*[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]*

Physicists believe they understand quite well how the universe works on large scales. There’s dark matter and there’s dark energy, and there’s the expansion of the universe that allows matter to cool and clump and form galaxies. The key assumption to this model for the universe is the cosmological principle, according to which the universe is approximately the same everywhere. But increasingly more observations show that the universe just isn’t the same everywhere. What are those observations? Why are they a problem? And what does it mean? That’s what we’ll talk about today.

Let’s begin with the cosmological principle, the idea that the universe looks the same everywhere. Well. Of course the universe does not look the same everywhere. There’s 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 noticed that too, so the cosmological principle more precisely says that matter in the universe is equally distributed when you average over sufficiently large distances.

To see what this means, forget about matter for a moment and suppose you have a row of detectors and they measure, say, temperature. Each detector gives you a somewhat different temperature but you can average over those detectors by taking a few of them at a time, let’s say 5, calculate the average value from the reading of those five detectors, and replace the values of the individual detectors with their average value. You can then ask how far away this averaged distribution is from one that’s the same everywhere. In this example it’s pretty close.

But suppose you have a different distribution, for example this one. If you average over sets of 5 detectors again, the result still does not look the same everywhere. Now, if you average over all detectors, then of course the average is the same everywhere. So if you want to know how close a distribution is to being uniform, you average it over increasingly large distances and ask from what distance on it’s very similar to just being the same everywhere.

In cosmology we don’t want to average over temperatures, but we want to average over the density of matter. On short scales, which for cosmologists is something like the size of the milky way, matter clearly is not uniformly distributed. If we average over the whole universe, then the average is uniform, but that’s uninteresting. What we want to know is, if we average over increasingly large distances, at what distance does the distribution of matter become uniform to good accuracy?

Yes, good question. One can calculate this distance using the concordance model, which is the currently accepted standard model of cosmology. It’s also often called ΛCDM, where Λ is the cosmological constant and CDM stands for cold dark matter. The distance at which the cosmological principle should be a good approximation to the real distribution of matter was calculated from the concordance model in a 2010 paper by Hunt and Sarkar.

They found that the deviations from a uniform distribution fall below one part in a hundred from an averaging distance of about 200-300 Mpc on. 300 Megaparsec are about 1 billion light years. And just to give you a sense of scale, our distance to the next closest galaxy, Andromeda, is about two and a half million light years. A billion light years is huge. But from that distance on at the latest, the cosmological principle should be fulfilled to good accuracy – if the concordance model is correct.

One problem with the cosmological principle is that astrophysicists have on occasion assumed it is valid already on shorter distances, down to about 100 Megaparsec. This is an unjustified assumption, but it has for example entered the analysis of supernovae data from which the existence of dark energy was inferred. And yes, that’s what the Nobel Prize in physics was awarded for in 2011.

Two years ago, I told you about a paper by Subir Sarkar and his colleagues, that showed if one analyses the supernovae data correctly, without assuming that the cosmological principle holds on too short distances, then the evidence for dark energy disappears. That paper has been almost entirely ignored by other scientists. Check out my earlier video for more about that.

Today I want to tell you about another problem with the cosmological principle. As I said, one can calculate the scale from which on it should be valid from the standard model of cosmology. Beyond that scale, the universe should look pretty much the same everywhere. This means in particular there shouldn’t be any clumps of matter on scales larger than about a billion light years. But. Astrophysicists keep on finding those.

Already in nineteen-ninety-one they found the Clowes-Campusano-Quasar group, which is a collection of thirty-four Quasars, about nine point five Billion light years away from us and it extends over two Billion Light-years, clearly too large to be compatible with the prediction from the concordance model.

Since 2003 astrophysicists know the „great wall“ a collection of galaxies about a billion light years away from us that extends over 1.5 billion light years. That too, is larger than it should be.

Then there’s the “Huge quasar group” which is… huge. It spans a whopping four Billion light-years. And just in July Alexia Lopez discovered the “Giant Arc” a collection of galaxies, galaxy clusters, gas and dust that spans 3 billion light years.

Theoretically, these structures shouldn’t exist. It can happen that such clumps appear coincidentally in the concordance model. That’s because this model uses an initial distribution of matter in the early universe with random fluctuations. So it could happen you end up with a big clump somewhere just by chance. But you can calculate the probability for that to happen. The Giant Arc alone has a probability of less than one in a hundred-thousand to have come about by chance. And that doesn’t factor in all the other big structures.

What does it mean? It means the evidence is mounting that the cosmological principle is a bad assumption to develop a model for the entire universe and it probably has to go. It increasingly looks like we live in a region in the universe that happens to have a significantly lower density than the average in the visible universe. This area of underdensity which we live in has been called the “local hole”, and it has a diameter of at least 600 million light years. This is the finding of a recent paper by a group of astrophysicists from Durham in the UK.

They also point out that if we live in a local hole then this means that the local value of the Hubble rate must be corrected down. This would be good news because currently measurements for the local value of the Hubble rate are in conflict with the value from the early universe. And that discrepancy has been one of the biggest headaches in cosmology in the past years. Giving up the cosmological principle could solve that problem.

However, the finding in that paper from the Durham group is only a mild tension with the concordance model, at about three sigma, which is not highly statistically significant. But Sarkar and his group had another paper recently in which they do a consistency check on the concordance model and find a conflict at four point nine sigma, that is a less than one in a million chance for it to be coincidence.

This works as follows. If we measure the temperature of the cosmic microwave background, it appears hotter into the direction which we move against it. This gives rise to the so-called CMB dipole. You can measure this dipole. You can also measure the dipole by inferring our motion from the observations of quasars. If the concordance model was right, the direction and magnitude of the dipoles should be the same. But they are not. You see this in this figure from Sarkar’s paper. The star is the location of the cmb dipole, the triangle that of the quasar dipole. In this figure you see how far away from the cmb expectation the quasar result is.

These recent developments make me think that in the next ten years or so, we will see a major paradigm shift in cosmology, where the current standard model will be replaced with another one. Just what the new model will be, and if it will still have dark energy, I don’t know. But I’ll keep you up to date. So don’t forget to subscribe, see you next week.

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