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16 Some Properties of the LCDM Universe
6. Large scale structures: The spectrum of the CMB is one example of large-scale
structure in the early universe. The distribution of matter at later times is another,
and depends on the contents and cosmological parameters of the universe. Intuition might lead us to expect that a random but roughly elliptical blob of matter
would first collapse toward a line, and then that line would collapse toward a
point. This is generally born out in the observations and simulations, but the
detailed large-scale structure is a very different matter. The results of computer
simulations can be found on the internet, and show a rich tapestry of filaments and
blobs and voids, which are observable in the real universe (Cosmicweb 2019).
Note that there is considerable overlap between the ideas of large-scale structure and gravitational lensing since the dark matter that produces the lensing
constitutes much of the mass of the universe.
7. Some possible further observations: In addition to the types of current observations above we discuss in the remainder of this section some potential
observations of future interest.
The light elements in the universe today were formed in the first few minutes
after the big bang; the heavier elements were formed later in the interiors of stars,
or in the collisions of neutron stars. The formation of the light elements is well
understood theoretically and accurately predicted as a function of properties such as
the ambient temperature and nucleon abundances (Weinberg 1988; WMAP 2010).
Thus observations of the present element abundances serve as a test of conditions in
the early universe. We will say a bit more about this big bang nucleosynthesis (BBN)
in Chap. 18.
The polarization pattern of the CMB is quite interesting, in addition to its spectrum
as noted above. Specifically, the pattern depends on processes that happened during
inflation and can thus give information about gravitational waves produced during
inflation and the inflationary energy scale (see Chap. 19). The detection and analysis
of the polarization patterns is quite difficult, specifically the so-called B modes, but
should be of great interest (cfa.harvard.edu 2019).
The universe is filled with extragalactic background light emitted by stars during
the lifetime of the universe. High energy photons interact with this light and it thus
attenuates gamma rays in their passage through space. The amount of attenuation
depends on the expansion rate of the universe and the matter content along the line of
travel of the gamma rays. As a result gamma ray telescopes can yield a measurement
of the Hubble constant and the present matter density of the universe (Dominguez
2019).
The rate of change of the redshift is an interesting quantity in both theory and
observations. The redshift z of receding galaxies is a fundamental property of the big
bang cosmology and is measured very accurately. It is defined in terms of the scale
factor in (13.22). The Friedmann equation (14.19) then determines the scale factor
in the current universe as a function of time, depending on the Hubble constant and
the present matter and vacuum energy densities. It is straightforward to calculate the
time derivative of z from (14.19), and that yields a surprisingly simple expression,
susceptible to testing. See Exercise 16.1. Given the present and expected accuracies
16 Some Properties of the LCDM Universe
6. Large scale structures: The spectrum of the CMB is one example of large-scale
structure in the early universe. The distribution of matter at later times is another,
and depends on the contents and cosmological parameters of the universe. Intuition might lead us to expect that a random but roughly elliptical blob of matter
would first collapse toward a line, and then that line would collapse toward a
point. This is generally born out in the observations and simulations, but the
detailed large-scale structure is a very different matter. The results of computer
simulations can be found on the internet, and show a rich tapestry of filaments and
blobs and voids, which are observable in the real universe (Cosmicweb 2019).
Note that there is considerable overlap between the ideas of large-scale structure and gravitational lensing since the dark matter that produces the lensing
constitutes much of the mass of the universe.
7. Some possible further observations: In addition to the types of current observations above we discuss in the remainder of this section some potential
observations of future interest.
The light elements in the universe today were formed in the first few minutes
after the big bang; the heavier elements were formed later in the interiors of stars,
or in the collisions of neutron stars. The formation of the light elements is well
understood theoretically and accurately predicted as a function of properties such as
the ambient temperature and nucleon abundances (Weinberg 1988; WMAP 2010).
Thus observations of the present element abundances serve as a test of conditions in
the early universe. We will say a bit more about this big bang nucleosynthesis (BBN)
in Chap. 18.
The polarization pattern of the CMB is quite interesting, in addition to its spectrum
as noted above. Specifically, the pattern depends on processes that happened during
inflation and can thus give information about gravitational waves produced during
inflation and the inflationary energy scale (see Chap. 19). The detection and analysis
of the polarization patterns is quite difficult, specifically the so-called B modes, but
should be of great interest (cfa.harvard.edu 2019).
The universe is filled with extragalactic background light emitted by stars during
the lifetime of the universe. High energy photons interact with this light and it thus
attenuates gamma rays in their passage through space. The amount of attenuation
depends on the expansion rate of the universe and the matter content along the line of
travel of the gamma rays. As a result gamma ray telescopes can yield a measurement
of the Hubble constant and the present matter density of the universe (Dominguez
2019).
The rate of change of the redshift is an interesting quantity in both theory and
observations. The redshift z of receding galaxies is a fundamental property of the big
bang cosmology and is measured very accurately. It is defined in terms of the scale
factor in (13.22). The Friedmann equation (14.19) then determines the scale factor
in the current universe as a function of time, depending on the Hubble constant and
the present matter and vacuum energy densities. It is straightforward to calculate the
time derivative of z from (14.19), and that yields a surprisingly simple expression,
susceptible to testing. See Exercise 16.1. Given the present and expected accuracies