122 research outputs found
A search for concentric rings with unusual variance in the 7-year WMAP temperature maps using a fast convolution approach
We present a method for the computation of the variance of cosmic microwave
background (CMB) temperature maps on azimuthally symmetric patches using a fast
convolution approach. As an example of the application of the method, we show
results for the search for concentric rings with unusual variance in the 7-year
WMAP data. We re-analyse claims concerning the unusual variance profile of
rings centred at two locations on the sky that have recently drawn special
attention in the context of the conformal cyclic cosmology scenario proposed by
Penrose (2009). We extend this analysis to rings with larger radii and centred
on other points of the sky. Using the fast convolution technique enables us to
perform this search with higher resolution and a wider range of radii than in
previous studies. We show that for one of the two special points rings with
radii larger than 10 degrees have systematically lower variance in comparison
to the concordance LambdaCDM model predictions. However, we show that this
deviation is caused by the multipoles up to order l=7. Therefore, the deficit
of power for concentric rings with larger radii is yet another manifestation of
the well-known anomalous CMB distribution on large angular scales. Furthermore,
low variance rings can be easily found centred on other points in the sky. In
addition, we show also the results of a search for extremely high variance
rings. As for the low variance rings, some anomalies seem to be related to the
anomalous distribution of the low-order multipoles of the WMAP CMB maps. As
such our results are not consistent with the conformal cyclic cosmology
scenario.Comment: 12 pages, 11 figures, 1 table. Published in MNRAS. This research was
supported by the Agence Nationale de la Recherche (ANR-08-CEXC-0002-01
Nucleosynthesis Constraints on Scalar-Tensor Theories of Gravity
We study the cosmological evolution of massless single-field scalar-tensor
theories of gravitation from the time before the onset of annihilation
and nucleosynthesis up to the present. The cosmological evolution together with
the observational bounds on the abundances of the lightest elements (those
mostly produced in the early universe) place constraints on the coefficients of
the Taylor series expansion of , which specifies the coupling of the
scalar field to matter and is the only free function in the theory. In the case
when has a minimum (i.e., when the theory evolves towards general
relativity) these constraints translate into a stronger limit on the
Post-Newtonian parameters and than any other observational
test. Moreover, our bounds imply that, even at the epoch of annihilation and
nucleosynthesis, the evolution of the universe must be very close to that
predicted by general relativity if we do not want to over- or underproduce
He. Thus the amount of scalar field contribution to gravity is very small
even at such an early epoch.Comment: 15 pages, 2 figures, ReVTeX 3.1, submitted to Phys. Rev. D1
How much will we learn from the CMB ?
The purpose of this article is to give a brief account of what we hope to
learn from the future CMB experiments, essentially from the point of view of
primordial cosmology. After recalling what we have already learnt, the
principles of parameter extraction from the data are summarized. The discussion
is then devoted to the information we could gain about the early universe, in
the framework of the inflationary scenario, or in more exotic scenarios like
brane cosmology.Comment: Invited Talk at "The Early Universe and Cosmological Observations: a
Critical Review", UCT, Cape Town, July 2001; to appear in Class. Quant. Gra
The Beginning and Evolution of the Universe
We review the current standard model for the evolution of the Universe from
an early inflationary epoch to the complex hierarchy of structure seen today.
