7,211 research outputs found
Galactic Punctuated Equilibrium: How to Undermine Carter's Anthropic Argument in Astrobiology
We investigate a new strategy which can defeat the (in)famous Carter's
"anthropic" argument against extraterrestrial life and intelligence. In
contrast to those already considered by Wilson, Livio, and others, the present
approach is based on relaxing hidden uniformitarian assumptions, considering
instead a dynamical succession of evolutionary regimes governed by both global
(Galaxy-wide) and local (planet- or planetary system-limited) regulation
mechanisms. This is in accordance with recent developments in both astrophysics
and evolutionary biology. Notably, our increased understanding of the nature of
supernovae and gamma-ray bursts, as well as of strong coupling between the
Solar System and the Galaxy on one hand, and the theories of "punctuated
equilibria" of Eldredge and Gould and "macroevolutionary regimes" of Jablonski,
Valentine, et al. on the other, are in full accordance with the regulation-
mechanism picture. The application of this particular strategy highlights the
limits of application of Carter's argument, and indicates that in the real
universe its applicability conditions are not satisfied. We conclude that
drawing far-reaching conclusions about the scarcity of extraterrestrial
intelligence and the prospects of our efforts to detect it on the basis of this
argument is unwarranted.Comment: 3 figures, 26 page
Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures
We propose that retinal-based phototrophy arose early in the evolution of
life on Earth, profoundly impacting the development of photosynthesis and
creating implications for the search for life beyond our planet. While the
early evolutionary history of phototrophy is largely in the realm of the
unknown, the onset of oxygenic photosynthesis in primitive cyanobacteria
significantly altered the Earth's atmosphere by contributing to the rise of
oxygen ~2.3 billion years ago. However, photosynthetic chlorophyll and
bacteriochlorophyll pigments lack appreciable absorption at wavelengths about
500-600 nm, an energy-rich region of the solar spectrum. By contrast, simpler
retinal-based light-harvesting systems such as the haloarchaeal purple membrane
protein bacteriorhodopsin show a strong well-defined peak of absorbance
centered at 568 nm, which is complementary to that of chlorophyll pigments. We
propose a scenario where simple retinal-based light-harvesting systems like
that of the purple chromoprotein bacteriorhodopsin, originally discovered in
halophilic Archaea, may have dominated prior to the development of
photosynthesis. We explore this hypothesis, termed the 'Purple Earth,' and
discuss how retinal photopigments may serve as remote biosignatures for
exoplanet research.Comment: Published Open Access in the International Journal of Astrobiology;
10 pages, 6 figure
The lithium problem, a phenomenologist's perspective
Thirty years after the first observation of the 7Li isotope in the atmosphere
of metal-poor halo stars, the puzzle about its origin persists. Do current
observations still support the existence of a "plateau": a single value of
lithium abundance, constant over several orders of magnitude in the metallicity
of the target star? If this plateau exists, is it universal in terms of
observational loci of target stars? Is it possible to explain such observations
with known astrophysical processes? Can yet poorly explored astrophysical
mechanisms explain the observations or do we need to invoke physics beyond the
standard model of Cosmology and/or the standard model of Particle Physics to
explain them? Is there a 6Li problem, and is it connected to the 7Li one? These
questions have been discussed at the Paris workshop Lithium in the Cosmos, and
I summarize here its contents, providing an overview from the perspective of a
phenomenologist.Comment: Proceedings of the workshop "Lithium in the Cosmos"; held at Institut
d'Astrophysique de Paris in February 27-29, 2012. To appear in Mem.S.A.It.
Suppl., Vol 2
Turbulence and turbulent mixing in natural fluids
Turbulence and turbulent mixing in natural fluids begins with big bang
turbulence powered by spinning combustible combinations of Planck particles and
Planck antiparticles. Particle prograde accretions on a spinning pair releases
42% of the particle rest mass energy to produce more fuel for turbulent
combustion. Negative viscous stresses and negative turbulence stresses work
against gravity, extracting mass-energy and space-time from the vacuum.
