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Three-dimensional mapping of dark matter reveals the expected filamentary scaffold 20
March 2007 Physics Today
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molecules and imaged the expanding
cloud. The newly unbound atoms sep-
arated from each other in opposite, cor-
related directions. Following Altman,
Demler, and Lukins recipe, the NIST
group found and measured those pair-
wise correlations in the shot noise of
the image.
7
Its hoped that an optical lattice filled
with fermionic atoms could mimic a
high-T
c
superconductor. Conceivably, a
NIST-like experiment in an optical lattice
could reproduce and reveal the mysteri-
ous correlations that lead to the emer-
gence of cuprate superconductivity.
In his autobiography, Hanbury
Brown recalled with bemusement the
theoretical controversies his and
Twisss effect stirred up. One suspects
hed be pleased to see it find another ex-
perimental application.
Charles Day
References
1. T. Rom, T. Best, D. van Oosten, U. Schnei-
der, S. Fölling, B. Paredes, I. Bloch, Nature
444, 733 (2006).
2. T. Jeltes, J. M. McNamara, W. Hogervorst,
W. Vassen, V. Krachmalnicoff, M.
Schellekens, A. Perrin, H. Chang, D.
Boiron, A. Aspect, C. I. Westbrook, Nature
445, 402 (2007).
3. E. Altman, E. Demler, M. D. Lukin, Phys.
Rev. A 70
, 013603 (2004).
4. M. Schellekens, R. Hoppeler, A. Perrin, J.
Viana Gomes, D. Boiron, A. Aspect, C. I.
Westbrook, Science 310, 648 (2005).
5. J. M. McNamara, T. Jeltes, A. S. Tychkov,
W. Hogervorst, W. Vassen, Phys. Rev. Lett.
97, 080404 (2006).
6. S. Fölling, F. Gerbier, A. Widera, O. Man-
del, T. Gericke, I. Bloch, Nature 434, 481
(2005).
7. M. Greiner, C. A. Regal, J. T. Stewart, D. S.
Jin, Phys. Rev. Lett. 94, 110401 (2005).
a
b
c
d
0.2
0.2
0.1
0.1
0
0
DENSITY
400
400
400
200
200
200
200
0
0
0
0
200
200
200
200
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400
x ( m) x ( m) x ( m) x ( m) 4
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4
CORRELA
TION
Figure 3. When released from an optical lattice, rubidium-87 atoms bunch, whereas potassium-40 atoms antibunch. Panels
(a) and (c) show the density profiles of the
87
Rb and
40
K clouds, respectively. Cross-correlating the
87
Rb density profile yields
the bunching signal in (b). Cross-correlating the
40
K density profile yields the antibunching signal in (d). (
87
Rb images
adapted from ref. 6;
40
K images adapted from ref. 1.)
Three-dimensional mapping of dark matter
reveals the expected filamentary scaffold
Model simulations of the large-scale distribution of galaxies have long suggested that galaxies
form on a filamentary network of dark matter. Now gravitational lensing has yielded a look at
that network.
In 2004 and 2005,
the Cosmic Evolu-
tion Survey was granted almost 1000
hours of observing time on the Hubble
Space Telescope
. COSMOS, an interna-
tional collaboration of some 90 as-
tronomers headed by Nick Scoville of
Caltech, used this extraordinary allot-
ment of scarce HST time to peer at very
distant galaxies in a patch of sky about
nine times as big as the full Moon.
Not far from the North Pole of our
own galaxy, this patch was chosen for its
relative freedom from obscuring fore-
ground stars, dust, and local galaxies.
The COSMOS exposure has yielded well
measured positions and shapes for half
a million galaxies out to a redshift z of 3.
Thats a glimpse all the way back to how
galaxies looked 11 billion years ago.
Having completed a gravitational-
lensing analysis of that prodigious ac-
cumulation of observational data, the
collaboration has now reported the
most extensive and detailed study to
date of how the distribution of dark
matter on a cosmological scale has been
evolving over the past 8 billion years.
1
The showpiece of the study is the three-
dimensional dark-matter map dis-
played in figure 1. Charting the distri-
bution of the dark matter lets the
COSMOS team examine how that dis-
tribution has governed the clustering of
ordinary matter into accumulations of
gas and stars.
Nonbaryonic dark matter made up
of still-unidentified weakly interacting
elementary particles is presumed in
standard cosmology to account for
about 85% of all matter. Because it nei-
ther emits nor reflects photons at any
wavelength, astronomers can map it on
large scales only through its gravita-
tional-lensing distortion of background
galaxies (see P
HYSICS
T
ODAY
, Novem-
ber 2006, page 21).
