6 ms·

Dynamical dark energy and the week that broke cosmology

54 points by rznicolet 2mo ago

ck2 1mo ago
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
- Joker_vD 1mo ago
  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".
  - bognition 1mo ago
    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.
    - MattPalmer1086 1mo ago
      No, that is not how science actually progresses, it is an idealised view of it at best. Thomas Kuhn wrote "The Structure of Scientific Revolutions" back in the 1960s. Plank's principle stated "Science progresses one funeral at a time". Scientists are also human and will tend to defend existing ideas. In cosmology the arguments over LCDM, dark matter and dark energy have been raging for decades. Dark matter people like to say that the Bullet Cluster is incontrovertible evidence of dark matter; MOND people don't agree and point to other aspects they claim refute dark matter! It's gonna take a long time for it all to shake out.
      - SubiculumCode 1mo ago
        Not an expert, but isn't your LCDM vs MOND example assymetric? Like different kinds of data supports each model very well, and so reconciliation isn't simple? Moreover, your objection that science progresses one funeral at a time is exaggerated, even if it is real, it's not necessarily all that strong: https://pmc.ncbi.nlm.nih.gov/articles/PMC6814193/ https://pmc.ncbi.nlm.nih.gov/articles/PMC6814193/
        MattPalmer1086 1mo ago
        Certainly true that the data can be viewed in many different ways. Often the same data is used by both sides! The funeral thing isn't really a position I strongly hold; it's a pithy comment that points out science is a human endeavor and maybe not quite as objective as it is often presented. But science as a whole is ultimately a self correcting methodology, even if it might take longer than we would want and doesnt always get it right.
      - layer8 1mo ago
        MOND people have become a small minority. In addition to the galaxy cluster observations, MOND doesn’t match the CMB power spectrum, while ΛCDM does, and various very precise measurements within the solar system also contradict MOND. This was different 30-40 years ago, but since then, as evidence has strongly mounted against MOND, a lot of “shaking out” has already taken place.
  - pdonis 1mo ago
    > 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.
    - suuuuuuuu 1mo ago
      Pedantic point, but in the time-evolving model ("model"), dark energy is not uniform across space - but the non-uniformity turns out to be negligible.
      - pdonis 1mo ago
        > in the time-evolving model ("model"), dark energy is not uniform across space What are you basing this on?
        suuuuuuuu 1mo ago
        Domain expertise. (I would ask the same of you!) Fluids always have perturbations, except in the special case of w = -1 (cosmological constant); otherwise, dropping them violates energy-momentum conservation and gauge invariance. Here are a few excerpts from DESI: * https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20cosmological,i.e.%20constant). https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20c... * https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20parameter,angular%20power%20spectrum. https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20pa...
        1mo ago
  - chowells 1mo ago
    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.)
  - ck2 1mo ago
    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
- undershirt 1mo ago
  yes galileo was forced to recant heliocentricism to the roman catholic church, because they had gone insane. also consider the dogmatic commitment that scientism has to methodological naturalism. this is the religious spirit of science today
MarkusQ 1mo ago
> 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?
- gjm11 1mo ago
  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.
layer8 1mo ago
On the general topic of dark energy and dark-energy theories, I can recommend the following two recent podcast episodes by Sean Carroll (theoretical physicist): https://preposterousuniverse.com/podcast/2026/06/22/358-solo-vacuum-energy-and-the-cosmological-constant/ https://preposterousuniverse.com/podcast/2026/06/22/358-solo... https://preposterousuniverse.com/podcast/2026/06/29/359-solo-theories-of-dark-energy/ https://preposterousuniverse.com/podcast/2026/06/29/359-solo...

54 points by rznicolet 2mo ago

ck2 1mo ago

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

Joker_vD 1mo ago
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".
- bognition 1mo ago
  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.
  - MattPalmer1086 1mo ago
    No, that is not how science actually progresses, it is an idealised view of it at best. Thomas Kuhn wrote "The Structure of Scientific Revolutions" back in the 1960s. Plank's principle stated "Science progresses one funeral at a time". Scientists are also human and will tend to defend existing ideas. In cosmology the arguments over LCDM, dark matter and dark energy have been raging for decades. Dark matter people like to say that the Bullet Cluster is incontrovertible evidence of dark matter; MOND people don't agree and point to other aspects they claim refute dark matter! It's gonna take a long time for it all to shake out.
    - SubiculumCode 1mo ago
      Not an expert, but isn't your LCDM vs MOND example assymetric? Like different kinds of data supports each model very well, and so reconciliation isn't simple? Moreover, your objection that science progresses one funeral at a time is exaggerated, even if it is real, it's not necessarily all that strong: https://pmc.ncbi.nlm.nih.gov/articles/PMC6814193/ https://pmc.ncbi.nlm.nih.gov/articles/PMC6814193/
      - MattPalmer1086 1mo ago
        Certainly true that the data can be viewed in many different ways. Often the same data is used by both sides! The funeral thing isn't really a position I strongly hold; it's a pithy comment that points out science is a human endeavor and maybe not quite as objective as it is often presented. But science as a whole is ultimately a self correcting methodology, even if it might take longer than we would want and doesnt always get it right.
    - layer8 1mo ago
      MOND people have become a small minority. In addition to the galaxy cluster observations, MOND doesn’t match the CMB power spectrum, while ΛCDM does, and various very precise measurements within the solar system also contradict MOND. This was different 30-40 years ago, but since then, as evidence has strongly mounted against MOND, a lot of “shaking out” has already taken place.
- pdonis 1mo ago
  > 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.
  - suuuuuuuu 1mo ago
    Pedantic point, but in the time-evolving model ("model"), dark energy is not uniform across space - but the non-uniformity turns out to be negligible.
    - pdonis 1mo ago
      > in the time-evolving model ("model"), dark energy is not uniform across space What are you basing this on?
      - suuuuuuuu 1mo ago
        Domain expertise. (I would ask the same of you!) Fluids always have perturbations, except in the special case of w = -1 (cosmological constant); otherwise, dropping them violates energy-momentum conservation and gauge invariance. Here are a few excerpts from DESI: * https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20cosmological,i.e.%20constant). https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20c... * https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20parameter,angular%20power%20spectrum. https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20pa...
        1mo ago
- chowells 1mo ago
  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.)
- ck2 1mo ago
  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
undershirt 1mo ago
yes galileo was forced to recant heliocentricism to the roman catholic church, because they had gone insane. also consider the dogmatic commitment that scientism has to methodological naturalism. this is the religious spirit of science today

MarkusQ 1mo ago

> 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?

gjm11 1mo ago
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.

layer8 1mo ago

On the general topic of dark energy and dark-energy theories, I can recommend the following two recent podcast episodes by Sean Carroll (theoretical physicist): https://preposterousuniverse.com/podcast/2026/06/22/358-solo-vacuum-energy-and-the-cosmological-constant/ https://preposterousuniverse.com/podcast/2026/06/22/358-solo... https://preposterousuniverse.com/podcast/2026/06/29/359-solo-theories-of-dark-energy/ https://preposterousuniverse.com/podcast/2026/06/29/359-solo...