arXiv subjects
Publications and source records attributed to Wei-Tou Ni.
From July 11 to July 13, 2012, Raman Research Institute (Bangalore, India) hosted the Fifth International ASTROD Symposium on Laser Astrodynamics, Space Test of Relativity and Gravitational-Wave Astronomy. This is a report on the symposium with an exposition of the outlook of direct gravitational-wave detection.
In order to attain the requisite sensitivity for LISA, laser frequency noise must be suppressed below the secondary noises such as the optical path noise, acceleration noise etc. In a previous paper (Dhurandhar et al., Class. Quantum Grav., 27, 135013, 2010), we have found a large family of second-generation analytic solutions of time delay interferometry with one arm dysfunctional, and we also estimated the laser noise due to residual time-delay semi-analytically from orbit perturbations due to Earth. Since other planets and solar-system bodies also perturb the orbits of LISA spacecraft and affect the time delay interferometry (TDI), we simulate the time delay numerically in this paper for all solutions with the generation number n \leq 3. We have worked out a set of 3-year optimized mission orbits of LISA spacecraft starting at January 1, 2021 using the CGC2.7 ephemeris framework. We then use this numerical solution to calculate the residual optical path differences in the second-generation solutions of our previous paper, and compare with the semi-analytic error estimate. The accuracy of this calculation is better than 1 cm (or 30 ps). The maximum path length difference, for all configuration calculated, is below 1 m (3 ns). This is well below the limit under which the laser frequency noise is required to be suppressed. The numerical simulation in this paper can be applied to other space-borne interferometers for gravitational wave detection with the simplification of having only one interferometer.
In this talk, we review the empirical status for modern gravitational theories with emphases on (i) Equivalence Principles; (ii) Lense-Thirring effects and the implications of Gravity Probe B experiment; (iii) Solar-System Tests of Cosmological Models.
We look at the foundations of electromagnetism in this 1st LeCosPA Symposium. For doing this, after some review (constraints on photon mass etc.), we use two approaches. The first one is to formulate a Parametrized Post-Maxwellian (PPM) framework to include QED corrections and a pseudoscalar photon interaction. PPM framework includes lowest corrections to unified electromagnetism-gravity theories based on connection approach. It may also overlap with corrections implemented from generalized uncertainty principle (GUP) when electromagnetism-gravity coupling is considered. We discuss various vacuum birefringence experiments - ongoing and proposed - to measure these parameters. The second approach - the chi-g framework is to look at electromagnetism in gravity and various experiments and observations to determine its empirical foundation. The SME (Standard Model Extension) and SMS (Standard Model Supplement) overlap with the chi-g framework in their photon sector. We found that the foundation is solid with the only exception of a potentially possible pseudoscalar-photon interaction. We discussed its experimental constraints and look forward to more future experiments.
Recently, there has been an interest in inflation and modified gravity with a Weyl term added to the general-relativistic action (N. Derulle, M. Sasaki, Y. Sendouda and A. Youssef, JCAP, 3, 040 (2011)). In this paper we study empirical constraint on this modified gravity from solar-system experiments/observations. We first derive linearized equation of motion in the weak field limit and solve it for isolated system in the slow motion limit. We then use it to derive the light propagation equations, and obtain the relativistic Shapiro time delay and the light deflection in one-body central problem. Applying these results to the solar-system measurements, we obtain constraints on the Weyl term parameter γ_W; the most stringent constraint, which comes from the Cassini relativistic time delay experiment, is for γ_W to be less than 0.0015 AU^2, or |γ_W|^(1/2) less than 0.039 AU (19 s). Analysis of precision laboratory gravity experiments put further limit on the Weyl term parameter γ_W to below the laboratory scale.
We look at the foundations of electromagnetism in this chapter of the "eletromagnetism" book. For doing this, after some review (constraints on photon mass etc.), we use two approaches. The first one is to formulate a Parametrized Post-Maxwellian framework to include QED corrections and a pseudoscalar photon interaction. We discuss various vacuum birefringence experiments - ongoing and proposed - to measure these parameters. The second approach is to look at electromagnetism in gravity and various experiments and observations to determine its empirical foundation. We found that the foundation is solid with the only exception of a potentially possible pseudoscalar-photon interaction. We discussed its experimental constraints and look forward to more future experiments.
Pseudoscalar-photon interactions were proposed in the study of the relations among equivalence principles. The interaction of pseudoscalar axion with gluons was proposed as a way to solve the strong CP problem. Subsequent proposal of axion as a dark matter candidate has been a focus of search. Motivation from superstring theories add to its importance. After a brief introduction and historical review, we present (i) the current status of our optical experiment using high-finesse Fabry-Perot resonant cavity - Q & A experiment - to detect pseudoscalar-photon interactions, (ii) the constraints on pseudoscalar-photon interactions from astrophysical and cosmological observations on cosmic polarization rotation, and (iii) theoretical models of non-minimal interactions of gravitational, electromagnetic and pseudoscalar (axion) fields, and their relevance to cosmology.