Turbulence mixes cooling temperatures until strong-force viscous stresses
freeze out turbulent mixing patterns as the first fossil turbulence. Cosmic
microwave background temperature anisotropies show big bang turbulence fossils
along with fossils of weak plasma turbulence triggered as plasma photon-viscous
forces permit gravitational fragmentation on supercluster to galaxy mass
scales. Turbulent morphologies and viscous-turbulent lengths appear as linear
gas-proto-galaxy-clusters in the Hubble ultra-deep-field at z~7. Proto-galaxies
fragment into Jeans-mass-clumps of primordial-gas-planets at decoupling: the
dark matter of galaxies. Shortly after the plasma to gas transition,
planet-mergers produce stars that explode on overfeeding to fertilize and
distribute the first life.Comment: 23 pages 12 figures, Turbulent Mixing and Beyond 2009 International
Center for Theoretical Physics conference, Trieste, Italy. Revision according
to Referee comments. Accepted for Physica Scripta Topical Issue to be
published in 201
Generation of an optimal target list for the Exoplanet Characterisation Observatory (EChO)
The Exoplanet Characterisation Observatory (EChO) has been studied as a space
mission concept by the European Space Agency in the context of the M3 selection
process. Through direct measurement of the atmospheric chemical composition of
hundreds of exoplanets, EChO would address fundamental questions such as: What
are exoplanets made of? How do planets form and evolve? What is the origin of
exoplanet diversity?
More specifically, EChO is a dedicated survey mission for transit and eclipse
spectroscopy capable of observing a large, diverse and well-defined planetary
sample within its four to six year mission lifetime.
In this paper we use the end-to-end instrument simulator EChOSim to model the
currently discovered targets, to gauge which targets are observable and assess
the EChO performances obtainable for each observing tier and time. We show that
EChO would be capable of observing over 170 relativity diverse planets if it
were launched today, and the wealth of optimal targets for EChO expected to be
discovered in the next 10 years by space and ground-based facilities is simply
overwhelming.
In addition, we build on previous molecular detectability studies to show
what molecules and abundances will be detectable by EChO for a selection of
real targets with various molecular compositions and abundances.
EChO's unique contribution to exoplanetary science will be in identifying the
main constituents of hundreds of exoplanets in various mass/temperature
regimes, meaning that we will be looking no longer at individual cases but at
populations. Such a universal view is critical if we truly want to understand
the processes of planet formation and evolution in various environments.
In this paper we present a selection of key results. The full results are
available online (http://www.ucl.ac.uk/exoplanets/echotargetlist/).Comment: Accepted for publication in Experimental Astronomy, 20 pages, 10
figures, 3 table
Neutrino reactions on La and Ta via charged and neutral currents by the Quasi-particle Random Phase Approximation (QRPA)
Cosmological origins of the two heaviest odd-odd nuclei, La and
Ta, are believed to be closely related to the neutrino-process. We
investigate in detail neutrino-induced reactions on the nuclei. Charged current
(CC) reactions, BaLa and HfTa, are calculated by the standard Quasi-particle Random Phase
Approximation (QRPA) with neutron-proton pairing as well as neutron-neutron,
proton-proton pairing correlations. For neutral current (NC) reactions,
La{La} and TaTa, we generate ground and excited states of odd-even target nuclei,
La and Ta, by operating one quasi-particle to even-even nuclei,
Ba and Hf, which are assumed as the BCS ground state. Numerical
results for CC reactions are shown to be consistent with recent semi-empirical
data deduced from the Gamow-Teller strength distributions measured in the
(He, t) reaction. Results for NC reactions are estimated to be smaller by
a factor about 4 5 rather than those by CC reactions. Finally, cross
sections weighted by the incident neutrino flux in the core collapsing
supernova are presented for further applications to the network calculations
for relevant nuclear abundances
Self-gravitating astrophysical mass with singular central density vibrating in fundamental mode
The fluid-dynamical model of a self-gravitating mass of viscous liquid with
singular density at the center vibrating in fundamental mode is considered in
juxtaposition with that for Kelvin fundamental mode in a homogeneous heavy mass
of incompressible inviscid liquid. Particular attention is given to the
difference between spectral formulae for the frequency and lifetime of -mode
in the singular and homogeneous models. The newly obtained results are
discussed in the context of theoretical asteroseismology of pre-white dwarf
stage of red giants and stellar cocoons -- spherical gas-dust clouds with dense
star-forming core at the center.Comment: Mod. Phys. Lett. A, Vol. 24, No. 40 (2009) pp. 3257-327
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