Model-Independent Probes of Cosmology & Gravitation

PhDCosmology & Gravitation are the fundamental studies in understanding the physics of the universe that we reside in. The approach in achieving the knowledge of the physical laws which govern our universe, is via the observations of the dynamics, geometries, and evolution of the astrophysical structures within it. Recent cosmological observations have tted well to a cosmological model known as CDM; where our universe's energy content is dominated a cosmological constant ( ) and Cold Dark Matter (CDM). The CDM model is based with gravity described by Einstein's theory of General Relativity (GR); where GR also provides the best description of gravity on all scales. However, the CDM plus GR model requires a cold, non-baryonic, non-visible CDM component, and DE to t the cosmological data. At present, there is no decisive detection of DM leaving an open window for Modi ed Gravity (MG) theories attempting to explain data without the inclusion of DM; and the CDM model contains loose constraints on the Epoch of Reionization (EoR), the epoch at which the rst galaxies and Super Massive Black Holes (SMBH) began to form. This thesis consists of model-independent probes of cosmology & gravitation. Part of this thesis involves searching for high redshift, z 6.5 quasars, with the VISTA Kilo-degree INfrared Galaxy (VIKING), where we expand on the search criteria used by Findlay et al. (2012) and Venemans et al. (2013), by applying various speci c cut methods, resulting in an extended search for these high redshift quasars within the VISTA Science Archive (VSA) database. These quasars can be used as cosmological probes of the EoR by constraining the redshift at which EoR begun, and the formation & evolution of the rst galaxies and SMBHs. Another part of this thesis is the prospects of testing gravity in very low acceleration regimes via Wide Binary (WB) stellar systems, with separations & 3 kAU. These WB systems can achieve low accelerations, on scales . 10 10 ms 2, which is comparable to the value where galaxy rotation curves attens due to DM or a MG. Thus, WBs can probe these low acceleration regimes without the presence of DM, hence making them `clean' and powerful probes of gravity. Our work consists of simulating a large sample of random orbits in various MG models, and predict the observed relative velocities and projected separations, comparing Newtonian prediction against other MG models. This work then follows into using the latest data release from GAIA to select a clean, unperturbed sample of WB systems, obtaining their projected velocities and separations. The selected WBs can then be followed-up with high-resolution ground-based spectroscopy, obtaining their radial velocities, allowing tests of gravity.This work was supported by the Science and Technology Facilities Council (STFC), grant number: ST/M503733/1

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.Comment: Latex, 14 pages, 13 figures. v2: accepted by MNRAS, 05 Jul 2019; 3 figures added, conclusions unchange

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.Comment: LaTeX, 20 pages, 17 figures. v2 is author-produced version as accepted by MNRAS; results similar to v1, moderate clarification

Testing Newton/GR, MoND and quantised inertia on wide binaries

Wide binary stars are within the low-acceleration regime in which galactic rotation curves deviate from Newtonian or general relativistic predictions. It has recently been observed that their rotation rates are similarly anomalous in a way that dark matter cannot explain, since it must be smooth on these small scales to fit galaxy rotation curves. Here, it is shown that Newtonian/GR models cannot predict these wide binaries since dark matter cannot be applied. It is also shown that MoND cannot predict these systems. However, a model which assumes that inertia is due to Unruh radiation made inhomogeneous in space by relativistic horizons (QI, quantised inertia) can predict these wide binaries, and it has the advantage of not needing an adjustable parameter.Comment: 9 pages, 1 figure. Accepted by Astrophysics and Space Science on 26/7/201

Strong constraints on the gravitational law from Gaia DR3 wide binaries

Funding: IB is supported by Science and Technology Facilities Council grant ST/V000861/1, which also partially supports HZ. IB acknowledges support from a “Pathways to Research” fellowship from the University of Bonn, during which the primary statistical analysis was largely coded. BF and RI acknowledge funding from the Agence Nationale de la Recherche (ANR projects ANR-18-CE31-0006 and ANR-19-CE31-0017) and from the European Research Council (ERC) under the European Union’s Horizon 2020 Framework programme (grant agreement number 834148).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 ≈ 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 αgrav, 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 αgrav = -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.Publisher PDFPeer reviewe