Implications if New Dark Matter Maps are Correct?

[Jarvis323]

There are reports that our cosmological models can't explain our up to date dark matter distribution maps, and that it calls general relativity into question.

An international team of researchers has created the largest and most detailed map of the distribution of so-called dark matter in the Universe.
The results are a surprise because they show that it is slightly smoother and more spread out than the current best theories predict.
The observation appears to stray from Einstein's theory of general relativity - posing a conundrum for researchers.
The results have been published by the Dark Energy Survey Collaboration.Dark Matter is an invisible substance that permeates space. It accounts for 80% of the matter in the Universe.
Astronomers were able to work out where it was because it distorts light from distant stars. The greater the distortion, the greater the concentration of dark matter.
Dr Niall Jeffrey, of École Normale Supérieure, in Paris, who pieced the map together, said that the result posed a "real problem" for physics.
"If this disparity is true then maybe Einstein was wrong," he told BBC News. "You might think that this is a bad thing, that maybe physics is broken. But to a physicist, it is extremely exciting. It means that we can find out something new about the way the Universe really is."
Prof Carlos Frenk, of Durham University, who was one of the scientists that built on the work of Albert Einstein and others to develop the current cosmological theory, said he had mixed emotions on hearing the news.
"I spent my life working on this theory and my heart tells me I don't want to see it collapse. But my brain tells me that the measurements were correct, and we have to look at the possibility of new physics," said Prof Frenk.
"Then my stomach cringes, because we have no solid grounds to explore because we have no theory of physics to guide us. It makes me very nervous and fearful, because we are entering a completely unknown domain and who knows what we are going to find."

https://www.bbc.com/news/science-environment-57244708

I guess the relevant publications can be found here?

https://scholar.google.com/citations?hl=en&user=xwYESZQAAAAJ&view_op=list_works&sortby=pubdate

What can we say about the likelihood the new maps are correct, and what can we say about the implications if they are (e.g. implications for general relativity, cosmological models, etc.)?

 

[PeterDonis]

There are reports

Translation: scientists are prematurely talking to the media about research that is still in progress and it's way too early to tell how it's going to play out. There is no scientific content to this article; it's just some scientists giving their unsupported opinions about how the research will end up playing out. Unsupported opinions are not science, even if they're stated by scientists.

I guess these the relevant publications can be found here?

You shouldn't have to guess. The BBC article, if it were an actual article about science, would give references to the particular papers being discussed.

What can we say about the likelihood the new maps are correct, and what can we say about the implications if they are (e.g. implications for general relativity, cosmological models, etc.)?

We can say what I said above: this is ongoing research and it's way too early to tell how it's going to play out.

 

[Jarvis323]

Translation: scientists are prematurely talking to the media about research that is still in progress and it's way too early to tell how it's going to play out. There is no scientific content to this article; it's just some scientists giving their unsupported opinions about how the research will end up playing out. Unsupported opinions are not science, even if they're stated by scientists.You shouldn't have to guess. The BBC article, if it were an actual article about science, would give references to the particular papers being discussed.We can say what I said above: this is ongoing research and it's way too early to tell how it's going to play out.

At Eureka Alert, they are saying it is results from 29 recent publications.

In 29 new scientific papers, the Dark Energy Survey examines the largest-ever maps of galaxy distribution and shapes, extending more than 7 billion light-years across the Universe. The extraordinarily precise analysis, which includes data from the survey's first three years, contributes to the most powerful test of the current best model of the Universe, the standard cosmological model...The recent DES results will be presented in a scientific seminar on 27 May 2021. Twenty-nine papers are available on the arXiv online repository and from the Dark Energy Survey website. The main paper is Dark Energy Survey Year 3 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing.

https://www.eurekalert.org/pub_releases/2021-05/aouf-des052721.php



https://www.darkenergysurvey.org/des-year-3-cosmology-results-papers/

 

[sqljunkey]

PeterDonis many experiments found that GR is incorrect. This is just another one that was published on BBC.

