A mys t e r i o u s
r e p u l s i ve fo r c e
seems to dominate
the Cosmos
Dark energy
VISIONS 19
B R I E F I N G P A P E R S F O R
P O L I C Y M A K E R S
The distribution of
galaxies as revealed
in the UK-supported
2dF Galaxy Redshift
Survey. The clustering
of matter supports the
dark energy scenario
The dark side of the Universe
I
Three of the most
distant supernovae
known, discovered with
the Hubble Space
Telescope and used to
trace the expansion
rate of the Universe
Before Supernova
Before Supernova
NASA and A Riess (STScl)
Before Supernova
n 1998, astronomers made a
shocking discovery that has
forced them to re-think their
ideas about the nature of the
Cosmos. Surveys of exploding
stars – supernovae – in distant
galaxies indicated that they were
much further away than
expected. We know that the
Universe has been expanding
ever since its birth in the Big
Bang, but the new observations
suggested that this expansion
was accelerating, starting about
6 billion years ago.
This was not what
cosmologists wanted to hear;
they had developed a robust
description of how the Universe
has evolved into the galaxies and
clusters of galaxies we see
today. A favoured idea was that
THANKS
GO TO
ROBERT CRITTENDEN
OF THE
the Universe contained a
critical density of matter and
energy to keep it ‘flat’: according
to Einstein’s General Theory of
Relativity, the gravitational force
exerted by matter makes space
curve round, which is then
counterbalanced by the energy
of expansion.
This critical density included
the visible stars and galaxies,
and a larger amount of invisible
dark matter. The acceleration of
expansion suggests, however,
that to retain a flat geometry
there must be another
component – a repulsive energy
field that is pushing everything
apart. This ‘dark energy’ actually
accounts for a huge 70 per cent
of the critical density, with about
25 per cent dark matter and
5 per cent ordinary matter.
The new results
The evidence for dark energy
was based on a certain type
of supernova, which emits a
characteristic amount of light,
and therefore acts as a ‘standard
candle’ for calculating distance.
Combining the supernova data
with measurements of the
redshifts of their host galaxies
UNIVERSITY
OF
allowed the survey teams to
calculate the expansion rate at
different epochs. (The light from
galaxies is shifted to the red end
of the spectrum depending on
how fast they are moving away
from us; the faster they recede,
the further away they are, and
the further back in time we are
seeing them.)
Although doubts remain about
the reliability of supernovas for
measuring cosmological
distances, other observations
provide supporting evidence for
dark energy. First, measurements
of fluctuations in the cool
radiation from the first structures
that formed after the Big Bang
and developed into galaxies –
the cosmic microwave
background (CMB) – have given
information about the Universe’s
geometry. The scaling of the
fluctuations indicates that the
Universe is flat. In addition,
large-scale surveys of galaxy
redshifts looking at the
distributions of galaxies and how
they cluster together over time
under the effects of gravity have
provided a measure of their total
average mass. Extrapolating the
results to the whole Universe
gives a mass density that is only
30 per cent of the critical density.
PORTSMOUTH
AND
ROBERTO TROTTA
OF
What is dark energy?
Faced with these uncomfortable
results, theorists have re-visited
the basics of modern physics –
relativity and quantum theory –
to find an explanation for dark
energy. The simplest idea lies in
Einstein’s gravitational equations
as applied to cosmology.
Einstein had introduced a
constant to keep the Universe
static, but later discarded it
when Edwin Hubble showed
that galaxies were, in fact, flying
apart. This Cosmological
Principle has been attributed to
the energy of virtual particles
popping in and out of empty
space – an intrinsic concept in
quantum theory. Unfortunately,
this simple interpretation
predicts an energy density that
is 120 orders of magnitude
larger than is observed and
would have been too large in the
early Universe to allow galaxies
to have formed under gravity.
Another candidate is a
dynamic form of energy called
quintessence which evolves
over time, working like a field of
springs to exert a negative
pressure on space. It is a gentler
version of a phenomenon called
‘inflation’ – when the Universe
blew up very rapidly just after
the Big Bang to become spatially
flat, with the structure we see
now. Other, more exotic ideas
have been proposed, such as
phantom energy which gets
stronger with expansion leading
to a ‘big rip’ when all matter is
just torn apart.
