9.2 The Big Bang
If we begin at the present day, we can measure a whole slew of properties of the
Universe as it is right now. There are specific temperature fluctuations that exist in
the cold (2.725 K) cosmic microwave background (Planck Collaboration et al.
2015a), there are patterns to the clustering of galaxies on the largest scales in the
Universe (Alam et al. 2017), and there are measurements of the expansion rate of the
Universe out to many billions of light years (Suzuki et al. 2012). Combined, this
gives us a concordance picture of the Universe, teaching us that it’s composed of a
specific mix of dark energy (68%), dark matter (27%), baryons (4.9%), neutrinos
(0.1%), and photons (~0.01%), where these and other various methods of inquiry all
produce measurements which point to the same, consistent cosmic picture (Freese
2017).
We also, when we look at the Universe nearby, find galaxies that are—on
average—intrinsically redder in color, with high levels of metallicity (heavy element
content), that are quite tightly clustered together, and that display relatively large
masses and an advanced, evolved morphology (Baillard et al. 2011). The Universe
shouldn’t have always been this way, however. Gravitational attraction is a runaway
process, pulling matter into structures slowly but increasingly when overdense
regions are only slightly greater than the mean density (Mészáros 1974), and
growing non-linearly once a certain density threshold is reached (Peacock and
Dodds 1994). Applying this to the Universe, where looking back to great distances
equates to looking back large amounts in time, we can reconstruct how more distant
galaxies ought to be different in the past. Specifically, they should appear less
clustered, with less power on large scales. For individual galaxies, they should be
intrinsically bluer in color, less evolved in morphology, smaller in size and mass,
with lower metallicities, and with different luminosity functions for their constituent
stellar populations (Kawamata et al. 2018).
This should progress more and more severely as we go back in time, culminating
in pristine populations of gas which have never yet formed any stars. This prediction
of the Big Bang dates back 70 years (Alpher et al. 1948) and was at last verified just a
few years ago (Fumagalli et al. 2011). As we continue to go back in time, we should
reach an epoch where there was so little starlight that the atoms in intergalactic space
remained neutral, since there was no ultraviolet radiation to ionize them (Gunn and
Peterson 1965). This light-blocking matter was detected in high-redshift quasars; the
Gunn-Peterson trough is now a solid part of observational cosmology (Becker et al.
2001). Prior even to this, there must be a time where no stars at all had formed, and
the Universe was entirely dark, save for the leftover radiation from even earlier
times. These cosmic “dark ages” are consistent with the timescale for reionization
measured in the optical depth of the cosmic microwave background (Bennett et al.
2013).
But we can go back even farther, to when the Universe was more uniform, less
clumped, and even smaller in size. Any radiation that existed would have its
wavelength severely compressed in comparison to what exists today, implying a
9 Before the Big Bang
87
If we begin at the present day, we can measure a whole slew of properties of the
Universe as it is right now. There are specific temperature fluctuations that exist in
the cold (2.725 K) cosmic microwave background (Planck Collaboration et al.
2015a), there are patterns to the clustering of galaxies on the largest scales in the
Universe (Alam et al. 2017), and there are measurements of the expansion rate of the
Universe out to many billions of light years (Suzuki et al. 2012). Combined, this
gives us a concordance picture of the Universe, teaching us that it’s composed of a
specific mix of dark energy (68%), dark matter (27%), baryons (4.9%), neutrinos
(0.1%), and photons (~0.01%), where these and other various methods of inquiry all
produce measurements which point to the same, consistent cosmic picture (Freese
2017).
We also, when we look at the Universe nearby, find galaxies that are—on
average—intrinsically redder in color, with high levels of metallicity (heavy element
content), that are quite tightly clustered together, and that display relatively large
masses and an advanced, evolved morphology (Baillard et al. 2011). The Universe
shouldn’t have always been this way, however. Gravitational attraction is a runaway
process, pulling matter into structures slowly but increasingly when overdense
regions are only slightly greater than the mean density (Mészáros 1974), and
growing non-linearly once a certain density threshold is reached (Peacock and
Dodds 1994). Applying this to the Universe, where looking back to great distances
equates to looking back large amounts in time, we can reconstruct how more distant
galaxies ought to be different in the past. Specifically, they should appear less
clustered, with less power on large scales. For individual galaxies, they should be
intrinsically bluer in color, less evolved in morphology, smaller in size and mass,
with lower metallicities, and with different luminosity functions for their constituent
stellar populations (Kawamata et al. 2018).
This should progress more and more severely as we go back in time, culminating
in pristine populations of gas which have never yet formed any stars. This prediction
of the Big Bang dates back 70 years (Alpher et al. 1948) and was at last verified just a
few years ago (Fumagalli et al. 2011). As we continue to go back in time, we should
reach an epoch where there was so little starlight that the atoms in intergalactic space
remained neutral, since there was no ultraviolet radiation to ionize them (Gunn and
Peterson 1965). This light-blocking matter was detected in high-redshift quasars; the
Gunn-Peterson trough is now a solid part of observational cosmology (Becker et al.
2001). Prior even to this, there must be a time where no stars at all had formed, and
the Universe was entirely dark, save for the leftover radiation from even earlier
times. These cosmic “dark ages” are consistent with the timescale for reionization
measured in the optical depth of the cosmic microwave background (Bennett et al.
2013).
But we can go back even farther, to when the Universe was more uniform, less
clumped, and even smaller in size. Any radiation that existed would have its
wavelength severely compressed in comparison to what exists today, implying a
9 Before the Big Bang
87