Although detailed designs for successor missions to the Great Observatories and Einstein Probes, would be premature, it is important to begin addressing some of the anticipated technology needs for the Einstein Vision Missions.

The ultimate goal of a Big Bang Observer is to directly observe gravitational waves with sufficient sensitivity to observe the background due to the quantum fluctuations during inflation. This must be accomplished in the face of a strong foreground of gravitational waves produced by all the binary stars and black holes in the Universe. Source-by-source removal of this foreground is practical at wave periods of 0.1-10 seconds.

To separate these foreground sources requires extraordinary sensitivity and angular resolution. One possible solution consists of four separate interferometers, each including three spacecraft separated by 50,000 km. These would be spaced in a triangle around the earth's orbit about the Sun (separations of 1.7 times the Earth-Sun distance), with two interferometers sharing one apex for independent correlation. Such a configuration imposes many technical challenges, including:

Strain Sensitivity. A significant improvement in strain sensitivity, about 10,000 times better than that of LISA is needed. This will require advances in mirror fabrication, laser power and stability, phase measurement, and instrument pointing.

Acceleration Noise. A gravitational reference sensor with acceleration noise performance 100 times lower than that planned for LISA is required.

This gravitational wave frequency band will not have been previously explored. To provide scientific guidance and to reduce the risk associated with making such large technical advances in one step, it would be desirable to first fly a pathfinder mission with fewer spacecraft and more modest improvements on LISA's technology.

The goal of the Black Hole Imager is to enable direct imaging of the distribution and motion of matter in the highly distorted spacetime near the event horizon of a black hole. This will require angular resolution better than 0.1 microarcsecond---a million times better than Hubble Space Telescope. An X-ray interferometer is naturally matched to this task, since accreting black holes are expected to have a high surface brightness in X-rays, and this, coupled with the short wavelength, allows an instrument of relatively modest aperture and baseline to be used.

An X-ray interferometer with 0.1 microarcsecond (mas) resolution poses technical challenges. At wavelengths near 1 nm, the required baselines are about 1 km, and focal distances must be 1,000-10,000 km to obtain reasonable detector scales. This means that separate spacecraft are needed with highly controlled formation flying. Nominal requirements are: position accuracy of a fraction of a nanometer, angles known to 0.1 mas, and optical surfaces figured to 2.5 nm.