Evidence For Modified Gravity Is Directly Show Against It.

Hamster. Not to scale.
Img src: Petopedia.

It’s solar daytime 12,805 inwards the nation of war betwixt modified gravity as well as black matter. That’s counting the days since the publication of Mordehai Milgrom’s 1983 paper. In this newspaper he proposed to alter Einstein’s theory of full general relativity rather than conjecturing invisible stuff.

Dark matter, to remind you, are hypothetical clouds of particles that hover about galaxies. We can’t meet them because they neither emit nor reverberate light, but nosotros do notice their gravitational describe because it affects the motion of the affair that nosotros can observe. Modified gravity, on the other hand, posits that normal affair is all in that place is, but the laws of gravity don’t operate equally Einstein taught us.

Which ane is right? We withal don’t know, though astrophysicists accept been on the illustration since decades.

Ruling out modified gravity is hard because it was invented to fit observed correlations, as well as this achievement is hard to improve on. The see which Milgrom came upwards amongst inwards 1983 was a uncomplicated model called Modified Newtonian Dynamics (MOND). It does a skillful chore plumbing equipment the rotation curves of hundreds of observed galaxies, as well as inwards contrast to particle black affair this model requires solely ane parameter equally input. That parameter is an acceleration scale which determines when the gravitational describe begins to go markedly different from that predicted past times Einstein’s theory of General Relativity. Based on his model, Milgrom also made some predictions which held upwards so far.

In a 2016 paper, McGaugh, Lelli, as well as Schomberg analyzed information from a railroad train of virtually 150 disk galaxies. They identified the best-fitting acceleration scale for each of them as well as constitute that the distribution is clearly peaked about a mean-value:

Histogram of best-fitting acceleration scale.
Blue: Only high lineament data. Via Stacy McGaugh.

McGaugh et al conclude that the information contains evidence for a universal acceleration scale, which is strong back upwards for modified gravity.

Then, a calendar month ago, Nature Astronomy published a newspaper titled “Absence of a telephone substitution acceleration scale inwards galaxies“ past times Rodrigues et al (arXiv-version here). The authors claim to accept ruled out modified gravity amongst at to the lowest degree 5 σ, ie amongst high certainty.

That’s pretty amazing given that 2 months agone modified gravity worked only fine for galaxies. It’s fifty-fifty to a greater extent than amazing ane time you lot notice that they ruled out modified gravity using the same information from which McGaugh et al extracted the universal acceleration scale that’s evidence for modified gravity.

Here is the key figure from the Rodrigues et al paper:

Figure 1 from Rodrigues et al

Shown on the vertical axis is their best-fit parameter for the (log of) the acceleration scale. On the horizontal axis are the private galaxies. The authors accept sorted the galaxies so that the best-fit value is monotonically increasing from left to right, so the growth is non relevant information. Relevant is that if you lot compare the error-margins marked past times the colors, as well as so the best-fit value for the galaxies on the really left side of the plot are incompatible amongst the best-fit values for the galaxies on the really correct side of the plot.

So what the heck is going on?

Influenza A virus subtype H5N1 commencement observation is that the 2 studies don’t role the same information analysis. The principal deviation is the priors for the distribution of the parameters which are the acceleration scale of modified gravity as well as the stellar mass-to-light ratio. Where McGaugh et al role Gaussian priors, Rodrigues et al role apartment priors over a finite bin. The prior is the supposition you lot brand for what the probable distribution of a parameter is, which you lot as well as so feed into your model to break the best-fit parameters. Influenza A virus subtype H5N1 bad prior tin forcefulness out give you lot misleading results.

Example: Suppose you lot accept an artificially intelligent infrared camera. One black it issues an alert: Something’s going on inwards the bushes of your garden. The AI tells you lot the best fit to the observation is a 300-pound hamster, the second-best fit is a span of humans inwards what seems a peculiar form of closed combat. Which pick do you lot recall is to a greater extent than likely?

I’ll go out on a limb as well as estimate the second. And why is that? Because you lot belike know that 300-pound hamsters are somewhat of a rare occurrence, whereas pairs of humans are not. In other words, you lot accept a different prior than your camera.

