456 CHAPTER 7. ASTROPHYSICS AND COSMOLOGY
Remark 7.18.The jets shown in Figures7.9and7.10are column-shaped. If the cell number
k≥3 for the latitudinal circulation of galaxy nucleus, then there are jets which are disc-
shaped. We don’t know if there exist such galaxy nuclei whichhave the disc-shaped jets in
the Universe. Theoretically, it appears to be possible.
Remark 7.19.Galactic nucleus are made up of plasm. The precise description of AGN
jets requires to take into consideration of the magnetic effect in the modeling. However the
essential mechanism does not change and an explosive magnetic energy as in (7.4.37) will
contribute to the supernovae explosion.
7.5 The Universe
7.5.1 Classical theory of the Universe
In this section, we recall some basic aspects of modern cosmology, including the Hubble
Law, the expanding universe, and the origin of our Universe,together with their experimental
justifications.
1.The Hubble Law. In 1929, American astronomer Edwin Hubble discovered an ap-
proximatively linear relation between the recession velocityvand the distanceRof remote
galaxies, which is now called the Hubble Law:
(7.5.1) v=HR,
whereHis called the Hubble constant, depends on time, and its present-time value is
(7.5.2) H=70 km/s·Mpc, Mpc= 106 pc(1 pc= 3 .26 ly).
Formula (7.5.1) implies that the farther away the galaxy is from our galaxy,the greater its
velocity is.
2.Expansion of the Universe.An important physical conclusion from the Hubble Law
(7.5.1)-(7.5.2) is that our Universe is expanding.
If we regard our Universe as a 3-dimensional sphere:
(7.5.3) M=S^3 r={x∈R^4 |x^21 +x^22 +x^23 +x^24 =r^2 }.
Each point on the sphereS^3 rcan be regarded as a center, as the radiusrincreases, all points
on the sphere are moving away from the point. Moreover, the farther away a remote object is
from the point, the faster the object appears to be moving. For example, for any two points
p 1 andp 2 on a sphereS^3 r,θis the angle betweenp 1 andp 2 ; see Figure7.12. As the radiusr
varies fromr 1 tor 2 , the distanceRbetweenP 1 andP 2 varies fromR 1 toR 2 :
R 1 =θr 1 →R 2 =θr 2.The velocity of separation of these two points is
(7.5.4) v=
dR
dt=θr ̇, (R=θr).