With one current value of the Hubble
Constant, about 68 km/s per megaparsec, the critical density
works out to be about 9 × 10−30 gm/cm3.
What is
SH0ES? Food for thought: We do not have to imagine
ourselves very
far
in the future to find a universe that would appear to
any observer who suddenly popped into existence (a “Boltzmann
Brain”) to be nothing other than empty space at 0 K.
[Any clumps of matter would be at 0 K and unimaginably
distant, and essentially undetectable.] Again, we are very
lucky to exist so relatively early in the history of the
universe, when things are still happening.
Historically, the Hubble
Constant has proven difficult to determine. The current best
value based on observation of the early universe, is
around 67.5 km/sec per megaparsec. However, various
independent methods for determining it DO NOT AGREE very well.
The disagreement
has persisted despite more and more careful measurements, and
it seems clear that some basic assumption made in standard
cosmology may be unrealistic. Time
will tell.
We live in a universe where ordinary
matter makes up only 5% of the mass-energy density!
Results from WMAP
Results from PLANCK.
How have we learned about the composition
of the universe so precisely and directly? The answer is, by
close observation of the complex energy-density variations in
the Last Scattering Surface, created at the moment the
universe became transparent to photons, at an age of about
380,000 years (temperature 3000 K). We will see how that works
a bit later in the class. These density variations grew from
random quantum fluctuations in the very early history of the
universe, just as the universe itself is presumably the result
of a random fluctuation in the field of quantum gravity... a
space-time boson! The density variations seen in the
Last Scattering Surface are the seeds and origin of all
structure in the universe existing today.
WMAP
PLANCK
A
super-telescope for an order-of-magnitude more detailed
study of the structure of the CMB!
