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Scientific American: Dark matter explanation flawed, but what should replace it?

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From Lee Billings at Scientific American:

Whatever dark matter is, it is not accounted for in the Standard Model of particle physics, a thoroughly-tested “theory of almost everything” forged in the 1970s that explains all known particles and all known forces other than gravity. Find the identity of dark matter and you illuminate a new path forward to a deeper understanding of the universe—at least, that is what physicists hope

“The desire is for dark matter to not only exist but also to solve other outstanding problems of the Standard Model,” says Jesse Thaler, a physicist at Massachusetts Institute of Technology. “Not every new discovery can be a revelation like the Higgs, where afterward theories suddenly fit together much better. Sometimes new particles just make you say, ‘Who ordered that?’ Do we live in a universe where each discovery leads to deeper, more fundamental insights or do we live in one where some parts have rhyme and reason but others don’t? Dark matter offers either possibility.”More.

See also: New class of galaxy mainly dark matter

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2 Replies to “Scientific American: Dark matter explanation flawed, but what should replace it?

  1. 1
    bornagain77 says:

    Of note: “some fine-tuning is required to explain the observations”

    The Dark Matter ‘conspiracy’ – Apr 30, 2015
    Excerpt: One of the most surprising scientific discoveries of the 20th century was that spiral galaxies, such as our own Milky Way, rotate much faster than expected, powered by an extra gravitational force of invisible ‘dark matter’.
    Since this discovery 40 years ago, we have learned this mysterious substance, which is probably an exotic elementary particle, makes up about 85 per cent of the mass in the known Universe, leaving only 15 per cent to be the ordinary stuff encountered in our everyday lives.
    Dark matter is central to our understanding of how galaxies form and evolve and is ultimately one of the reasons for the existence of life on Earth – yet we know almost nothing about it.
    “One of the surprising findings of this study was that spiral galaxies maintain a remarkably constant rotation speed throughout their disks,” Dr Cappellari said. “This means stars and dark matter conspire to redistribute themselves to produce this effect, with stars dominating in the inner regions of the galaxies, and a gradual shift in the outer regions to dark matter dominance.”
    But the ‘conspiracy’ does not come out naturally from the models, and some fine-tuning is required to explain the observations.

    Dark Matter:
    Despite comprehensive maps of the nearby universe that cover the spectrum from radio to gamma rays, we are only able to account for 10% of the mass that must be out there, “It’s a fairly embarrassing situation to admit that we can’t find 90 percent of the universe.”
    Astronomer Bruce H. Margon

    Dark matter halo
    Excerpt: The dark matter halo is the single largest part of the Milky Way Galaxy as it covers the space between 100,000 light-years to 300,000 light-years from the galactic center. It is also the most mysterious part of the Galaxy. It is now believed that about 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy’s matter and energy in any way except through gravity. The dark matter halo is the location of nearly all of the Milky Way Galaxy’s dark matter, which is more than ten times as much mass as all of the visible stars, gas, and dust in the rest of the Galaxy.

    Milky Way – image

    Besides Dark matter being fine tuned and having a hand in sculpting the Milky Way into its beautiful spiral shape, Scientific American also refers to the ‘Invisible Hand’ of Dark Energy, (which is fine tuned to within 1 part to 10^120), as also having a hand in shaping our galaxy into its spiral shape.

    The Universe’s Invisible Hand – Christopher J. Conselice – 4 August 2014
    Dark energy does more than hurry along the expansion of the universe. It also has a stranglehold on the shape and spacing of galaxies
    Excerpt: it now appears that dark energy may be the key link among several aspects of galaxy and cluster formation that not long ago appeared unrelated. The reason is that the formation and evolution of these systems is partially driven by interactions and mergers between galaxies, which in turn may have been driven strongly by dark energy.

