Science is a story of coming up with theories then doing our best to disprove them. That is especially true for theories on a grand, cosmological scale, though disproving them can be particularly hard. One of the most famous examples of a hard to disprove theory is that of dark energy and dark matter. In large parts of space we see unequivocal evidence that something is messing with general relativity. But down at the scale of our own solar system, there’s no evidence of it whatsoever, at least as far as we can see. A new paper from Slava Turyshev, a physicist at NASA’s Jet Propulsion Laboratory, discusses a way scientists might be able to deal with this discrepancy - by being very, very selective with the way we test for evidence of dark matter and energy in our solar system.
The fundamental problem the paper is trying to solve is the “Great Disconnect” between the physics we see in cosmology and that which we see in our own solar system. In regions with little to no density of matter (i.e. no gravitational force), the effects of dark energy and modified gravity that doesn’t obey Einstein’s Theory of General Relativity are most evident. However, in areas where there’s a lot of matter, and hence a lot of gravity, that discrepancy completely disappears, at least to the instruments that we currently use to observe it.
Planets orbit as expected. The curve of spacetime around the Sun from radio signals of the probes we spend there is right in line with predictions. All of the probes we have sent throughout the solar system act as if regular general relativity and gravity are acting on them. There’s no evidence for anything different at all.
Fraser discusses how we know dark energy is real - or at least something is making the universe behave differently.But at larger scales, like that between galaxies, the evidence is hard to miss. The universe itself seems to be expanding, and while there’s some debate about how quickly it is, we currently don’t have any other way to describe it other than to say that something is messing with our understanding of either relativity or gravity itself.
Physicists think that might have something to do with a “screening” process, where whatever is causing this discrepancy changes its physical properties when is areas of increasing density. There are two main categories of “screening” models. One is known as a “chameleon” model, where a theoretical fifth force of nature (other than gravity, electromagnetism, and the two nuclear forces) changes its effect whether or not there are large amounts of other matter around. In large, low density areas, it's very strong and causes the effect that we attribute currently to dark energy. But in highly dense areas, it is extraordinarily weak, to a point where it’s essentially undetectable to modern instruments, though it is still there. In highly dense environments, like the Sun, it might only be noticeable in a “thin shell” around the object, but at least in theory it would still be detectable there.
An alternative model for this discrepancy is the Vainshtein screening model. In this case, instead of the force itself changing its properties, it is essentially paralyzed by the gravity surrounding massive objects, making it look weak but not really changing its own physical properties. In this model, there is an idea called a Vainshtein Radius, where the fifth force returns back to normal outside the influence of a massive object. However, for our Sun, its Vainshtein radius is estimated to be 400 light years, an area which includes many, many other stars, so in effect the fifth force would be suppressed entirely until you reach some distance past the edge of the galaxy.
Fraser explains the difficulty in understanding dark energy.Each of these models would have “hints” in the data sets collected by large cosmological missions like Euclid and The Dark Energy Spectroscopic Instrument (DESI). However, since they are only looking at far-away space and large numbers of galaxies, they wouldn’t be able to prove how the fifth force would change when only interacting with objects in the solar system. That would require a specific mission in the solar system, and more importantly, a falsifiable theory that makes a prediction about what that mission should see.
According to Dr. Turyshev, without the theoretical backing of a falsifiable theory, there’s no point in continuing to conduct experiments in our own solar system - we’ve already proved that our best efforts aren’t able to detect anything out of the realm of general relativity. But, if theoreticians can extrapolate out testable hypotheses from the data collected by the big cosmological surveys that can be tested in the solar system, then we should design a mission to do so.
Admittedly it might be a while before we can develop instruments sensitive enough to disprove the theory. So, if we aren’t able to yet, then we should focus on missions to incrementally develop those instruments. But if there is a testable hypothesis based on “hints” from cosmological surveys that can be falsified by an experiment we can actually build, then we should do it - and potentially fundamentally change our understanding of how the universe works.
Learn More:
S. Turyshev - Solar--System Experiments in the Search for Dark Energy and Dark Matter
UT - A New Supernova Study Suggests Dark Energy Might be Weakening
UT - New Study of Supernovae Data Suggests That Dark Energy is an Illusion
UT - Fresh Findings Strengthen the Case for Dark Energy's Evolution
