Combined Supernova Data
Constrain Consensus Cosmology
by Simon Mitton
Cosmology continues to top the Physics Top Ten (papers #1,
#4, #8, and #9), but the superconductivity renaissance means that
condensed matter physics now has five contenders (#2, #3, #5, #7,
and #10). Paper #1, however, on the 5-year Wilkinson Microwave
Anisotropy Probe (WMAP) dataset, has roared so far ahead (138
citations this period) that it would appear unlikely that
laboratory physics can capture pole position any time soon. But who
knows what mighty shake-up the Large Hadron Collider could achieve
in a year or so?
Paper #9, a new entrant, is from the Supernova Cosmology Project, a
consortium headed by Saul Perlmutter (Lawrence Berkeley National
Laboratory, California); this team won a half share in the 2007
Gruber Cosmology Prize, the other laureate being the High-z
Supernova Search Team led by Brian Schmidt (Australian National
University). The supernova cosmology game compares the distance of
Type Ia supernovae (SN Ia) with the redshifts of their parent
galaxies. The supernova data provide the best fix on how fast the
universe was expanding at different times in its history. Good
results depend upon observing many SN Ia, both near and far. And in
the quest for good results, #9 revisits old data, combines that
with new data, and puts another turn on the ratchet that constrains
the values of the cosmological parameters.
The Progenitor of a Type Ia Supernova.
Historically, observational cosmology has been plagued by
systematic errors. A new kind of telescope or detector may have
instrumental bias that is initially poorly understood. More
commonly, unknown bias creeps in when data obtained at different
epochs or with different instruments are binned together. Since the
purpose of observational cosmology is to detect how the universe
has changed over time, it is essential that surveys do not
introduce bias. Dealing with such error is mainly what #9 is all
about: taking a heterogeneous compilation, laboriously winnowing
out a reliable dataset, and then refining the cosmological models.
The backstory of #9 starts in 1998, when new surveys of SN Ia at
high redshift (z ~ 0.5) were compared to data on
low-redshift objects (z ~ 0.05). The literature for 1998 to 2007
includes several unrelated high-redshift samples that were analyzed
independently of each other. Paper #9 sieves the original data to
arrive at a more uniform sample. Selection cuts reduced the initial
sample of 414 SN Ia to a union sample of 307.
The investigation achieved several major goals. Importantly, it
includes a new sample of low-redshift SN Ia to complement the
growing sample of high-redshift SN Ia. This is non-trivial because
the volume of the nearby universe is far smaller than the distant
universe: therefore local SN are rare, and the statistics get
complicated by the paucity of the sample. It is still the case that
most low-redshift SN are in a sample made 16 years ago. Nearby and
faraway SN are both needed to constrain cosmological parameters.
The paper is highly cited because it reflects the current best
knowledge of the world’s SN Ia datasets, thanks to the
addition of new datasets from the local universe. The sample is
sufficiently large to permit the exclusion of outliers, and
that’s the main reason why the associated errors have been
reduced. The future of SN cosmology is that high-quality data will
improve our understanding of the accelerating universe, and that
should provide insights into
dark energy and dark
matter.
Elsewhere in our current list, #8 is getting plenty of attention.
This is a report on antiparticles in cosmic rays, and it looks at a
relatively simple question: what is the source of cosmic ray
positrons? Traditionally the answer has been that antiparticles
result from a secondary source: collisions between cosmic ray
particles and atoms in the interstellar medium. Paper #8 reports
data from a satellite-borne experiment that detected electrons and
positrons with energies 1.5 to 100 GeV between July 2006 and
February 2008.
As reported in #8, the measured fraction of higher- energy
positrons cannot be entirely explained by secondary sources; there
have to be primary sources. There are several interesting
candidates for the primary component, including the annihilation of
dark matter particles in the vicinity of our galaxy or contribution
from astrophysical sources, such as pulsars. Right now the data
cannot tell the difference between dark matter annihilation or
astrophysical sources. However, ongoing space missions are slowly
improving the statistics. And then there’s always the Large
Hadron Collider...
Dr. Simon Mitton is a Fellow of St. Edmund’s College,
Cambridge, U.K.
Physics
Top 10 Papers
Rank
Paper
Citations
This Period
(Jul-Aug 09)
Rank
Last Period
(May-Jun 09)
1
E. Komatsu,
et al., "Five-year
Wilkinson Microwave
Anisotropy Probe
observations: Cosmological
interpretation," Astrophys.
J. Suppl. Ser., 180(2):
330-76, February 2009. [14
institutions worldwide] *406EI
138
1
2
X.H. Chen, et al.,
"Superconductivity at 43K in
SmFeAsO1-xFx,"
Nature, 453(7196): 761-2,
5 June 2008. [U. Sci. & Tech.,
Hefei, China] *308UK
62
8
3
Z.A. Ren, et al.,
"Superconductivity at 55 K in
iron-based F-doped layered
quaternary compound
Sm[O1-xFx]FeAs,"
Chinese Phys. Lett.,
25(6): 2215-6, June 2008. [Chinese
Acad. Sci, Beijing] *306MN
56
9
4
J. Dunkley, et al.,
"Five-year Wilkinson Microwave
Anisotropy Probe observations:
Likelihoods and parameters from the
WMAP data," Astrophys.
J. Suppl. Ser., 180(2):
306-29, February 2009. [14 U.S. and
Canadian institutions] *406EI
51
5
5
F.-C. Hsu, et al.,
"Superconductivity in the PbO-type
structure alpha-FeSe,"
PNAS, 105(38): 14262-4, 23
September 2008. [Acad. Sinica,
Taipei, Taiwan; Natl. Tsing Hua U.,
Hsinchu, Taiwan; Duke U., Durham,
NC] *353TY
46
†
6
F. Schedin, et al.,
"Detection of individual gas
molecules adsorbed on
graphene," Nature
Mater., 6(9): 652-5, September
2007. [U. Manchester, U.K.; Inst.
Microelectronics Tech.,
Chernogolovka, Russia; U. Nijmegen,
Netherlands] *207FE
41
†
7
H. Ding, et al.,
"Observation of
Fermi-surface-dependent nodeless
superconducting gaps in
Ba0.6K0.4Fe
2As2,"
EPL-Europhys. Lett.,
83(4): no. 47001, August 2008.
[Chinese Acad. Sci, Beijing; Adv.
Inst. Mater. Res., Tohoku, Japan;
Tohoku U., Japan; Boston Coll., MA]
*345VP
40
†
8
O. Adriani, et al., "An
anomalous positron abundance in
cosmic rays with energies 1.5-100
GeV," Nature, 458(7238):
607-9, 2 April 2009. [17
institutions worldwide] *427RK
35
†
9
M. Kowalski, et al.,
"Improved cosmological constraints
from new, old, and combined
supernova data sets,"
Astrophys. J., 686(2):
749-78, 20 October 2008. [41
institutions worldwide] *364YB
32
†
10
J. Dong, et al.,
"Competing orders and
spin-density-wave instability in
La(O1-xFx)FeAs,"
EPL-Europhys. Lett.,
83(2): no. 27006, July 2008.
[Beijing Natl. Lab. Condensed
Matter Phys., Chinese Acad. Sci.]
*345TZ