TrickyDicky said:
When you said that if we find a new observation that the LCDM couldn't fit you wouldn't be terribly surprised if we started talking about xLCDM, that is defining a mechanic, a model producing process, so that when we'd find another observation that would make xLCDM not work we could add a new parameter so that we'd start talking about $xLCDM.
That's not what I'm saying. Again it's all about observation, and you have to go into the details of the observations.
Right now we have very good expansion curve observations for relatively late periods, but we don't have that sort of information about the early universe. It so happens given our knowledge about the early universe, we *could* parameterize everything with a single number for lambda and still hit observational constraints. It may turn out that we can't once the observations are better, at which point we have to put in a function for lambda.
Can't you see the pattern?, it's this pattern that has the plasticity, but you should be able this is the pattern of the ptolemaic epicycles. Nothing wrong with this pattern, in a way it works, it adapts to empirical observations, but eventually it comes to a point where the number of fudge factors is absurdly high, then you have two options: either you come up with a different model that explains observations without the fudge factors,and that probably make you reconsider something you thought it was obvious, or if you don't find the alternative you keep the model with the alphabet soup but with a suspicious rat smell you can't get rid of.
Sure but reality is messy. However...
1) If you can figure out something with fewer fudge factors that explains the observations, then people will go gaga over this. The problem is that people can't. It's not that people aren't looking or questioning models. The problem is that if you reduce the number of parameters, you just don't fit the data.
Also once you have something that works with a number of fudge factors, you don't give up. At that point you look at each of the numbers that you put in and ask why that number is that number.
2) The number of parameters in cosmological models really isn't that large. Twelve or so numbers and you've explained the universe. Think of the number of parameters you need to model the aerodynamics of a car or ocean currents. You typical solar model has dozens of parameters.
But these parameters aren't just random numbers. They *mean* something. If you have to set lambda to something to make everything work, then you scratch your head and think about what lambda is. And you can't set those numbers to anything. If you have to set the baryon mass of the universe to -1 to get it to work, then you have a problem.
3) Talking about Ptolemy is interesting because if you look at any model of the solar system, what you end up it quite more complex than anything Ptolemy every came up with, and it turns out that any model of the solar system has a lot of parameters that you have to set to make everything work (and curiously a solar system model has more parameters than a cosmology model).
It turns out that those aren't *free* parameters, but they correspond to physically relevant qualities (namely the mass of all of the planets). Same with LCDM. People aren't putting random numbers into the models. Each of the numbers *means* something, and then you can figure out the implications of the parameters being what they are.
It's not a matter of not looking. If you can get rid of a parameter, great! It's a matter of getting things to work. It's not that people aren't very actively looking for alternatives.
Also more data makes the problem worse. Our solar system models are a lot more complicated than anything that Ptolemy came up but we can predict the location of the planets to within 1 meter and within fractions of seconds, and that's nothing that he was able to do.
Ultimately you have to deal with the data. If you can come up with a simple model to deal with the data, then GREAT! But if you have to come up with something really messy and complex to deal with the data because you can't come up with anything better, than that's the just the universe works. And if the universe turns out to be more complicated than you like, that's something you have to take up with God since I can't help you with that.