 

[Jarvis323]

Forgive me if this is a naive question, but I thought that GR and some particular assumptions about the distribution of matter used by the standard model, are what we use in order to derive interpretations of cosmological observations and justify that they are sufficiently unambiguous and can be used to compare the model with the observations.

If the observations, conditioned on the model/assumptions/physics, disagree with the model/assumptions/physics, then where would that leave us?

In other words, can we rule out a model with evidence that is only valid if the model is correct?

I'm sure this isn't realt the case, as probably the parts of the model/assumptions/physics that are needed to interpret the observations are still probably solid enough as far as we know.

Basically, does the data not agreeing with the model imply more uncertainty in the interpretation of the data in the first place?

 

[PeterDonis]

many experiments found that GR is incorrect.

Please give specific references.

This is just another one that was published on BBC.

Nothing in the BBC article refers to an experiment that found that GR was incorrect. If you disagree, please give specific references to published, peer-reviewed papers.

 

[PeterDonis]

I thought that GR and some particular assumptions about the distribution of matter used by the standard model, are what we use in order to derive interpretations of cosmological observations

That's correct. In order to build any model with GR, you have to make some assumption about the distribution of matter (or more generally, of stress-energy).

The "dark matter" issue, at least as it relates to galaxy rotation curves (not that this is by no means the only area in which the dark matter hypothesis plays a role--a fact which is notoriously ignored by critics of dark matter, who talk as if explaining galaxy rotation curves was the only issue), is that, if we build a GR model of a galaxy using a matter distribution that is based on just what we can see, the model predicts rotation curves that don't agree with what we actually observe. The simplest way to fix those models is to assume that there is some additional matter present which is not visible, and add that to the matter distribution in the model so that its predictions of rotation curves match observations. That is the dark matter hypothesis.

The alternative hypothesis is that GR is the wrong theory of gravity; alternatives such as MOND propose a different theory of gravity which, it is claimed, predicts galaxy rotation curves that agree with observations when the only matter distribution included in the model is that based on just what we can see.

can we rule out a model with evidence that is only valid if the model is correct?

The galaxy rotation curve observations are not model-dependent; both alternatives (the dark matter hypothesis and the different theory of gravity hypothesis) agree on what the observations are that have to be accounted for. So what you describe is not an issue here.

 

[kimbyd]

There are reports that our cosmological models can't explain our up to date dark matter distribution maps, and that it calls general relativity into question.
https://www.bbc.com/news/science-environment-57244708

I guess the relevant publications can be found here?

https://scholar.google.com/citations?hl=en&user=xwYESZQAAAAJ&view_op=list_works&sortby=pubdate

What can we say about the likelihood the new maps are correct, and what can we say about the implications if they are (e.g. implications for general relativity, cosmological models, etc.)?

The anomalous distribution of dark matter in the universe is an issue that has been known about for a very long time, and the solution is unresolved. For me, when it comes to the likely physics at work, one specific observation stands out as being of critical importance: the CMB. It's very, very difficult for CMB observations to be fit with anything but dark matter, and the relevant physics producing that signal are far simpler and cleaner than anything that comes after. This is why I lean very strongly on the likely resolution to this problem being that it's telling us something interesting about the nature of dark matter, not whether or not it exists at all.

As for possible resolutions to this problem, one is simply that dark matter isn't entirely cold. The simplest dark matter models suggest a heavy particle (hundreds of proton masses) with near-zero temperature. If the dark matter particles were lighter (say, a fraction of the electron mass) and had some non-zero temperature, it could smooth out the distribution of dark matter.

There are a bunch of other dark matter models which have been proposed that could resolve this discrepancy in other ways.

