About Quintessence: the time-evolving form of energy which drives the expansion of the universe

In this weeks continuation article of Dark Energy and what it is, we will be looking at Quintessence: which could be what dark matter is made of.

Quintessence. It is a “time-evolving and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion” (Cladwell R.R. and Steinhardt P.J., 2000a, para 41).

Since it has negative pressure, it also has negative gravity. This negative gravity could explain the expansion of the universe. There are many models to describe quintessence, the simplest being, the fact that quintessence might be a quantum field with very long wavelength stretching across the universe. Negative gravity arises in this field by the negative pressure, and we can calculate the pressure by subtracting the Kinetic and Potential energies of the rate of oscillations in the field strength. This model is also successful, because it explains how the density of Dark Energy, or quintessence, changed over time, and fits in with the idea that dark energy must have been insignificant during the early universe to allow the large scale structures to form.

Cladwell, R.R. and Steinhardt, P.J. (2000b) shows how the energy-density of dark energy changed as time proceeded. We can take a look at the different phases of the graph. The end of inflation corresponded to a radiation dominated area, where the universe was hot and glowing and large scale-structures hadn’t formed. The radiation-matter transition, this is where the energy density of the matter equals the energy density of radiation and matter becomes dominant, here large scale structures can finally form. We can also see that the green line, which talks about the quintessence fields, is still not dominating, allowing matter to form clumps. But in the last phase where q begins to dominate, we can see that quintessence has overtaken matter and thus, expansion takes place. This graph overall describes the energy densities of quintessence, matter and radiation over a period of time.

Although this model seems great, it does not address the cosmological constant problem. Which is the difference in the theoretical and physical values of the constant. The quintessence model cannot explain why the observed value of Dark Energy is much smaller than the predicted values in accordance with quantum field theory (Brax P. et al., 2002).

But do not be discouraged, this ‘predicted value’ might not be correct at all, because we do not yet know if there are any other fields involved in quantum field theory which are other than those of the standard model. Further more, why are we even using quantum field theory to predict a gravitational effect on the universe (expansion) when quantum field theory does not have a graviton or its field which we could account for, perhaps quantum gravity might be able to that. And also maybe there’s other unknown particles such as a Dark matter particle in the standard model which we don’t know of and aren’t accounting into the picture.

I can write a whole article about the problems in this ‘predicted vacuum energy value’ and why it might not even be correct, and you know what maybe I will write an article about it explaining the quantum fields and how the value was even calculated, so stay tuned for that if you’re interested.

Well, anyways, thank you for reading, and as always, next week, we will have something even more interesting to read about, another potential candidate for dark energy: Dark Fluid. Sounds cool doesn’t it?

Here are the references:

1. Cladwell, R.R. and Steinhardt, P.J. (2000a) Quintessence. Available at: https://physicsworld.com/a/quintessence/
2. Cladwell, R.R. and Steinhardt, P.J. (2000b) Tracker Fields and k-essence, Quintessence. Physics World. Available at: https://physicsworld.com/wp-content/uploads/2000/11/pw1311085.gif
3. Brax, P. et al. (2002) ‘Quintessence model building’, Non-Perturbative QCD [Preprint]. doi:10.1142/9789812778352_0043.