6.3. A TIME-LINE TO NOW 91
6.3 A TIME-LINE TO NOW
Following is a condensed timeline of key events in the early history of the universe, summarized
in Table 6.1, expressed in terms of what is probably our best current overall cosmological theory,
the so-called ƒ-CDM model. We will consider the details of the ƒ-CDM model more fully in
Section 17.1; here we simply describe its implications for the history of the universe.
e variable t refers to how much time has passed since t D 0. e variable T stands
for temperature, that is, how hot the universe is. e temperature is expressed in the Kelvin
scale, which tells how many degrees Centigrade above absolute zero. But first off, we must be
clear regarding what we mean by t and T . Who is measuring these variables? In particular,
we have already seen that time is relative. And so who’s time do we mean? e answer is that
we describe the universe in terms of a co-moving observer—a hypothetical person riding along
with the expansion of the universe, describing the conditions as they happen for them. And so
t represents the time ticking along on their clock, but we can generalize this result; because of
the cosmological principal, we assume that any other observer would experience the same.
For the first 380,000 years or so, the baryonic (ordinary) matter and electromagnetic radi-
ation (light) in the universe are locked together, and so both are at the same temperature. After
this time of decoupling, baryonic matter and light go there separate ways, and so the temperature
of the universe is less meaningful as a concept for the matter. But it still has some meaning for
the radiation, and so that is what T refers to after that time. e redshift, z, is also listed, but
it is important to note that for many of the earlier entries we have no way to directly measure
such a redshift for any observable object. Notice that for smaller t (longer ago), z is bigger. In
the sections that follow, we briefly consider each part of this chronology.
6.3.1 THE PLANCK ERA
Before this time, ending at about t D 10
´43
s, the universe is too hot and dense for our estab-
lished physics to apply. To understand what is going on at this high temperature and density,
we would need to know how all the forces of nature are connected to each other. e Standard
Model of Particle Physics explains most of the relations between electricity, magnetism and the
forces at work within the nucleus of an atom. But it does not include gravity in a unified way,
and it would surely need to in order to describe the conditions at temperatures this high.
6.3.2 INFLATION
At about 10
´36
s it is thought that the so-called strong force, that holds protons and neutrons
together in the nucleus of an atom, separated from the other fundamental forces. is may have
caused a brief and sudden, exponential expansion of the universe called inflation, an idea first
proposed by Alan Guth [1981].
Inflation is an attractive proposal for three reasons. First, it has some theoretical justifi-
cation from the standpoint of fundamental physics, albeit physics that is at best only partially
92 6. THE PAST
Table 6.1: A chronology of key events in the history of the universe. e table shows the
redshift, time from the beginning of the big bang in both seconds and years, the time in
years from the present, and the approximate temperature of the radiation in the universe.
(e specific values are approximate, and derived from a particular cosmological model, taken mostly from
en.wikipedia.org/wiki/Chronology_of_the_universe.)
Event
z t (s) t (yrs) t -t
0
(yrs) T (K)
Beginning of Big Bang
∞ 0 0
13.8 × 10
9
∞
End of Planck era 1 × 10
-43
13.8 × 10
9
10
32
End of infl ation era 1 × 10
-32
13.8 × 10
9
10
22
Protons and neutrons form 1 × 10
-6
13.8 × 10
9
10
9
Proton-neutron ratio fi xed
1.0
13.8 × 10
9
10
8
End of fusion era
180
13.8 × 10
9
10
7
Time of decoupling
1,100
1.2 × 10
13
3.8 × 10
5
13.8 × 10
9
4,000
Re-ionization begins
10
2 × 10
16
5 × 10
8
13.3 × 10
9
60
Formation of Milky Way
7,6
2.4 × 10
16
6.8 × 10
8
13.1 × 10
9
40
Formation of Sun
0.423
2.81 × 10
17
9.17 × 10
9
4.63 × 10
9
4.1
Formation of Earth
0.418
2.89 × 10
17
9.23 × 10
9
4.57 × 10
9
4.1
Dark energy era begins
0.4
3 × 10
17
9.8 × 10
9
4 × 10
9
4
e present (t
0
)
0
4.33 × 10
17
13.8 × 10
9
0 2.7
understood. Second, it solves the so-called horizon problem. Finally, inflation solves the so-called
flatness problem. We discuss these issues further in Chapter 17.
