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The image to the left shows a plot of the scale factor R(t) of the Universe from the start of the Big Bang until today, rotated about the time axis to create the roughly conical shape. Note the strange behavior near the beginning of the Universe where it shows a huge rate of expansion (an era known as Inflation). |
Timeline for the Universe: From Today to the BeginningToday, we are bathed in the all pervasive 2.7 Kelvin Cosmic Microwave Background (CMB) that fills our Universe, a Universe composed of 31.5 % matter (normal matter and dark matter) and 68.5 % dark energy. When these facts and the homogeneity and isotropy of the CMB is combined with the Friedman models for the Universe, we have the tools to detemine how the Universe will evolve into the distant future and what the Universe was like when it was younger (and smaller) in the distant past. When we run the clock backward from today, we find that the Universe was hotter and denser in the past and we encounter almost unimaginably extreme conditions at early times. Despite this we have a reasonably good handle on the way things work because, in a sense, the Universe actually becomes simpler as we go back in time. To see why I say this, let's think about the ways in which things interact with each other in the Universe.
As far as we know, these 4 forces are the only ways in which objects in the Universe may affect each other. If we can understand how these four forces work, we would be unable to understand how everything in the Universe works. In the past, the situation becomes even simpler as the different forces become indistinguishable from each other until, eventually, we may wind up with only one force to consider. At this point we would have the Theory of Everything.
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Using what we know, we find that:
In the sections that follow, we discuss the evolution of the Universe in more detail. |
We consider each era in the evolution of the Universe in turn. The history is uncertain in the beginning, but after 10-11 seconds, we are on very solid ground as by this time conditions in the Universe are such that much of the relevant physics may be probed in Terrestrial laboratories. |
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A. 0 → 10-43 seconds, the Planck Era |
The Planck era when the four forces of the Universe -- the gravitational, electro-magnetic, strong (nuclear), and weak forces -- may have been unified. This is the era of quantum gravity -- the time when a theory which encompasses both quantum mechanics and gravity must be used. The size of the current Universe at this time is less than 10-50 centimeters.
B. 10-43 → 10-36 seconds, Era of the GUT and Beginning of Inflation |
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Era of the Grand Unified Theories (GUTs), which is after gravity separates from the other 3 forces (i.e., gravity becomes an interaction distinct from the electromagnetic, strong (nuclear) and weak (nuclear) forces). The GUT era ends after around 10-32 seconds have elapsed after the beginning of the Universe. At this time the Universe has a temperature of 1027-1028 Kelvin. Inflation begins at the end of the GUT stage -- inflation is driven by an, as of now, unknown repulsive force. We describe inflation in the next section. |
C. 10-36 → 1 second, Inflation, the Quark Epoch, and On to Element Production, Nucleosynthesis |
The strong (nuclear) force becomes a distinct interaction at the beginning of this stage and inflation stops at the beginning of this stage. During inflation, the Universe increases in size by a huge factor. The size of the current Universe is currently believed to have increased in size by arond a factor of 1050 during inflation. A region around the size of the Planck length (~ 4x10-33 cm) expands enough during inflation to encompass more than the currently observable Universe.
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This means that high-energy radiation can transform into particles if the radiation is energetic enough. Interestingly, the production always produces pairs of particles, one an ordinary particle and the other its anti-matter twin. In the inverse process, matter particles and their anti-matter twins annihilate and release energy (radiation)!
The matter particle and its anti-matter twin are nearly identical, the primary difference lying in their electrical properties. For example, an electron has a negative charge while its anti-matter twin, the anti-electron or positron, has a positive charge.
When the Universe is very hot, pair production of the fundamental particles quarks and leptons occurs.
