Table 1
Various perspective effects.
| Effect | Origin | Description | Measurement method | Expected magnitude | Redshift dependence |
|
|
|||||
| Acceleration of | o | Acceleration of the solar system assumed to be towards | Astrometric | ~4.3 μas yr-1 | None |
| the solar system | the Galactic centre resulting in patterns of proper motion. | ||||
| However, the local group of galaxies and clusters of local | |||||
| supercluster will also contribute to the effect. | |||||
|
|
|||||
| Cosmological | o | Instantaneous velocity of the solar system with respect | CMB and astrometric | 1–2 μas yr-1 | Increases at low z |
| proper motion | to the CMB can cause distant extragalactic sources to | ||||
| undergo an apparent systematic proper motion. | |||||
|
|
|||||
| Gravitational waves | o | Primordial gravitational waves produce systematic | Dedicated detectors/ | Unknown but | None |
| proper motions over the sky. | possibly astrometric | probably <1 μas yr-1 | |||
|
|
|||||
| Transverse redshift drift/ | c | A temporal shift of the angular separation of distant | Astrometric | 0.2 μas yr-1(Bianchi) | Increases at high z, |
| cosmic parallax | sources can be used to detect an anisotropic expansion | 0.02 μas yr-1(LTB) | decreases at very high z | ||
| of the Universe and results in a pattern of proper motions. | |||||
|
|
|||||
| Peculiar proper motion | s | Proper acceleration is the observed transverse acceleration | Astrometric | Can be 10μas yr-1 | None |
| of an object due to the local gravitational field. | for Galactic clusters | ||||
|
|
|||||
| Quasar microlensing | s | Weak microlensing can induce apparent motions of quasars. | Astrometry | 10’s of μas yr-1 but | None |
| photometry | is extremely rare | ||||
Notes. o refers to an effect that is due to the observer’s motion. s refers to an effect that is due to the source’s motion, and c refers to an effect that is of cosmological origin. We arbitrarily define the boundary between low and high z as a redshift of 1.0.
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