JONATHAN SILVERLIGHT said in reply to ALISTAIR:

What sort of particle density are you
assuming - how many per m^3 for instance?
It looks to me as though you're proposing some sort of relativistic
particle, a new sort of cosmic ray, and I'm sure they would have been
detected in other ways. I don't see how they could be blocked by
anything in the outer solar system to produce the sort of localised
flux
you need, or how they could deliver momentum to the Pioneers.
It gets worse! I've already mentioned Galileo, and the "Pioneer
effect"
has also been reported for Ulysses. That's four probes in different
parts of the solar system.

ALISTAIR writes:

I'm assuming 10^42 dark energy particles per cubic metre.
This is why:
If dark energy particles carry electric charge and spin and exist in a
magnetic field associated with the universe as a whole,
then some of those particles will be in a higher energy spin state
than the others.
Since the universe is 10^18 seconds old and using E x t = hbar
we get E = x 10^18 = 10^ -34
E = 10^ -52 J.

Assuming dark energy particles were put in high energy spin states at
the time of the Big Bang and are still in high energy states, this
means that the maximum energy a dark energy particle can emit is
10^-52 J.It can't emit more energy than is associated with its own
rest mass.So a dark energy particle would have a rest energy of at
least 10^-52 J.This is at least 10^-69 kg (using E=mc^2).
Now since dark energy has a density of 10^-27 kg / m^3 this means
there is a maximum of 10^-27 / 10^-69 dark energy particles per cubic
metre.
i.e 10^42 particles per cubic metre.