The Hubble tension or crisis in cosmology is the discrepancy in different methods of measuring the speed of the universe's expansion between methods using observations of the early universe and modeling, vs those using a "ladder" or different distance measurement techniques of stars.

This market will resolve when there is a single, widely agreed upon estimate of Hubble's constant with tight confidence intervals (s.d. <1).

Current best estimates:

Cosmic ladder: SH0ES - 73.04 +- 1.04

CMB: Planck - 67.66 +-0.42

Wrong here means that the current best estimates using either method are 3 standard deviations or more away from the agreed estimate.

If there is scientific consensus that one cannot put a single number estimate on the Hubble constant is wrong, for example in case there are meaningful differences across space, then this market will resolve to "both are wrong". Temporal variation in the Hubble constant does not count, as the Hubble constant is already regarded as a point in time estimate, and the consensus view is that it varies by time.

## Related questions

For a consensus to form, you would need a third method of estimating H0 that is completely independent of the other two *and* the confidence interval for this estimate has to be small enough so it's consistent with *one and only one* of the two candidates. Standard sirens are the only option I'm aware of. While estimates of H0 using gravitational waves will probably improve over this decade, it's unlikely they will be good enough to select either ΛCDM or the distance ladder. (And that's assuming it *does* select a candidate—it could end up being inconsistent with both, which would prevent any consensus from forming!)

As such this market is likely to resolve N/A in 2030. I'm betting Ṁ10 just to track the question; I'm going for the ladder being wrong due to the common rule of thumb in cosmology that "you don't bet against ΛCDM" when it comes to other things in the field, but this doesn't represent any deep analysis on my part.

@BerF That’s not true at all! A consensus that one of the two estimates has a particular methodological flaw or misunderstanding of a physical phenomenon, leading to a reassessment of that data that brings it within the other’s confidence interval would be the most likely way for a consensus on H0 to form!

Two likely ways this would happen are

1) New physics is discovered which changes the calculation for one of the two estimates

2) Some clever physicists write a paper pointing out an obvious (in hindsight) methodological flaw in one of the two estimates, that when corrected, brings the two estimates into agreement

@BerF So far there's been several independent methods tried, we just don't have the confidence in those methods, and they usually don't have tight confidence intervals. Mostly they just tend to add to the confusion, since some of them get estimates close to the cosmic ladder, and some are close to ΛCDM. It's a mess.

My bet is that the tension will only be resolved once someone finds a flaw in either method. So far the cosmic ladder has withstood a very high level of scrutiny, and is consistent with new JWST data, so I'm crossing my fingers for new physics that would explain why ΛCDM is wrong.

This isn't a timed market. Close date may be extended.

@benshindel Ah, yes, I forgot to say, I don't expect any new physics to be relevant for this question before 2030. Because you have to first detect it somewhere else not related to H0, then theorists have to understand it well enough to apply it to H0, then both the previous CMB and/or standard candle data has to be re-assessed with the new theory. This would take forever.

I see now the market won't actually close in 2030, though. 🤷♂️

Wrong here means that the current best estimates using either method are 3 standard deviations or more away from the agreed estimate.

This means that (rounding stuff to zero digits after the point) H_0 between 66 and 69 km/s/Mpc would resolve "Current cosmic ladder estimates are wrong", H_0 between 70 and 76 km/s/Mpc would resolve "Current CMB estimates are wrong", and H_0 below 66, between 69 and 70, or above 76 km/s/Mpc would resolve "Both are wrong", right?

By "Hubble constant" you mean the present-day one, so that if cosmic ladder results are right about the recent expansion rate and CMB results are right about the ancient expansion rate (and merely wrong about the way to extrapolate them to today) it would resolve as "current CMB estimates are wrong", right?

@ArmandodiMatteo we had this discussion in another thread. Both estimates are for the current day rate, H0. This market is not concerned with past rates.

What if both methods are correct, and Hubble constant is not a constant? (Or something similar)

@Shump My dad is a Distinguished Professor of cosmology at University of California Davis who publishes papers on this topic. I asked him “the Hubble constant isn’t actually a constant right?” And he responded with “It's a measure of the expansion rate, which changes over time. The value today is constant, but we define the same quantity at other times, and those values are different from the value today.”

