much hotter temperature under these early conditions. At some sufficiently early
time, the wavelength of the most energetic photons would be short enough to enable
the spontaneous ionization of neutral atoms: the photoelectric effect of this ultraviolet radiation would have prevented the stable formation of bound states of electrons
and nuclei. In the face of an ionized plasma, this radiation would experience rapid
Thomson scattering off of the free electrons, while after sufficient stretching and
cooling, neutral atoms will form and the radiation will free-stream, creating a
leftover radiation bath—a primeval fireball—whose remnants should exist with a
low temperature and blackbody spectrum today (Dicke et al. 1965). The detection of
this radiation (Penzias and Wilson 1965) and the subsequent measurement of its
spectral properties (Fixsen et al. 1996) have thoroughly verified this cornerstone of
the Big Bang. We’ve even come so far as to measure the minuscule temperature
fluctuations in the spectrum of this radiation (Planck Collaboration et al. 2015a),
corresponding to the density fluctuations that gave rise to the large-scale structure of
the Universe today in great detail (Alam et al. 2017).
Even before that, the wavelength of photons would have been so short that atomic
nuclei themselves would have been blasted apart. High-energy radiation can dissociate protons and neutrons from one another, creating a sea of unbound particles. The
first step towards binding protons and neutrons together towards the formation of
heavier nuclei is to create deuterium: a heavy isotope of hydrogen with a binding
energy of just 2.2 MeV. When there are sufficient densities of photons in excess of
those energies, no nucleosynthesis of heavier elements can proceed; the “deuterium
bottleneck” delays those interactions due to the fragility of the products of this first,
necessary step (Peacock 1999). Only when photons cool below this critical threshold
can the synthesis of the first elements proceed. In this framework of the Big Bang,
the only parameter that determines the relative element abundances is the baryon-tophoton ratio (Steigman 2006). The direct measurement of that ratio (Bennett et al.
2013) aligns spectacularly with the observations of the abundances of helium-4,
helium-3, deuterium, and lithium-7, many of which are seen in distant quasar
absorption lines (Riemer-Sørensen and Jennsen 2017).
So far, all of these predictions of the Big Bang have been spectacularly verified,
but we can continue to extrapolate even farther back, to early times and high
temperatures where no verifiable signatures remain. Before nucleosynthesis, particles should have high enough energies to spontaneously produce matter-antimatter
pairs. Electrons and positrons remain the longest, but at even earlier times, any-andall standard model particles and antiparticles are produced in great abundance, as
defined by their fermionic or bosonic statistics. After the Universe cools and the
particle/antiparticle pairs annihilate away, a tiny fraction of leftover matter remains:
this baryon asymmetry is one of the greatest unsolved mysteries in all of physics.
When temperatures are high enough and the density of the Universe exceeds
certain limits, protons, neutrons, and any type of baryon cease to exist, as they
become replaced with a quark-gluon plasma. At even higher temperatures, exotic
physics is expected to exist, particularly as we approach the theorized grand unification (GUT) scales. Particles mediating proton decay may appear; leptoquarks may
exist; the couplings of the fundamental forces may run, and possibly run together; a
88
E. R. Siegel
time, the wavelength of the most energetic photons would be short enough to enable
the spontaneous ionization of neutral atoms: the photoelectric effect of this ultraviolet radiation would have prevented the stable formation of bound states of electrons
and nuclei. In the face of an ionized plasma, this radiation would experience rapid
Thomson scattering off of the free electrons, while after sufficient stretching and
cooling, neutral atoms will form and the radiation will free-stream, creating a
leftover radiation bath—a primeval fireball—whose remnants should exist with a
low temperature and blackbody spectrum today (Dicke et al. 1965). The detection of
this radiation (Penzias and Wilson 1965) and the subsequent measurement of its
spectral properties (Fixsen et al. 1996) have thoroughly verified this cornerstone of
the Big Bang. We’ve even come so far as to measure the minuscule temperature
fluctuations in the spectrum of this radiation (Planck Collaboration et al. 2015a),
corresponding to the density fluctuations that gave rise to the large-scale structure of
the Universe today in great detail (Alam et al. 2017).
Even before that, the wavelength of photons would have been so short that atomic
nuclei themselves would have been blasted apart. High-energy radiation can dissociate protons and neutrons from one another, creating a sea of unbound particles. The
first step towards binding protons and neutrons together towards the formation of
heavier nuclei is to create deuterium: a heavy isotope of hydrogen with a binding
energy of just 2.2 MeV. When there are sufficient densities of photons in excess of
those energies, no nucleosynthesis of heavier elements can proceed; the “deuterium
bottleneck” delays those interactions due to the fragility of the products of this first,
necessary step (Peacock 1999). Only when photons cool below this critical threshold
can the synthesis of the first elements proceed. In this framework of the Big Bang,
the only parameter that determines the relative element abundances is the baryon-tophoton ratio (Steigman 2006). The direct measurement of that ratio (Bennett et al.
2013) aligns spectacularly with the observations of the abundances of helium-4,
helium-3, deuterium, and lithium-7, many of which are seen in distant quasar
absorption lines (Riemer-Sørensen and Jennsen 2017).
So far, all of these predictions of the Big Bang have been spectacularly verified,
but we can continue to extrapolate even farther back, to early times and high
temperatures where no verifiable signatures remain. Before nucleosynthesis, particles should have high enough energies to spontaneously produce matter-antimatter
pairs. Electrons and positrons remain the longest, but at even earlier times, any-andall standard model particles and antiparticles are produced in great abundance, as
defined by their fermionic or bosonic statistics. After the Universe cools and the
particle/antiparticle pairs annihilate away, a tiny fraction of leftover matter remains:
this baryon asymmetry is one of the greatest unsolved mysteries in all of physics.
When temperatures are high enough and the density of the Universe exceeds
certain limits, protons, neutrons, and any type of baryon cease to exist, as they
become replaced with a quark-gluon plasma. At even higher temperatures, exotic
physics is expected to exist, particularly as we approach the theorized grand unification (GUT) scales. Particles mediating proton decay may appear; leptoquarks may
exist; the couplings of the fundamental forces may run, and possibly run together; a
88
E. R. Siegel