Are intergalactic voids inertial frames? I know earth is a non-inertia frame, so is the solar system, the milky way, the local group and even our super cluster. But what about all the space, the void that is left outside of the superclusters.
From wikipedia "Interspersed among superclusters are large voids of space in which few galaxies exist. Superclusters are frequently subdivided into groups of clusters called galaxy clouds."
When we talk of the voids the only movement we can talk about is the space expansion, not even a real movement as I have learnt after several debates and discussions.
So are these voids actually inertial frames? 
If they are not, why are they not? 
 A: In special relativity the definition of an inertial frame is straightforward because acceleration is absolute, that is you can always tell if you're in an accelerated frame because you feel a force.
However this is not true in general relativity. Indeed, in GR every frame is locally inertial. For example, the international space station is accelerating towards the Earth (at about 8.8 m/sec$^2$) but as we know from the video footage the astronauts on board are weightless. If you painted over all the windows on the ISS the astronauts inside wouldn't be able to tell whether they were orbiting the Earth or at the centre of an intergalactic void.
Well, not quite.
The way you tell if you are in an inertial frame is to surround yourself with a sphere of test masses and watch them to see what happens. If you did this in the ISS then you'd find the sphere gradually became stretched into an ellipsoid due to the tidal forces from the Earth. The edge of the sphere nearest the earth orbits very slightly faster then the edge farthest from the Earth, so the sphere ends up being stretched. This effect gets bigger as you increase the size of your test sphere, so if you make the sphere big you'll see the effect quickly. But if you make the sphere small enough the changes will be smaller than your experimental errors and you won't be able to tell you're not in an inertial frame.
Now back to your question. There is nowhere in the universe that you are infinitely far from all masses, so everywhere in the universe there will some gravitational field to perturb your test masses. In other words there are no inertial frames anywhere! But the region over which your frame looks inertial, i.e. the region within which you can't detect any change in your test masses, will vary from place to place. It will be much, much bigger in an intergalactic void than in the ISS.
So to summarise:


*

*there are no perfectly inertial frames anywhere

*there are approximately inertial frames everywhere

*the difference is the size of the approximately inertial region

A: To answer the question of your exact words "actual inertial frames", I would say no. There will always be some small gravitational effects, even far out in the vast nothingness of space, they would just be very small perhpas.
