5,252 research outputs found
The Search for a Primordial Magnetic Field
Magnetic fields appear wherever plasma and currents can be found. As such,
they thread through all scales in Nature. It is natural, therefore, to suppose
that magnetic fields might have been formed within the high temperature
environments of the big bang. Such a primordial magnetic field (PMF) would be
expected to arise from and/or influence a variety of cosmological phenomena
such as inflation, cosmic phase transitions, big bang nucleosynthesis, the
cosmic microwave background (CMB) temperature and polarization anisotropies,
the cosmic gravity wave background, and the formation of large-scale structure.
In this review, we summarize the development of theoretical models for
analyzing the observational consequences of a PMF. We also summarize the
current state of the art in the search for observational evidence of a PMF. In
particular we review the framework needed to calculate the effects of a PMF
power spectrum on the CMB and the development of large scale structure. We
summarize the current constraints on the PMF amplitude and the
power spectral index and discuss prospects for better determining these
quantities in the near future.Comment: 40 pages, 13 figures, Accepted for Physics Reports 23 Feb 2012.
Available online 3 March 2012. In press, corrected proo
Galaxy alignments: Observations and impact on cosmology
Galaxy shapes are not randomly oriented, rather they are statistically
aligned in a way that can depend on formation environment, history and galaxy
type. Studying the alignment of galaxies can therefore deliver important
information about the physics of galaxy formation and evolution as well as the
growth of structure in the Universe. In this review paper we summarise key
measurements of galaxy alignments, divided by galaxy type, scale and
environment. We also cover the statistics and formalism necessary to understand
the observations in the literature. With the emergence of weak gravitational
lensing as a precision probe of cosmology, galaxy alignments have taken on an
added importance because they can mimic cosmic shear, the effect of
gravitational lensing by large-scale structure on observed galaxy shapes. This
makes galaxy alignments, commonly referred to as intrinsic alignments, an
important systematic effect in weak lensing studies. We quantify the impact of
intrinsic alignments on cosmic shear surveys and finish by reviewing practical
mitigation techniques which attempt to remove contamination by intrinsic
alignments.Comment: 52 pages excl. references, 16 figures; minor changes to match version
published in Space Science Reviews; part of a topical volume on galaxy
alignments, with companion papers arXiv:1504.05456 and arXiv:1504.0554
Constraining the cosmic radiation density due to lepton number with Big Bang Nucleosynthesis
The cosmic energy density in the form of radiation before and during Big Bang
Nucleosynthesis (BBN) is typically parameterized in terms of the effective
number of neutrinos N_eff. This quantity, in case of no extra degrees of
freedom, depends upon the chemical potential and the temperature characterizing
the three active neutrino distributions, as well as by their possible
non-thermal features. In the present analysis we determine the upper bounds
that BBN places on N_eff from primordial neutrino--antineutrino asymmetries,
with a careful treatment of the dynamics of neutrino oscillations. We consider
quite a wide range for the total lepton number in the neutrino sector, eta_nu=
eta_{nu_e}+eta_{nu_mu}+eta_{nu_tau} and the initial electron neutrino asymmetry
eta_{nu_e}^in, solving the corresponding kinetic equations which rule the
dynamics of neutrino (antineutrino) distributions in phase space due to
collisions, pair processes and flavor oscillations. New bounds on both the
total lepton number in the neutrino sector and the nu_e -bar{nu}_e asymmetry at
the onset of BBN are obtained fully exploiting the time evolution of neutrino
distributions, as well as the most recent determinations of primordial 2H/H
density ratio and 4He mass fraction. Note that taking the baryon fraction as
measured by WMAP, the 2H/H abundance plays a relevant role in constraining the
allowed regions in the eta_nu -eta_{nu_e}^in plane. These bounds fix the
maximum contribution of neutrinos with primordial asymmetries to N_eff as a
function of the mixing parameter theta_13, and point out the upper bound N_eff
< 3.4. Comparing these results with the forthcoming measurement of N_eff by the
Planck satellite will likely provide insight on the nature of the radiation
content of the universe.Comment: 17 pages, 9 figures, version to be published in JCA
Gone after one orbit: How cluster environments quench galaxies
The effect of galactic orbits on a galaxy's internal evolution within a
galaxy cluster environment has been the focus of heated debate in recent years.
To understand this connection, we use both the Gpc) and the
Gpc boxes from the cosmological hydrodynamical simulation set Magneticum
Pathfinder. We investigate the velocity-anisotropy, phase space, and the
orbital evolution of up to resolved satellite galaxies
within our sample of 6776 clusters with at low redshift, which we also trace back in time. In
agreement with observations, we find that star-forming satellite galaxies
inside galaxy clusters are characterised by more radially dominated orbits,
independent of cluster mass. Furthermore, the vast majority of star-forming
satellite galaxies stop forming stars during their first passage. We find a
strong dichotomy both in line-of-sight and radial phase space between
star-forming and quiescent galaxies, in line with observations. The tracking of
individual orbits shows that the star-formation of almost all satellite
galaxies drops to zero within after in-fall. Satellite
galaxies that are able to remain star-forming longer are characterised by
tangential orbits and high stellar mass. All this indicates that in galaxy
clusters the dominant quenching mechanism is ram-pressure stripping.Comment: 22 pages, 16 figures, accepted by MNRA
A comparison of the galaxy populations in the Coma and distant clusters: the evolution of k+a galaxies and the role of the intracluster medium
The spectroscopic properties of galaxies in the Coma cluster are compared
with those of galaxies in rich clusters at , to investigate the
evolution of the star formation history in clusters. Luminous galaxies with
and post-starburst/post-starforming (k+a) spectra which
constitute a significant fraction of galaxies in distant cluster samples are
absent in Coma, where spectacular cases of k+a spectra are found instead at
and represent a significant proportion of the cluster dwarf galaxy
population. A simple inspection of their positions on the sky indicates that
this type of galaxy does not show a preferential location within the cluster,
but the bluest and strongest-lined group of k+a's lies in projection towards
the central 1.4 Mpc of Coma and have radial velocities significantly higher
than the cluster mean. We find a striking correlation between the positions of
these young and strong post-starburst galaxies and substructure in the hot
intracluster medium (ICM) identified from {\it XMM-Newton} data, with these
galaxies lying close to the edges of two infalling substructures. This result
strongly suggests that the interaction with the dense ICM could be responsible
for the quenching of the star formation (thus creating the k+a spectrum), and
possibly, for any previous starburst. The evolution with redshift of the
luminosity distribution of k+a galaxies can be explained by a ``downsizing
effect'', with the maximum luminosity/mass of actively star-forming galaxies
infalling onto clusters decreasing at lower redshift. We discuss the possible
physical origin of this downsizing effect and the implications of our results
for current scenarios of environmental effects on the star formation in
galaxies.Comment: 21 pages, 7 figures, to appear in ApJ, version after referee's
change
Living on the edge of stability, the limits of the nuclear landscape
A first-principles description of nuclear systems along the drip lines
presents a substantial theoretical and computational challenge. In this paper,
we discuss the nuclear theory roadmap, some of the key theoretical approaches,
and present selected results with a focus on long isotopic chains. An important
conclusion, which consistently emerges from these theoretical analyses, is that
three-nucleon forces are crucial for both global nuclear properties and
detailed nuclear structure, and that many-body correlations due to the coupling
to the particle continuum are essential as one approaches particle drip lines.
In the quest for a comprehensive nuclear theory, high performance computing
plays a key role.Comment: Contribution to proceedings of Nobel Symposium 152: Physics with
radioactive beams, June 2012, Gothenburg, Swede
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