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1. Experimental Progress Towards Testing the Behavior of Gravity at the 20-micron Distance Scale

   Ross, M.P. ; Johnson, J.S. ; Guerrero, I.S. ; Leopardi, H.F. ; Hoyle, C.D. ( October 2018 , Journal of undergraduate research and scholarly excellence)

   Rykhus, R. ; Brown, K. (Ed.)

   Due to discrepancies between the Standard Model and General Relativity, questions have arisen about the fundamental behavior of gravity. Many theories have speculated that gravity behaves fundamentally different at short ranges with respect to the predictions of Newtonian theory. These discrepancies have led the Humboldt State Gravitational Research Lab to begin constructing an experiment that will test the behavior of gravity at distances that have yet to be explored. The experiment has been improved upon in many aspects and has entered an initial data acquisition phase. 

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2. General relativistic pulsations of ultra-massive ZZ Ceti stars

   https://doi.org/10.1093/mnras/stad2248  

   Córsico, Alejandro H. ; Boston, S. Reece ; Althaus, Leandro G. ; Kilic, Mukremin ; Kepler, S. O. ; Camisassa, María E. ; Torres, Santiago ( July 2023 , Monthly Notices of the Royal Astronomical Society)

   Ultra-massive white dwarf stars are currently being discovered at a considerable rate, thanks to surveys such as the Gaia space mission. These dense and compact stellar remnants likely play a major role in Type Ia supernova explosions. It is possible to probe the interiors of ultra-massive white dwarfs through asteroseismology. In the case of the most massive white dwarfs, general relativity could affect their structure and pulsations substantially. In this work, we present results of relativistic pulsation calculations employing relativistic ultra-massive ONe-core white dwarf models with hydrogen-rich atmospheres and masses ranging from 1.29 to $1.369 \ \mathrm{M}_{\odot }$ with the aim of assessing the impact of general relativity on the adiabatic gravity (g)-mode period spectrum of very high mass ZZ Ceti stars. Employing the relativistic Cowling approximation for the pulsation analysis, we find that the critical buoyancy (Brunt–Väisälä) and acoustic (Lamb) frequencies are larger for the relativistic case, compared to the Newtonian case, due to the relativistic white dwarf models having smaller radii and higher gravities for a fixed stellar mass. In addition, the g-mode periods are shorter in the relativistic case than those in the Newtonian computations, with relative differences of up to ∼$50$ per cent for the highest mass models ($1.369 \ \mathrm{M}_{\odot }$) and for effective temperatures typical of the ZZ Ceti instability strip. Hence, the effects of general relativity on the structure, evolution, and pulsations of white dwarfs with masses larger than ∼$1.29 \ \mathrm{M}_{\odot }$ cannot be ignored in the asteroseismological analysis of ultra-massive ZZ Ceti stars.

    

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3. Post-Newtonian gravitational and scalar waves in scalar-Gauss–Bonnet gravity

   https://doi.org/10.1088/1361-6382/ac4196  

   Shiralilou, Banafsheh ; Hinderer, Tanja ; Nissanke, Samaya M ; Ortiz, Néstor ; Witek, Helvi ( January 2022 , Classical and Quantum Gravity)

   Abstract Gravitational waves emitted by black hole binary inspiral and mergers enable unprecedented strong-field tests of gravity, requiring accurate theoretical modeling of the expected signals in extensions of general relativity. In this paper we model the gravitational wave emission of inspiralling binaries in scalar Gauss–Bonnet gravity theories. Going beyond the weak-coupling approximation, we derive the gravitational waveform to relative first post-Newtonian order beyond the quadrupole approximation and calculate new contributions from nonlinear curvature terms. We also compute the scalar waveform to relative 0.5PN order beyond the leading −0.5PN order terms. We quantify the effect of these terms and provide ready-to-implement gravitational wave and scalar waveforms as well as the Fourier domain phase for quasi-circular binaries. We also perform a parameter space study, which indicates that the values of black hole scalar charges play a crucial role in the detectability of deviation from general relativity. We also compare the scalar waveforms to numerical relativity simulations to assess the impact of the relativistic corrections to the scalar radiation. Our results provide important foundations for future precision tests of gravity. 

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4. Observation of the effect of gravity on the motion of antimatter

   https://doi.org/10.1038/s41586-023-06527-1  

   Anderson, E. K. ; Baker, C. J. ; Bertsche, W. ; Bhatt, N. M. ; Bonomi, G. ; Capra, A. ; Carli, I. ; Cesar, C. L. ; Charlton, M. ; Christensen, A. ; et al ( September 2023 , Nature)

   Einstein’s general theory of relativity from 1915¹remains the most successful description of gravitation. From the 1919 solar eclipse²to the observation of gravitational waves³, the theory has passed many crucial experimental tests. However, the evolving concepts of dark matter and dark energy illustrate that there is much to be learned about the gravitating content of the universe. Singularities in the general theory of relativity and the lack of a quantum theory of gravity suggest that our picture is incomplete. It is thus prudent to explore gravity in exotic physical systems. Antimatter was unknown to Einstein in 1915. Dirac’s theory⁴appeared in 1928; the positron was observed⁵in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted⁶by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter^7–10. In the general theory of relativity, the weak equivalence principle (WEP) requires that all masses react identically to gravity, independent of their internal structure. Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive ‘antigravity’ is ruled out in this case. This experiment paves the way for precision studies of the magnitude of the gravitational acceleration between anti-atoms and the Earth to test the WEP.

    

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5. What Are Observables in Hamiltonian Einstein-Maxwell Theory?

   https://doi.org/10.1007/s10701-019-00284-w  

   Pitts, J. Brian ( August 2019 , Foundations of physics)

   null (Ed.)

   Is change missing in Hamiltonian Einstein–Maxwell theory? Given the most common definition of observables (having weakly vanishing Poisson bracket with each first-class constraint), observables are constants of the motion and nonlocal. Unfortunately this definition also implies that the observables for massive electromagnetism with gauge freedom (‘Stueckelberg’) are inequivalent to those of massive electromagnetism without gauge freedom (‘Proca’). The alternative Pons–Salisbury–Sundermeyer definition of observables, aiming for Hamiltonian–Lagrangian equivalence, uses the gauge generator G, a tuned sum of first-class constraints, rather than each first-class constraint separately, and implies equivalent observables for equivalent massive electromagnetisms. For General Relativity, G generates 4-dimensional Lie derivatives for solutions. The Lie derivative compares different space-time points with the same coordinate value in different coordinate systems, like 1 a.m. summer time versus 1 a.m. standard time, so a vanishing Lie derivative implies constancy rather than covariance. Requiring equivalent observables for equivalent formulations of massive gravity confirms that G must generate the 4-dimensional Lie derivative (not 0) for observables. These separate results indicate that observables are invariant under internal gauge symmetries but covariant under external gauge symmetries, but can this bifurcated definition work for mixed theories such as Einstein–Maxwell theory? Pons, Salisbury and Shepley have studied G for Einstein–Yang–Mills. For Einstein–Maxwell, both 𝐹𝜇𝜈 and 𝑔𝜇𝜈 are invariant under electromagnetic gauge transformations and covariant (changing by a Lie derivative) under 4-dimensional coordinate transformations. Using the bifurcated definition, these quantities count as observables, as one would expect on non-Hamiltonian grounds. 

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