Darwin Proven Wrong?

[Alex K]

Because one can never detect a force itself, only its effect. Detecting a force itself is a meaningless term. That's what detecting means: you have a detecting apparatus, and it shows an effect.

Fair enough.

I guess the fact that it's referred to as 'energy' and many of the energies that we interact with we are capable of observing led to my misunderstanding.

So, beyond this, does anyone have any opinion on whether this "force" possibly affects anything else in our reality other than the expansion of the universe?

So we're getting somewhere here.

The thing with Energy is exactly the same as with Forces. You may have more or less intuition for the one than for the other, but they are both theoretical concepts which are not identical to the phenomena you observe, but only constructs which can be used to describe them.

Energy has Units Force x Distance, which means that it describes a quantity which more or less measures the capability to exert force on something while it moves a distance. This quantity is conserved (cannot be destroyed and created at least in flat spacetime), and therefore it a useful theoretical quantity to consider.

Anyways, coming back to dark energy. If it indeed has the form of the cosmological constant (for which there is no evidence against), it is difficult to come up with *other* effects. It is basically the energy level of empty space as seen by gravity. As you may know, the Einstein Field Equation basically says

Spacetime Curvature = some numbers * Energy-Momentum density of stuff.

If you add a constant on either side of this equation, it amounts to a constant energy density of empty space (=dark Energy). I am not aware of any other observable that could yield an independent measurement for the forseeable future. That's a pity, isn't it. Even though it seemingly only yields one single observable, it does make predictions, namely *how* exactly this accelerated expansion should be accelerated. One can therefore in principle test it by looking at the rate of relative expansion over time, and in a Universe in which dark energy dominates, this rate should be constant. If it changes, it's a more complicated beast, which can for example be described by quintessence models for dynamical dark energies. But, again, there's no evidence for that yet.

The cosmological constant is not a fudge either - it was a free parameter in the theory from the beginning, and Einstein first put it in out of prejudice, and then removed it again out of prejudice. From a modern perspective, the only thing that is surprising about it is how little of it there is. The standard model of particle physics produces vacuum energy density from quantum effects which are much larger than the observed dark energy by at least, say, 40 orders of magnitude, which has to cancel with the cosmological constant input parameter. This so-called hierarchy problem is the thing that confuses people, not that there should be dark energy at all. People were aware of this theoretical weirdness and were therefore hoping that the total cosmo constant would be set to exactly zero by some symmetry principle. This was of course famously proven wrong around the 2000s.