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Register a new userAsteroseismic constraints on the cosmic-time variation of the gravitational constant from an ancient main-sequence star
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We investigate the variation of the gravitational constant $G$ over the history of the Universe by modeling the effects on the evolution and asteroseismology of the low-mass star KIC 7970740, which is one of the oldest (~11 Gyr) and best-observed solar-like oscillators in the Galaxy. From these data we find $dot{G}/G = (1.2 pm 2.6) times 10^{-12}~text{yr}^{-1}$, that is, no evidence for any variation in $G$. We also find a Bayesian asteroseismic estimate of the age of the Universe as well as astrophysical S-factors for five nuclear reactions obtained through a 12-dimensional stellar evolution Markov chain Monte Carlo simulation.
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Cosmological constraints on the scalar-tensor theory of gravity by analyzing the angular power spectrum data of the cosmic microwave background (CMB) obtained from the Planck 2015 results are presented. We consider the harmonic attractor model, in which the scalar field has a harmonic potential with curvature ($beta$) in the Einstein frame and the theory relaxes toward the Einstein gravity with time. Analyzing the {it TT}, {it EE}, {it TE} and lensing CMB data from Planck by the Markov chain Monte Carlo method, we find that the present-day deviation from the Einstein gravity (${alpha_0}^2$) is constrained as ${alpha_0}^2<2.5times10^{-4-4.5beta^2} (95.45% {rm C.L.})$ and ${alpha_0}^2<6.3times10^{-4-4.5beta^2} (99.99% {rm C.L.})$ for $0<beta<0.4$. The time variation of the effective gravitational constant between the recombination and the present epochs is constrained as $G_{rm rec}/G_0<1.0056 (95.45% {rm C.L.})$ and $G_{rm rec}/G_0<1.0115 (99.99 %{rm C.L.})$. We also find that the constraints are little affected by extending to nonflat cosmological models because the diffusion damping effect revealed by Planck breaks the degeneracy of the projection effect.
We present cosmological constraints on the scalar-tensor theory of gravity by analyzing the angular power spectrum data of the cosmic microwave background obtained from the Planck 2015 results together with the baryon acoustic oscillations (BAO) data. We find that the inclusion of the BAO data improves the constraints on the time variation of the effective gravitational constant by more than $10%$, that is, the time variation of the effective gravitational constant between the recombination and the present epochs is constrained as $G_{rm rec}/G_0-1 <1.9times 10^{-3} (95.45% {rm C.L.})$ and $G_{rm rec}/G_0-1 <5.5times 10^{-3} (99.99 % {rm C.L.})$. We also discuss the dependence of the constraints on the choice of the prior.
We propose a method to constrain the variation of the gravitational constant $G$ with cosmic time using gravitational-wave (GW) observations of merging binary neutron stars. The method essentially relies on the fact that the maximum and minimum allowed masses of neutron stars at a particular cosmic epoch has a simple dependence on the value of $G$ at that epoch. GWs carry an imprint of the value of $G$ at the time of the merger. Thus, if the value of $G$ at merger is significantly different from its current value, the masses of the neutron stars inferred from the GW observations will be inconsistent with the theoretically allowed range. This enables us to place bounds on the variation of $G$ between the merger epoch and the present epoch. Using the observation of the binary neutron star system GW170817, we constrain the fractional difference in $G$ between the merger and the current epoch to be in the range $-1 lesssim Delta G/G lesssim 8$. Assuming a monotonic variation in $G$, this corresponds to a bound on the average rate of change of $-7 times 10^{-9}~mathrm{yr}^{-1} le dot{G}/G le 5 times 10^{-8}~mathrm{yr}^{-1}$ between these epochs. Future observations will put tight constraints on the deviation of $G$ over vast cosmological epochs not probed by other observations.
Gravitational waves (GWs) are one of the key signatures of cosmic strings. If GWs from cosmic strings are detected in future experiments, not only their existence can be confirmed but also their properties might be probed. In this paper, we study the determination of cosmic string parameters through direct detection of GW signatures in future ground-based GW experiments. We consider two types of GWs, bursts and the stochastic GW background, which provide us with different information about cosmic string properties. Performing the Fisher matrix calculation on the cosmic string parameters, such as parameters governing the string tension $Gmu$ and initial loop size $alpha$ and the reconnection probability $p$, we find that the two different types of GW can break degeneracies in some of these parameters and provide better constraints than those from each measurement.
White dwarf atmospheres are subjected to gravitational potentials around $10^5$ times larger than occur on Earth. They provide a unique environment in which to search for any possible variation in fundamental physics in the presence of strong gravitational fields. However, a sufficiently strong magnetic field will alter absorption line profiles and introduce additional uncertainties in measurements of the fine structure constant. Estimating the magnetic field strength is thus essential in this context. Here we model the absorption profiles of a large number of atomic transitions in the white dwarf photosphere, including first-order Zeeman effects in the line profiles, varying the magnetic field as a free parameter. We apply the method to a high signal-to-noise, high-resolution, far-ultraviolet HST/STIS spectrum of the white dwarf G191-B2B. The method yields a sensitive upper limit on its magnetic field of $B < 2300$ Gauss at the $3sigma$ level. Using this upper limit we find that the potential impact of quadratic Zeeman shifts on measurements of the fine structure constant in G191-B2B is 4 orders of magnitude below laboratory wavelength uncertainties.
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