> A recent analysis by the Dark Energy Survey Supernova Program also finds a preference for evolving dark energy, in the same direction as DESI DR2, but with a slightly lower statistical significance (bringing the combined significance down from 4.2 sigma to only 3.2).
Is this right? It would seem that adding another finding "in the same direction" should always increase the statistical significance, even if only slightly if the new evidence is week.
Under what circumstances could additional evidence for X reduce the estimated likelihood of X?
It is not true that another finding in the same direction should always increase the statistical significance.
Suppose you've got a coin; null hypothesis is that it comes up heads and tails equally often. I flip the coin 5 times and get heads every time. Probability of at least this many heads on the null hypothesis is 1/32. Now I flip the coin another 9 times and get 5 heads / 4 tails: evidence in the same direction. Between the two experiments I have 10 heads / 4 tails. Probability of at least this many heads on the null hypothesis is [(14 choose 0) + ... + (14 choose 4)] / 2^14 ~= 0.09, much bigger than 1/32.
There are also other circumstances in which even additional strong evidence for X can reduce the probability of X.
Suppose I have three hypotheses A,B,C which initially I think are all equally likely. Then two things happen that both have probability 1/2 if A is true, probability 1/4 if B is true, and probability 0 if C is true. After one of them, I should think A is true with probability 2/3. After both, I should be more confident, right?
Nope. Suppose e.g. what I'm doing is pulling balls out of a bag. Hypothesis A is "I have either a bag of red balls or a bag of blue balls, with equal probability". Hypothesis B is "I have a bag with 25% red balls, 25% blue balls, and 50% green balls". Hypothesis C is "I have a bag containing only green balls".
So I pull out a ball from the bag and it's red. That happens half the time in scenario A, 1/4 the time in scenario B, and never in scenario C, like I claimed.
I put the ball back and shake things up so I'm starting afresh, and pull out another ball. This one's blue. Again: half the time in scenario A, 1/4 the time in scenario B, never in scenario C.
But those two things can't ever both happen in scenario A, because in that scenario I have a monochromatic bag. They can both happen in scenario B. And of course neither of them can happen in scenario C.
So I got a result that (on its own) was evidence for A over the other two hypotheses, and then another result that (on its own) was evidence for A over the other two hypotheses, and the effect of both together is that I know A is false and B is true.
to be fair, cosmology has been broken every several months lately (Hubble Tension, etc.)
I mean that's how science works, discover something new, rewrite knowledge
not like religion where you make stuff up before you know anything about anything and then force everything new to adapt to the legacy of decisions thousands of years prior
Is it? Because 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".
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.)
> 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.
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.
Is this right? It would seem that adding another finding "in the same direction" should always increase the statistical significance, even if only slightly if the new evidence is week.
Under what circumstances could additional evidence for X reduce the estimated likelihood of X?
Suppose you've got a coin; null hypothesis is that it comes up heads and tails equally often. I flip the coin 5 times and get heads every time. Probability of at least this many heads on the null hypothesis is 1/32. Now I flip the coin another 9 times and get 5 heads / 4 tails: evidence in the same direction. Between the two experiments I have 10 heads / 4 tails. Probability of at least this many heads on the null hypothesis is [(14 choose 0) + ... + (14 choose 4)] / 2^14 ~= 0.09, much bigger than 1/32.
There are also other circumstances in which even additional strong evidence for X can reduce the probability of X.
Suppose I have three hypotheses A,B,C which initially I think are all equally likely. Then two things happen that both have probability 1/2 if A is true, probability 1/4 if B is true, and probability 0 if C is true. After one of them, I should think A is true with probability 2/3. After both, I should be more confident, right?
Nope. Suppose e.g. what I'm doing is pulling balls out of a bag. Hypothesis A is "I have either a bag of red balls or a bag of blue balls, with equal probability". Hypothesis B is "I have a bag with 25% red balls, 25% blue balls, and 50% green balls". Hypothesis C is "I have a bag containing only green balls".
So I pull out a ball from the bag and it's red. That happens half the time in scenario A, 1/4 the time in scenario B, and never in scenario C, like I claimed.
I put the ball back and shake things up so I'm starting afresh, and pull out another ball. This one's blue. Again: half the time in scenario A, 1/4 the time in scenario B, never in scenario C.
But those two things can't ever both happen in scenario A, because in that scenario I have a monochromatic bag. They can both happen in scenario B. And of course neither of them can happen in scenario C.
So I got a result that (on its own) was evidence for A over the other two hypotheses, and then another result that (on its own) was evidence for A over the other two hypotheses, and the effect of both together is that I know A is false and B is true.
I mean that's how science works, discover something new, rewrite knowledge
not like religion where you make stuff up before you know anything about anything and then force everything new to adapt to the legacy of decisions thousands of years prior
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.)
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.
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.