We summarize and provide key references for the following topics: observations
of the expanding Universe; the hot early Universe and nucleosynthesis; theory
and observations of the cosmic microwave background; Big Bang cosmology;
inflation; dark matter and dark energy; theory of structure formation; the cold
dark matter model; galaxy formation; cosmological simulations; observations of
galaxies, clusters, and quasars; statistical measures of large-scale structure;
and measurement of cosmological parameters. We conclude with discussion of some
open questions in cosmology. This review is designed to provide a graduate
student or other new worker in the field an introduction to the cosmological
literature.Comment: 69 pages. Invited review article for Publications of the Astronomical
Society of the Pacific. Supplementary references, tables, and more concise
PDF file at http://www.physics.drexel.edu/univers
“Excellence R Us”: university research and the fetishisation of excellence
The rhetoric of “excellence” is pervasive across the academy. It is used to refer to research outputs as well as researchers, theory and education, individuals and organisations, from art history to zoology. But does “excellence” actually mean anything? Does this pervasive narrative of “excellence” do any good? Drawing on a range of sources we interrogate “excellence” as a concept and find that it has no intrinsic meaning in academia. Rather it functions as a linguistic interchange mechanism. To investigate whether this linguistic function is useful we examine how the rhetoric of excellence combines with narratives of scarcity and competition to show that the hypercompetition that arises from the performance of “excellence” is completely at odds with the qualities of good research. We trace the roots of issues in reproducibility, fraud, and homophily to this rhetoric. But we also show that this rhetoric is an internal, and not primarily an external, imposition. We conclude by proposing an alternative rhetoric based on soundness and capacity-building. In the final analysis, it turns out that that “excellence” is not excellent. Used in its current unqualified form it is a pernicious and dangerous rhetoric that undermines the very foundations of good research and scholarship
Can the Universe Create Itself?
The question of first-cause has troubled philosophers and cosmologists alike.
Now that it is apparent that our universe began in a Big Bang explosion, the
question of what happened before the Big Bang arises. Inflation seems like a
very promising answer, but as Borde and Vilenkin have shown, the inflationary
state preceding the Big Bang must have had a beginning also. Ultimately, the
difficult question seems to be how to make something out of nothing. This paper
explores the idea that this is the wrong question --- that that is not how the
Universe got here. Instead, we explore the idea of whether there is anything in
the laws of physics that would prevent the Universe from creating itself.
Because spacetimes can be curved and multiply connected, general relativity
allows for the possibility of closed timelike curves (CTCs). Thus, tracing
backwards in time through the original inflationary state we may eventually
encounter a region of CTCs giving no first-cause. This region of CTCs, may well
be over by now (being bounded toward the future by a Cauchy horizon). We
illustrate that such models --- with CTCs --- are not necessarily inconsistent
by demonstrating self-consistent vacuums for Misner space and a multiply
connected de Sitter space in which the renormalized energy-momentum tensor does
not diverge as one approaches the Cauchy horizon and solves Einstein's
equations. We show such a Universe can be classically stable and
self-consistent if and only if the potentials are retarded, giving a natural
explanation of the arrow of time. Some specific scenarios (out of many possible
ones) for this type of model are described. For example: an inflationary
universe gives rise to baby universes, one of which turns out to be itself.
Interestingly, the laws of physics may allow the Universe to be its own mother.Comment: 48 pages, 8 figure
Stellar structure and compact objects before 1940: Towards relativistic astrophysics
Since the mid-1920s, different strands of research used stars as "physics
laboratories" for investigating the nature of matter under extreme densities
and pressures, impossible to realize on Earth. To trace this process this paper
is following the evolution of the concept of a dense core in stars, which was
important both for an understanding of stellar evolution and as a testing
ground for the fast-evolving field of nuclear physics. In spite of the divide
between physicists and astrophysicists, some key actors working in the
cross-fertilized soil of overlapping but different scientific cultures
formulated models and tentative theories that gradually evolved into more
realistic and structured astrophysical objects. These investigations culminated
in the first contact with general relativity in 1939, when J. Robert
Oppenheimer and his students George Volkoff and Hartland Snyder systematically
applied the theory to the dense core of a collapsing neutron star. This
pioneering application of Einstein's theory to an astrophysical compact object
can be regarded as a milestone in the path eventually leading to the emergence
of relativistic astrophysics in the early 1960s.Comment: 83 pages, 4 figures, submitted to the European Physical Journal
Australian Aboriginal Ethnometeorology and Seasonal Calendars
This paper uses a cultural anthropological approach to investigate an indigenous Australian perspective on atmospheric phenomena and seasons, using data gained from historical records and ethnographic fieldwork. Aboriginal people believe that the forces driving the weather are derived from Creation Ancestors and spirits, asserting that short term changes are produced through ritual. By recognizing signals such as wind direction, rainfall, temperature change, celestial movements, animal behaviour and the flowering of plants, Aboriginal people are able to divide the year into seasons. Indigenous calendars vary widely across Australia and reflect annual changes within Aboriginal lifestyles
Does Lateral Transmission Obscure Inheritance in Hunter-Gatherer Languages?
