As most of you have probably already noticed, I am not particularly fond of the standard model of cosmology… More precisely, I have a problem with dark matter and dark energy, what I call the “dark side” of the Universe. Many times, I was asked why, so I will give an answer here, and explain why my preference goes to alternative theories.
According to the standard model of cosmology, our Universe was born some 13.7 billion years ago, when it started to expand from an insanely hot and dense state. Very early, after 10-34 second approximately, the Universe expanded at an extremely fast rate for a very short period of time, smoothing out its original lumpiness: this period is known as inflation. The Universe we see today is homogeneous and isotropic, and it is still expanding and cooling.
The standard model describes the present composition of the Universe as follows:
- about 5% of matter: stars, gas, neutrinos and various heavy elements
- about 25% of cold dark matter: an unknown form of matter , inferred to exist from gravitational effects on visible matter
- about 70% of dark energy: an unknown form of energy, thought to be responsible for the apparent acceleration of the expansion of the Universe
In short, 95% of our Universe is made of… unknown dark stuff. And the problem is that there is not a shred of evidence that either dark energy or dark matter exist. Nothing. So far, every attempt to detect dark matter particles has failed. You might have read articles saying ‘Dark energy is real‘ or ‘Dark matter confirmed‘, which would be cool if it was true, but it’s not. These are titles or headlines intended to hook the reader: the only things that are confirmed in such cases are the fact that the Universe appears to be expanding at an accelerating rate, or that some mass appears to be missing.
Just to be clear, I am not saying that dark matter and dark energy don’t exist, I am saying that we still don’t know if they do. Both of them help describe our observable Universe, and they do it very well: that’s why they are part of the standard model that most cosmologists agree upon.
However, there are a few alternative theories that attempt to describe the Universe, without the addition of any dark component. And some of them seem to reproduce observations pretty well too! I am not going to give an exhaustive list of the alternatives, but I’ll give a short description of the most interesting ones, which I have already addressed on the blog.
What if dark matter was just an illusion? Dragan S. Hajdukovic suggests that the quantum vacuum could be a dipolar fluid. Assuming that antimatter has a negative gravitational charge (there would be a gravitational repulsion between matter and antimatter), a gravitational polarization of the quantum vacuum caused by baryonic matter would produce the same effect as dark matter. The cancellation of gravitational charges in the vacuum may also produce a similar effect to that of dark energy. If future experiments don’t reveal any gravitational repulsion between matter and antimatter, this hypothesis will be ruled out. Otherwise, if a gravitational repulsion between matter and antimatter is observed, then dark matter and dark energy might be an effect of interactions between the quantum vacuum and matter.
Spacetime torsion is another extremely interesting alternative. The Einstein–Cartan–Sciama–Kibble (ECSK) theory of gravity naturally extends general relativity to account for the intrinsic angular momentum (spin) of elementary particles. It causes spacetime to exhibit a geometric property called torsion. Torsion manifests itself as a force countering gravitational attraction, accounting for the flatness of the Universe without requiring inflation. It could also explain the nature of dark energy, as well as dark matter which would actually be made of antimatter.
Another alternative theory, modified gravity (MOG) may also offer a description of the Universe without any dark component. MOG postulates the existence of a massive vector field, introducing a repulsive modification of the law of gravitation at short range. Like the standard model, MOG can account for a variety of key cosmological observations like gravitational lensing, rotation curves of galaxies, the distribution of mass in the Universe… Because some of the predictions of MOG differ from the standard model, further observations with higher resolutions will tell which model is the best.
So this is the current situation: whether you accept the standard model and try to identify the dark side of the Universe, or you try to find alternatives without adding any unknown dark substance. And between “The standard model describes our Universe, and it’s almost entirely made of stuff we don’t know” and “Maybe the standard model is incomplete, maybe we need to rethink our theories”, I wouldn’t say that the first option is obviously the best one…
Maybe the Universe is mostly made of dark energy and dark matter, but there is still no evidence supporting their existence, and none of them has been confirmed to be “real” yet. There are alternative theories trying to describe our world without them, and they deserve at least as much attention. Further experiments and studies will help confirm, or rule out, the wrong theories. Maybe the answer is something no one has even thought of yet…
Finally, here are a few words by a fictional green creature living in a galaxy far, far away, that I find quite appropriate:
“Hard to see, the dark side is. We must investigate further before drawing a conclusion to the identity of your adversary.”