255 research outputs found

### Kinetic field theory for cosmic structure formation

Kinetic field theory is a new analytic approach to cosmic structure formation which avoids some of the notorious problems in this field. I will briefly review the foundations of the theory and then highlight four recent results, concerning the asymptotic behaviour of power spectra, a mean-field approximation to particle interactions, perturbation theory, and structure formation in modified gravity.</p

### Triaxiality in galaxy clusters: Mass versus Potential reconstructions

Accounting for the triaxial shapes of galaxy clusters will become important
in the context of upcoming cosmological surveys. We show that, compared to the
gas density distribution, the cluster gravitational potential can be better
characterised by a simple 3D model and is more robust against fluctuations.
Perturbations in the gas density distribution can have a substantial influence
on the derived thermodynamic properties, while cluster potentials are smooth
and well-approximated by a spheroidal model. We use a statistical sample of 85
galaxy clusters from a large cosmological hydrodynamical simulation to
investigate cluster shapes as a function of radius. In particular, we examine
the shape of isodensity and isopotential shells and analyze how it is affected
by the choice of component (gas vs. potential), substructure removal (for the
gas density) and the definition of the computation domain (interior vs.
shells). We find that the orientation and axis ratios of gas isodensity
contours are degenerate with the presence of substructures and unstable against
fluctuations. We observe that, as the derived cluster shape depends on the
method used for removing the substructures, thermodynamic properties extracted
from e.g. X-ray emissivity profiles suffer from this additional, often
underestimated bias. In contrast, the shape reconstruction of the potential is
largely unaffected by these factors and converges towards simple geometric
models for both relaxed and dynamically active clusters. The observation that
cluster potentials are better represented by simple geometrical models and
reconstructed with a low level of systematics for both dynamically active and
relaxed clusters suggests that by characterising galaxy clusters by their
potential rather than by their mass, dynamically active and relaxed clusters
could be combined in cosmological studies, improving statistics and lowering
scatter.Comment: Updated with referee's comments, revised version submitted to A&A, 16
pages, 14 figure

### Kinetic Field Theory: Perturbation theory beyond first order

We present recent improvements in the perturbative treatment of particle
interactions in Kinetic Field Theory (KFT) for inertial Zel'dovich
trajectories. KFT has been developed for the systematic analytical calculation
of non-linear cosmic structure formation on the basis of microscopic
phase-space dynamics. We improve upon the existing treatment of the interaction
operator by deriving a more rigorous treatment of phase-space trajectories of
particles in an expanding universe. We then show how these results can be
applied to KFT perturbation theory by calculating corrections to the late-time
dark matter power spectrum at second order in the interaction operator. We find
that the modified treatment of interactions w.r.t. inertial Zel'dovich
trajectories improves the agreement of KFT with simulation results on
intermediate scales compared to earlier results. Additionally, we illustrate
that including particle interactions up to second order leads to a systematic
improvement of the non-linear power spectrum compared to the first-order
result.Comment: 26 pages, 4 figure

### On the asymptotic behaviour of cosmic density-fluctuation power spectra

We study the small-scale asymptotic behaviour of the cosmic
density-fluctuation power spectrum in the Zel'dovich approximation. For doing
so, we extend Laplace's method in arbitrary dimensions and use it to prove that
this power spectrum necessarily develops an asymptotic tail proportional to
$k^{-3}$ , irrespective of the cosmological model and the power spectrum of the
initial matter distribution. The exponent $-3$ is set only by the number of
spatial dimensions. We derive the complete asymptotic series of the power
spectrum and compare the leading- and next-to-leading-order terms to derive
characteristic scales for the onset of non-linear structure formation,
independent of the cosmological model and the type of dark matter. Combined
with earlier results on the mean-field approximation for including particle
interactions, this asymptotic behaviour is likely to remain valid beyond the
Zel'dovich approximation. Due to their insensitivity to cosmological
assumptions, our results are generally applicable to particle distributions
with positions and momenta drawn from a Gaussian random field. We discuss an
analytically solvable toy model to further illustrate the formation of the
$k^{-3}$ asymptotic tail.Comment: 22 pages, 4 figues, to be submitted to SciPost Physics Added
arguments to section 1 and section 5, results unchange

### On the asymptotic behaviour of cosmic density-fluctuation power spectra of cold dark matter

We study the small-scale asymptotic behaviour of the cold dark matter density
fluctuation power spectrum in the Zel'dovich approximation, without introducing
an ultraviolet cut-off. Assuming an initially correlated Gaussian random field
and spectral index $0 < n_s < 1$, we derive the small-scale asymptotic
behaviour of the initial momentum-momentum correlations. This result is then
used to derive the asymptotics of the power spectrum in the Zel'dovich
approximation. Our main result is an asymptotic series, dominated by a $k^{-3}$
tail at large wave-numbers, containing higher-order terms that differ by
integer powers of $k^{n_s-1}$ and logarithms of $k$. Furthermore, we show that
dark matter power spectra with an ultraviolet cut-off develop an intermediate
range of scales, where the power spectrum is accurately described by the
asymptotics of dark matter without a cut-off. These results reveal information
about the mathematical structure that underlies the perturbative terms in
kinetic field theory and thus the non-linear power spectrum. We also discuss
the sensitivity of the small-scale asymptotics to the spectral index $n_s$.Comment: 21 pages, 1 table, 6 figures; to be submitted to SciPost Physic

### Kinetic field theory: Non-linear cosmic power spectra in the mean-field approximation

We use the recently developed Kinetic Field Theory (KFT) for cosmic structure
formation to show how non-linear power spectra for cosmic density fluctuations
can be calculated in a mean-field approximation to the particle interactions.
Our main result is a simple, closed and analytic, approximate expression for
this power spectrum. This expression has two parameters characterising
non-linear structure growth which can be calibrated within KFT itself. Using
this self-calibration, the non-linear power spectrum agrees with results
obtained from numerical simulations to within typically $\lesssim10\,\%$ up to
wave numbers $k\lesssim10\,h\,\mathrm{Mpc}^{-1}$ at redshift $z = 0$. Adjusting
the two parameters to optimise agreement with numerical simulations, the
relative difference to numerical results shrinks to typically $\lesssim 5\,\%$.
As part of the derivation of our mean-field approximation, we show that the
effective interaction potential between dark-matter particles relative to
Zel'dovich trajectories is sourced by non-linear cosmic density fluctuations
only, and is approximately of Yukawa rather than Newtonian shape

### Model Independent Analysis of Supernova Data, Dark Energy, Trans-Planckian Censorship and the Swampland

In this Letter, we consider the model-independent reconstruction of the
expansion and growth functions from the Pantheon supernova data. The method
relies on developing the expansion function in terms of shifted Chebyshev
polynomials and determining the coefficients of the polynomials by a
maximum-likelihood fit to the data. Having obtained the expansion function in a
model-independent way, we can then also determine the growth function without
assuming a particular model. We then compare the results with the predictions
of two classes of Dark Energy models, firstly a class of quintessence scalar
field models consistent with the trans-Planckian censorship and swampland
conjectures, and secondly a class of generalized Proca vector field models. We
determine constraints on the parameters which appear in these models.Comment: 11 pages, 6 figure

- â€¦