DARK MATTER!


Jan Oort (1900 - 92) [left] and Fritz Zwicky (1898 - 1974) [above]


Vera Rubin (1928 - 2016).   Note her calculator, the same kind I used in my first theoretical calculations around 1962!


Jan Oort and Fritz Zwicky in the early 1930s independently noticed problems when the virial theorem was applied to stars in galaxies, or galaxies in clusters. Zwicky coined the term “dark matter.” Vera Rubin addressed the question of invisible mass in a long series of careful measurements beginning in the 1960s. In 1975 she reported that the speeds of stars in each of the galaxies that she had studied were independent of the distance from the core of the galaxy, rather than depending on the inverse square root of the distance to the central condensation. Many other studies confirmed this result and indicated consistently that about 95% of the mass of most galaxies is in an invisible cloud extending out to typically more than 10 galactic radii.
Since then, a few galaxies have been found that have no dark matter, and there is a hint of a galaxy or two consisting almost entirely of dark matter. In clusters of galaxies, the dark matter distributions tend to merge and blend. A number of possible culprits have been eliminated from consideration over the years, and the only surviving good candidate for dark matter is massive particles that interact gravitationally, and (very unlikely) weakly, but not electromagnetically or via the strong interaction. No particle in the Standard Model has the characteristics that detailed observations require for dark matter particles. The currently accepted best cosmological model is known as the ΛCDM model, where Λ is the Einsteinian cosmological constant (dark energy) and CDM means "cold dark matter."


Ways to Study Dark Matter:

• Rotation curves for stars in individual galaxies,
• Gravitational micro-lensing (LSST),
• Speeds of galaxies in large clusters,
• X-ray emission by hot gas in clusters,
• Power spectrum of temperature variations in the Big Flash,
• Study of baryonic acoustic oscillations in the Big Flash, as they survive in the present universe.
• Direct detection of Dark Matter particles on earth. [Here are the two largest facilities with underground DM detectors, LNGS, and SURF.]






The last scattering surface is currently more than 40 billion light years away.





The power spectrum of temperature variations observed in the Big Flash at high resolution is extraordinarily sensitive to the precise composition of the early universe, and gives us what is probably our most accurate look at the composition of the universe at a temperature of 3000 K.




A quote from the literature: “On sub-degree scales, the rich structure in the anisotropy spectrum is the consequence of gravity-driven acoustic oscillations occurring before the atoms in the universe became neutral. Perturbations inside the horizon at last scattering have been able to evolve causally and produce anisotropy at the last scattering epoch which reflects that evolution. The frozen-in phases of these sound waves imprint a dependence on the cosmological parameters, which gives CMB anisotropies their great constraining power.

“The underlying physics can be understood as follows. When the proton-electron plasma was tightly coupled to the photons, these components behaved as a single ‘photon-baryon fluid’, with the photons providing most of the pressure and the baryons the inertia. Perturbations in the gravitational potential, dominated by the dark matter component, are steadily evolving. They drive oscillations in the photon-baryon fluid, with photon pressure providing the restoring force. The perturbations are quite small, O(10-5), and so evolve linearly. That means each Fourier mode evolves independently and is described by a driven harmonic oscillator, with frequency determined by the sound speed in the fluid. Thus, there is an oscillation of the fluid density, with velocity π/2 out of phase and having amplitude reduced by the sound speed.

“After the Universe recombined the baryons and radiation decoupled, and the radiation could travel freely towards us. At that point the phases of the oscillations were frozen-in, and projected on the sky as a harmonic series of peaks. The main peak is the mode that went through 1/4 of a period, reaching maximal compression. The even peaks are maximal under-densities, which are generally of smaller amplitude because the rebound has to fight against the baryon inertia. The troughs, which do not extend to zero power, are partially filled because they are at the velocity maxima.

“An additional effect comes from geometrical projection. The scale associated with the peaks is the sound horizon at last scattering, which can be confidently calculated as a physical length scale. This scale is projected onto the sky, leading to an angular scale that depends on the background cosmology. Hence the angular position of the peaks is a sensitive probe of the spatial curvature of the Universe (i.e., Ωtot), with the peaks predicted to lie at higher ℓ in open universes and lower in closed geometry.”




Baryonic acoustic oscillations are turning into a major and important tool in determining precise distances to the most remote galactic clusters and systems.




A search for dark matter particles produced in collisions was asserted to be the number one priority of the previous run cycles of the LHC.  The masses of the particles could be anywhere from 10 to 1000 GeV, and their signature would be processes where a good part of the final energy and momentum seem to be missing.  The data from the last run cycles are not completely analyzed, but so far no trace of dark matter is seen.  The new run cycle is so far also not providing any evidence of observation of dark matter.


[Presumably, galaxies, stars and planets may have dark matter cores. Does this matter??]


One sadly likely explanation for why dark matter is not present in the Standard Model is that the dark matter particles feel only the gravitational force. If this is true, there may be no possibility whatsoever of detecting dark matter particles directly, so that we can study them only via the gravitational effects of large assemblies of such particles. Thus, dark matter particles would be an entirely new category of pointlike fermions, neither quarks nor leptons. And their detailed understanding would presumably require a quantum theory of gravity. The vital role of dark matter in our universe is, of course, to stamp the density variations caused by quantum fluctuations in the very early universe onto the expanding later universe, and therefore to provide the seeds of all large-scale structure that now exists!

Final reports on the results of the Planck Satellite observations have been released.


The oldest known galaxies currently, based on red shifts, were previously said to have existed about 480 million years after the Big Bang. Astronomers had previously though the so-called Cosmic Dark Ages ended about 450 million years after the Big Bang, but final results from the Planck satellite indicate that they did not end until about 550 million years afterward. Before that the universe should supposedly have been too dense, turbulent and hot for significant amounts of star formation. The earliest galaxies should supposedly not have appeared much before 550 million years. However, a galaxy is already known (GN-z11) which is seen as it appeared at 400 million years, and it looks “surprisingly mature.” The new, hard-working James Webb Space Telescope is our best hope for studying the First Galaxies.




COMPUTER SIMULATIONS OF EARLY UNIVERSE!

LOCAL EXPERTS!

DARK ENERGY

MAKING MATTER?