Finally, it may be that
Einstein’s theory of gravity
needs to be modified over large
scales. One suggestion is that
our four-dimensional Universe is
embedded in higher dimensions;
gravity leaks into them, so that
its grip on matter weakens and
causes the cosmic expansion
rate to increase. Another
THE
UNIVERSITY
OF
OXFORD
speculative idea is that gravity
could vary in different parts of
the Universe and that we are
just lucky to be in a region
where the conditions are
suitable for our existence.
Further observations
The next steps must be to
measure the expansion in more
detail and establish how it
evolves with time. Recent
supernova surveys and also
X-ray studies of hot gas in galaxy
clusters suggest that dark
energy does behave like the
Cosmological Constant. But
what is needed is a variety of
independent tests. For example,
sound waves generated in the
primordial gas that comprised
the early Universe leave their
fossil imprint on the clustering of
galaxies. The scale of these
acoustic oscillations at different
epochs offers a complementary
yardstick for expansion. Another
approach is to measure the
distortion in images of distant
galaxies, which is created when
their light is bent by the gravity
of galaxies lying in front of them.
The statistics of this weak
gravitational lensing depend on
the rate at which galaxies
clustered – and thus the
expansion rate. Finally, subtle
variations in the CMB, arising
from the effects of a repulsive
energy on the interplay between
light and matter in the early
Universe, are also a useful probe.
The UK is committed to
playing a leading role in such
studies. Dedicated international
programmes such as the Dark
Energy Survey will observe
supernovas, galaxy clustering
and weak lensing over the
evolution of the Universe. Galaxy
clustering will be measured with
a new spectrograph, WFMOS,
built for installation on one of the
world’s 8-metre telescopes, and
also by a giant radiotelescope,
the Square Kilometre Array, now
being planned as a global project.
Dark energy is also being
investigated at the subatomic
scale. For example, some
theories predict a value for dark
energy similar to the masses of
neutrinos – wispy particles
thought to fill the Universe –
so their role is being explored.
Furthermore, the new Large
Hadron Collider shortly to start
up at Europe’s main particle
physics laboratory, CERN, will be
able to probe modified gravity
ideas by searching for evidence
of hidden dimensions.
In fact, dark energy causes
just as many headaches for
particle physicists as it does for
cosmologists, since it is not
easily explained by theories of
particles and forces such as
superstrings. The discovery of
this mysterious new energy
opens a new window on our
understanding of existence and
there are clearly exciting, if
unpredictable, times ahead.
LSST
Dark Energy Survey
WFMOS
Pan-STARRS
SNAP
P R O P O S E D D E D I C AT E D D A R K E N E R G Y P R O J E C T S
Large Synoptic Survey Telescope (LSST) – US collaboration
Dark Energy Survey (DES) – international collaboration
Wide-field Fibre Multi-Object Spectrograph (WFMOS) – international collaboration
Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) – University of Hawaii
Joint Dark Energy Mission (JDEM) – NASA. JDEM includes the Supernova Acceleration Probe
(SNAP), the Advanced Dark Energy Physics Telescope (ADEPT) and the DESTINY space telescope
FOR THEIR HELP WITH THIS PAPER
Vi s i o n s i s a s e r i e s o f p a p e r s w h i ch
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FOR FURTHER INFORMATION CONTACT:
Department of Higher Education and Research
The Institute of Physics
76 Portland Place, London W1B 1NT, UK
e-mail: [email protected]
Institute website: http://www.iop.org
For more information on dark energy, go to:
www-astro.physics.ox.ac.uk/darksector
www.scienceatstake.com
http://en.wikipedia.org/wiki/Dark_energy
http://physicsweb.org/articles/world/17/5/7
www.darkenergysurvey.org
http://snap.lbl.gov
www.lsst.org/lsst_home.shtml
A computer graphic
showing the simulated
distribution of dark
matter in a galaxy cluster
formed in the Universe
with dark energy
Courtesy of Andrey Kravtsov /
University of Chicago
Inset:
The infant Universe
seen in microwaves from
WMAP. The temperature
fluctuations (colour
differences) correspond
to the seeds that
became galaxies
Below left:
Studies of the cosmic
microwave background
reveal the evolution of
the Cosmos