# I Dynamical dark energy

1. Oct 3, 2017

2. Oct 4, 2017

### Orodruin

Staff Emeritus
They are not ruling out a cosmological constant
The claim is that the best fit is a dynamical DE model and the ΛCDM is disfavoured (which is not the same as saying that it is excluded).

Last edited: Oct 4, 2017
3. Oct 4, 2017

### George Jones

Staff Emeritus
Making explicit what Orodruin wrote, the last sentence of the abstact reads
According to their analysis of currently available data, dynamical dark energy is slightly favoured, but the difference is not yet statistically significant. The situation, however, could be changed dramatically by data from a planned upcoming survey.

Last edited: Oct 4, 2017
4. Oct 4, 2017

### kimbyd

This turns out to be one of those things that is remarkably difficult to estimate well. The problem is that there's no one good model for how dark energy might vary, so that scientists end up comparing the cosmological constant to what essentially amounts to fitting a curve. That curve fitting will always produce a tighter fit to the data, so it becomes a question of how much better a fit is meaningful.

A number of potential data issues can complicate matters. For instance, different data sets tend to measure expansion rates at different redshifts, which means that small differences in calibration between the data sets can show up as an apparent change in dark energy over time.

Personally, I'm partial to this kind of work (though it is a few years older and doesn't include as much data):
https://arxiv.org/abs/1212.6644

Disclaimer: I did work on this as a graduate student and consulted with the authors of this paper as they were writing it.

The traditional way to try to measure how dark energy behaves over time is to measure what is known as the equation of state parameter, $w$. This parameter has a very physical meaning: it is the relationship between pressure and energy density. For non-relativistic matter, $w = 0$ (which means that the pressure is negligible, which is the same as saying that galaxies do not exert pressure on one another). For radiation and relativistic matter, $w = 1/3$, as radiation exerts a pressure equal to one third its energy density. If dark energy is actually a field, then it will have a relationship between pressure and energy density determined by this parameter that is a function of the physics of the field. For most fields that might explain dark energy, the parameter $w$ changes over time as the universe expands, so it can be expressed as a function $w(z)$.

The problem is, $w(z)$ is essentially a derivative of the density of the field over time (there's more than just a derivative involved, but the derivative is the essential point here). And derivatives are inherently more difficult to measure than the functions they are derived from, as noise is amplified by the derivative. The solution that some theorists (sadly, a minority) have settled on is to measure the dark energy density itself* and see if it varies. The specific mathematical details of how this is done are a bit complicated, but the data are separated out into components where one of the components is forced to be a constant, and the rest are ordered by how well they are measured by the data. In principle, if there is variation, it should show up very cleanly in the very first non-constant component. So far, this doesn't appear to happen. The first few components are consistent to within 95% confidence with zero.