What if the earth's core goes cold? What effects would occur if the earth's core goes cold? Would the planet stay liveable after this happens?
 A: Once the core has solidifed it can no longer generate a magnetic field. There may be some frozen in field, but I would guess that the strength of the magnetic field with decrease as more and more of the core freezes, so any residual field is likely to be small. The core of Mars is thought to be frozen and it's magnetic field is negligable.
Anyhow, the main problem would be that without the magnetosphere to protect it the atmosphere will gradually be stripped by the solar wind and we'll be left with nothing to breathe. In addition, without an atmosphere the surface will be subject to wide temperature swings; just as the Moon is in fact.
So no, by any reasonable definition of the word liveable once the core has frozen the Earth will not be liveable. I wouldn't worry for a while yet though.
A: I am not educated in this field but I suggest that the moon acts upon the liquid earth outer core much like it does upon our oceans, acting as a mixer therefore constantly stirring the molten pot if you will.  The liquid is thus ever moving within the mantle causing a more consistent and uniform magnetic field.
