DARK ENERGY

Another theory proposes that dark energy is something called quintessence, which is defined rather generally and encompasses multiple models. The core difference between quintessence and the cosmological constant is that the theory of quintessence proposes that dark energy is a scalar field. While the cosmological constant is just that--constant--a scalar field can vary in time and space.  The theory of quintessence, meaning ‘fifth essence,’ proposes that dark energy is not an intrinsic property of space, but a massive energy field that encompasses the whole universe and has varied in its properties over the billions of years our universe has existed. In our own lifetimes, however, we would not see the effects of quintessence vary because it’s thought to change so slowly. Nonetheless, we live in a time period in which the ratio of dark energy density to matter density in our universe is transitioning from very small--infinitesimally small--to almost unfathomably large. This begs the question of why the brief period in which dark energy and matter have similar densities in our universe and  mankind’s evolution have closely coincided, and quintessence offers a few different answers.

The general theory proposes a simple “time-dependent, evenly distributed scalar field,” but there are three key modifications that have been proposed to answer the cosmological coincidence. The first of these is tracking solutions, solutions to the equation of motion that cause the current state of the quintessence field to be virtually independent of starting conditions. Tracking solutions explain how cosmic acceleration may be inevitable, but need yet another parameter to explain why it’s happening right now. The theory of k-essence, or kinetic essence, claims to be more natural because it eliminates the need for some of the more complex parameters of tracking solutions (potentials). K-essence can behave similarly to tracking solutions in the early universe, but eventually, when the universe transitions to being matter-dominated, behaves similarly to a cosmological constant. Since matter domination is linked to the transition to dark energy domination, it makes sense that we observe this transition at the present time. Finally, we have phantom energy, quintessence in its most exotic form. Phantom energy requires a so-called ‘open universe’ in which the equation of state parameter w is less than negative one. A common (and reasonable) assumption in cosmology is that w equals -1, but the current available data constrains it to roughly 1.02 plus or minus about .02, meaning w seems to be very near 1 but may not be exactly one. The model of phantom energy is defined by w being less than negative one, which leads to some bizarre possibilities. First of all, not only gravitationally bound systems like galaxies and solar systems be eventually ripped apart by dark energy as it becomes stronger and stronger, but eventually even atomic nuclei will be gradually ripped apart and the universe will end in what’s referred to as a ‘big rip’. This 'cosmic doomsday’ scenario in the distant future essentially nullifies the cosmic coincidence since we have to live at some point between the beginning and the ‘end’ of the universe.