Correct, e.g.
The combined effect is a net loss of material, estimated at 5.5×107 kg (5.4×104 long tons) per year.
What this means
in GR is that we can compare a clock floating in space to s a clock on Earth and measure that the Earth clock is slower than the space one. For example GPS satellites have atomic clocks that would get
45 microseconds per day ahead of Earth atomic clocks if they were not adjusted to tick slower.
This has nothing to do with the
Rindler horizon which is part of SR.
Any change in mass of the Earth or Sun has no effect on the mass of the universe in cosmology. The mass is still inside the universe!
There is enormous evidence that the universe is expanding, not just the CMBR.
Merely writing "some unruh radiation" does not give the
cosmic microwave background.
Thus your ATM idea is invalid.
ETA: This is the
Unruh effect which is caused by accelerating detectors. We have detected the CMBR with detectors under
very different accelerations to have the same temperature. The first detections were ground-based so an acceleration of 1 g. Other detections from orbit (COBE), balloons and WMAP and Planck at Lagrange 2. Unruh radiation temperature is directly related to acceleration. Not getting different CMBR temperature for different accelerations also makes your ATM idea invalid.