26 research outputs found
Model of Dark Matter and Dark Energy Based on Gravitational Polarization
A model of dark matter and dark energy based on the concept of gravitational
polarization is investigated. We propose an action in standard general
relativity for describing, at some effective or phenomenological level, the
dynamics of a dipolar medium, i.e. one endowed with a dipole moment vector, and
polarizable in a gravitational field. Using first-order cosmological
perturbations, we show that the dipolar fluid is undistinguishable from
standard dark energy (a cosmological constant Lambda) plus standard dark matter
(a pressureless perfect fluid), and therefore benefits from the successes of
the Lambda-CDM (Lambda-cold dark matter) scenario at cosmological scales.
Invoking an argument of "weak clusterisation" of the mass distribution of
dipole moments, we find that the dipolar dark matter reproduces the
phenomenology of the modified Newtonian dynamics (MOND) at galactic scales. The
dipolar medium action naturally contains a cosmological constant, and we show
that if the model is to come from some fundamental underlying physics, the
cosmological constant Lambda should be of the order of a0^2/c^4, where a0
denotes the MOND constant acceleration scale, in good agreement with
observations.Comment: 38 pages, 4 figures; to appear in Phys. Rev.
Three little pieces for computer and relativity
Numerical relativity has made big strides over the last decade. A number of
problems that have plagued the field for years have now been mostly solved.
This progress has transformed numerical relativity into a powerful tool to
explore fundamental problems in physics and astrophysics, and I present here
three representative examples. These "three little pieces" reflect a personal
choice and describe work that I am particularly familiar with. However, many
more examples could be made.Comment: 42 pages, 11 figures. Plenary talk at "Relativity and Gravitation:
100 Years after Einstein in Prague", June 25 - 29, 2012, Prague, Czech
Republic. To appear in the Proceedings (Edition Open Access). Collects
results appeared in journal articles [72,73, 122-124
Self-force: Computational Strategies
Building on substantial foundational progress in understanding the effect of
a small body's self-field on its own motion, the past 15 years has seen the
emergence of several strategies for explicitly computing self-field corrections
to the equations of motion of a small, point-like charge. These approaches
broadly fall into three categories: (i) mode-sum regularization, (ii) effective
source approaches and (iii) worldline convolution methods. This paper reviews
the various approaches and gives details of how each one is implemented in
practice, highlighting some of the key features in each case.Comment: Synchronized with final published version. Review to appear in
"Equations of Motion in Relativistic Gravity", published as part of the
Springer "Fundamental Theories of Physics" series. D. Puetzfeld et al.
(eds.), Equations of Motion in Relativistic Gravity, Fundamental Theories of
Physics 179, Springer, 201
Spin and quadrupole contributions to the motion of astrophysical binaries
Compact objects in general relativity approximately move along geodesics of
spacetime. It is shown that the corrections to geodesic motion due to spin
(dipole), quadrupole, and higher multipoles can be modeled by an extension of
the point mass action. The quadrupole contributions are discussed in detail for
astrophysical objects like neutron stars or black holes. Implications for
binaries are analyzed for a small mass ratio situation. There quadrupole
effects can encode information about the internal structure of the compact
object, e.g., in principle they allow a distinction between black holes and
neutron stars, and also different equations of state for the latter.
Furthermore, a connection between the relativistic oscillation modes of the
object and a dynamical quadrupole evolution is established.Comment: 43 pages. Proceedings of the 524. WE-Heraeus-Seminar "Equations of
Motion in Relativistic Gravity". v2: fixed reference. v3: corrected typos in
eqs. (1), (57), (85
Exploring new physics frontiers through numerical relativity
The demand to obtain answers to highly complex problems within strong-field gravity has been met with significant progress in the numerical solution of Einstein's equations - along with some spectacular results - in various setups. We review techniques for solving Einstein's equations in generic spacetimes, focusing on fully nonlinear evolutions but also on how to benchmark those results with perturbative approaches. The results address problems in high-energy physics, holography, mathematical physics, fundamental physics, astrophysics and cosmology
Transconductance and mobility behaviors in UTB SOI MOSFETs with standard and thin BOX
In this paper, we analyze the effects of the front and back interfaces on the transport properties in undoped ultra-thin body (UTB) SOI MOSFETs with standard and ultra-thin buried oxides (BOX), using measurements of the transconductance, gate-to-channel capacitance and carrier mobility at various back gate biases
SAGE: finding IMBH in the black hole desert
International audienceSAGE (SagnAc interferometer for Gravitational wavE) is a project for a space observatory based on multiple 12-U CubeSats in geosynchronous equatorial orbit. The objective is a fast track mission which would fill the observational gap between LISA and ground based observatories. With albeit a lower sensitivity, it would allow early investigation of the nature and event rate of intermediate-mass black hole (IMBH) mergers, constraining our understanding of the universe formation by probing the building up of IMBH up to supermassive black holes (SMBH). Technically, the CubeSats would create a triangular Sagnac interferometer with 140.000 km roundtrip arm length, optimised to be sensitive to gravitational waves at frequencies between 10 mHz and 2 Hz. The nature of the Sagnac measurement makes it almost insensitive to position error, a feature enabling the use of spacecrafts in ballistic trajectories instead of perfect free fall. The light source and recombination units of the interferometer are based on compact fibered technologies without bulk optics. A peak sensitivity of 23 pm ()â1 is expected at 1 Hz assuming a 200 mW internal laser source and 10-centimeter diameter apertures. Because of the absence of a test mass, the main limitation would come from the non-gravitational forces applied on the spacecrafts. However, conditionally upon our ability to partially post-process the effect of solar wind and solar pressure, SAGE would allow detection of gravitational waves with strains as low as a few 10â19 within the 0.1 to 1 Hz range. Averaged over the entire sky, and including the antenna gain of the Sagnac interferometer, the SAGE observatory would sense equal mass black hole mergers in the 104 to 106 solar masses range up to a luminosity distance of 800 Mpc. Additionally, coalescence of stellar black holes (10âM) around SMBH (IMBH) forming extreme (intermediate) mass ratio inspirals could be detected within a sphere of radius 200 Mpc
Special Features of the Back-Gate Effects in UTB SOI MOSFETs
Ultra-thin body silicon-on-insulator (SOI) MOSFET is considered to be a strong candidate for ultimate scaling of CMOS technologies, because of its excellent suppression of the short-channel effects, even without the use of channel doping. Apart from undoped ultra-thin silicon body, nowadays SOI MOSFETs also feature ultra-thin gate high-k gate dielectrics and thin buried oxides. These innovating features bring about special electrical properties. In this work, we describe some of these properties revealed via the back-gate effects, including special behaviors of interface coupling, transport properties and gate tunneling currents, which may be beneficial for the back-gate control schemes