On 31st October night I took part to an outreach event within the Dark Matter Day initiative, in French, to discuss the Dark Universe at the Café du Pont Neuf for a special 'Bar des sciences'. About 40 people attended and could ask questions about theory and observations to me and three colleagues expert in different methods used to detect Dark Matter. We started discussing Dark Matter, direct and indirect detection, accelerators, and continued with Dark Energy and cosmic acceleration. Here some of the FAQ:
Q1: What is the difference between Dark Matter and Dark Energy?
A1: Dark Matter feels gravitational attraction and is the glue that allows galaxies and clusters to form. Dark Energy causes cosmic acceleration, and effectively contrasts gravity (as if it was a fluid with negative pressure). A possible definition is to consider Dark Matter as the part that clusters and Dark Energy as the part which does not cluster.
Q2: Are Dark Energy and Dark Matter connected?
A2: We don't know. In the simplest model, seen as standard, they are not. In general models of Dark Energy, that include a fifth force in addition to gravity, they can be connected (coupled dark energy). In these models there is an exchange of energy between Dark Matter and Dark Energy fluids.
Q3: If Dark Matter is decreasing and Dark Energy is constant, then energy is not conserved?
A3: No, it is not conserved; if you do an (isolated) experiment on Earth, colliding particles, energy is conserved. But in cosmology, within General Relativity, you need to take into account that the universe is expanding: in this framework energy is not conserved. What is conserved is the total 'stress-energy tensor', i.e. a combination of pressure (or momentum) and energy (total, for all species). Here is a link to a blog post on this topic.
After the event I got various feedback: some found it interesting, some people found part of the discussion too technical, some felt as impostors as if everyone else in the room was more expert than themselves (but the same people were actually able to list the points they had understood in a clear and precise way). Someone else decided to write me afterwards to explain how current theories and experiments in physics are all wrong.
The challenge in explaining complex concepts in clear words (first time in French!) is not only in avoiding to be too technical; it is rather in finding the right balance that avoids both excess of technicalities and, at least at the same level, excess of simplifications.
Below more info on the event, happy to had the chance to meet and talk as well with the other colleagues invited to join.