6.3.3 THE FORMATION OF PROTONS AND NEUTRONS
After inflation ended at about 10
´32
s, the universe resumed its previous much-slower (but still
rapid!) expansion. At this point it was a sea of fundamental particles: quarks, leptons, and pho-
tons. Quarks (the name was pulled by physicist Murray Gell-Mann from James Joyce’s novel,
Finnegan’s Wake) are the basic building blocks of protons and neutrons, which reside at the
nuclei of atoms. At these temperatures, however, quarks cannot hold together long enough to
form these familiar particles. Leptons are another class of particles, the most familiar of which
is the electron. Photons are particles of light.
By about 1 µs the universe had cooled enough that protons and neutrons could exist. But
it was still too hot and dense for them to be stable, and so they rapidly changed back and forth
into each other. at is, protons changed into neutrons, but neutrons also changed into protons.
Both processes would have occurred simultaneously, but in a lopsided way: it is more likely for a
6.3. A TIME-LINE TO NOW 93
neutron to change into a proton than the reverse. is means that there was more of a tendency
to make protons than to make neutrons.
As the universe cools further, by around t D 1 s it is no longer hot enough for neutrons
and protons to change into each other, and so we are now stuck with the results. Computer
calculations show that when all was said and done, there should have been about seven protons
for every neutron. Since the universe, after this point in time, is too cool to change that situation,
it is what we should have now. And so Big Bang cosmology here makes a fundamental prediction
regarding the universe today: there should be seven protons for every neutron in the universe.
at is, unless parts of the universe later find some way to heat itself back up to these enormous
temperatures.
6.3.4 THE FUSION ERA
Immediately after stable neutrons and protons have formed, it is too hot for them to get together
to form the nuclei of atoms. But as the temperature drops, protons and neutrons begin to join
up and form familiar and stable nuclei. After about 3 min have passed, the universe has cooled
to below 10 million Kelvin, and it is thereafter too cool for protons and neutrons to rearrange
their combinations; we are then stuck with whatever has formed by that time.
is is called the fusion era because it is similar to the process of nuclear fusion that occurs
in the hot and dense centers of stars such as the Sun. In a sense, during the fusion era the
conditions everywhere throughout the universe are not unlike the conditions found in the centers
of stars.
It turns out that of all the possible ways for protons and neutrons to hook up with each
other, there are two ways that are far more likely than any other.
1. Single protons: a proton all by itself is the nucleus of a normal hydrogen atom.
2. Two protons joined with two neutrons: this is the nucleus of the most common form of
helium.
And so the neutrons and the protons hitch up almost entirely in these two ways. To make
one helium nucleus, two neutrons are needed. Since 7 protons were made for every neutron,
there must be 7 ˆ 2 D 14 protons for these 2 neutrons. Two of them will be needed to complete
that helium nucleus, leaving 12 with no neutrons to hitch up with. is means we get one helium
nucleus (2 protons with 2 neutrons) for every 12 hydrogen nuclei (single protons). But a helium
nucleus weighs four times as much as a hydrogen nucleus, so by weight we have the result in
Table 6.2: 75% hydrogen, 25% helium.
Since the universe is cooling, after these first 3 min, it is too cool to break apart nuclei or
fuse them together. And so we are stuck with a universe made of 3/4 hydrogen and 1/4 helium.
us, Big Bang cosmology makes another prediction about the universe, and we have already
seen that this is roughly the observed composition of the universe today.
94 6. THE PAST
Table 6.2: Hydrogen and helium nucleosynthesis
1
12
Total
Helium
Hydrogens
weighs
weigh
weighs
4
12
16
Helium/total is 4/16 = 25%
Hydrogen/total is 12/16 = 75%
But again, if the universe can later find some way to heat parts of itself up to these same
temperatures, other nuclei could be formed. I have already hinted that this does happen, and
stars such as the Sun are the key.
