DARK MATTER!
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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.
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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 Last Scattering
Surface,
• Study of baryonic
acoustic oscillations in the Last Scattering Surface, 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.]
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The last scattering surface
is currently more than 40 billion light years away.
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The power spectrum of
temperature variations observed in the Last Scattering Surface
at high angular 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.
“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.
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.
GRAVITATIONAL RADIATION
COMPUTER SIMULATIONS OF EARLY
UNIVERSE!
LOCAL EXPERTS!
DARK ENERGY
MAKING MATTER?
