- 3350 - DARK MATTER - some ideas to explain it. - Even though we know that ordinary matter accounts for only about one-twentieth of the universe’s energy (5%) and a sixth of the total energy carried by matter (with dark energy constituting the remaining portion), we nonetheless consider “ordinary matter” to be the truly “important” constituent.
--------------------- 3350 - DARK MATTER - some ideas to explain it.
- With the exception of cosmologists, almost everyone’s attention is focused on the ordinary matter component, which you might have thought to be largely insignificant according to the energy accounting.
-
- We of course care more about ordinary matter because we are made of the stuff, as is the tangible world in which we live. Ordinary matter interacts through the electromagnetic, the weak, and the strong nuclear forces helping the visible matter of our world to form complex, dense systems.
-
- Not only stars, but also rocks, oceans, plants, and animals owe their very existence to the non-gravitational forces of nature through which ordinary matter interacts. Ordinary matter, though carrying a small percentage of the energy density, influences itself and its surroundings much more noticeably than something that just passes through.
-
- Familiar visible matter can be thought of as the privileged percent, actually more like 15 %, of matter. In business and politics, the interacting 1% dominates decision making and policy, while the remaining 99% of the population provides less widely acknowledged infrastructure and support, maintaining buildings, keeping cities operational, and getting food to people’s tables.
-
- Similarly, “ordinary matter” dominates almost everything we notice, whereas “dark matter“, in its abundance and ubiquity, helped create clusters and galaxies and facilitated star formation, but has only limited influence on our immediate surroundings today.
-
- It seems very odd to assume that all of ”dark matter” is composed of only one type of particle. For nearby structure, ordinary matter is in charge. It is responsible for the motion of our bodies, the energy sources that drive our economy, the computer screen or paper on which you are reading this, and basically anything else you can think of or care about. If something has measurable interactions, it is worth paying attention to, as it will have far more immediate effects on whatever is around.
-
- Dark matter lacks this type of interesting influence and structure. The common assumption is that dark matter is the “glue” that holds together galaxies and galaxy clusters, but resides only in amorphous clouds around them.
-
- The “Standard Model of particle physics contains six types of quarks, three types of charged leptons (including the electron), three species of neutrinos, all the particles responsible for forces, as well as the newly discovered Higgs boson.
-
- What if most dark matter interacts only negligibly, but a small component of dark matter would interact under forces reminiscent of those in ordinary matter.
-
- In that case, just as ordinary matter consists of different types of particles and these fundamental building blocks interact through different combinations of charges, dark matter would also have different building blocks, and , at least one of those distinct new particle types would experience non-gravitational interactions.
-
- Neutrinos in the Standard Model don’t interact under the strong or electric force yet the six types of quarks do.
-
- Despite cosmology’s enormous progress in the last 50 years, we still know very little about our universe. General relativity, the theory which allowed modern cosmology to start in first place, can account for our observations, but it comes with a cost: It introduces two unknown components in the energy-matter content of the universe, dark energy and dark matter, which supposedly have to account for 95 percent of our universe.
-
- Focusing on dark matter has proved to be extremely successful in accounting for the observed history of the cosmos, from its very beginning to present day, and from small galactic scales up to the boundaries of the visible universe. It even seems natural, as many extensions of the Standard Model of particle physics do predict very weakly interacting particles with the right properties.
-
- However, no observational evidence came from direct searches for these new particles, not in astrophysics or at the Large Hadron Collider at CERN.
-
- Dark matter properties at galactic scales, as inferred from observations, have proven to be not quite the same as those predicted by numerical simulations. Dark matter halos, clouds in which galaxies are supposed to be immersed, are observed to be much less dense at their centers and endowed with fewer and lighter satellites than predicted. All of these facts pose a real challenge for the idea of dark matter.
-
- Maybe dark matter undergoes a phase transition into something akin to a superfluid, in cold and dense enough environments.
-
- One would not expect the total baryonic matter in a galaxy (atoms of any sort) to correlate with its asymptotic rotation velocity, which probes the total mass, luminous and dark, independently of any other property of the galaxy.
