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Fundamental Physics

Fundamental Physics

Fundamental Physics

With very accurate positional measurements the effects of stellar aberration and general relativistic effects (treatments of light-bending due to the gravitational field of the Sun and Earth) must be included. Given the dramatic increase in the precision of the Gaia measurements above that achieved by Hipparcos, the Gaia data reduction requires a more accurate inclusion of relativistic effects. This also provides the opportunity to test General Relativity in new observational domains and with much improved precision.

Light Bending in the Solar System

One of the most important contributions which Gaia will make to the area of fundamental physics is in the area of gravitational light bending and measurement of the Parameterized Post-Newtonian parameter gamma. gamma represents the deviation of the gravitational light-bending from Newtonian theory, which according to General Relativity = 1.

Light deflection due to gravitational effects has been observed with various degrees of precision, on distance scales of 109 to 1021 m, and on mass scales from 1 to 1013 Msolar. Gaia will extend the domain of observations by two orders of magnitude in length-scale, and six orders of magnitude in mass.

The Gaia measurements will provide a precision of about 5 x 10-7 for gamma, based on multiple observations of about 107 stars brighter than V=13 mag at wide angles from the Sun, with individual measurement accuracies better than 10 microarcsec.

Gravitational Waves

Gravitational waves passing over a freely falling telescope will cause a time-varying shift in the apparent position of the source, i.e. the waves cause apparent proper motions. Gaia will be most sensitive to gravity waves in the frequency range from 10-11 to 10 8 Hz.

The bounds from Big Bang nucleosynthesis models indicate that gravitational wave proper motions, for any individual source, will be less than 0.1 microarcsec per year, and pulsar timing measurements strongly suggest that they will be less than about 0.002 microarcsec per year. Thus gravitational waves will not significantly affect any individual position measured by Gaia. However, the entire Gaia data set could be used to put the strongest limit on Omegagw in the band 10-12 < f < 4 x 10-9 Hz.

Gaia could set an upper limit of roughly Omegagw <10 -6 to 10-7 in this frequency band. That is better than the Big Bang nucleosynthesis limit, and much better than the limit from VLBI or the binary pulsar.

Omegagw is the ratio of energy density in gravitational waves to the energy density needed to close the Universe.

Perihelion Precession of Minor Planets

Gaia will discover and observe several hundred thousand minor planets during its five year mission. Most of these planets will belong to the asteroidal main belt and will have small orbital eccentricities and semi-major axes close to 3AU. These conditions are not favourable for seeing significant relativistic effects in their motion. This is true for the main belt asteroids but not so for the Apollo and Aten groups, which are all Earth-orbit crossers, and which include objects with semi-major axes of the order of 1AU and with large eccentricities.

The relativistic effect and the solar quadrupole cause the orbital perihelion of a solar-system body to precess at a rate which depends on the PPN precession coefficient. In the case of 3 Earth-crossing asteroids (Icarus, Talos and Phaeton) the favourable combination of distance and eccentricity means that a determination of the PPN precession coefficient with an accuracy of 10-4 may reasonably be expected.

Rate of change of Gravitational Constant

The possibility of a time variation of the constant of Gravitation (G) was first considered by Dirac, and subsequently developed by Brans & Dicke. Interest in the concept of a time variable gravitational constant has been revived with the development of superstring theories where G is considered to be a dynamical quantity. The Gaia census of white dwarfs will impact on the determination of time-dependent changes in G. White dwarfs are relevant to this problem for two reasons: when they are cool enough, tehir energy is entirely of gravitational and thermal origin, and any changes in G modify the energy balance which in turn modifies the luminosity; sinc ethey are long-lived objects, with lifetimes of the order of 10 Gyr, even very small values of the rate of change of G can become prominent.

Last Update: 1 September 2019
29-Mar-2024 00:19 UT

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