MOTIVATION FOR PRECISION RELATIVE ASTROMETRY (from arxiv.org/abs/2010.09100)
It is impossible to foresee all the scientific opportunities offered by an instrument that would enable orders of magnitude better resolution compared to current instruments. Here we consider a few example cases.
Testing theories of gravity by direct imaging of black hole accretion discs: The power of intereferometry has recently been demonstrated by the direct imaging of the black hole event horizon in M87 by the Event Horizon Telescope. This telescope used the Earth-sized array of telescopes operating in radio bands at 1.1mm to achieves resolution of 25 microarcseconds. Since the telescopes were already spread around earth as much as possible, it is only possible to increase the resolution by using telescopes in space or observing at a smaller wavelength. The quantum-improved techniques advocated here will allow, in principle, for arbitrary baselines, and so by re- peating this observation in optical wavelengths it would be possible to increase the resolution by three orders of magnitude (ratio of wavelengths between 1 mm radio and 1 micron optical), bringing about a game changing improvement in resolution. This would open completely new avenues in study of theories of modified gravity that could potentially have large impacts on our understanding of dark energy.
Precision parallax and cosmic distance ladder: there is presently a tension in determination of the ex- pansion rate of the Universe, also known as Hubble pa- rameter H0, between those based on distance ladder and those based on indirect extrapolation from higher red- shift measurements of Baryonic Acoustic Oscillations and Cosmic Microwave Background. The distance ladder method uses a set of probes to bootstrap distance calibra- tion from local measurements to cosmological distances. Parallaxes are used to calibrate distance to the Cepheid variable stars, which have a fixed period-luminosity relation. Cepheid calibration is then transferred from our own galaxy to other galaxies where supernovae Type Ia are observed, and supernovae Type Ia in somewhat more distant galaxies are then used for H0 measurement [8, 9].
Naturally, however, errors in any one step affect the entire ladder. Direct parallax measurements are systematically very robust, but are necessarily limited by the achievable astrometric precision. The most sensitive as- trometric data with precision of few dozens microarcsec is provided by the recent Gaia space mission [3]. The use of Cepheids as standard candles in the distance ladder is complicated by a number of systematic uncertainties in their period-luminosity dependence. An improvement in the astrometric precision by several orders of magnitude proposed here should allow us to completely sidestep the Cepheids and use parallax directly on galaxies with supernovae Type Ia, providing a landmark advance in H0 measurements. In practice this will be done by measuring the fringe changes from a pair of nearby objects composed of a “background object” such as a distant quasar that is essentially fixed on the celestical sphere and a “foreground” object that is subject to parallactic correction as the Earth orbits the sun.
Mapping microlensing events: the nature of dark matter (DM) remains one of the greatest mysteries of the Universe. One possibility is that DM exists in the form of compact objects the size of planets or stars, perhaps as black holes, or just extended virialized subhalos of dark matter particles. Such objects act as gravitational mi- crolenses both in the Galaxy and in extragalactic lens systems. Traditionally, microlensing has been observed photometrically by looking at the apparent change in brightness of object during passage of the lens in front of it. However, the main signature would be measurement of the change in position and appearance of the object, which has so far eluded astrometric measurements due to lack of precision [10]. With lensing a star’s image would split in two images and evolve while the star moves behind the lens [11]. Improving the astrometric precision of the measurements will allow to decrease the detection thresholds, dramatically increasing the statistics hence the sensitivity to the DM subhalos. The astrometric ap- proach is also more straightforward to interpret in terms of the lens mass and its spatial distribution. An inter- esting novel possibility here would be to constrain astro- metric jitter that would in turn constrain the presence of a population of small microlenses in a statistical manner.
Peculiar motions and dark matter: it is well known that the dynamics of our Galaxy is affected by the DM distribution in the Galaxy. The redshifts and blueshifts of stars measure their radial velocities and are technically feasible for all bright stars across the Galaxy. The transverse velocities, on the other hand, are probed by measuring peculiar motions of stars through astro- metric measurements and are currently available only in the vicinity of the Earth. Thus the reconstruction of the truly 3D velocities for a substantial sample of stars in the Galaxy is not possible now. Measurement of the full 3D velocity vector for a significant portion of the stars across the Galaxy would allow us to infer the gravitational po- tential for the galactic halo and would be transformative. It will give us a census of merging events in the history of the milky way halo and it would directly probe dark matter self-interaction, its interactions with baryons and other exciting possibilities (see e.g. [12]). It may also allow to detect populations of dark matter subhalos, as well as density fluctuations sourced by the dark matter [13, 14], and to measure clumping on a range of scales not available through other means, giving direct constraints on the coldness of dark matter [15].
Further: Much improved astrometric precision also will offer large gains in other areas of astrophysics, which are very important for modern science. For example, it could revolutionize searches for exoplanets and interpretation of their properties through direct observation of disturbed trajectories for their host stars, or even by di- rectly resolving the star-planet binary systems. Precision astrometry can also be used for the detection of gravita- tional waves as coherent movements of stars. Many more applications can also be imagined.