WIMPS vs. Axions: What is dark matter?

Dark matter rules every galaxy. But what exactly is it? Astronomers believe it to be some kind of new, exotic particle. You may have heard some terms tossed around, like WIMPs or axions. Let’s explore what those terms actually mean.

First off, there’s a few things that we know about dark matter. Astronomers believe that dark matter is some kind of particle, previously unknown to physics. Whatever it is, it makes up about 80% of the mass of the universe. It barely interacts with light, if at all. It barely interacts with normal matter, if at all. It barely interacts with itself, if at all. We also know that it’s “cold”, which means that the individual particles don’t have very high velocities.

It basically just sits there and gravitates. But that gravity is essential: it keeps galaxies glued together and provides the scaffolding for the entire large-scale structure of the universe.

One of the earliest candidates for the dark matter particle are the WIMPs, for weakly-interacting massive particles. It’s not so much a name as a catch-all category. In this case, “weakly-interacting” means “interacts via the weak nuclear force” (although that interaction is also literally weak). WIMPs would be a new kind of particle that only talk to normal matter via the weak force, which would explain why we only rarely see it. In this scenario, WIMPs flood the universe – and might even be traveling through you right now, though you would never know except for their gravity.

The axion, on the other hand, is another hypothetical particle (or really, category of possible particles) that was motivated by theoretical explorations of various symmetry laws in the universe. It just so turned out that this hypothetical particle, if it existed in sufficient number, would operate exactly like we know the dark matter should.

Beyond WIMPs and axions, theoretical physicists and cosmologists have come up with all sorts of more complicated possibilities. Maybe the dark matter interacts with itself to some degree. Maybe there are multiple species of dark matter particles. Maybe new forces of physics are involved. Maybe axions can clump together in strange ways. Maybe this, maybe that.

Yes, dark matter is a big mystery. We know that something funky is going on with the universe, but we’re not exactly sure what. But we do know that cracking the dark matter mystery will illuminate a whole new universe of physics.

10 Replies to “WIMPS vs. Axions: What is dark matter?”

  1. Since the basis for WIMPs is mostly supersymmetry string theory AFAIU and that in turn has been mostly rejected in both LHC and ACME as a natural physics just above the standard particle scale there are now dark matter possibilities that isn’t “more complicated”.

    If they ever where since it seems for instance sterile neutrinos – who wouldn’t be weakly interacting as active neutrinos are – are natural consequences of standard model field theory.

    I quote at length since I’m no particle physicist so I would need that text myself, as well as for the axion implications. But I take it that neutrino oscillations that are caused by active neutrino masses can be taken as indicative that they easily would have a smidgen of Majorana corrections needed for (self-annihilating) sterile neutrinos. (It would be interesting to know from a particle physicist if the observed CP conservation may be accidental as well, also doing away with axions. I gather that if at least one of the quarks had been zero mass, it would fulfill Weinberg’s condition for chiral symmetry becoming “an accidental exact symmetry of the strong interactions” on its own, since “? would become unobservable; i.e. it would vanish from the theory.” [“Strong CP problem”, Wikipedia.])

    “Indeed, quite apart from the development of effective field theory, one of the great things about the Standard Model was that it explained various symmetries that could not be fundamental, because we already knew they were only partial or approximate symmetries. This included flavor conservation, such as strangeness conservation, a symmetry of the strong and electromagnetic interactions that was manifestly violated in the weak interactions. Another example was charge conjugation invariance—likewise a good symmetry of strong and electromagnetic but violated by weak interactions. The same was true of parity, although in this case you have to make special accommodations for nonperturbative effects. All these were accidental symmetries, imposed by the simplicity of the Standard Model necessary for renormalizability plus other symmetries like gauge symmetries and Lorentz invariance that seem truly exact and fundamental. Chiral symmetry itself is such an accidental symmetry, though only approximate. It becomes an accidental exact symmetry of the strong interactions in the limit in which the up and down quark masses are zero, as does isotopic spin symmetry. Since these masses are not zero, but relatively small chiral symmetry is an approximate accidental symmetry.

    Now, coming back to effective field theory, there are other symmetries within the Standard Model that are accidental symmetries of the whole renormalizable theory of weak, strong and electromagnetic interactions: In particular, baryon conservation and lepton conservation are respected aside from very small non-perturbative effects (well, very small at least in laboratories, though maybe not so small cosmologically). If baryon and lepton conservations are only accidental properties of the Standard Model, maybe they are not symmetries of nature. In this case, there is no reason why baryon and lepton conservation should be respected by nonrenormalizable corrections to the Lagrangian, and so you would expect terms of O(E/M) or O((E/M)^2) or higher order as corrections to the Standard Model that violate these symmetries.

    Wilczek and Zee and I independently did a catalog of the leading terms of this type. Some of them—those involving baryon number non-conservation—give you corrections of O((E/M)^2). They have not been yet been discovered experimentally. But there are other terms that produce corrections of O(E/M) that violate lepton conservation, and they apparently have been discovered, in the form of neutrino masses.”

