arXiv subjects

Michael Doran

Publications and source records attributed to Michael Doran.

At least 19 recordsLinked to original sources

arXiv2026

iFAN Ecosystem: A Unified AI, Digital Twin, Cyber-Physical Security, and Robotics Environment for Advanced Nuclear Simulation and Operations

As nuclear facilities experience digital transformation and advanced reactor development, AI integration, cyber-physical security, and other emerging technologies such as autonomous robot operations are increasingly developed. However, evaluation and deployment is challenged by the lack of dedicated virtual testbeds. The Immersive Framework for Advanced Nuclear (iFAN) ecosystem is developed, a comprehensive digital twin framework with a realistic 3D environment with physics-based simulations. The iFAN ecosystem serves as a high-fidelity virtual testbed for plant operation, cybersecurity, physical security, and robotic operation, as it provides real-time data exchange for pre-deployment verification. Core features include virtual reality, reinforcement learning, radiation simulation, and cyber-physical security. In addition, the paper investigates various applications through potential operational scenarios. The iFAN ecosystem provides a versatile and secure architecture for validating the next generation of autonomous and cyber-resilient nuclear operations.

cs.RO↗

arXiv2007

Does Planck mass run on the cosmological horizon scale?

Einstein's theory of general relativity, which contains a universal value of the Planck mass, has been so far successfully invoked to explain gravitational dynamics from sub-millimeter scales to the scale of the cosmological horizon. However, one may envisage that in alternative theories of gravity, the effective value of the Planck mass (or Newton's constant), which quantifies the coupling of matter to metric perturbations, can run on the cosmological horizon scale. In this letter, we study the consequences of a glitch in the Planck mass from sub-horizon to super-horizon scales. We first give three examples of models that naturally exhibit this feature, and then show that current cosmological observations severely constrain this glitch to less than 1.2%. This is the strongest constraint to date, on natural (i.e. non-fine-tuned) deviations from Einstein gravity on the cosmological horizon scale.

astro-ph↗

arXiv2007

Cuscuton Cosmology: Dark Energy meets Modified Gravity

In a companion paper (hep-th/0609150), we have introduced a model of scalar field dark energy, Cuscuton, which can be realized as the incompressible (or infinite speed of sound) limit of a k-essence fluid. In this paper, we study how Cuscuton modifies the constraint sector of Einstein gravity. In particular, we study Cuscuton cosmology and show that even though Cuscuton can have an arbitrary equation of state, or time dependence, and is thus inhomogeneous; its perturbations do not introduce any additional dynamical degree of freedom and only satisfy a constraint equation, amounting to an effective modification of gravity on large scales. Therefore, Cuscuton can be considered to be a minimal theory of evolving dark energy, or a minimal modification of a cosmological constant, as it has no internal dynamics. Moreover, this is the only modification of Einstein gravity to our knowledge that does not introduce any additional degrees freedom (and is not conformally equivalent to the Einstein gravity). We then study two simple Cuscuton models, with quadratic and exponential potentials. The quadratic model has the exact same expansion history as LCDM, and yet contains an early dark energy component with constant energy fraction, which is constrained to < 2%, mainly from WMAP Cosmic Microwave Background (CMB) and SDSS Lyman-alpha forest observations. The exponential model has the same expansion history as the DGP self-accelerating braneworld model, but generates a much smaller Integrated Sachs-Wolfe (ISW) effect, and is thus consistent with the CMB observations. Finally, we show that the evolution is local on super-horizon scales, implying that there is no gross violation of causality, despite Cuscuton's infinite speed of sound.

astro-ph↗

arXiv2007

Baryon Acoustic Oscillations and Dynamical Dark Energy

We compute the impact of dark energy at last scattering on measurements of baryon acoustic oscillations (BAOs). We show that an early dark energy component can contribute a systematic uncertainty to BAO measurements of up to 2.5%. Whilst this effect turns out to only slightly affect current BAO surveys, the results of future BAO surveys might become biased. We find that BAO surveys alone appear unable to resolve this systematic uncertainty, so supplementary measurements are necessary.

astro-ph↗

arXiv2007

Impact of three years of data from the Wilkinson Microwave Anisotropy Probe on cosmological models with dynamical dark energy

The first three years of observation of the Wilkinson Microwave Anisotropy Probe (WMAP) have provided the most precise data on the anisotropies of the cosmic microwave background (CMB) to date. We investigate the impact of these results and their combination with data from other astrophysical probes on cosmological models with a dynamical dark energy component. By considering a wide range of such models, we find that the constraints on dynamical dark energy are significantly improved compared to the first year data.