The ultra-precise Gravity Probe B experiment measured the frame-dragging effect and geodetic precession on four quartz gyros. We use this result to test WEP II (Weak Equivalence Principle II) which includes rotation in the universal free-fall motion. The free-fall Eötvös parameter eta for rotating body is < = 10**(-11) with four-order improvement over previous results. The anomalous torque per unit angular momentum parameter lambda is constrained to (-0.05 +- 3.67) \times 10**(-15) s-1, (0.24 +- 0.98) \times 10**(-15) s-1, and (0 +- 3.6) \times 10**(-13) s-1 respectively in the directions of geodetic effect, frame-dragging effect and angular momentum axis; the dimensionless frequency-dependence parameter κ is constrained to (1.75 +- 4.96) \times 10**(-17), (1.80 +- 1.34) \times 10**(-17), and (0 +- 3) \times 10**(-14) respectively.
This paper on ASTROD I is based on our 2010 proposal submitted for the ESA call for class-M mission proposals, and is a sequel and an update to our previous paper [Experimental Astronomy 23 (2009) 491-527; designated as Paper I] which was based on our last proposal submitted for the 2007 ESA call. In this paper, we present our orbit selection with one Venus swing-by together with orbit simulation. In Paper I, our orbit choice is with two Venus swing-bys. The present choice takes shorter time (about 250 days) to reach the opposite side of the Sun. We also present a preliminary design of the optical bench, and elaborate on the solar physics goals with the radiation monitor payload. We discuss telescope size, trade-offs of drag-free sensitivities, thermal issues and present an outlook. ASTROD I is a planned interplanetary space mission with multiple goals. The primary aims are: to test General Relativity with an improvement in sensitivity of over 3 orders of magnitude, improving our understanding of gravity and aiding the development of a new quantum gravity theory; to measure key solar system parameters with increased accuracy, advancing solar physics and our knowledge of the solar system; and to measure the time rate of change of the gravitational constant with an order of magnitude improvement and the anomalous Pioneer acceleration, thereby probing dark matter and dark energy gravitationally. It is envisaged as the first in a series of ASTROD missions. ASTROD I will consist of one spacecraft carrying a telescope, four lasers, two event timers and a clock. Two-way, two-wavelength laser pulse ranging will be used between the spacecraft in a solar orbit and deep space laser stations on Earth, to achieve the ASTROD I goals.
In this paper we first present a complete classification of gravitational waves according to their frequencies: (i) Ultra high frequency band (above 1 THz); (ii) Very high frequency band (100 kHz - 1 THz); (iii) High frequency band (10 Hz - 100 kHz); (iv) Middle frequency band (0.1 Hz - 10 Hz); (v) Low frequency band (100 nHz - 0.1 Hz); (vi) Very low frequency band (300 pHz - 100 nHz); (vii) Ultra low frequency band (10 fHz - 300 pHz); (viii) Hubble (extremely low) frequency band (1 aHz - 10 fHz); (ix) Infra-Hubble frequency band (below 1 aHz). After briefly discussing the method of detection for different frequency bands, we review the concept and status of space gravitational-wave missions --- LISA, ASTROD, ASTROD-GW, Super-ASTROD, DECIGO and Big Bang Observer. We then address to the determination of dark energy equation, and probing the inflationary physics using space gravitational wave detectors.
We establish a new self-consistent system of equations accounting for a non-minimal interaction of gravitational, electromagnetic and axion fields. The procedure is based on a non-minimal extension of the standard Einstein-Maxwell-axion action. The general properties of a ten-parameter family of non-minimal linear models are discussed. We apply this theory to the models with pp-wave symmetry and consider propagation of electromagnetic waves non-minimally coupled to the gravitational and axion fields. We focus on exact solutions of electrodynamic equations, which describe quasi-minimal and non-minimal optical activity induced by the axion field. We also discuss empirical constraints on coupling parameters from astrophysical birefringence and polarization rotation observations.
Dark matter is a focused issue in galactic evolution and cosmology. Axion is a viable particle candidate for dark matter. Its interaction with photon is an effective way to detect it, e.g., pseudoscalar-photon interaction will generate vacuum dichroism in a magnetic field. Motivated to measure the QED vacuum birefringence and to detect pseudoscalar-photon interaction, we started to build up the Q & A experiment (QED [Quantum Electrodynamics] and Axion experiment) in 1994. In this talk, we first give a brief historical account of planet hunting and dark matter evidence. We then review our 3.5 m Fabry-Perot interferometer together with our results of measuring vacuum dichroism and gaseous Cotton-Mouton effects. Our first results give (-0.2 $\pm$ 2.8) $\times$ 10$^{-13}$ rad/pass, at 2.3 T with 18,700 passes through a 0.6 m long magnet, for vacuum dichroism measurement. We are upgrading our interferometer to 7 m arm-length with a new 1.8 m 2.3 T permanent magnet capable of rotation up to 13 cycles per second. We aim at 10 nrad/Hz$^{1/2}$ optical sensitivity with 532 nm cavity finesse around 100,000. When achieved, vacuum dichroism would be measured to 8.6 $\times$ 10$^{-17}$ rad/pass in about 50 days, and QED birefringence would be measured to 28 %.
