13.1 Basic Observations and Assumptions
205
Thus 1/H 0 is a characteristic time scale for expansion of the universe and c/H 0 is a
characteristic distance scale. The scenario in which all the material in the universe
was close together in the past is now very well-known as the big bang, and is a
fundamental part of cosmology.
There was once large uncertainty in the value of the Hubble constant. Early estimates from galaxy observations varied over the range 50 − 100 (km/s)/Mpc because
of the difficulties involved in measuring cosmological distances. Because of this,
astronomers have often expressed results which depend on H 0 in terms of a dimensionless number h 0 , defined by H 0 = h 0 100 (km/s)/Mpc. As noted above the value
of the Hubble constant is now much better determined, but there remain important
questions regarding its value. In particular, another method of obtaining H 0 using
the CMB discussed below, gives a value that is not quite consistent with the classic
Hubble method; this is discussed below and in Appendix 1 and also in Chap. 17
(Reiss 2019; Planck 2018; Chalinor 2012).
The linear Hubble law (13.1) only holds for galaxies relatively close to us on the
cosmological scale. For more distant objects, galaxies and supernovae, observations
show a deviation from linearity, and this gives information on the material of the
universe. Specifically the data indicate that the universe is not only expanding but
that the expansion is accelerating, as we will later discuss (Reiss 1998; Perlmutter
1999; Kirshner 2004). This in turn indicates the presence of dark energy as a large
part of the total energy content of the universe as we will discuss in Chap. 15. The
observed acceleration is quite important as it is one of the bases of the presently
favored cosmological model, the CDM or LCDM model, where CDM stands for
cold dark matter and or L refers to the cosmological constant lambda.
B. Black body radiation fills the Universe. This radiation, called the cosmic
microwave background or CMB, is observed directly with satellite and groundbased microwave and infrared detectors and fits the Planck black body spectrum
for a temperature 2.75 K extremely well. It is also very isotropic in direction. The
standard interpretation is that it is the remnant of the thermal radiation produced
by the very hot big bang explosion, now greatly cooled by the expansion of the
universe. The CMB existence and nature leave little doubt that the general big
bang and expanding universe scenario are correct (Kirshner 2004). Indeed we
can think of the CMB pattern on the sky as a photo of the big bang.
Theoretical models of the early universe predict the detailed spectrum of the
CMB, that is the very small deviations from a perfect isotropic black body spectrum,
of order 10
−5
. For example the LCDM theory of the early universe with inflation
explains some detailed properties of the CMB spectrum, so the spectrum has become a
standard and useful tool for testing the LCDM model and the early universe (Chalinor
2012; NASA 2019).
For example the CMB spectrum has become an important tool to measure the
Hubble constant independently of the classic Hubble diagram technique discussed
above (Planck 2018; NASA 2019). The result of the CMB measurements and theoretical analysis is a value of about H 0 = 67 (km/s)/Mpc. It is gratifying that this
value is in rough agreement with the classic technique of Hubble that we mentioned
205
Thus 1/H 0 is a characteristic time scale for expansion of the universe and c/H 0 is a
characteristic distance scale. The scenario in which all the material in the universe
was close together in the past is now very well-known as the big bang, and is a
fundamental part of cosmology.
There was once large uncertainty in the value of the Hubble constant. Early estimates from galaxy observations varied over the range 50 − 100 (km/s)/Mpc because
of the difficulties involved in measuring cosmological distances. Because of this,
astronomers have often expressed results which depend on H 0 in terms of a dimensionless number h 0 , defined by H 0 = h 0 100 (km/s)/Mpc. As noted above the value
of the Hubble constant is now much better determined, but there remain important
questions regarding its value. In particular, another method of obtaining H 0 using
the CMB discussed below, gives a value that is not quite consistent with the classic
Hubble method; this is discussed below and in Appendix 1 and also in Chap. 17
(Reiss 2019; Planck 2018; Chalinor 2012).
The linear Hubble law (13.1) only holds for galaxies relatively close to us on the
cosmological scale. For more distant objects, galaxies and supernovae, observations
show a deviation from linearity, and this gives information on the material of the
universe. Specifically the data indicate that the universe is not only expanding but
that the expansion is accelerating, as we will later discuss (Reiss 1998; Perlmutter
1999; Kirshner 2004). This in turn indicates the presence of dark energy as a large
part of the total energy content of the universe as we will discuss in Chap. 15. The
observed acceleration is quite important as it is one of the bases of the presently
favored cosmological model, the CDM or LCDM model, where CDM stands for
cold dark matter and or L refers to the cosmological constant lambda.
B. Black body radiation fills the Universe. This radiation, called the cosmic
microwave background or CMB, is observed directly with satellite and groundbased microwave and infrared detectors and fits the Planck black body spectrum
for a temperature 2.75 K extremely well. It is also very isotropic in direction. The
standard interpretation is that it is the remnant of the thermal radiation produced
by the very hot big bang explosion, now greatly cooled by the expansion of the
universe. The CMB existence and nature leave little doubt that the general big
bang and expanding universe scenario are correct (Kirshner 2004). Indeed we
can think of the CMB pattern on the sky as a photo of the big bang.
Theoretical models of the early universe predict the detailed spectrum of the
CMB, that is the very small deviations from a perfect isotropic black body spectrum,
of order 10
−5
. For example the LCDM theory of the early universe with inflation
explains some detailed properties of the CMB spectrum, so the spectrum has become a
standard and useful tool for testing the LCDM model and the early universe (Chalinor
2012; NASA 2019).
For example the CMB spectrum has become an important tool to measure the
Hubble constant independently of the classic Hubble diagram technique discussed
above (Planck 2018; NASA 2019). The result of the CMB measurements and theoretical analysis is a value of about H 0 = 67 (km/s)/Mpc. It is gratifying that this
value is in rough agreement with the classic technique of Hubble that we mentioned