astro-ph↗

arXiv2006

Early Quintessence in Light of WMAP

We examine the cosmic microwave background (CMB) anisotropy for signatures of early quintessence dark energy -- a non-negligible quintessence energy density during the recombination and structure formation eras. Only very recently does the quintessence overtake the dark matter and push the expansion into overdrive. Because the presence of early quintessence exerts an influence on the clustering of dark matter and the baryon-photon fluid, we may expect to find trace signals in the CMB and the mass fluctuation power spectrum. In detail, we demonstrate that suppressed clustering power on small length-scales, as suggested by the combined Wilkinson Microwave Anisotropy Probe (WMAP) / CMB / large scale structure data set, is characteristic of early quintessence. We identify a set of concordant models, and map out directions for further investigation of early quintessence.

astro-ph↗

arXiv2006

Early Dark Energy Cosmologies

We propose a novel parameterization of the dark energy density. It is particularly well suited to describe a non-negligible contribution of dark energy at early times and contains only three parameters, which are all physically meaningful: the fractional dark energy density today, the equation of state today and the fractional dark energy density at early times. As we parameterize Omega_d(a) directly instead of the equation of state, we can give analytic expressions for the Hubble parameter, the conformal horizon today and at last scattering, the sound horizon at last scattering, the acoustic scale as well as the luminosity distance. For an equation of state today w_0 < -1, our model crosses the cosmological constant boundary. We perform numerical studies to constrain the parameters of our model by using Cosmic Microwave Background, Large Scale Structure and Supernovae Ia data. At 95% confidence, we find that the fractional dark energy density at early times Omega_early < 0.06. This bound tightens considerably to Omega_early < 0.04 when the latest Boomerang data is included. We find that both the gold sample of Riess et. al. and the SNLS data by Astier et. al. when combined with CMB and LSS data mildly prefer w_0 < -1, but are well compatible with a cosmological constant.

astro-ph↗

arXiv2006

What measurable zero point fluctuations can(not) tell us about dark energy

We show that laboratory experiments cannot measure the absolute value of dark energy. All known experiments rely on electromagnetic interactions. They are thus insensitive to particles and fields that interact only weakly with ordinary matter. In addition, Josephson junction experiments only measure differences in vacuum energy similar to Casimir force measurements. Gravity, however, couples to the absolute value. Finally we note that Casimir force measurements have tested zero point fluctuations up to energies of ~10 eV, well above the dark energy scale of ~0.01 eV. Hence, the proposed cut-off in the fluctuation spectrum is ruled out experimentally.

astro-ph↗

arXiv2006

CMBEASY:: an Object Oriented Code for the Cosmic Microwave Background

We have ported the cmbfast package to the C++ programming language to produce cmbeasy, an object oriented code for the cosmic microwave background. The code is available at www.cmbeasy.org. We sketch the design of the new code, emphasizing the benefits of object orientation in cosmology, which allow for simple substitution of different cosmological models and gauges. Both gauge invariant perturbations and quintessence support has been added to the code. For ease of use, as well as for instruction, a graphical user interface is available.

astro-ph↗

arXiv2005

Speeding Up Cosmological Boltzmann Codes

We introduce a novel strategy for cosmological Boltzmann codes leading to an increase in speed by a factor of \sim 30 for small scale Fourier modes. We (re-)investigate the tight coupling approximation and obtain analytic formulae reaching up to the octupoles of photon intensity and polarization. This leads to accurate results reaching optimal precision, while still being simple. Damping rapid oscillations of small scale modes at later times, we simplify the integration of cosmological perturbations. We obtain analytic expressions for the photon density contrast and velocity as well as an estimate of the quadrupole from after last scattering until today. These analytic formulae hold well during re-ionization and are in fact negligible for realistic cosmological scenarios. However, they do extend the validity of our approach to models with very large optical depth to the last scattering surface.

astro-ph↗

arXiv2005

Observational constraints on the dark energy density evolution

We constrain the evolution of the dark energy density from Cosmic Microwave Background, Large Scale Structure and Supernovae Ia measurements. While Supernovae Ia are most sensitive to the equation of state $w_0$ of dark energy today, the Cosmic Microwave Background and Large Scale Structure data best constrains the dark energy evolution at earlier times. For the parametrization used in our models, we find $w_0 < -0.8$ and the dark energy fraction at very high redshift $Ω_{early} < 0.03$ at 95 per cent confidence level.

