Narrow Results

We discuss prospects for the most stable space-qualified atomic clocks to test general relativity in potential future satellite missions in Earth orbit. By comparing the tick rate of a clock on a satellite to ground clocks, the redshift is obtained. Choosing a highly eccentric orbit will boost relativistic effects at pericenter due to low altitude and high velocity. We find that with a clock having a fractional timing instability of 10⁻¹⁵ to 10⁻¹⁶ on such an orbit, one can measure a host of relativistic effects. These include frame-dragging and the Shapiro delay of the signal light pulses. In optimistic scenarios, higher order (spin-squared) effects are measurable. Additionally, this kind of mission tests alternative theories of gravity in the neighborhood of the Earth. We find that the PPN parameters γ and β can be constrained to the 10⁻⁶ level. Current constraints are at the 10⁻⁵ level coming from radio signals of the Cassini mission traveling through the Sun’s gravitational field and planetary ephemerides. It is important to probe gravitation around different central objects since some alternative theories predict different behavior around e.g. the Earth and the Sun due to screening mechanisms.

We observe the past and present of the universe, but can we predict the far future? Observations suggest that in thousands of billions of years from now most matter and radiation will be absorbed by the cosmological horizon. As it absorbs the contents of the universe, the cosmological horizon is pushed further and further away. In time, the universe asymptotes towards an equilibrium state of the gravitational field. Flat Minkowski space is the limit of this process. It is indistinguishable from a space with an extremely small cosmological constant (Λ → 0) and thus has divergent entropy.