Back to the galaxies. As we’ve seen, if you lot start amongst an unmotivated prior, you lot tin forcefulness out halt upwards amongst a “best fit” (the 300 pound hamster) that’s unlikely for reasons your software didn’t work concern human relationship for. At the really least, therefore, you lot should cheque that whatever the resulting best-fit distribution of your parameters is doesn’t contradict other data. The Rodrigues et al analysis hence raises the concern that their best-fit distribution for the stellar mass-to-light ratio doesn’t agree usually observed distributions. The McGaugh newspaper on the other manus starts amongst a Gaussian prior, which is a reasonable expectation, as well as hence their analysis makes to a greater extent than physical sense.

Having said this though, it turns out the priors don’t brand much of a deviation for the results. Indeed, for what the numbers are concerned the results inwards both papers are pretty much the same. What differs is the decision the authors describe from it.

Let me tell you lot a storey to illustrate what’s going on. Suppose you lot are Isaac Newton as well as an apple tree only banged on your head. “Eureka,” you lot call as well as need that the gravitational potential fulfils the Poisson-equation.^* Smart equally you lot are, you lot assume that the world is some a homogeneous sphere, solve the equation as well as break an inverse-square law. It contains ane gratis parameter which you lot modestly telephone phone “Newton’s constant.”

You as well as so move about the globe, Federal Reserve annotation downwardly your height as well as mensurate the acceleration of a falling test-body. Back dwelling you lot plot the results as well as extract Newton’s constant (times the majority of the Earth) from the measurements. You break that the measured values cluster about a mean. You declare that you lot accept constitute evidence for a universal police push clitoris of gravity.

Or accept you?

Influenza A virus subtype H5N1 calendar week later on your skillful former friend Bob knocks on the door. He points out that if you lot aspect at the mensuration errors (which you lot accept of class recorded), as well as so some of the mensuration results are incompatible amongst each other at 5 sigma certainty. There, Bob declares, I accept ruled out your police push clitoris of gravity.

Same data, different conclusion. How does this brand sense?

“Well,” Newton would nation to Bob, “You accept forgotten that also the mensuration dubiety in that place is theoretical uncertainty. The world is neither homogeneous nor a sphere, so you lot should aspect a spread inwards the information that exceeds the mensuration uncertainty.” – “Ah,” Bob says triumphantly, “But inwards this illustration you lot can’t brand predictions!” – “Sure I can,” Newton speaks as well as points to his inverse foursquare law, “I did.” Bob frowns, but Newton has run out of patience. “Look,” he says as well as shoves Bob out of the door, “Come dorsum when you lot accept a amend theory than I.”

Back to 2018 as well as modified gravity. Same difference. In the Rodrigues et al paper, the authors dominion out that modified gravity’s one-parameter police push clitoris fits all disk galaxies inwards the sample. This shouldn’t come upwards equally much of a surprise. Galaxies aren’t disks amongst bulges whatever to a greater extent than than the world is a homogeneous sphere. It’s such a unsmooth oversimplification it’s remarkable it industrial plant at all.

Indeed, it would go an interesting practise to quantify how good modified gravity does inwards this railroad train of galaxies compared to particle black affair amongst the same number of parameters. Chances are, you’d break that particle black affair too is ruled out at 5 σ. It’s only that no ane is dumb plenty to brand such a claim. When it comes to particle black matter, astrophysicists volition go quick to tell you lot galaxy dynamics involves loads of complicated astrophysics as well as it’s rather unrealistic that ane parameter volition work concern human relationship for the multifariousness inwards whatever sample.

Without the comparing to particle black matter, therefore, the solely thing I acquire from the Rodrigues et al newspaper is that a non-universal acceleration scale fits the information amend than a universal one. And that I could accept told you lot without fifty-fifty looking at the data.

Summary: I’m non impressed.

It’s solar daytime 12,805 inwards the nation of war betwixt modified gravity as well as black affair as well as black affair enthusiasts withal haven’t constitute the battle field.

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^*Dude, I know that Newton isn’t Archimedes. I’m telling a storey non giving a history lesson.