Ordinary attractive gravity exists between two masses. With energy relating to mass in Einsteins famous E=mc2 equation, energy also introduces an attractive gravity. But with pressure also being a form of energy, pressure can create an attractive gravity too. The opposite of pressure is tension, this is a kind of negative energy. The tension in a stretched elastic band is negative energy, and negative energy generates a repulsive gravitational field. But here on Earth, we cannot just have a stretched elastic band on it's own without something else stretching it, and this something then also has a pressure on it equal and opposite to the tension, so the two forces cancel and no net gravitational field, attractive or repulsive, is created. Such puny forces will generate only extremely feeble and immeasurably small gravitational fields anyway. If something could be found to hold a worm-hole in space open, then the profound tension in such a structure would produce extremely high anti-gravitational repulsive forces, and offer a way to travel backwards in time through the Worm-hole, to arrive at another point in the Universe extremely distant from the point of entry in a very short time. But no known force can hold a Worm-Hole open long enough, and even if one could, then the pressure holding it open would surely cancel the tension trying to shut it! That is, even if Worm-Holes do briefly exist. A Worm Hole is tunnel through space that connects different parts of the Univese, and is very similar to a Black Hole, but has no singularity which would always stop everything from passing.
All the particles in the Ghost Condensate straddle the whole Universe, which itself has been exanding since its inception at the Big Bang. This stretches the particles more and more, like an elastic band, which stores more and more tension which generates more and more repulsive gravity - anti-gravity. As the Universe gets larger and the Ghost particles get stretched further, the repulsive gravitational force gets stronger and stronger, pushing the Universe further and further apart.
The Ghost Condensate fluid need not be uniformly distributed throughout the Universe, it may have patches where there is a greater density, and places where there is a lower density. This would come about by quantum fluctuations in the Big Bang. The places with higher density would repel more matter and become voids in the Universe, whereas the places with less density would accrete more matter becoming giant stars in the early Universe. Thus the Ghost Condensate fulfills three functions at once; it accounts for dark energy (the tension in the Universe), dark matter (where the less dense condensate accretes more mass) and it accounts for the inflation of the Universe. These three things, previously thought to be conveyed by three separate entities may all be explained by the Ghost Condensate. Researchers need to make sure that the Ghost Condensate can get dense enough to fulfill the role of dark matter. [Dark Matter accounts for more than 90% of the attractive force holding galaxies together, and it has been proved that dark energy cannot be ordinary (baryonic) matter].
The Universe underwent a period of extremely rapid exponential expansion about 10-35 seconds after the Big Bang, this is called cosmic inflation. Various theories have been proposed to account for this inflation, including a sudden phase-change where a great deal of energy was suddenly absorbed when the Universe cooled (through expansion) releasing an extremely large repulsive field which propelled the Universes expansion in a sudden fit for a very brief moment. The Ghost Condensate could perform the same function, giving the young Universe a fierce kick before settling down to a more gradual expansion.
There are some predictions of the Ghost Condensate. Ordinary attractive gravity due to matter should oscillate in strength, but the period of oscillation is longer than the present age of the Universe, consequently that prediction cannot be tested. Another test is more promising. Local variations in inflation would have created local variations in the temperature of the omni-present microwave background radiation, as measured by WMAP. Most inflation theories predict that this temperature variation should have have a symmetrical and gaussian-shaped temperature distribution, but the Ghost Condensate theory predicts a skewed non-gaussian temperature variation. Further measurements to be performed shortly will determine the exact shape of the cosmic microwave temperature distribution and whether it fits the curve predicted by the Ghost Condensate theory.
The Ghost Condensate may also interact with matter by forces other than gravity, for instance, by the electromagnetic force. If it does, it would generate a tiny extra force between electrons and this extra force would be dependant upon the electrons spin, rather like the electrons magnetic force is. Although very weak compared to the ordinary electromagnetic force, this extra force would decrease with distance slower than that of the electromagnetic force, and beyond a distance of 10km should be stronger. However, it can be appreciated that measuring the electromagnetic force of an electron at a distance greater than 10km is no mean feat!
Another researchers calculations seem to show that the Ghost Condensate would also be attracted by Black Holes. His calculations suggest that a supermassive Black Hole, such as the one at the centre of our galaxy, would suck up the condensate at a rate of 1/10th of the suns mass every second, which if true would have given our supermassive Black Hole twice the mass it is observed to have, which is clearly wrong. Yet another researcher points out that a supermassive Black Hole would attract mainly local Ghost Condensate; Ghost Condensate far removed from the supermassive Black Hole would be virtually un-affected for it would take aeons to be so sucked up. Yer pays your money and yer takes your pick...
The Ghost Condensate might also not be un-conditionally stable; there may be an extremely improbable, but finite, way for it to decay, and if it did so, our whole Universe, including us, would disappear in a blink. Hmmm. Whoooops!