    [“On the development of effective field theory”, Steven Weinberg, Eur. Phys. J. H (2021) 46:6]

  2. Dark matter is now understood to be smoothly distributed and to be pushed out of stars. The smoothly distributed dark matter displaced by the quarks a star consists of, pushing back and exerting pressure toward the star, causes gravity.

    Curved spacetime is a geometrical representation of gravity.

    Displaced dark matter is the physical manifestation of gravity.

    What is referred to geometrically as curved spacetime physically exists as displaced dark matter.

    1. This is made up claims with no reference and going against what we know for a fact: “understood to be smoothly distributed and to be pushed out of stars.”

      That isn’t what is observed and it is opposite of what is understood as the Lambda Cold Dark Matter cosmology – cold dark matter being clumpy and at best slightly denser density expectance in stars by gravity (i.e. the density gradient over the solar system, if not the clumps, is exceedingly small).

      “Using NASA’s Hubble Space Telescope and a new observing technique, astronomers have found that dark matter forms much smaller clumps than previously known. This result confirms one of the fundamental predictions of the widely accepted “cold dark matter” theory.

      All galaxies, according to this theory, form and are embedded within clouds of dark matter. Dark matter itself consists of slow-moving, or “cold,” particles that come together to form structures ranging from hundreds of thousands of times the mass of the Milky Way galaxy to clumps no more massive than the heft of a commercial airplane. (In this context, “cold” refers to the particles’ speed.)”

      “Using this method, the team uncovered dark matter clumps along the telescope’s line of sight to the quasars, as well as in and around the intervening lensing galaxies. The dark matter concentrations detected by Hubble are 1/10,000th to 1/100,000th times the mass of the Milky Way’s dark matter halo. Many of these tiny groupings most likely do not contain even small galaxies, and therefore would have been impossible to detect by the traditional method of looking for embedded stars.”

      [“Hubble Detects Smallest Known Dark Matter Clumps”, NASA]

      Dark matter forms the massive backbone of cosmic filaments of gas and galaxies – see the article on its dominance – and without its clumping effect there would be few galaxies in the universe.

      The existence of observed LCDM cosmology rejects that dark matter is part of general relativity underlying the model but instead show it is explicitly part of the matter-energy content of the universe. This is not rocket science.

      1. ‘Dark Matter More Ubiquitous Than We Ever Thought’
        https://www.inverse.com/article/24863-dark-matter-might-be-smoother-than-we-thought

        > “dark matter is smooth, distributed more evenly throughout space than we thought”

        ‘Dark matter is on the move: Scientists find the elusive material can be pushed out of a galaxy’s center by star formation’
        https://www.dailymail.co.uk/sciencetech/article-6554999/Dark-matter-Scientists-material-pushed-galaxys-center.html

        > “‘The dark matter at the centres of star-forming dwarfs appears to have been “heated up” and pushed out.’”

        Dark matter is smoothly distributed and pushed out by stars

      2. ‘Dark Matter More Ubiquitous Than We Ever Thought’

        > “dark matter is smooth, distributed more evenly throughout space than we thought”

        ‘Dark matter is on the move: Scientists find the elusive material can be pushed out of a galaxy’s center by star formation’

        > “‘The dark matter at the centres of star-forming dwarfs appears to have been “heated up” and pushed out.’”

        Dark matter is smoothly distributed and pushed out of stars.

  3. Reference to the small dark matter density gradient within the solar system:

    “And for any orbiting object in general, its orbit is determined by the total mass enclosed by an imaginary sphere centered on the Sun, with that object at the edge of the sphere. … Because we know the mass of the Milky Way, the relative densities of normal and dark matter, and we have simulations that tell us how the dark matter density ought to behave, we can come up with some very good estimates. When you do these calculations, you find that about 10^13 kg of dark matter ought to be felt by Earth’s orbit, while around 10^17 kg would be felt by a planet like Neptune.

    But these values are tiny compared to the other masses of consequence! The Sun has a mass of 2 × 10^30 kg, while Earth is more like 6 × 10^24 kg. Values like the one we came up with, in the 10^13 – 10^17 kg range, are the mass of a single modest asteroid. Someday, we may understand the Solar System well enough that such tiny differences will be detectable, but we’re a good factor of 100,000+ away from that right now.”

    [“Ask Ethan: If Dark Matter Is Everywhere, Why Haven’t We Detected It In Our Solar System?”, Forbes]

  4. ‘Dark Matter More Ubiquitous Than We Ever Thought’
    https://www.inverse.com/article/24863-dark-matter-might-be-smoother-than-we-thought

    > “dark matter is smooth, distributed more evenly throughout space than we thought”

    ‘Dark matter is on the move: Scientists find the elusive material can be pushed out of a galaxy’s center by star formation’
    https://www.dailymail.co.uk/sciencetech/article-6554999/Dark-matter-Scientists-material-pushed-galaxys-center.html

    > “‘The dark matter at the centres of star-forming dwarfs appears to have been “heated up” and pushed out.’”

    Dark matter is smoothly distributed and pushed out by stars

  5. You say “there are a few things we know about matter”. Then you say “Astronomers believe”. Believing is NOT knowing. We know NOTHING about dark matter other than a few things we believe.

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