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posted by Fnord666 on Wednesday January 30 2019, @11:37AM   Printer-friendly
from the mini-halos-are-for-really-small-angels dept.

Missing Galaxies? Now There's Too Many:

Gaze skyward from the Southern Hemisphere and it's hard to miss the Large Magellanic Cloud. The fact that it looks like one of the Milky Way's spiral arms, albeit smaller, reveals that it's a small galaxy roughly 30,000 light-years across with a few billion stars. Indeed, any small telescope will show that it's scattered with glowing nebulae that are punctured by dark dollops of dust.

And it isn't the only satellite galaxy that slowly swirls around the Milky Way. By 1999, astronomers had detected a dozen companions, many of them invisible to the unaided eye. But at that time, computer simulations of the evolution of the universe had predicted that the Milky Way's neighborhood should be bustling with activity — hosting not a dozen, but thousands, of tiny companions. So where were the missing satellites?

That astronomical riddle went on to bedevil astronomers for nearly two decades. Researchers came up with a number of potential explanations. Some involved speculative new ideas about how galaxies evolve. Others proposed the existence of exotic forms of dark matter — the mysterious substance that makes up 84 percent of the matter in the universe.

But within the past few years something strange happened. New surveys allowed astronomers to find more satellite galaxies that had previously been hidden. At the same time, updated computer simulations predicted the existence of far fewer galaxies than their predecessors did.

In fact, the estimates of galaxy numbers from observational studies and from theoretical simulations converged so quickly that they ended up overshooting each other. Whereas in the early 2000s astronomers worried that there were too few satellites, by 2018 there appeared to be too many. The missing satellites problem had been turned inside out.

The story dives into ultra faint dwarf galaxies, dark matter halos, mini-halos, tiny little ghost galaxies, as well as computer simulations of the Milky Way galaxy having different results depending on whether they were based on dark matter or on our everyday baryonic matter.

But [University of California, Irvine astronomer James] Bullock and his colleagues didn't merely outline the problem, they also proposed a solution. Simulations have long suggested that lots of dark-matter mini-halos formed around the Milky Way. But astronomers argued that these halos didn't form galaxies. There's a threshold, the argument went, below which these halos simply didn't have enough gravity to hold on to the gas necessary to form stars. They were thus star-free and invisible.

For nearly 20 years, astronomers thought that threshold for the mass of a dark-matter halo that could form a galaxy rested around 500 million times the mass of the sun. But Bullock's team suspects that it's much lower, around 30 million times the mass of the sun.

If such small globs of dark matter can grab onto enough ordinary matter to create stars (and thus galaxies), then simulations start to match observations. Indeed, Bullock's team was able to model galaxies that are eerily real. Not only do the numbers of simulated mini-halos match the numbers that are predicted by observations, but the shapes of the galaxies' orbits even look like the ones we have already detected.


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  • (Score: 3, Interesting) by JoeMerchant on Wednesday January 30 2019, @01:23PM (7 children)

    by JoeMerchant (3937) on Wednesday January 30 2019, @01:23PM (#794034)

    When theory said more galaxies but observation was lacking, theory drove in the direction of observation and observation drove in the direction of theory... they overshot. Wonder if it will ever reach equilibrium, and if it does - does theory+observation have any correlation with reality?

    When we learn to observe better theory will remake itself again.

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  • (Score: 2, Insightful) by Anonymous Coward on Wednesday January 30 2019, @02:20PM (2 children)

    by Anonymous Coward on Wednesday January 30 2019, @02:20PM (#794051)

    This commingling of theory and observation is very dangerous in my opinion. It is far easier than most people realize to come up with a model that is extremely intricate yet self-consistent, and wrong.

    • (Score: 1, Informative) by Anonymous Coward on Wednesday January 30 2019, @03:02PM

      by Anonymous Coward on Wednesday January 30 2019, @03:02PM (#794063)

      In machine learning, we refer to that as overfitting.