astro-ph↗

arXiv2005

Non-linear Structure Formation in Cosmologies with Early Dark Energy

We argue that a few per cent of "Early Dark Energy" can be detected by the statistics of nonlinear structures. The presence of Dark Energy during linear structure formation is natural in models where the present tiny Dark-Energy density is related to the age of the Universe rather than a new fundamental small parameter. Generalisation of the spherical collapse model shows that the linear collapse parameter delta_c is lowered. The corresponding relative enhancement of weak gravitational lensing on arc-minute scales lowers the value of sigma_8 inferred from a given lensing amplitude as compared to Lambda-CDM. In presence of Early Dark Energy, structures grow slower, such that for given sigma_8 the number of galaxies and galaxy clusters is substantially increased at moderate and high redshift. For realistic models, the number of clusters detectable through their thermal Sunyaev-Zel'dovich effect at redshift unity and above, e.g. with the Planck satellite, can be an order of magnitude larger than for Lambda-CDM.

astro-ph↗

arXiv2005

Can we test Dark Energy with Running Fundamental Constants ?

We investigate a link between the running of the fine structure constant $α$ and a time evolving scalar dark energy field. Employing a versatile parameterization for the equation of state, we exhaustively cover the space of dark energy models. Under the assumption that the change in $α$ is to first order given by the evolution of the Quintessence field, we show that current Oklo, Quasi Stellar Objects and Equivalence Principle observations restrict the model parameters considerably stronger than observations of the Cosmic Microwave Background, Large Scale Structure and Supernovae Ia combined.

astro-ph↗

arXiv2005

Testing Dark Energy and Light Particles via Black Hole Evaporation at Colliders

We show that collider experiments have the potential to exclude a light scalar field as well as generic models of modified gravity as dark energy candidates. Our mechanism uses the spectrum radiated by black holes and can equally well be applied to determine the number of light degrees of freedom. We obtain the grey body factors for massive scalar particles and calculate the total emissivity. While the Large Hadron Collider (LHC) may not get to the desired accuracy, the measurement is within reach of next generation colliders.

astro-ph↗

arXiv2005

Dark-Energy Evolution Across the Cosmological-Constant Boundary

We explore the properties of dark energy models for which the equation-of-state, w, defined as the ratio of pressure to energy density, crosses the cosmological-constant boundary w = -1. We adopt an empirical approach, treating the dark energy as an uncoupled fluid or a generalized scalar field. We describe the requirements for a viable model, in terms of the equation-of-state and sound speed. A generalized scalar field cannot safely traverse w = -1, although a pair of scalars with w > -1 and w < -1 will work. A fluid description with a well-defined sound speed can also cross the boundary. Contrary to expectations, such a crossing model does not instantaneously resemble a cosmological constant at the moment w = -1 since the density and pressure perturbations do not necessarily vanish. But because a dark energy with w < -1 dominates only at very late times, and because the dark energy is not generally prone to gravitational clustering, then crossing the cosmological-constant boundary leaves no distinct imprint.

astro-ph↗

arXiv2004

Analyze This! A Cosmological Constraint Package for CMBEASY

We introduce a Markov Chain Monte Carlo simulation and data analysis package that extends the CMBEASY software. We have taken special care in implementing an adaptive step algorithm for the Markov Chain Monte Carlo in order to improve convergence. Data analysis routines are provided which allow to test models of the Universe against measurements of the cosmic microwave background, supernovae Ia and large scale structure. We present constraints on cosmological parameters derived from these measurements for a $Λ$CDM cosmology and discuss the impact of the different observational data sets on the parameters. The package is publicly available as part of the CMBEASY software at www.cmbeasy.org.

astro-ph↗

arXiv2004

Dark-Matter Electric and Magnetic Dipole Moments

We consider the consequences of a neutral dark-matter particle with a nonzero electric and/or magnetic dipole moment. Theoretical constraints, as well as constraints from direct searches, precision tests of the standard model, the cosmic microwave background and matter power spectra, and cosmic gamma rays, are included. We find that a relatively light particle with mass between an MeV and a few GeV and an electric or magnetic dipole as large as $\sim 3\times 10^{-16}e$ cm (roughly $1.6\times10^{-5} μ_B$) satisfies all experimental and observational bounds. Some of the remaining parameter space may be probed with forthcoming more sensitive direct searches and with the Gamma-Ray Large Area Space Telescope.

astro-ph↗

arXiv2003

Recent Constraints on Models of Quintessence

We perform a comparison of the WMAP measurements with the predictions of quintessence cosmological models of dark energy. We consider a wide range of quintessence models, including: a constant equation-of-state; a simply-parametrized, time-evolving equation-of-state; a class of models of early quintessence; scalar fields with an inverse-power law potential. We also provide a joint fit to the CBI and ACBAR CMB data, and the type 1a supernovae.

astro-ph↗