88 research outputs found
Constraining fundamental constants of physics with quasar absorption line systems
We summarize the attempts by our group and others to derive constraints on
variations of fundamental constants over cosmic time using quasar absorption
lines. Most upper limits reside in the range 0.5-1.5x10-5 at the 3sigma level
over a redshift range of approximately 0.5-2.5 for the fine-structure constant,
alpha, the proton-to-electron mass ratio, mu, and a combination of the proton
gyromagnetic factor and the two previous constants, gp(alpha^2/mu)^nu, for only
one claimed variation of alpha. It is therefore very important to perform new
measurements to improve the sensitivity of the numerous methods to at least
<0.1x10-5 which should be possible in the next few years. Future
instrumentations on ELTs in the optical and/or ALMA, EVLA and SKA pathfinders
in the radio will undoutedly boost this field by allowing to reach much better
signal-to-noise ratios at higher spectral resolution and to perform
measurements on molecules in the ISM of high redshift galaxies.Comment: 11 pages, 3 figure
Search for varying constants of nature from astronomical observation of molecules
The status of searches for possible variation in the constants of nature from
astronomical observation of molecules is reviewed, focusing on the
dimensionless constant representing the proton-electron mass ratio
. The optical detection of H and CO molecules with large
ground-based telescopes (as the ESO-VLT and the Keck telescopes), as well as
the detection of H with the Cosmic Origins Spectrograph aboard the Hubble
Space Telescope is discussed in the context of varying constants, and in
connection to different theoretical scenarios. Radio astronomy provides an
alternative search strategy bearing the advantage that molecules as NH
(ammonia) and CHOH (methanol) can be used, which are much more sensitive to
a varying than diatomic molecules. Current constraints are
for redshift , corresponding to
look-back times of 10-12.5 Gyrs, and for
, corresponding to half the age of the Universe (both at 3
statistical significance). Existing bottlenecks and prospects for future
improvement with novel instrumentation are discussed.Comment: Contribution to Workshop "High Performance Clocks in Space" at the
International Space Science Institute, Bern 201
The fundamental constants and their variation: observational status and theoretical motivations
This article describes the various experimental bounds on the variation of
the fundamental constants of nature. After a discussion on the role of
fundamental constants, of their definition and link with metrology, the various
constraints on the variation of the fine structure constant, the gravitational,
weak and strong interactions couplings and the electron to proton mass ratio
are reviewed. This review aims (1) to provide the basics of each measurement,
(2) to show as clearly as possible why it constrains a given constant and (3)
to point out the underlying hypotheses. Such an investigation is of importance
to compare the different results, particularly in view of understanding the
recent claims of the detections of a variation of the fine structure constant
and of the electron to proton mass ratio in quasar absorption spectra. The
theoretical models leading to the prediction of such variation are also
reviewed, including Kaluza-Klein theories, string theories and other
alternative theories and cosmological implications of these results are
discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy
X-Ray Synchrotron Emission from 10-100 TeV Cosmic-Ray Electrons in the Supernova Remnant SN 1006
We present the results of a joint spectral analysis of RXTE PCA, ASCA SIS,
and ROSAT PSPC data of the supernova remnant SN 1006. This work represents the
first attempt to model both the thermal and nonthermal X-ray emission over the
entire X-ray energy band from 0.12 to 17 keV. The thermal flux is described by
a nonequilibrium ionization model with an electron temperature kT = 0.6 keV, an
ionization timescale n0t = 9 x 10^9 s / cm^3, and a relative elemental
abundance of silicon that is 10-18 times larger than the solar abundance. The
nonthermal X-ray spectrum is described by a broken power law model with low-
and high-energy photon indices Gamma_1 = 2.1 and Gamma_2 = 3.0, respectively.
Since the nonthermal X-ray spectrum steepens with increasing energy, the
results of the present analysis corroborate previous claims that the nonthermal
X-ray emission is produced by synchrotron radiation. We argue that the magnetic
field strength is significantly larger than previous estimates of about 1 x
10^-5 G and arbitrarily use a value of 4 x 10^-5 G to estimate the parameters
of the cosmic-ray electron, proton, and helium spectra of the remnant. The
results for the ratio of the number densities of protons and electrons (R = 160
at 1 GeV), the total energy in cosmic rays (E_cr = 1 x 10^50 ergs), and the
spectral index of the electrons at 1 GeV (Gamma_e = 2.14 +/- 0.12) are
consistent with the hypothesis that Galactic cosmic rays are accelerated
predominantly in the shocks of supernova remnants. Yet, the remnant may or may
not accelerate nuclei to energies as high as the energy of the "knee,"
depending on the reason why the maximum energy of the electrons is only 10 TeV.Comment: 41 pages, 9 figures, accepted for publication in ApJ in Septembe
Universal health coverage from multiple perspectives: a synthesis of conceptual literature and global debates
Background: There is an emerging global consensus on the importance of universal health coverage (UHC), but no unanimity on the conceptual definition and scope of UHC, whether UHC is achievable or not, how to move towards it, common indicators for measuring its progress, and its long-term sustainability. This has resulted in various interpretations of the concept, emanating from different disciplinary perspectives. This paper discusses the various dimensions of UHC emerging from these interpretations and argues for the need to pay attention to the complex interactions across the various components of a health system in the pursuit of UHC as a legal human rights issue. Discussion: The literature presents UHC as a multi-dimensional concept, operationalized in terms of universal population coverage, universal financial protection, and universal access to quality health care, anchored on the basis of health care as an international legal obligation grounded in international human rights laws. As a legal concept, UHC implies the existence of a legal framework that mandates national governments to provide health care to all residents while compelling the international community to support poor nations in implementing this right. As a humanitarian social concept, UHC aims at achieving universal population coverage by enrolling all residents into health-related social security systems and securing equitable entitlements to the benefits from the health system for all. As a health economics concept, UHC guarantees financial protection by providing a shield against the catastrophic and impoverishing consequences of out-of-pocket expenditure, through the implementation of pooled prepaid financing systems. As a public health concept, UHC has attracted several controversies regarding which services should be covered: comprehensive services vs. minimum basic package, and priority disease-specific interventions vs. primary health care. Summary: As a multi-dimensional concept, grounded in international human rights laws, the move towards UHC in LMICs requires all states to effectively recognize the right to health in their national constitutions. It also requires a human rights-focused integrated approach to health service delivery that recognizes the health system as a complex phenomenon with interlinked functional units whose effective interaction are essential to reach the equilibrium called UHC
The Formation of the First Stars in the Universe
In this review, I survey our current understanding of how the very first
stars in the universe formed, with a focus on three main areas of interest: the
formation of the first protogalaxies and the cooling of gas within them, the
nature and extent of fragmentation within the cool gas, and the physics -- in
particular the interplay between protostellar accretion and protostellar
feedback -- that serves to determine the final stellar mass.
In each of these areas, I have attempted to show how our thinking has
developed over recent years, aided in large part by the increasing ease with
which we can now perform detailed numerical simulations of primordial star
formation. I have also tried to indicate the areas where our understanding
remains incomplete, and to identify some of the most important unsolved
problems.Comment: 74 pages, 4 figures. Accepted for publication in Space Science
Review
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