-
- Astronomers found how the gravitational force, due to the luminous matter, are tightly correlated among very different galaxies. Rather than extend the reality of particle physics, perhaps we should have looked for a modification of gravity instead.
-
- For example, Modified Newtonian Dynamics, MOND in short, is a model in which no exotic matter component is present but the underlying gravitational law differs from Newtonian mechanics when accelerations are very tiny, as it occurs in galaxies.
-
- This proposal nicely performs at galactic scales but it fails in reproducing observations on larger scales.
-
- Of course, it is just a modification of the Newtonian theory of gravity, so per se it is not a competitor to general relativity. However, since the introduction of MOND, several relativistic theories of gravity, extensions of general relativity, have been proposed that reduce to MOND in the same limit in which general relativity reduces to Newtonian gravity.
-
- So it seems, then, that we have two mutually excluding patches to cover our universe: on the scale of galaxy clusters, dark matter fits the data with exquisite accuracy, while on smaller scales modified gravity seems to be favored. In particular, one can try to join the strengths of the two models to forge a better one. This is the idea of superfluid dark matter.
-
- The basic idea is pretty simple: Dark matter does indeed exist in the form of a particle beyond the standard model, as normally conjectured, but undergoes a phase transition, the condensation into something akin to a superfluid, in cold and dense enough environments, like for example in galactic halos.
-
- On the other hand, on larger scales, where the condensation is not taking place, this form of dark matter behaves exactly as the standard cold dark matter. So this model will perform as nicely as standard dark matter on cosmological scales, but will introduce new physics at small (galactic) ones.
-
- In some models, the condensation of dark matter into a fluid generates a special coupling of dark matter to gravity. The point is that the dark matter condensate is endowed with a characteristic coherence length (as any condensate or superfluid in the lab does).
-
- This provides a sort of “gravitational foot” of dark matter that makes it sensible not only to the strength of the gravitational field at a given point but also to how much rapidly it changes around that point.
-
- Dark matter also couples to the curvature of spacetime, and this makes it appear as an extra component of the gravitational field. This in turn makes the standard matter “feel” the dark matter as a modification to spacetime, in essence, as modified gravity!
-
- Even more strikingly, the obtained form of modified gravity strictly resembles the kind of theories introduced, so they reduce to MOND in weak gravity/slow velocities situations.
-
- The idea of a dark matter condensate immediately provides an answer to some puzzles of the standard framework. In other approaches, the excitations over this dark matter condensate, the so called “phonons“, mediate an additional interaction between standard matter particles, which is universal in nature and, hence, appears again to modify the expected effects of gravity.
-
- So the superfluid dark matter idea nicely conjugates the strengths of the standard dark matter at large scales with those of modified gravity at short ones. However, it is not identical to both proposals, so it is in principle falsifiable!
-
- For example, galaxies still has a contribution to the gravitational field coming from the dark matter forming the condensate/superfluid phase. So there will be deviations from exact MOND-like behavior when reproducing rotation curves in galaxies.
-
- A Bose-Einstein condensate is characterized by a few parameters: the strength of the interaction among its constituents, the number density, and the mass of them. It is still unclear if any reasonable choice of these parameters can lead to a suitable fit of galaxy rotation curves and satellites as well as cluster dynamics.
-
- How to explain the Bullet Cluster, for example? Further work is needed on the theoretical side to see if any extension of the Standard Model could provide the right elementary particle able to act as the dark matter of cosmology but at the same time perform such condensation at late times on small scales.
-
- We do not yet know if the superfluid dark matter idea will turn out to be right. But it is of course intriguing if the solution to such a long standing riddle could find a solution through an elegant marriage of three branches of modern physics: the gravitational, particle, and condensed matter ones.
-
November 26, 2021 DARK MATTER - some ideas to explain it. 3350
----------------------------------------------------------------------------------------
----- Comments appreciated and Pass it on to whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for copies or Index of all reviews
--- to: ------ jamesdetrick@comcast.net ------ “Jim Detrick” -----------
--------------------- --- Saturday, November 27, 2021 ---------------------------
No comments:
Post a Comment