    • (Score: 2) by Gaaark on Wednesday January 30 2019, @04:03PM

      by Gaaark (41) on Wednesday January 30 2019, @04:03PM (#794095) Journal

      And that is what they have found with dark matter, except it is not even self-consistent.

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  • (Score: 5, Interesting) by AthanasiusKircher on Wednesday January 30 2019, @02:52PM (3 children)

    by AthanasiusKircher (5291) on Wednesday January 30 2019, @02:52PM (#794058) Journal

    does theory+observation have any correlation with reality?

    Not necessarily. "Reality" is a tricky thing. The entire rationale behind modern Science -- the essence of the Newtonian Revolution -- was the acceptance that mathematical models were potentially valid without justification in "reality." Newton modeled the universe effectively with gravity -- an unseen force acting at a distance that his critics characterized as mystical or even magical. But the model fit the data, and Newton went to the trouble in later editions of his Principia to argue that that's what's important for science now. Modeling data and making predictions. The model may locate an actual mechanism, or it may leave something unexplained (as gravity basically was for centuries after Newton) but the model worked.

    When we learn to observe better theory will remake itself again.

    While you have a valid point about confirmation bias and the way it influences studies, data gathering in physics and astronomy tends to be a lot more careful than, say, in designs of medical studies or psychology studies or whatever. Astronomers and cosmologists do pretty much go the way the data leads them.

    And yes, that means they will reconsider the theory if they discover more data that contradicts it. What else are they supposed to do?

    Honestly, I thought I was a jaded skeptic about most things, but then I read comments here and it put things in perspective. What are cosmologists supposed to do exactly? If they develop theory purely without regard to empirical data to support it (as the advanced forms of string theory go), people complain here about theory without empirical grounding... no falsifiability, therefore no science. If scientists on the other hand respond to empirical data by modifying their theories and even throwing out previous assumptions, you accuse them of confirmation bias. (And the AC who replied to you even thinks it's "dangerous" for scientists to actually compare their theories to empirical data and then correct them! How exactly is science supposed to work?!)

    Perhaps we should stop using the word "theory" and just consider the concept of "modeling." That's what scientists do. It's how they make sense of data. Otherwise it's just a bunch of numbers. Sure, confirmation bias is a problem when the sample space is small and/or data collection (and analysis) procedures are created. But here we have a clear example of scientists being SURPRISED by both models and data multiple times. People engaging in confirmation bias don't get "surprised" -- they just see what they want to. And here scientists are responding to new data by trying to figure out how it works... and that involves creating models. (Models which may be flawed and require revision, or even rejection... or they may turn out to be partly right and partly wrong.)

    Can I ask how else science is supposed to work for you?

    • (Score: 3, Insightful) by DeathMonkey on Wednesday January 30 2019, @06:41PM (1 child)

      by DeathMonkey (1380) on Wednesday January 30 2019, @06:41PM (#794175) Journal

      Can I ask how else science is supposed to work for you?

      For them, science is supposed to confirm whatever it is they already believe regardless of whether it's true or not.

      • (Score: 0) by Anonymous Coward on Wednesday January 30 2019, @07:29PM

        by Anonymous Coward on Wednesday January 30 2019, @07:29PM (#794196)

        Fake news, grant chasing money grubbing "scientists", deep state conspiracy to sell carbon credits!

    • (Score: 2) by JoeMerchant on Thursday January 31 2019, @01:57AM

      by JoeMerchant (3937) on Thursday January 31 2019, @01:57AM (#794333)

      What are cosmologists supposed to do exactly?

      Exactly what they are doing, with an extra NdGT dose of "what we think we know is..."

      Cosmologists are better than the scientists sponsored by industries with vested interests about admitting that they don't know what they don't know, but, when you lump all "Scientists" together, they come across as a smug bunch of "truth" peddlers who are little more trustworthy than television evangelists.

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