billion-years-ago
Note for editors
The Royal Astronomical Society National Astronomy Meeting (NAM 2015, http://nam2015.org)
will take place in Llandudno, Wales, from 5-9 July. NAM 2015 will be
held in conjunction with the annual meetings of the UK Solar Physics
(UKSP) and Magnetosphere Ionosphere Solar-Terrestrial physics (MIST)
groups. The conference is principally sponsored by the Royal
Astronomical Society (RAS) and the Science and Technology Facilities
Council (STFC). Follow the conference on Twitter via @RASNAM2015
The Royal Astronomical Society (RAS, www.ras.org.uk),
founded in 1820, encourages and promotes the study of astronomy,
solar-system science, geophysics and closely related branches of
science. The RAS organises scientific meetings, publishes international
research and review journals, recognizes outstanding achievements by the
award of medals and prizes, maintains an extensive library, supports
education through grants and outreach activities and represents UK
astronomy nationally and internationally. Its more than 3800 members
(Fellows), a third based overseas, include scientific researchers in
universities, observatories and laboratories as well as historians of
astronomy and others. Follow the RAS on Twitter via @royalastrosoc
The Science and Technology Facilities Council (STFC, www.stfc.ac.uk)
is keeping the UK at the forefront of international science and
tackling some of the most significant challenges facing society such as
meeting our future energy needs, monitoring and understanding climate
change, and global security. The Council has a broad science portfolio
and works with the academic and industrial communities to share its
expertise in materials science, space and ground-based astronomy
technologies, laser science, microelectronics, wafer scale
manufacturing, particle and nuclear physics, alternative energy
production, radio communications and radar. It enables UK researchers to
access leading international science facilities for example in the area
of astronomy, the European Southern Observatory. Follow STFC on Twitter
via @stfc_matters
About Durham University
• The Durham astronomy group, including the Centre for
Extragalactic Astronomy, as well as the Institute for Computational
Cosmology, is one of the leading research centres in the world and was
ranked first in Europe and 6th in the world based on impact in the Space
Sciences.
• A world top 100 university with a global reputation and performance in research and education
• A member of the Russell Group of leading research-intensive UK universities
• Research at Durham shapes local, national and
international agendas, and directly informs the teaching of our students
• Ranked in the world’s top 100 universities for
reputation (Times Higher Education World Reputation Review rankings
2015).
• Ranked in the world top 25 for the employability of its
students by blue-chip companies world-wide (QS World University
Rankings 2014/15)
• In the global top 50 for Arts and Humanities (THE World University Rankings 2013/14)
• In the 2016 Complete University Guide, Durham was ranked fifth in the UK.
Durham was named as The Times and Sunday Times 'Sports University of
the Year 2015' in recognition of outstanding performance in both the
research and teaching of sport, and student and community participation
in sport at all levels.