For testing gravity and detecting gravitational waves in space, deep-space laser ranging using drag-free spacecraft is a common method. Deep space provides a large arena and a long integration time. Laser technology provides measurement sensitivity, while drag-free technology ensures that gravitational phenomenon to be measured with least spurious noises. In this talk, we give an overview of motivations and methods of various space missions/proposals testing relativistic gravity and detecting gravitational waves, and refer to various references.
Spin is fundamental in physics. Gravitation is universal. Searches for the role of spin in gravitation dated before the firm establishment of the electron spin in 1925. Since mass and spin or helicity in the case of zero mass are the only invariants of the Poincare group and mass participates in universal gravitation, these searches are natural steps to pursue. Here we review both the theoretical and experimental efforts in searching for the role of spin/polarization in gravitation. We discuss torsion, Poincare gauge theories, teleparallel theories, metric-affine connection theories and pseudoscalar (axion) theories. We discuss laboratory searches for electron and nucleus spin-couplings -- the weak equivalence principle experiments for polarized-bodies, the finite-range spin-coupling experiments, the spin-spin coupling experiments and the cosmic-spin coupling experiments. The role played by angular momentum and rotation is explicitly discussed. We discuss astrophysical and cosmological searches for photon polarization coupling. Investigation in the implications and interrelations of equivalence principles led to a possible pseudoscalar or vector interaction, and led to the proposal of WEP II (Weak Equivalence Principle II) which include rotation in the universal free-fall motion. Evidences for WEP II are discussed and compiled.
Motivated to measure the QED birefringence and to detect pseudoscalar-photon interaction, we started to build up the Q & A experiment (QED [Quantum Electrodynamics] and Axion experiment) in 1994. In this talk, we first review our 3.5 m Fabry-Perot interferometer together with our results of measuring Cotton-Mouton effects of gases. We are uprading our interferometer to 7 m armlength with a new 1.8 m 2.3 T permanent magnet capable of rotation up to 13 cycles per second. We will use 532 nm Nd:YAG laser as light source with cavity finesse around 100,000, and aim at 10 nrad/Hz^{1/2} optical sensitivity. With all these achieved and the upgrading of vacuum, QED birefringence would be measured to 28% in about 50 days. Along the way, we should be able to improve on the dichroism detection significantly.
Effective pseudoscalar-photon interaction(s) would induce a rotation of linear polarization of electromagnetic wave propagating with cosmological distance in various cosmological models. Pseudoscalar-photon interaction is proportional to the gradient of the pseudoscalar field. From phenomenological point of view, this gradient could be neutrino number asymmetry, other density current, or a constant vector. In these situations, Lorentz invariance or CPT may effectively be violated. CMB polarization observations are superb tests of these models and have the potential to discover new fundamental physics. In this paper, we review the constraints on pseudoscalar-photon interaction from CMB polarization observation.
Deep space laser ranging missions like ASTROD I (Single-Spacecraft Astrodynamical Space Test of Relativity using Optical Devices) and ASTROD, together with astrometry missions like GAIA and LATOR will be able to test relativistic gravity to an unprecedented level of accuracy. More precisely, these missions will enable us to test relativistic gravity to $10^{-7}-10^{-9}$, and will require 2nd post-Newtonian approximation of relevant theories of gravity. The first post-Newtonian approximation is valid to $10^{-6}$ and the second post-Newtonian is valid to $10^{-12}$ in the solar system. The scalar-tensor theory is widely discussed and used in tests of relativistic gravity, especially after the interests in inflation, cosmological constant and dark energy in cosmology. In the Lagrangian, intermediate-range gravity term has a similar form as cosmological term. Here we present the full second post-Newtonian approximation of the scalar-tensor theory including viable examples of intermediate-range gravity. We use Chandrasekhar's approach to derive the metric coefficients and the equation of the hydrodynamics governing a perfect fluid in the 2nd post-Newtonian approximation in scalar-tensor theory; all terms inclusive of $O(c^{-4})$ are retained consistently in the equation of motion.
Inflationary cosmology is successful in explaining a number of outstanding cosmological issues including the flatness, the horizon and the relic issues. More spectacular is the experimental confirmation of the structure as arose from the inflationary quantum fluctuations. However, the physics in the inflationary era is unclear. Polarization observations of Cosmic Microwave Background (CMB) missions may detect the tensor mode effects of inflationary gravitational waves (GWs) and give an energy scale of inflation. To probe the inflationary physics, direct observation of gravitational waves generated in the inflationary era is needed. In this essay, we advocate that the direct observation of these GWs with sensitivity Omega-gw down to 10**(-23) is possible using present projected technology development if foreground could be separated.