Let's start with an analogy: imagine a bus that always makes the same route, from the station to the home of our protagonists. These two have set themselves a challenge: to find out what the speed of the bus is; each has decided to do it in different ways. The first one is going to do it by photographing the bus when it comes from far away, before braking, he takes a few pictures with his camera, and from the change of position in the images he deduces the speed of the bus.
The second one opts for another method: he knows the city well and knows the possible routes the bus can take to get to the station, so he assumes that he is taking the most reasonable one. He also has some witnesses who have seen it pass by. So, knowing the total distance he is going to travel, he only has to press his stopwatch when the bus starts the round and stop it when it returns. Divide the distance by time and you get the speed.
What happens later when the two of them put together the figure they have found, they realize that they don't match. It's nothing to pull your hair out over, but it's a noticeable discrepancy. At first they think it's a matter of accuracy: the first tells the second that the watch and the map he uses to make the estimate are rubbish and that if he used GPS measurements and a better stopwatch, he would probably get his number. In turn, the second tells the first that if he invested in a camera, a better lens and went out more days to take pictures, he would be able to deduce the speed much more accurately, surely getting closer to his number.
Let's call this group the Near Team, as they try to measure objects in an environment close to us. On the other hand we have the opposite strategy of Team Far Away: instead of seeing what the nearest Universe tells us, they analyze what the farthest reaches of the cosmos tell us. And, to look into the depths of space is to look into the past, it is to receive the light of events that happened billions of years ago, it is to see the Universe when it was in a larval state, compressed into a gigantic plasma. And we have a picture of the final phases of this plasma: the famous Cosmic Microwave Background.
The problem: after years of research, it turns out that the results of the two teams differ. Historically, the two coexisted, although they looked at each other out of the corner of their eyes. The errors of the two were so large that their bars overlapped: in the uncertainty there was a place for concord. But in recent years the errors have become so small that war has broken out. The difference, as you see, is not barbaric.... But it is there. And it reveals that at least one of the teams is getting it wrong. There is a tension, the near versus the far, the modern versus the ancient.
Some researchers of the Far Team think that the problem is that the Near Team needs to map many more objects and better distributed by the celestial vault. It is suggested that some measurements have been too local and may have been clouded by gravitational phenomena that have nothing to do with the expansion of the Universe. Meanwhile, some researchers of the Near Team believe that it is more likely that the theoretical assumptions of the Far Team are flawed. To predict the Hubble Constant of the Microwave Background, one has to start from a good understanding of what happened in the ancestral plasma: what substances were present, what processes took place... If something is wrong or missing here, the prediction is compromised. It is as if you had to design a bridge thinking that you are going to use a particular cement when you then use three very different ones, most likely that bridge would collapse because your calculations have not taken it into account. That is, this determination of the rate of expansion is what is called "Model Dependent", it is not a method in which you take an instrument and simply measure a quantity; it is an indirect way that requires that the physics that we suppose happened in the first moments of the Universe is right.
We have only been observing the Universe for a few decades now, on the large scales, we can find anything! It is precisely this sense of mystery that makes all scientists, no matter what team they belong to, put all their cards on the table to find out what the solution is. I don't want you to finish the reading thinking that this is like a debate between politicians, in which no one recognizes their weaknesses and rewards leaving the opponent wrong. No, this is more like a friendly soccer match, where both teams give everything they have so that the best team wins, so that the truth wins, even if it means that someone is wrong. Mainly because being wrong... That's precisely what scientists are looking for! This is a super nice thing about science: we love change! We love it when Mother Nature comes along and challenges what we know about her, posing us new challenges, problems and mysteries.
The world is still full of them and the depths of space have just opened their doors... On top of all that, if it turns out that the whole cosmology is wrong... Someone is going to have to MAKE PUBLICATIONS ABOUT IT. What do you guys think? What does your inner self say? Do you align yourself with Team Near or Team Far? Leave it for me in the comments and when there is an update on the topic I assure you that you will see it here.