Dynamical dark energy and the week that broke cosmology

A couple of other remarks.

1. In the first of those cases, with the biased coin, suppose that in the second phase you flip it _thousands_ of times and it comes up with N+1 heads and N tails. Then the statistical significance of the overall bias can become arbitrarily small, and if you had a concrete alternate hypothesis along the lines of "at least 5% more likely to come up H than T" the overall evidence can be extremely strong evidence against that.

2. In the second case, with the coloured balls, note that we can readily make each bit of evidence for A > B > C much stronger than the 2 : 1 : 0 above; the bag in hypothesis B could be, say, 1% red, 1% blue, 98% green. We'd still end up with B being the only one of our viable possibilities after both observations.

3. There are real-world situations with the sort of structure we see in that second case. Suppose hypothesis A is "there is a god who not infrequently works miracles", hypothesis B is "there are no gods, but people not infrequently fabricate evidence of miracles", and hypothesis C is "nothing at all miraculous-looking ever happens". Then you see what looks like a convincing case of evidence for one god, and then what looks like a convincing case of evidence for a different one. The two gods are from different religions each of which adamantly insists that the other one is pure invention.

4. The real world is never as neat as these examples. E.g., in the gods scenario you also need to consider possibilities like "actually both those religions have some truth to them, and rivalry between their gods is why each one's followers insist that the other religion is completely imaginary" and "there is a miracle-working god, but also followers of other gods fabricate miracles" and "there are no gods and very little fabrication, but sometimes incredibly unlikely things happen just by chance" and "there are no gods but there are super-advanced aliens who enjoy messing with us" and so forth. Even in the balls-from-bags scenario -- if it happens in the real world rather than in a textbook -- you have to consider things like "I was mistaken about what ball I picked the first time" and "actually sometimes one blue ball gets into a big bag of red balls by accident" and so forth.

(A lot of the most important work in, e.g., science consists not of testing rival hypotheses but of figuring out viable hypotheses to test.)

Thank you, that makes a lot of sense!

> it all really looks awfully close to "okay, fine, the dark energy/matter apparently also can do/be this stuff as well, just so we can write in whatever correction factors we need to make the theory fit the observations".

The dark energy density is already a parameter in the model. The simplest case for such a parameter is that it's just a constant, so in the absence of evidence to the contrary, Occam's Razor led cosmologists to adopt it.

But now we have evidence that suggests that it's not a constant, so we're looking at the next simplest case, a function of time (but still constant everywhere in space at each instant of time). The article describes how the DESI data suggest that it's a slowly decreasing function of time.

What has not happened is people making up models and continuing to insist on them even after the data says otherwise. That's what the GP was saying religion does.

Yes, you're describing cosmology. We can't create separate test universes, so all we can do is argue over which model is the best way to explain our observations. The difference from ad-hoc reasoning is the exact same as any other science. You make predictions based on models and then try to find places where those predictions hold - or are broken. In either case, you're testing the validity of a model. And when something isn't good enough, you either adjust the model or find a new one that works better.

This isn't some slapdash random patching. Everyone involved knows that assuming that the λ in λCDM is a constant is shaky. It could be a function of time, or even location in some way. But it's simplest if it's a constant. So you start by modeling the universe as if it is. Then you determine what sorts of observations would support or contradict that, and you start making them. When you get results, you start examining what version of the model best explains those observations. And someone somewhere goes off to try find a better model than any version of λCDM. If they succeed, their model will eventually supplant it. This is how science progresses, even if the experiments are less under the control of the experimentors than they'd like. The important part that you make revisions in response to observations.

(FWIW, particle physicists are constantly frustrated that they can't find counterexamples to the Standard Model. They know it has to be incomplete, but the lack of contradictory observation leaves them no direction to try to improve it.)

Its fundamentally different. Science doesn't ask you to accept unknowns on faith, its provides evidence that there is something we dont yet understand and that evidence has predictive value.

If science operated like religion it would say "We dont know what dark energy is, but you have to accept it or your wrong".

Instead the argument is "Hmm... our current best understanding of the universe is lacking, when we add in these extra variables many of our predictions go from wildly inaccurate to nearly accurate. There must be something going on here we cannot observe, so lets call it "dark"."

Dark energy is "Wow, there's a factor at play here that we dont understand, without it our equations do not predict what we see, with it the equations are very accurate. Additionally we can predict multiple different things with this."

And the most important part, as soon as there is evidence that refutes the Dark Energy/Matter hypothesis scientists will quickly (over a few years, a decade at the most) walk away from it. Versus religious systems take centuries for people to shift their beliefs.

math is simply a working model of the observed

math doesn't try to explain why

math also allows predictions without understanding why

it's a placeholder to get things done

ie. correction factors

Einstein thought the universe was static but his math would not fit

So he made a "cosmological constant" to make static universe math fit

Hubble later used science to figure out the universe was expanding, not static

Einstein said "whoops" and threw out the constant

Except his math was right even without understanding why, it was just a model of the observed

The "cosmological constant" actually measures dark energy without even knowing what it was

Pretty amazing IMHO