The Big Bang Theory with Inflation: The Tale of Our Universe (circa 2007)
In this class, we’ve gradually peeled the curtain back on the latest explanations for
the history of the Universe, navigating through twists and wrong turns until we arrived at
a hypothesis consistent with most of our present day observations. Scientists certainly
have a lot more work to do: specifying particular details, pondering the reasons for yetunexplained properties (such as the relative weakness of the gravitational force, or the
values of fundamental quantities like the speed of light), and constantly testing our best
theories with new observations. But for this conclusion to your introductory course about
the Universe, it’s good to look back at what we have so far.
First (just so we get the most basic picture straight), a super-brief summary of the
inflationary Big Bang theory:
Age of Universe
0
Event
Big Bang
Now, let’s
consider
10-37 – 10-35 seconds
Inflation
how
various
A few minutes
Nucleosynthesis
observatio
ns about
400,000 years
Recombination
our
Universe
105 years to present
Evolution Under Gravity and
are
(13.7 billion years)
Expansion
explained
in the context of this theory. After all, what’s the point of a really interesting story that
has no connection to the real world?
Below I’ve listed some observed features of the Universe that are important to
consider. You should try to add your own, and see how the inflationary Big Bang theory
and other phenomena we’ve discussed in Astro 10 help explain them.
* High abundances of hydrogen and helium, distributed uniformly, versus lower and less
uniform abundances of heavier elements
* Much more matter than anti-matter
* Galaxies and galaxy clusters, separated by vast cosmic voids
* In spite of many, many stars and galaxies, the night sky is dark
* A radiation field in all directions (the CMB) that is nearly uniform even over vast
distances
* Flat space (within our best measurements), corresponding to a matter/energy density
very close to the unstable value  = 1
* The existence of complex life forms, including intelligent human life
* Your own . . .
* High abundances of hydrogen and helium, distributed uniformly, versus lower and less
uniform abundances of heavier elements
Hydrogen and helium were created early in the Universe, from particles formed
during the Big Bang and after inflation. Protons were created from quarks in the first
second of the Universe from, and captured electrons to make hydrogen atoms during
Recombination. Helium nuclei were fused around 100 s (when the Universe was hot
enough for fusion, but not so hot that particles' speeds overwhelmed attractive nuclear
forces) and also captured electrons during Recombination to become neutral atoms.
Because of their primordial origins, we see nearly uniform abundances of hydrogen and
helium in today's universe.
Heavier elements, as we discussed a while ago, are formed in the centers of stars
and in supernovae. The relative abundances of these elements depend on local properties
when the Universe was more evolved, so they are not as uniform across space.
* Much more matter than anti-matter
One would expect the Universe to be formed with the same amount of matter as
antimatter. But if this equilibrium continued, then all mass would be annihilated, and
there would be no present-day Universe to speak of! Instead, a slight excess of matter
formed (physicists aren’t yet sure why) so that when matter and antimatter annihilated
each other, a small fraction of matter was left over. This remainder evolved into the
matter that is distributed throughout the Universe today. Because the initial small excess
of matter occurred when the Universe was very dense and photons, particles, and antiparticles were in equilibrium (less than 10-6 seconds after the Big Bang), the subsequent
excess of matter after annihilation was roughly the same throughout the Universe.
* Galaxies and galaxy clusters, separated by vast cosmic voids
From observing the Cosmic Microwave Background, we know that the Universe
had small density fluctuations (corresponding to the small temperature fluctuations in the
CMB) at the time of Recombination. These tiny differences in density across space
probably originated at or shortly after the Big Bang. As the Universe continued to
evolve, gravity caused the denser regions to attract more mass and become still denser.
These regions eventually became large clusters and superclusters of galaxies. In
between, the less dense regions were overwhelmed by dark energy (they didn’t have
sufficient self-gravity to counteract the accelerating expansion), and they expanded into
cosmic voids.
* In spite of many, many stars and galaxies, the night sky is dark
We do not know whether the Universe is infinite in size, but observations (the
CMB, for instance) clearly indicate that it has a finite age, which the Big Bang theory and
associated calculations set at 13.7 billion years. As a result, we can only see objects
within 13.7 billion light years of us: the distance light could have traveled since the Big
Bang. If the Universe were infinite in size and age, we would expect every line of sight
to encounter the surface of a star, and the night sky to therefore be bright.
* A radiation field in all directions (the CMB) that is nearly uniform even over vast
distances
The Cosmic Microwave Background originates from Recombination, when the
initially hot and dense Universe cooled enough to allow electrons to become bound to
atomic nuclei. This created photons which, due to the newfound transparency of the
neutral Universe, could travel freely over long distances.
Yet the near-perfect uniformity of the CMB is not explicable without inflation.
At some point, the Universe must have expanded dramatically from a state when it was
small enough to be in thermal equilibrium. Without inflation the Universe would have
always been too big for signals to travel across it in its age.
Note that the current observed expansion of the Universe does not count as
inflation. Without early inflation it would be too big to reach thermal equilibrium, in the
past as well as the present.
* Flat space (within our best measurements), corresponding to a matter/energy density
very close to the unstable value  = 1
Likewise, inflation is a necessary component to explain the likelihood of flat
space. With a non-inflationary Big Bang theory it is technically possible to observe W =
1 now, but the initial conditions at the Big Bang must have then been extremely finely
tuned to allow this property to last. On the other hand, inflation immensely expands the
early Universe, causing all apparent curvature to disappear (even if the Universe’s global
curvature is still slightly greater than or smaller than zero). This allows our present-day
observations of flat space to result from a variety of possible initial conditions.
* The existence of complex life forms, including intelligent human life
Clearly, if our Universe had evolved to conditions that could not support any
life, you and I would never be able to read/write about it on this worksheet. Our
Universe is very special (not too dense to last billions of years, not too sparse for matter
to gather into stars and planets, appropriately balanced between matter and antimatter,
capable of producing heavy elements via supernovae, etc., etc.). This may be an amazing
freak coincidence, or maybe ours is one of many universes, most of which are never able
to foster life. To date, science has not found a way to answer whether there is a greater
context outside our own Universe.
Your own observations above: how are they explained?