Physicists believe they understand pretty well how the universe works on large scales. There is dark matter and dark energy, and there is the expansion of the universe that allows matter to cool down and clump together and form galaxies. The central assumption of this model for the universe is the cosmological principle according to which the universe is roughly the same everywhere. But more and more observations show that the universe is not the same everywhere. What kind of observations are these? Why are they a problem? And what does that mean? That’s what we’ll talk about today.
Let’s start with the cosmological principle, the idea that the universe looks the same everywhere. Spring. Of course, the universe doesn’t look the same everywhere. There is more matter under your feet than above your head and more matter in the Milky Way than in intergalactic space and so on. Physicists have also noticed this, so the cosmological principle says more precisely that matter in the universe is evenly distributed if one averages over sufficiently large distances.
To see what that means, forget about matter for a moment and assume you have a number of detectors that measure temperature, for example. Each detector gives you a slightly different temperature, but you can average over these detectors by taking a few at a time, say 5, averaging the readings from those five detectors and going through the values from each detector. replace their average value. You can then ask how far this averaged distribution is from the same everywhere. In this example, it’s pretty close.
But let’s say you have another distribution, for example this one. If you average over sets of 5 detectors again, the result still does not look the same everywhere. If you now make an average over * all * detectors, then the average is of course the same everywhere. So if you want to know how close a distribution is to uniformity, average it over increasing distances and ask at what distance it is very similar if it is the same everywhere.
In cosmology we don’t want to average over temperatures, but rather over the density of matter. On short scales, which for cosmologists roughly correspond to the size of the Milky Way, the matter is clearly not evenly distributed. If we average over the entire universe, the average is uniform, but that is uninteresting. What we want to know is, if we average over ever greater distances, at what distance does the distribution of matter become uniform with good accuracy?
Yes, good question. This distance can be calculated with the help of the concordance model, which is the currently accepted standard model of cosmology. It is also often referred to as LambdaCDM, where Lambda is the cosmological constant and CDM stands for cold dark matter. The distance at which the cosmological principle should be a good approximation of the real distribution of matter was calculated from the concordance model in a 2010 article by Hunt and Sarkar.
They found that the deviations from a uniform distribution decrease to less than a hundredth from an averaging distance of around 200-300 Mpc. 300 megaparsecs is about 1 billion light years. And just to give you a sense of size, our distance to the nearest galaxy, Andromeda, is about two and a half * million light years. A billion light years is huge. But from this distance at the latest, the cosmological principle should be met with good accuracy – if the concordance model is correct.
One problem with the cosmological principle is that astrophysicists have occasionally assumed that it is already valid at shorter distances, down to about 100 megaparsecs. This is an unjustified assumption, but one that has flowed into the analysis of supernova data, for example, from which the existence of dark energy was concluded. And yes, the 2011 Nobel Prize in Physics was awarded for this.
Two years ago I told you about a work by Subir Sarkar and his colleagues that showed that if you analyze the supernovae data correctly, without assuming that the cosmological principle applies to too short distances, then the evidence for dark energy disappear. This paper has been almost completely ignored by other scientists. Check out my previous video to learn more about it.
Today I want to tell you about * another problem with the cosmological principle. As I said, the standard model of cosmology can be used to calculate from which scale it should be valid. Beyond that scale, the universe should look pretty much the same everywhere. In particular, this means that there should be no lumps of matter on scales greater than about a billion light years. But. Astrophysicists find these again and again.
Already in ninety-one they found the Clowes-Campusano quasar group, which is a collection of thirty-four quasars, about nine point five billion light-years from us and spanning two billion light-years, clearly too large to be compatible with the prediction from the concordance model be.
Astrophysicists have known the “Great Wall” since 2003, a collection of galaxies about a billion light years away from us, which extends over 1.5 billion light years. That too is bigger than it should be.
Then there is the “Giant Quasar Group” which is … huge. It extends over / ˈwɒpɪŋ / four billion light years. And it wasn’t until July that Alexia Lopez discovered the “Giant Arc”, a collection of galaxies, galaxy clusters, gas and dust that extends over 3 billion light years.
In theory, these structures shouldn’t exist. It can happen that such clumps appear randomly in the concordance model. This is because this model uses an initial distribution of matter in the early universe with random fluctuations. So it can happen that you happen to have a large lump somewhere. But you can calculate the probability that that will happen. The Giant Arc alone has a chance of less than one in a hundred thousand that it came about by chance. And that doesn’t take into account all other large structures.
What does it mean? This means that the evidence is mounting that the cosmological principle is a bad assumption to model the entire universe, and it must likely go away. It is increasingly looking as if we live in a region of the universe that happens to have a significantly lower density than the average in the visible universe. This underdense area in which we live is known as the “local hole” and is at least 600 million light years in diameter. This is the result of a recent article by a group of astrophysicists based in Durham, UK.
They also point out that if we live in a local dump it means that the local value of the Hubble course needs to be revised downwards. This would be good news because currently the measurements for the local value of the Hubble rate conflict with the value from the early universe. And that discrepancy has been one of the biggest headaches in cosmology in recent years. Giving up the cosmological principle could solve this problem.
However, the result in this Durham Group paper is only a slight tension with the concordance model, around three sigma, which is not highly statistically significant. But Sarkar and his group recently had another paper doing a consistency check of the concordance model and finding a conflict at four point nine sigma, which is less than a one in a million chance that it was a coincidence.
It works like this. When we measure the temperature of the cosmic microwave background, it appears hotter in the direction we are moving against it. This creates the so-called CMB dipole. You can measure this dipole. You can also measure the dipole by inferring our motion from observations of quasars. If the concordance model was correct, the direction and size of the dipoles should be the same. But they are not. You can see this in this illustration from Sarkar’s paper. The star is the location of the cmb dipole, the triangle that of the quasar dipole. In this figure you can see how far the quasar result is from the cmb expectation.
These recent developments lead me to suspect that in the next ten years we will see a major paradigm shift in cosmology in which the current Standard Model is replaced by another. I don’t know what the new model will be and whether it will still have dark energy. But I’ll keep you posted. So don’t forget to subscribe until next week.