I want to talk to you about a current problem, a mystery that has astrophysicists and cosmologists from all over the planet fighting and working on a solution. It may even be the threshold that triggers a new revolution in how we understand the Universe.

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.

So over the years they both improve their instrumentation and become more skilled at doing their homework, waiting for one of the possible speeds to win out. Until a time comes when each realizes that they are so highly skilled, their technology so accurate and so low in error, that there can be no understanding between the two methods. There is no middle ground: both ways of finding the speed of the bus are giving different values with certainty. And this is when the battle begins: the latter accuses the former that his method is full of errors that he is completely underestimating, that his calibration is wrong, that he doesn't have enough pictures and that measuring distances so far away is much more delicate. But the former accuses the latter that his entire method relies on an assumption that may be wrong: that the bus travels the most reasonable path. And who knows! Maybe the driver takes a detour to get a coffee and the route could be even more bizarre than we are assuming.

From the small spots in this photo one can deduce what this plasma was made of and once you know this you can predict how space will react to these substances, at what rate it will expand throughout its history. This prediction is not at all easy and requires extra information from other imprints left by ancestral processes; all to have an accurate answer. But, roughly speaking, the study of the microwave background and the early universe allows us to predict the rate of expansion today; it allows us to deduce what the Hubble Constant is.

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.

So, are we on the verge of a revolution? Well, if we look at the past, the big changes in physics started with small inconveniences like this. But we'll know more soon enough. New methods of finding the Hubble Constant have been in the making for some time: the red giant tip, mega-masers, even the use of gravitational waves as standard sirens; totally different ways with their good and bad. In essence they will make this battle no longer just two teams, shedding a lot of light on which is the correct figure. Hey, if in the end it turns out that Team Close is right, that ACDM is not the end of the road and that we have to turn our overall picture of the Universe upside down? Welcome! In fact it would not be unexpected at all.

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.

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