In recent years, linguists have begun to increasingly rely on quantitative phylogenetic approaches to examine language evolution. Some linguists have questioned the suitability of phylogenetic approaches on the grounds that linguistic evolution is largely reticulate due to extensive lateral transmission, or borrowing, among languages. The problem may be particularly pronounced in hunter-gatherer languages, where the conventional wisdom among many linguists is that lexical borrowing rates are so high that tree building approaches cannot provide meaningful insights into evolutionary processes. However, this claim has never been systematically evaluated, in large part because suitable data were unavailable. In addition, little is known about the subsistence, demographic, ecological, and social factors that might mediate variation in rates of borrowing among languages. Here, we evaluate these claims with a large sample of hunter-gatherer languages from three regions around the world. In this study, a list of 204 basic vocabulary items was collected for 122 hunter-gatherer and small-scale cultivator languages from three ecologically diverse case study areas: northern Australia, northwest Amazonia, and California and the Great Basin. Words were rigorously coded for etymological (inheritance) status, and loan rates were calculated. Loan rate variability was examined with respect to language area, subsistence mode, and population size, density, and mobility; these results were then compared to the sample of 41 primarily agriculturalist languages in [1]. Though loan levels varied both within and among regions, they were generally low in all regions (mean 5.06%, median 2.49%, and SD 7.56), despite substantial demographic, ecological, and social variation. Amazonian levels were uniformly very low, with no language exhibiting more than 4%. Rates were low but more variable in the other two study regions, in part because of several outlier languages where rates of borrowing were especially high. High mobility, prestige asymmetries, and language shift may contribute to the high rates in these outliers. No support was found for claims that hunter-gatherer languages borrow more than agriculturalist languages. These results debunk the myth of high borrowing in hunter-gatherer languages and suggest that the evolution of these languages is governed by the same type of rules as those operating in large-scale agriculturalist speech communities. The results also show that local factors are likely to be more critical than general processes in determining high (or low) loan rates
Big Bang nucleosynthesis and physics beyond the Standard Model
The Hubble expansion of galaxies, the 2.73\dK blackbody radiation
background and the cosmic abundances of the light elements argue for a hot,
dense origin of the universe --- the standard Big Bang cosmology --- and enable
its evolution to be traced back fairly reliably to the nucleosynthesis era when
the temperature was of \Or(1) MeV corresponding to an expansion age of
\Or(1) sec. All particles, known and hypothetical, would have been created at
higher temperatures in the early universe and analyses of their possible
effects on the abundances of the synthesized elements enable many interesting
constraints to be obtained on particle properties. These arguments have
usefully complemented laboratory experiments in guiding attempts to extend
physics beyond the Standard SU(3)_{\c}{\otimes}SU(2)_{\L}{\otimes}U(1)_{Y}
Model, incorporating ideas such as supersymmetry, compositeness and
unification. We first present a pedagogical account of relativistic cosmology
and primordial nucleosynthesis, discussing both theoretical and observational
aspects, and then proceed to examine such constraints in detail, in particular
those pertaining to new massless particles and massive unstable particles.
Finally, in a section aimed at particle physicists, we illustrate applications
of such constraints to models of new physics.Comment: 156 pages LaTeX, including 18 PostScript figures; uses ioplppt.sty,
epsf, and personal style file (incl.); Revised and updated to include, e.g.
implications of new deuterium observations in primordial clouds; 2-up
PostScript version (78 pages) available at
ftp://ftp.physics.ox.ac.uk/pub/local/users/sarkar/BBNreview.ps.gz ; to appear
in Reports on Progress in Physic
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