Matter content
The observable universe contains about 3 to 100 × 10
22
stars
(30
sextillion
to a
septillion
stars),
[37]
[38]
[39]
[40]
organized in more than 80 billion
galaxies
, which themselves form
clusters
and
superclusters
.
[41]
Two approximate calculations give the number of
atoms
in the observable universe to be close to
10
80
.
1. Observations of the cosmic microwave background from the
Wilkinson Microwave
Anisotropy Probe
suggest that the
spatial curvature
of the universe is very close to zero, which in
current cosmological models implies that the value of the
density parameter
must be very close
to a certain critical value. A NASA page gives this density, which includes dark energy, dark
matter and ordinary matter all lumped together, as 9.9×10
−27
kg/m
3
,
[42]
although the figure has not
been updated since 2005
[43]
and a number of new estimates of the
Hubble parameter
have been
made since then. The present value of the Hubble parameter
H
0
is important because it is related
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This note was uploaded on 12/15/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.
 Fall '10
 EmilyHoward
 Astronomy, Galaxies

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