Exploiting the Hubble
Pioneering gravitational-lensing studies
of dark matter on large scales have been
carried out in recent years with ground-
based telescopes.
2
But atmospheric blur-
ring makes it difficult for ground-based
telescopes to measure the typically www.physicstoday.org
March 2007 Physics Today
21
small lensing distortions with the requi-
site precision, especially for the faint
galaxies at the higher redshifts. The
HST
was essential to our charting of the
dark matter with resolution good
enough for interesting comparison with
theory, says Richard Massey (Caltech),
who led the gravitational-lensing analy-
sis of the COSMOS field.
The ellipticity of the distortion im-
parted to a distant galaxys image by
any massive foreground system not
precisely along the line of sight is a few
percent at most. Because thats well
within the range of true galactic ellip-
ticities, only a painstaking statistical
analysis of the nonrandom distribution
of ellipse orientations can yield the so-
called gravitational-shear field from
which one deduces the distribution of
all foreground lensing matter.
But complementary data from large
ground-based telescopes, and indeed
from the European Space Agencys or-
biting XMM-Newton x-ray telescope,
were also essential to the COSMOS un-
dertaking. Gravitational lensing, like its
optical analogue, is strongest when the
lensing mass is halfway between the ob-
server and the object being lensed. In
the absence of distance information
about the distorted background galax-
ies, the shear field yields only an inte-
gral of all the lensing masses along each
line of sight.
Therefore, to get 3D information
about the dark-matter distribution one
needs some measure of how far away
each background galaxy is. Thats
where the ground-based telescopes
come in. Follow-up photometric meas-
urements of almost all the half-million
galaxies found in the COSMOS field by
the HST were carried out in 15 wave-
length bands by the 8-meter Subaru tel-
escope in Hawaii and other large tele-
scopes. Those observations measured
each galaxys redshift, and thus its dis-
tance or, equivalently, its look-back
time. (Galaxies with redshifts less than
0.03 were discarded as being too local.)
Cosmic tomography
To deduce the dependence of the dark-
matter distribution on distance, the
group resorted to whats been called
cosmic tomography. Massey and com-
pany compiled separate gravitational-
shear maps for 12 different redshift bins
of background galaxies. Then, knowing
how the distortion of galaxies at a given
distance depends on the distance of
foreground lensing matter, they were
able to create tomographic slices
through the dark-matter distribution at
different redshift distances.
Figure 2 shows three such slices, at
distances corresponding roughly to
look-back times ranging from 3.5 to 6.5
billion years. The contour lines show
the COSMOS fields distribution of
lensing matter (mostly dark) in each
slice. As expected from the standard
cold-dark-matter scenariowhich as-
sumes that the dark-matter particles are
too cold to stream freely out of deep
Figure 1. Three-dimensional map of the distribution of dark matter in a 1.6-square-
degree patch of sky to a redshift depth of z = 1, which means a look-back time of
almost 8 billion years. The dark matter reveals itself by its gravitational-lensing dis-
tortion of the images of background galaxies. The surface is an isodensity contour
chosen to illustrate the evolving fragmentation of the dark matter by gravitational col-
lapse into filaments and clumps. The interior white shading suggests the filamentary
structure. The smooth continuity of the dark matter in z is probably somewhat exag-
gerated by the limited redshift resolution of the mapping. (Adapted from ref. 1.)
2.9
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149.4
DECLINA
TION
(degrees)
RIGHT
ASCENSION
(degrees)
0
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0.8
1.0
REDSHIFT
z
2.8
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DECLINA
TION
(degrees)
150.6 150.4 150.2 150.0 149.8 149.6
RIGHT ASCENSION (degrees)
z = 0.3
z = 0.5
z = 0.7
62 million light-years
85 million light-years
100 million light-years
Figure 2. Tomographic slices
of the COSMOS fields evolving
distribution of dark matter and
galaxies at three different red-
shifts z, corresponding to look-
back times (with increasing z)
of about 3.5, 5.0, and 6.5 bil-
lion years. The contour lines
show the density variation of
total lensing matter, dark and
baryonic. The colors show the
distribution of galaxies near
each redshift. Green shading
indicates the number density of
galaxies, and blue shading
weights that density in favor of
galaxies with large stellar mass-
es. (Adapted from ref. 1.) 22
March 2007 Physics Today
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gravitational potential wellsthe dark-