Chapter 18
A Brief Historical Overview
of the Universe
Abstract We may use our theories and presently observed properties of the universe
to run the cosmic clock backwards and view the history of the universe in reverse.
This takes us from the present universe of stars and galaxies, to the radiation era of hot
plasma when the presently observed matter formed, and back to the era commonly
described by inflation. We may finally run the clock all the way back to a putative
Planck era when spacetime itself was likely fundamentally different from what it is
at present, and for which we have no accepted theory.
18.1 Overview
This chapter is a very brief and superficial account of the contents, temperature and
size of the universe during its history (Freedman 2006). We have already traced the
history from the present back into the radiation era in the preceding chapters. As we
noted in Sect. 17.1 we can track and understand the history with some confidence
back to about a microsecond, which is a rather remarkable claim (Peebles 1993;
Weinberg 1988).
We now live in a universe of stars and galaxies made of atoms, ionized plasma,
and a great deal of dark matter of unknown nature. The vacuum also appears to have
a nonzero energy density, although we do not understand why it has the magnitude it
does; the vacuum or dark energy density is the dominant constituent in the universe
as we discussed in previous chapters. We are also bathed in the CMB radiation sea,
and almost certainly a similar sea of neutrinos, which has yet to be detected. As
we discussed in Chap. 17 the energy density in matter is about 10,000 times the
energy density in the CMB radiation at present, but we know the radiation density
was much greater in the radiation era. The young universe was hot and had a different
composition than at present as we discussed in the preceding chapter. In this chapter
we will run the cosmic clock of the universe further backwards to even earlier times,
and thus a smaller scale factor and higher temperature; we want to see what the
important contents of the universe were, based on the physics of atoms, nuclei,
nucleons, and quarks and leptons.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
R. J. Adler, General Relativity and Cosmology, Graduate Texts in Physics,
https://doi.org/10.1007/978-3-030-61574-1_18
275
A Brief Historical Overview
of the Universe
Abstract We may use our theories and presently observed properties of the universe
to run the cosmic clock backwards and view the history of the universe in reverse.
This takes us from the present universe of stars and galaxies, to the radiation era of hot
plasma when the presently observed matter formed, and back to the era commonly
described by inflation. We may finally run the clock all the way back to a putative
Planck era when spacetime itself was likely fundamentally different from what it is
at present, and for which we have no accepted theory.
18.1 Overview
This chapter is a very brief and superficial account of the contents, temperature and
size of the universe during its history (Freedman 2006). We have already traced the
history from the present back into the radiation era in the preceding chapters. As we
noted in Sect. 17.1 we can track and understand the history with some confidence
back to about a microsecond, which is a rather remarkable claim (Peebles 1993;
Weinberg 1988).
We now live in a universe of stars and galaxies made of atoms, ionized plasma,
and a great deal of dark matter of unknown nature. The vacuum also appears to have
a nonzero energy density, although we do not understand why it has the magnitude it
does; the vacuum or dark energy density is the dominant constituent in the universe
as we discussed in previous chapters. We are also bathed in the CMB radiation sea,
and almost certainly a similar sea of neutrinos, which has yet to be detected. As
we discussed in Chap. 17 the energy density in matter is about 10,000 times the
energy density in the CMB radiation at present, but we know the radiation density
was much greater in the radiation era. The young universe was hot and had a different
composition than at present as we discussed in the preceding chapter. In this chapter
we will run the cosmic clock of the universe further backwards to even earlier times,
and thus a smaller scale factor and higher temperature; we want to see what the
important contents of the universe were, based on the physics of atoms, nuclei,
nucleons, and quarks and leptons.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
R. J. Adler, General Relativity and Cosmology, Graduate Texts in Physics,
https://doi.org/10.1007/978-3-030-61574-1_18
275