what is quintessence made of?
No one knows for sure. Radiation, ordinary matter and likely dark matter all have positive pressure. They therefore exert a gravitationally attractive force. Anything with negative pressure, the general theory of relativity dictates, would have a gravitationally repulsive force. For quintessence, the quantum field would have a very long wavelength, about the size of the universe. Its kinetic energy depends on the rate of oscillations in the field strength; its potential energy depends on the interaction of the field with matter. The more kinetic energy, the more positive the pressure – which isn’t so likely for a universe-long wavelength. So for now, potential energy and negative pressure dominates. Hence, quintessence is a repulsive force. This can change, Steinhardt says. Quintessence interacts with matter and evolves over time. Quintessence can also decay into new forms of hot matter or radiation. So we are not necessarily doomed to a universe that expands forever, stretching every
