Quintessence is the title of chapter #18 from the course “Dark Matter, Dark Energy: The Dark Side of the Universe” I just mentioned in the previous post. A course taught by Prof. Sean Carroll. According to that course’s page,

Another idea about dark energy is that it results from a new field in nature, analogous to the electromagnetic field but remaining persistent as the universe expands. This field is called quintessence. It would be observationally distinguishable from the cosmological constant.

That is, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe (Wikipedia’s definition). Real or not, it is a way of explaining one of the hottest open topics in Cosmology nowadays. A good article about the subject can be found here. According to Prof. Paul J. Steinhardt, from Princeton University,

Quintessence encompasses a wide class of possibilities. It is a dynamic, time-evolving and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion.

Related concepts that appear in the aforementioned chapter are:

  • The “Coincidence Scandal”: it points out the coincidence between the observed vacuum energy and the current matter density. That is to say, we live in the era when vacuum and matter are comparable. Pretty strange.
  • The Phantom Field: scalar field with negative kinetic energy to realize the late time acceleration.
  • The “Big Rip”: The Big Rip is a cosmological hypothesis first published in 2003, about the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, is progressively torn apart by the expansion of the universe at a certain time in the future (from Wikipedia).
  • The Fifth Force of Nature: generally believed to have roughly the strength of gravity and to have a range of anywhere from less than a millimeter to cosmological scales.
  • Tangled Strings, Variable Mass Particles…: speculative ideas that try to explain dynamic dark energy.
  • The equation-of-state parameter (w): parameter that relates the energy density to the pressure, closely related to the thermodynamic equation of state and ideal gas law.

Although I would recommend purchasing the original videos from The Teaching Company, this chapter can be seen on YouTube here (part 1) and here (part 2).

Dark Matter, Dark Energy: The Dark Side of the Universe

Dark Matter, Dark Energy: The Dark Side of the Universe (image by The Teaching Company)