20 research outputs found
The role of the tachyonic instability in Horndeski gravity
The tachyonic instability is associated with the unboundedness of the
Hamiltonian from below and results in an unstable low- regime. In the
cosmological exploration of modified gravity, it is seldom taken into account,
with more focus given to the popular no-ghost and no-gradient conditions. The
latter though are intrinsically high- statements. Here we combine all three
conditions into a full set of requirements that we show to guarantee stability
on the whole range of cosmological scales. We then explore the impact of the
different conditions on the parameter space of scalar-tensor gravity, with
particular emphasis on the no-tachyon one. We focus on Horndeski gravity and
also consider separately the two subclasses of and Generalized Brans
Dicke theories. We identify several interesting features, for instance in the
parameter space of designer on a CDM background, shedding light on
previous findings. When looking at the phenomenological functions and
, associated to the weak lensing and clustering potential respectively, we
find that in the case of Generalized Brans Dicke the no-tachyon condition
clearly cuts models with . This effect is less prevalent in
the Horndeski case due to the larger amount of free functions in the theory.Comment: 9 pages, 4 figures - accepted version by JCA
Do current cosmological observations rule out all Covariant Galileons?
We revisit the cosmology of Covariant Galileon gravity in view of the most
recent cosmological data sets, including weak lensing. As a higher derivative
theory, Covariant Galileon models do not have a CDM limit and predict
a very different structure formation pattern compared with the standard
CDM scenario. Previous cosmological analyses suggest that this model
is marginally disfavoured, yet can not be completely ruled out. In this work we
use a more recent and extended combination of data, and we allow for more
freedom in the cosmology, by including a massive neutrino sector with three
different mass hierarchies. We use the Planck measurements of Cosmic Microwave
Background temperature and polarization; Baryonic Acoustic Oscillations
measurements by BOSS DR12; local measurements of ; the joint light-curve
analysis supernovae sample; and, for the first time, weak gravitational lensing
from the KiDS collaboration. We find, that in order to provide a reasonable
fit, a non-zero neutrino mass is indeed necessary, but we do not report any
sizable difference among the three neutrino hierarchies. Finally, the
comparison of the Bayesian Evidence to the CDM one shows that in all
the cases considered, Covariant Galileon models are statistically ruled out by
cosmological data.Comment: 8 pages, 5 figures, 2 tables. The Covariant Galileon patch of EFTCAMB
is released in the EFTCAMB developers version - accepted version by PR
Latest evidence for a late time vacuum -- geodesic CDM interaction
We perform a reconstruction of the coupling function between vacuum energy
and geodesic cold dark matter using the latest observational data. We bin the
interaction in seventeen redshift bins but use a correlation prior to prevent
rapid, unphysical oscillations in the coupling function. This prior also serves
to eliminate any dependence of the reconstruction on the binning method. We use
two different forms of the correlation prior, finding that both give similar
results for the reconstruction of the dark matter -- dark energy interaction.
Calculating the Bayes factor for each case, we find no meaningful evidence for
deviation from the null interacting case, i.e. CDM, in our
reconstruction.Comment: 14 pages, 7 figures. Version 2 matches published version in Physics
of the Dark Universe (Figure 2 updated to better show H0 and sigma 8
tensions, additional discussion of results added in section 4.1
EPIC-KITCHENS-100 Unsupervised Domain Adaptation Challenge: Mixed Sequences Prediction
This report presents the technical details of our approach for the
EPIC-Kitchens-100 Unsupervised Domain Adaptation (UDA) Challenge in Action
Recognition. Our approach is based on the idea that the order in which actions
are performed is similar between the source and target domains. Based on this,
we generate a modified sequence by randomly combining actions from the source
and target domains. As only unlabelled target data are available under the UDA
setting, we use a standard pseudo-labeling strategy for extracting action
labels for the target. We then ask the network to predict the resulting action
sequence. This allows to integrate information from both domains during
training and to achieve better transfer results on target. Additionally, to
better incorporate sequence information, we use a language model to filter
unlikely sequences. Lastly, we employed a co-occurrence matrix to eliminate
unseen combinations of verbs and nouns. Our submission, labeled as 'sshayan',
can be found on the leaderboard, where it currently holds the 2nd position for
'verb' and the 4th position for both 'noun' and 'action'.Comment: 2nd place in the 2023 EPIC-KITCHENS-100 Unsupervised Domain
Adaptation Challenge for Action Recognitio
Phenomenology of the generalized cubic covariant Galileon model and cosmological bounds
We investigate the generalized cubic covariant Galileon model, a kinetically
driven dark energy model within the Horndeski class of theories. The model
extends the cubic covariant Galileon by including power laws of the field
derivatives in the K-essence and cubic terms which still allow for tracker
solutions. We study the shape of the viable parameter space by enforcing
stability conditions which include the absence of ghost, gradient and tachyon
instabilities and the avoidance of strong coupling at early time. We study here
the relevant effects of the modifications induced by the model on some
cosmological observables such as the cosmic microwave background (CMB), the
lensing potential auto-correlation and the matter power spectrum. For this
goal, we perform parameter estimation using data of CMB temperature and
polarization, baryonic acoustic oscillations (BAO), redshift-space distortions
(RSD), supernovae type Ia (SNIa) and Cepheids. Data analysis with CMB alone
finds that the today's Hubble parameter is consistent with its
determination from Cepheids at , resolving the famous tension of the
cosmological standard models. The joint analysis of CMB, BAO, RSD and SNIa sets
a lower bound for the sum of neutrino masses which is eV
at 1, in addition to the usual upper limit. The model selection
analysis based on the effective and Deviance Information
Criterion is not able to clearly identify the statistically favored model
between CDM and the generalized cubic covariant Galileon, from which
we conclude that the latter model deserves further studies.Comment: 13 pages, 10 figures, 3 table
Constraints on the interacting vacuum -- geodesic CDM scenario
We investigate an interacting dark sector scenario in which the vacuum energy
is free to interact with cold dark matter (CDM), which itself is assumed to
cluster under the sole action of gravity, i.e. it is in free fall (geodesic),
as in CDM. The interaction is characterised by a dimensionless
coupling that we constrain using cosmic microwave background data
from the Planck 2015 data release, along with baryon acoustic oscillation,
redshift space distortion and Type Ia supernova measurements. We present the
full linear perturbation theory of this interacting scenario and use MCMC
sampling to study five different cases: two cases in which we have CDM
evolution in the distant past, until a set redshift , below
which the interaction switches on and is the single sampled
parameter, with fixed at and respectively; a case where we allow this transition redshift to
vary along with ; a case in which the vacuum energy is zero for
and then begins to grow once the interaction switches on; and
the final case in which we bin in four redshift bins to
investigate the possibility of a dynamical interaction, reconstructing the
redshift evolution of the function using Gaussian processes. We find that, in
all cases where the high redshift evolution is not modified, the results are
compatible with a vanishing coupling, thus finding no significant deviation
from CDM.Comment: 17 pages, 14 figures. Version 2 aligns with the published MNRAS
article (some model comparison discussion added with respect to version 1