To make it all worse, there remains some possibility that our simulations of dark matter are simply wrong. The issue at hand is that the biggest discrepancy happens where dark matter is at its densest. And dark matter is at its densest in places where there's a lot of normal matter too. Which means that the behavior of normal matter might be mucking things up, which is a problem because normal matter's behavior is incredibly complicated and hard to simulate.

In the end, we really don't know what's going on here, and I wouldn't be at all surprised if it took decades to resolve the problem. It is very frustrating, but the fact is that it's just entirely too easy to write down plausible theoretical models which are unbelievably hard to test experimentally, so difficult that it might actually be impossible as a practical matter to find the answers definitively. Which really sucks, but it calls for patience. This science is hard, and we shouldn't jump to conclusions just because the answers aren't quick or easy.

 

[kimbyd]

So, I decided to go back and track down the scientific papers for these. Two came up of note:
https://www.darkenergysurvey.org/wp-content/uploads/2021/05/desy3_3x2_results.pdf
https://www.darkenergysurvey.org/wp-content/uploads/2021/05/desy3_cosmic_shear_cosmology1.pdf

The first one is the main cosmological constraint paper that gives an overview. The second looks more precisely at the amount of clustering of dark matter.

I haven't read a lot of them, but so far my take-aways are:
1) There is no Hubble tension between this data and the Planck measurement. The DES results here produced the tightest measure of the Hubble constant yet, and it generally agrees with the CMB measurement.
2) There is a noticeable but not dramatic difference in the measurement of dark matter clustering between Planck and this survey (just over $2\sigma$).
3) The survey relies upon photomoetric redshift measurements, which is fascinating to me because a bunch of my colleagues in grad school were working on how to figure out how to do that kind of survey well (at the time it was largely a pipe dream for various reasons).

It looks like that second point is what all these news articles are referring to when they say it challenges dark matter models.

The problem is that the statistics are weak. A $2\sigma$ discrepancy is really nothing to write home about, and can easily disappear if any new systematic errors are uncovered. In this case, the DES results face huge challenges with regard to making sure their weak lensing models and photometric redshift models are properly-calibrated. Both of these challenges are easily an order of magnitude more difficult to account for than anything that needs to be done with the CMB temperature data (CMB polarization is harder, but these results rely almost entirely on temperature).

It's very clear to me from skimming these papers that some really dramatic strides have been made in the last decade with regards to using large-scale optical surveys for cosmology, but with only a $2\sigma$ discrepancy, there is much more work that remains to say that any interesting physics here are happening at all.

 

[kimbyd]

As for why photometric redshifts are interesting, a bit of background.

The most accurate way to measure redshift is through spectroscopy: you use a special instrument on the telescope that extracts the detailed spectrum of light coming from a galaxy. This takes a lot of telescope time: you have to get enough photons coming from this galaxy to cover enough of the spectrum to see the emission/absorption lines.

Photometric redshift is different. You use a satellite detector which doesn't look at the detailed spectrum, but instead sees broader bands of color (e.g., red, blue, green). This is far faster: you don't need nearly as much telescope time to just get the brightness of a galaxy in a few bands of color. But you can't see the emission lines. Being able to use this faster method is extremely desirable because you can look at far, far more galaxies this way, dramatically increasing the statistical power of your analysis.

But if you want to interpret the redshift from color bands, you have to have some idea of the unredshifted color of the galaxy. And different galaxies can have remarkably different colors, because the color depends upon how much recent star formation there has been. When there's a burst of star formation, stars of a bunch of different masses form. But the brightest ones are the most massive ones, and also the bluest ones. As the group of stars ages, the more massive stars die out and the overall color shifts towards the red end of the spectrum.

So you must have a way of determining how "old" a given galaxy's stars are if you want to use photometric redshifts. And that creates all sorts of really difficult challenges. Because galaxies in the early universe behaved very differently from the way they behave today.

I didn't look into all the ways in which they accounted for the really dramatic systematic errors associated with a photometric redshift survey, but they must have done a huge amount of work given the tiny error bars they're reporting.