arXiv subjects

Charalambos Pittordis

Publications and source records attributed to Charalambos Pittordis.

8 recordsLinked to original sources

arXiv2026

Strong constraints on the gravitational law from $Gaia$ DR3 wide binaries

We test Milgromian dynamics (MOND) using wide binary stars (WBs) with separations of $2-30$ kAU. Locally, the WB orbital velocity in MOND should exceed the Newtonian prediction by $\approx 20\%$ at asymptotically large separations given the Galactic external field effect (EFE). We investigate this with a detailed statistical analysis of $Gaia$ DR3 data on 8611 WBs within 250 pc of the Sun. Orbits are integrated in a rigorously calculated gravitational field that directly includes the EFE. We also allow line of sight contamination and undetected close binary companions to the stars in each WB. We interpolate between the Newtonian and Milgromian predictions using the parameter $α_{\rm{grav}}$, with 0 indicating Newtonian gravity and 1 indicating MOND. Directly comparing the best Newtonian and Milgromian models reveals that Newtonian dynamics is preferred at $19σ$ confidence. Using a complementary Markov Chain Monte Carlo analysis, we find that $α_{\rm{grav}} = -0.021^{+0.065}_{-0.045}$, which is fully consistent with Newtonian gravity but excludes MOND at $16σ$ confidence. This is in line with the similar result of Pittordis and Sutherland using a somewhat different sample selection and less thoroughly explored population model. We show that although our best-fitting model does not fully reproduce the observations, an overwhelmingly strong preference for Newtonian gravity remains in a considerable range of variations to our analysis. Adapting the MOND interpolating function to explain this result would cause tension with rotation curve constraints. We discuss the broader implications of our results in light of other works, concluding that MOND must be substantially modified on small scales to account for local WBs.

astro-ph.SR↗

arXiv2026

A Quality Framework for Testing Gravity with Wide Binaries: No Evidence for MOND

Wide binaries (WBs) offer a unique opportunity to test gravity in the low-acceleration regime, where modifications such as Milgromian dynamics (MOND) predict measurable deviations from Newtonian gravity. We construct a rigorous framework for conducting the wide binary test (WBT), emphasizing high quality sample selection, filtering of poor astrometric solutions, contamination mitigation, and uncertainty propagation. We show that undetected close binaries, chance alignments, and improper treatment of projection effects can mimic MOND-like signals. We introduce a checklist of best practices to identify and avoid these pitfalls. Applying this framework to Gaia DR3 data, we compile a high-purity sample of WBs within 130 pc with projected separations of 1 - 30 kAU, spanning the transition between the Newtonian and MOND regimes. We find that the scaled relative velocity distribution of wide binaries does not exhibit the 20% enhancement expected from MOND and is consistent with Newtonian gravity across all separations. A meta-analysis of previous WBTs shows that apparent MOND signals diminish as methodological rigour improves. We conclude that when stringent quality controls are applied, there is no observational evidence for MOND-induced velocity boosts in wide binaries. Our results place strong empirical constraints on modified gravity theories operating between a0/10 and 200 a0, where a0 is the MOND acceleration scale. Across this range of internal accelerations, Newtonian gravity is up to 1500x more likely than MOND for our cleanest sample.

astro-ph.GA↗

arXiv2025

Wide Binaries from Gaia DR3 : testing GR vs MOND with realistic triple modelling

We provide an updated test for modifications of gravity from a sample of wide-binary stars from Gaia DR3, and their sky-projected relative velocities. Here we extend on our earlier 2023 study, using several updated selection cuts aimed at reducing contamination from triple systems with an undetected third star. We also use improved mass estimates from Gaia FLAME, and we add refinements to previous modelling of the triple and other populations and the model-fitting. We fit histograms of observed vs Newtonian velocity differences to a flexible mixture of binary + triple populations with realistic eccentricity distributions, plus unbound flyby and random-chance populations. We find as before that Newtonian models provide a significantly better fit than MOND, though improved understanding of the triple population is necessary to make this fully decisive.

astro-ph.GA↗

arXiv2023

Wide Binaries from GAIA EDR3: preference for GR over MOND ?

Several recent studies have shown that velocity differences of very wide binary stars, measured to high precision with GAIA, can potentially provide an interesting test for modified-gravity theories which attempt to emulate dark matter. These systems should be entirely Newtonian according to standard dark-matter theories, while the predictions for MOND-like theories are distinctly different, if the various observational issues can be overcome. Here we provide an updated version of our 2019 study using the recent GAIA EDR3 data release: we select a large sample of 73 159 candidate wide binary stars with distance <300 parsec and magnitudes G<17 from GAIA EDR3, and estimate component masses using a main-sequence mass-luminosity relation. We then examine the frequency distribution of pairwise relative projected velocity (relative to circular-orbit value) as a function of projected separation, compared to simulations; as before, these distributions show a clear peak at a value close to Newtonian expectations, along with a long 'tail' which extends to much larger velocity ratios and may well be caused by hierarchical triple systems with an unresolved or unseen third star. We then fit these observed distributions with a simulated mixture of binary, triple and flyby populations, for GR or MOND orbits, and find that standard gravity is somewhat preferred over one specific implementation of MOND; though we have not yet explored the full parameter space of triple population models and MOND versions. Improved data from future GAIA releases, and followup of a subset of systems to better characterise the triple population, should allow wide binaries to become a decisive test of GR vs MOND in the future.

astro-ph.GA↗

arXiv2023

Wide Binaries as a Modified Gravity test: prospects for detecting triple-system contamination

