Could a new, fifth force of nature provide some answers to our biggest questions about dark matter and dark energy? We’re working on it.
The Standard Model is, for all intents and purposes, the supreme accomplishment of modern physics. It describes four forces of nature, a zoo of particles, and how they all interact. It is perhaps the most successful scientific theory of all time.
And it’s fantastically incomplete.
It turns out that the Standard Model is able to account for less than 5% of all the matter and energy in the cosmos. Another 25% or so is Dark Matter, an unknown kind of matter that is for all intents and purposes invisible. The rest is known as Dark Energy, a mysterious entity that is causing the expansion of the universe to accelerate.
One of the first things astronomers noticed when they first discovered dark matter and dark energy was their apparent similarity. Why in the world are the two dark components of our universe roughly the same strength? I know, 25% and 70% don’t sound very similar, but when it comes to astronomy – and especially cosmology – they’re basically the exact same number.
Maybe it’s just a coincidence that they have about the same strength, and we’re overthinking it.
Or maybe it’s something else. Clever physicists have proposed connections within the “dark sector” of the universe, where dark matter and dark energy talk to each other. This would allow them to follow each other’s evolution, ensuring that they have roughly equal contributions to the energy budget of the universe for long periods of time.
To make them talk to each other, you need a force. But this force can’t be any of the known ones, otherwise dark matter and/or dark energy must also interact with normal matter, and we would have seen more directly evidence of them already.
So it has to be a new force, a fifth force of nature, completely different from electromagnetism, gravity, strong nuclear, and weak nuclear. While ideas like this remain only in the realm of hypothesis, some of the ideas already have names.
One name is quintessence, the fifth essence of the universe. Another is dark photons, a particle that travels the cosmos like a photon but is, as its name suggests, dark.
To test these ideas we have to turn to the cosmos for answers. If a fifth force exists, it must be very subtle. Stronger manifestations of the fifth force have already been ruled out by observations of galaxy clusters, the expansion of the universe, and even the behaviors of neutron stars. So we have our work cut out for us – it will take a truly massive amount of data to tease out some signal that differs from expectations.
The description of the Standard Model jumps over the fact that it already predicts a fifth force, the Higgs force between Standard Model particles. As a particle physicist describes it:
“As of 2012, we have a new force to think about: the force between two particles induced by the Higgs field! This is not to be confused with the effect by which the Higgs field gives the known elementary particles their masses; the Higgs field can do this to a single, isolated particle. That’s not a force; it doesn’t push or pull. But the Higgs field can also induce a force between two particles; this happens in much the same way that electromagnetic forces are created. However, as far as its effect on ordinary matter, this force is very, very hard to detect. At short distances, for particles like electrons and the up and down quarks that dominate the proton, the Higgs force is very weak (much weaker than electromagnetism, but much much stronger than gravity). At long distances, like the weak nuclear force, the Higgs force becomes extremely weak, because the Higgs particle, like the W particle, has a mass.”
“When might we actually observe this new force? It’s effects will first be observed either in the scattering of W and Z particles off each other (which will eventually be done, indirectly, within the proton-proton collisions at the Large Hadron Collider) or in the interaction between a top quark and a top anti-quark (which can be observed at an electron-positron collider — in fact I wrote my first particle physics paper [see in particular Figure 11 of the paper] about this very phenomenon.)” [link omitted; The Strengths of the Known Forces, Matt Strassler.]
“But this [presumably sixth fundamental] force can’t be any of the known ones, otherwise dark matter and/or dark energy must also interact with normal matter, and we would have seen more directly evidence of them already.”
This arguments smells funny.
If the roughly equal mass energies would be explained that way, why wouldn’t interactions with standard matter which is about as much mass energy as 10 % of dark energy and 25 % of dark matter be in order?
Meanwhile, radiation used to be 100 % of the mass energy and now it is less than 0.1 %. All the while interacting with normal matter.