@SoniaAlbrecht well that's funny because MY dad is a Distinguished Professor of cosmology at University of California Berkeley who had Edwin Hubble himself on his thesis committee at Cambridge. HE says that the Hubble constant IS in fact a constant.

All jokes aside, you seemed to have bet that option up to 80% on the idea that if the Hubble constant could change over time, this would resolve as "both are wrong", and I guess it's up to @Shump but I don't think that's the point of the question, and doesn't really get at the nature of the Hubble tension.

@SoniaAlbrecht also this paper authored by your dad seems to imply that the quintessence model he's proposing would give a value for H0 that is roughly within 1 std deviation of the Planck data, lending support to the "Current Cosmic Ladder estimates are wrong" option on this market, not the "both estimates are wrong" option.

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.063521

I don’t think the Hubble constant not being a constant means the current cosmic ladder estimates are wrong or the cosmic microwave background estimates are wrong. It’s just that the person who made the market thinks it does.

@SoniaAlbrecht Let me clarify. Both is wrong only resolve in case it doesn't make sense to put a single number estimate on the Hubble constant. If it changes by time, you can still treat it as a constant in the present. I will update the description.

Wikipedia says:

The parameter H is commonly called the "Hubble constant", but that is a misnomer since it is constant in space only at a fixed time; it varies with time in nearly all cosmological models, and all observations of far distant objects are also observations into the distant past, when the “constant” had a different value. "Hubble parameter" is a more correct term, with H0 denoting the present-day value.

I don't think your dads actually disagree, there's just some confusion about definitions here.

@Shump Suppose Hubble constant changes over time, but is otherwise constant. Cosmic ladder correctly estimates average Hubble constant in the last 5 billion years, and CMB measurement correctly estimates the average over 13 billion years. Would this resolve as “both wrong”?

@OlegEterevsky That's not how it works. CMB calculations still give an estimate for today, not for the average of 13 billion years.

@SoniaAlbrecht I was joking about my dad, lol, there's a long-running Manifold joke of the form "my dad works at _____ (Time Magazine / Capitol Hill) and he says ______ (Xi Jinping is gonna win POTY / the next speaker of the house will be...)" but you ACTUALLY have a cosmologist dad, which was funny

@Shump From my understanding of https://arxiv.org/abs/1502.01589, they are assuming that Hubble constant is constant and then fit a model to the data. So strictly speaking, yes, if Hubble constant were variable, this assumption would be wrong and the method invalid.

That said, if the constant were to change a bit, this method *would* probably approximate something like the average value of Hubble constant since CMB was emitted. (But here I'm just speculating. I agree that if the Hubble constant is non-constant than the CMB method is invalid.)

@Shump My bad, you are right. It's because they include dark energy in their model, right? So the potential error of this model is that our model of dark energy is somehow incorrect?

@OlegEterevsky Yes. If the CMB estimate is wrong, it probably means something is missing in the current model of the universe, or "new physics" as the paper authors call it. If the Hubble estimate is wrong, it's probably because of some errors in the calculation. My favorite hypothesis is sterile neutrinos, which might solve the discrepancy, can also explain dark matter, and I think are relatively likely to exist from theoretical concerns (I mean, why would neutrinos be the only particle without a right-handed variant?)

@Shump Since mine was a fair assumption to make given the original wording of the question, could I get a refund on the mana I spent? I don’t know how or if this is possible, but I figured I’d ask.

@SoniaAlbrecht I could send you some mana, but honestly it seems to me like you didn't misunderstand the question, you misunderstood the Hubble constant. Both the Planck estimate and the Hubble one refer to present day values (that's the 0 in H0). I don't see how this can make both estimates wrong.

I'll send you the profit I made, I don't like making profit from others' mistakes.

@Shump Thanks so much! I really appreciate your kindness! I did understand that this doesn’t make both estimates wrong, I just thought you had misunderstood the concept when I made that bet (I’ve experienced people who make markets doing that before)