10,658 research outputs found
Improving the precision matrix for precision cosmology
The estimation of cosmological constraints from observations of the large
scale structure of the Universe, such as the power spectrum or the correlation
function, requires the knowledge of the inverse of the associated covariance
matrix, namely the precision matrix, . In most analyses,
is estimated from a limited set of mock catalogues. Depending
on how many mocks are used, this estimation has an associated error which must
be propagated into the final cosmological constraints. For future surveys such
as Euclid and DESI, the control of this additional uncertainty requires a
prohibitively large number of mock catalogues. In this work we test a novel
technique for the estimation of the precision matrix, the covariance tapering
method, in the context of baryon acoustic oscillation measurements. Even though
this technique was originally devised as a way to speed up maximum likelihood
estimations, our results show that it also reduces the impact of noisy
precision matrix estimates on the derived confidence intervals, without
introducing biases on the target parameters. The application of this technique
can help future surveys to reach their true constraining power using a
significantly smaller number of mock catalogues.Comment: 9 pages, 7 figures, minor changes to match version accepted by MNRA
Measuring work and heat in ultracold quantum gases
We propose a feasible experimental scheme to direct measure heat and work in
cold atomic setups. The method is based on a recent proposal which shows that
work is a positive operator valued measure (POVM). In the present contribution,
we demonstrate that the interaction between the atoms and the light
polarisation of a probe laser allows us to implement such POVM. In this way the
work done on or extracted from the atoms after a given process is encoded in
the light quadrature that can be measured with a standard homodyne detection.
The protocol allows one to verify fluctuation theorems and study properties of
the non-unitary dynamics of a given thermodynamic process.Comment: Published version in the Focus Issue on "Quantum Thermodynamics
Communicating via ignorance: Increasing communication capacity via superposition of order
Classically, no information can be transmitted through a depolarising, that
is a completely noisy, channel. We show that by combining a depolarising
channel with another channel in an indefinite causal order---that is, when
there is superposition of the order that these two channels were applied---it
becomes possible to transmit significant information. We consider two limiting
cases. When both channels are fully-depolarising, the ideal limit is
communication of 0.049 bits; experimentally we achieve
bits. When one channel is fully-depolarising,
and the other is a known unitary, the ideal limit is communication of 1 bit. We
experimentally achieve 0.640.02 bits. Our results offer intriguing
possibilities for future communication strategies beyond conventional quantum
Shannon theory
Shapes of clusters and groups of galaxies: Comparison of model predictions with observations
We study the properties of the 3-dimensional and projected shapes of haloes
using high resolution numerical simulations and observational data where the
latter comes from the 2PIGG (Eke et al. 2004) and SDSS-DR3GC group catalogues
(Merchan & Zandivarez 2005). We investigate the dependence of halo shape on
characteristics such as mass and number of members. In the 3-dimensional case,
we find a significant correlation between the mass and halo shape; massive
systems are more prolate than small haloes. We detect a source of strong
systematics in estimates of the triaxiality of a halo, which is found to be a
strong function of the number of members; LCDM haloes usually characterised by
triaxial shapes, slightly bent toward prolate forms, appear more oblate when
taking only a small subset of the halo particles. The ellipticities of observed
2PIGG and SDSS-DR3GC groups are found to be strongly dependent on the number of
group members, so that poor groups appear more elongated than rich ones.
However, this is again an artifact caused by poor statistics and not an
intrinsic property of the galaxy groups, nor an effect from observational
biases. We interpret these results with the aid of a GALFORM mock 2PIGG
catalogue. When comparing the group ellipticities in mock and real catalogues,
we find an excellent agreement between the trends of shapes with number of
group members. When carefully taking into account the effects of low number
statistics, we find that more massive groups are consistent with more elongated
shapes. Finally, our studies find no significant correlations between the shape
of observed 2PIGG or SDSS-DR3GC groups with the properties of galaxy members
such as colour or spectral type index.Comment: 9 pages, 10 figures, submitted to MNRA
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