    Conditions for life may hinge on how fast the universe is expanding – Ilima Loomis – Feb. 29, 2016
    Excerpt: it turns out that the unknown forces behind the rate of this accelerating expansion—a mathematical value called the cosmological constant—may play a previously unexplored role in creating the right conditions for life.
    That’s the conclusion of a group of physicists who studied the effects of massive cosmic explosions, called gamma ray bursts, on planets. They found that when it comes to growing life, it’s better to be far away from your neighbors—and the cosmological constant helps thin out the neighborhood.
    “In dense environments, you have many explosions, and you’re too close to them,” says cosmologist and theoretical physicist Raul Jimenez of the University of Barcelona in Spain and an author on the new study. “It’s best to be in the outskirts, or in regions that have not been highly populated by small galaxies—and that’s exactly where the Milky Way is.”
    Jimenez and his team had previously shown that gamma ray bursts could cause mass extinctions or make planets inhospitable to life by zapping them with radiation and destroying their ozone layer. The bursts channel the radiation into tight beams so powerful that one of them sweeping through a star system could wipe out planets in another galaxy.,,,
    As it turns out, our universe seems to get it just about right. The existing cosmological constant means the rate of expansion is large enough that it minimizes planets’ exposure to gamma ray bursts, but small enough to form lots of hydrogen-burning stars around which life can exist. (A faster expansion rate would make it hard for gas clouds to collapse into stars.)
    Jimenez says the expansion of the universe played a bigger role in creating habitable worlds than he expected. “It was surprising to me that you do need the cosmological constant to clear out the region and make it more suburbanlike,” he says.,,,
    In theory, Heavens explains, either the constant should be hundreds of orders of magnitude higher than it appears to be, or it should be zero, in which case the universe wouldn’t accelerate. But this would disagree with what astronomers have observed. “The small—but nonzero—size of the cosmological constant is a real puzzle in cosmology,” he says, adding that the research shows the number is consistent with the conditions required for the existence of intelligent life that is capable of observing it.

  2. 2
    BrianFraser says:

    Here is the view from “Beyond Einstein: non-local physics” by Brian Fraser (2015):
    Einstein recognized that a “static” Universe (the accepted view in the early 1900s) could not be a stable one. The Universe would eventually collapse due to the action of gravitation. To counter that problem, Einstein introduced the “cosmological constant” to General Relativity in 1917. It represents what is now viewed as “dark energy” which keeps the Universe from collapsing. However, it was recognized that its inclusion still did not lead to a static Universe, because the equilibrium was unstable. If stars moved closer, the gravitational force would increase, moving closer still. If stars moved farther apart, then the gravitational effect would be lessened, and “dark energy” would more readily move them even farther apart. The whole situation was unstable, and to this day the cosmological constant is still regarded as an “outstanding theoretical challenge” in cosmology.

    . . .

    The view that is gaining currency today is that space itself expands or is “emergent” (new spatial units are being generated by some unknown process). It is like time, in that it progresses. But it progresses in three dimensions, and we call that an expansion.

    Opposing the expansion is gravitation, which is centered on an object (planet, star, galaxy). We interpret the resulting motions in terms of forces, the cosmological expansion force, which is not affected by distance, and the gravitational force, which has a 1/d^2 dependence. Because of this, there is necessarily a distance where the forces are at equilibrium . . . . For stars it is apparently a few light years, and for galaxies it is apparently a few million light years. Inside this distance, objects come together, and outside this distance, objects move apart. This is the “beyond Einstein” view that reconciles the issues of stability and instability. It explains why globular clusters are stable, even though they do not rotate sufficiently to keep them from collapsing. It explains why stars are separated by light years, but not by light weeks. It may explain some of the problems in calculating the Hubble constant, because the “constant” would be dependent on the location from which the observations are made (a large versus small galaxy).

    Gravitation seems to have three regions. Gravitational force near a star starts out strong but declines rapidly with distance (the 1/d^2 region). At the gravipause, gravitation is still present, but falls off less rapidly (the 1/d^1 region, or “Hubble space” as it could be called). Beyond that, quantization causes the gravitation to disappear completely (the 1/d^0 region, where it does not decrease at all, because there isn’t any).
    The free 22 page paper can be downloaded from: The .html file gives a link to the .pdf file but the former has additional information, and many more links and insights.

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