Recent studies have shown that velocity differences of very wide binary stars, measured to high precision with GAIA, can provide an interesting test for modified-gravity theories which emulate dark matter; in essence, MOND-like theories (with external field effect included) predict that wide binaries (wider than ~ 7 kAU) should orbit ~ 15% faster than Newtonian for similar orbits; such a shift is readily detectable in principle in the sample of 9,000 candidate systems selected from GAIA EDR3 by Pittordis and Sutherland (2022; PS22). However, the main obstacle at present is the observed "fat tail" of candidate wide-binary systems with velocity differences at ~1.5 - 6x circular velocity; this tail cannot be bound pure binary systems, but a possible explanation of the tail is triple or quadruple systems with unresolved or undetected additional star(s). While this tail can be modelled and statistically subtracted, obtaining an accurate model for the triple population is crucial for a robust test for modified gravity. In this paper we explore prospects for observationally constraining the triple population: we simulate a population of hierarchical triple systems "observed" as in PS22 at random epochs and viewing angles; for each simulated system we evaluate various possible methods for detecting the third star, including GAIA astrometry, radial velocity drift, and several imaging methods from direct Rubin images, speckle imaging and coronagraphic imaging. Results are generally encouraging, in that typically 90% of the triple systems in the key regions of parameter space are detectable; there is a moderate "dead zone" of cool brown-dwarf companions at ~ 25-100 AU separation which are not detectable with any of our baseline methods. A large but feasible observing campaign can greatly clarify the effect of the triple/quadruple population and make the gravity test decisive.

astro-ph.GA↗

arXiv2021

Detailed numerical implementation of the wide binary test

The observed flat rotation curves of galaxies are among a number of astrophysical phenomena which require a larger acceleration than can be provided by the Newtonian gravity of the detected baryons. The main proposed solutions are additional undetected mass in the form of dark matter, or a low-acceleration modification to Newtonian gravity known as Milgromian dynamics (MOND). It is possible to directly test MOND using wide binary stars in the Solar neighbourhood, as these systems should contain a dynamically insignificant amount of dark matter even if it comprises most of the Galaxy. However, local wide binaries in MOND should orbit each other $\approx 20\%$ faster than in Newtonian dynamics. We describe the detailed plan for how this wide binary test will be conducted, focusing especially on stages with a high numerical cost. The computational costs and memory requirements are estimated for the main stages in the plan. Our overall assessment is that the critically important cost function can be evaluated deterministically at a marginal cost of a few seconds, giving the absolute binomial likelihood of a model. This will allow the cost function to be embedded within a Markov Chain Monte Carlo sampler, or a less expensive gradient descent stage designed to reveal the best-fitting model parameters. Therefore, the wide binary test should be feasible using currently available technology.

astro-ph.GA↗

arXiv2019

Testing Modified Gravity with Wide Binaries in GAIA DR2

Several recent studies have shown that very wide binary stars can potentially provide an interesting test for modified-gravity theories which attempt to emulate dark matter; these systems should be almost Newtonian according to standard dark-matter theories, while the predictions for MOND-like theories are distinctly different, if the various observational issues can be overcome. Here we explore an observational application of the test from the recent GAIA DR2 data release: we select a large sample of $\sim 24,000$ candidate wide binary stars with distance $< 200$ parsec and magnitudes $G < 16$ from GAIA DR2, and estimated component masses using a main-sequence mass-luminosity relation. We then compare the frequency distribution of pairwise relative projected velocity (relative to circular-orbit value) as a function of projected separation; these distributions show a clear peak at a value close to Newtonian expectations, along with a long `tail' which extends to much larger velocity ratios; the `tail' is considerably more numerous than in control samples constructed from DR2 with randomised positions, so its origin is unclear. Comparing the velocity histograms with simulated data, we conclude that MOND-like theories without an external field effect are strongly inconsistent with the observed data since they predict a peak-shift in clear disagreement with the data; testing MOND-like theories with an external field effect is not decisive at present, but has good prospects to become decisive in future with improved modelling or understanding of the high-velocity tail, and additional spectroscopic data.

astro-ph.CO↗

arXiv2018

Testing Modified Gravity Theories via Wide Binaries and GAIA

The standard LambdaCDM model based on General Relativity (GR) including cold dark matter (CDM) is very successful at fitting cosmological observations, but recent non-detections of candidate dark matter (DM) particles mean that various modified-gravity theories remain of significant interest. The latter generally involve modifications to GR below a critical acceleration scale $\sim 10^{-10} \, m \, s^{-2}$. Wide-binary (WB) star systems with separations $> 5 \, kAU$ provide an interesting test for modified gravity, due to being in or near the low-acceleration regime and presumably containing negligible DM. Here, we explore the prospects for new observations pending from the GAIA spacecraft to provide tests of GR against MOND or TeVes-like theories in a regime only partially explored to date. In particular, we find that a histogram of (3D) binary relative velocities against circular velocity predicted from the (2D) projected separations predicts a rather sharp feature in this distribution for standard gravity, with an 80th (90th) percentile value close to 1.025 (1.14) with rather weak dependence on the eccentricity distribution. However, MOND/TeVeS theories produce a shifted distribution, with a significant increase in these upper percentiles. In MOND-like theories {\em without} an external field effect, there are large shifts of order unity. With the external field effect included, the shifts are considerably reduced to $\sim 0.04 - 0.08$, but are still potentially detectable statistically given reasonably large samples and good control of contaminants. In principle, followup of GAIA-selected wide binaries with ground-based radial velocities accurate to < 0.03 km/s should be able to produce an interesting new constraint on modified-gravity theories.

astro-ph.CO↗