6.3.5 TIME OF DECOUPLING
Before about 380,000 years the universe was too hot for electrons to join up with the hydrogen
and helium nuclei to make neutral atoms. But when the universe cooled to about 3000 K, free
electrons could join up with hydrogen and helium nuclei to make neutral atoms. is is called
the epoch of recombination. It is something of a misnomer; electrons are not re-combining with
nuclei, they are combining for the first time!
Before this, the universe was opaque. Photons of light interact very strongly with free
electrons, so light could not travel very far before scattering off in another direction. When the
universe was half a million years old, the free electrons were no longer free; they joined up with
hydrogen and helium nuclei to make neutral hydrogen and helium atoms. us, there was no
longer anything for light to scatter off of, and the universe rather suddenly became transparent.
Before the epoch of recombination, the intense radiation would have kept the protons,
neutrons, and electrons at a uniform density; it would have been difficult for a denser clump
here, a less-dense clump there to form. After this time, when we talk about the temperature of
the universe, we are talking about the temperature of the leftover radiation. Matter, meanwhile,
is free to do its own thing, while the radiation continues to cool.
6.3.6 TIME OF RE-IONIZATION: STARS AND GALAXIES
e central idea of Big Bang cosmology is that the universe began extremely hot and dense, and
rapidly expanded and cooled. But meanwhile, gravity was working on matter, pulling little bits
together to make small parts of the universe hot and dense again. And so the universe—as a
whole, on the largest scales—expanded and cooled. But on the small scale, gravity pulled bits
together. A star is one such gravity-induced concentration of matter.
ink of a star as a place where gravity pulled a small piece of the universe together,
where it heated that tiny place up to high temperatures again—high enough to knock electrons
6.3. A TIME-LINE TO NOW 95
off atoms once again. And so parts of the now much expanded universe re-ionizes because of
the formation of stars. is added light is detectable today; it very slightly alters the cosmic
microwave background in ways that can just barely be untangled. And this allows us to estimate
approximately when the first stars formed: the answer is about one half billion years into the
13.8-billion-year expansion of the universe.
ese stars did more than re-ionize the matter and make light. ey also—at the ex-
treme temperatures of their centers—reignited nuclear fusion. And so the original Big-Bang-
originated ratio of 3/4 hydrogen, 1/4 helium was gradually altered. Galaxies also formed, again
with gravity pulling things together—taking the uniform smoothness of the early universe and
giving it the lumpiness we see today.
6.3.7 FORMATION OF THE MILKY WAY, SUN, AND EARTH
e Milky Way was not one of the earliest of galaxies to form. But like most galaxies, it did
form relatively early on in the history of the universe, at roughly 680 million years since the Big
Bang—about 13.1 billion years ago.
e Earth formed as part of the process which formed the Sun—although the formation
of the Sun was completed slightly before that of Earth. But this happened much after the for-
mation of the Galaxy. e Sun began to form only about 4.60 billion years ago, and the Earth
was mostly formed soon after—about 4.54 billion years ago.
By the time the Sun formed, the Milky Way Galaxy had been around a while—about
8.5 billion years. During these eons, multiple generations of stars formed, and used their ex-
treme gravity to heat hydrogen and helium to the enormous temperatures and densities first
encountered in the first few minutes of the Big Bang itself. ese conditions allow for lighter
elements—hydrogen and helium—to undergo nuclear fusion and thus make heavier elements.
ese stars then added heavier elements to the original primoridal mix of 3/4 hydrogen, 1/4
helium produced by the big bang itself. As later generations of stars formed, they were made in
part from these heavier elements produced by earlier generations of stars.
us, the solar system contains about 2% of “metals”—elements heavier than hydrogen
and helium. is is a lucky thing for me! Otherwise, this book would never have been published
and you would not be reading it; the complexity of life depends utterly on the elements heavier
than hydrogen and helium.
6.3.8 TIME OF DARK ENERGY DOMINATION
“Normal” gravity is attractive, and it is stronger at smaller distances. But there is evidence of
another effect that in some ways acts in the opposite sense. It is negligible at small distances,
only becoming noticeable at very large, cosmologlcal distances. And its effect is repulsive; it
makes the universe expand faster and faster. We call this effect, somewhat whimsically, dark
energy.
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