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After inflation the Universe is dominated by a quark-gluon plasma. What are quarks and gluons? To the left is a schematic picture of what is known as a baryon. Baryons are a class of fundamental particles which includes protons and neutrons (see rigtht); protons and neutrons are composed of three quarks held together by the intermediary particles known as gluons. When the Universe is very hot and energetic, the quarks and gluons are free and are not tied up in baryons. |
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As the Universe expands it cools. After around around ~ 10-11 seconds hve elapsed, the temperature is around ~ 1015 Kelvin and the weak nuclear force and the electromagnetic force become distinct (a further symmetry breaking. We now have a universe dominated by the four known forces of the Universe, the gravitational force, the strong nuclear force, the weak nuclear force, and the electromagnetic force. From this time on, we are rather secure in our understanding of the Universe. The conditions in the Universe from here onward are attainable in Terrestrial laboratories and so amenable to detailed experimental study.
Around 10-6 seconds after the birth of the Universe, quarks and gluons combine to form baryons (neutrons and protons). This marks the end to the Quark Epoch. The production of baryons continues until the temperature drops below 1013 Kelvin where the Universe is no longer energetic enough to produce baryonic matter/anti-matter pairs. Unfortunately, there is no barrier to annihilations of matter/anti-matter pairs and the annihilation process continues quite efficiently. As noted earlier, we are lucky in that there was a slight asymmetry between matter and anti-mattter quarks and leptons which allows some matter baryons to survive the annihilation! The production of the matter baryons is referred to as baryogenesis while the existence of the slight excess of matter over anti-matter referred to as the Matter-Anti-Matter Asymmetry Problem.
Because electrons are less massive than baryons, the similar event for electrons and their anti-matter twins, the positrons, occurs later, around ~1 second after the birth of the Universe when the temperature has dropped below 1 billion Kelvin.
D. 1 second → 100 seconds, Nucleosynthesis and the Nuclear Epoch |
 
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 | In Big Bang Nucleosynthesis, only hydrogen, helium, lithium and the hydrogen isotope deuterium are produced in significant amounts (see the figure to the left). The amount of helium produced in the Big Bang is insensitive to the mass contained in baryons in the Universe, but the amount of deuterium is very sensitive to the mass in baryons in the Universe. Based on current measurements of the primordial abundances of hydrogen, helium, and deuterium, we infer that the amount of normal matter which can be contained in the Universe is 3-4 % of the critical density (the gray bar in the figure to the left)!After the finish of the era of nucleosynthesis, the Universe continues to expand and cool but no new chemical elements are created until stars form and die. |
E. 380,000 years → Formation of Cosmic Microwave Background Radiation |
The Universe becomes cool and rarefied enough for electrons and protons
to form neutral hydrogen atoms. This occurs around when the temperature of
the Universe has dropped to around 3,000 Kelvin (z ~ 1,100).
If the Universe was only normal
matter, the observed variations in the CMB are too small
to explain the observed structure in the Universe. This is reconciled
by the existence of Dark
Matter.
This means that the overdense regions could start
growing earlier in the evolution of the Universe without showing up in the
temperature fluctuations; the origin of the
seeds which eventually lead to the current structure in the Universe in fact
has its origins in Inflation.
Currently,
the largest structures in the Universe are consistent with the
imperfections seen in the CMB in this scenario.
If we see much larger structures, then there will be trouble.
Recent results
from the Webb telescope (see Current Events on
course homepage) finds massive young galaxies. It is not that the galaxies
are forming too fast, The problem is that these
galaxies are so massive that nearly every available normal atom must go into
galaxy formation.
Current models suggeset that at most around 10 % of the
atoms go into galaxy formation. This could be a problem for how we
believe galaxies form.
Quasars and some radio galaxies become detectable
The Big Rip is the ultimate fate of the Universe
if the dark energy density increases
as the Universe expands.
Also, around the time one trillion years, the Universe will have expanded so
much that the peak of the CMB will be in the meters range and diluted by a
factor of 1020 and likely will be unobservable
At this time the temperature
of the Universe is around one-trillionth of a Kelvin (10-13
Kelvin) certainly making the CMB undetectable.
The largest supermassive black holes found in the cores of galaxies are about
100 billion times the mass of the Sun. Such supermassive black holes would take about
2x10100 years to evaporate!
Because of the Uncertainty Principle,
spontaneous production of matter/anti-matter twins continuously occurs
in the Universe, even in vacuum(Top left panel)!
These particles are not directly detectable
because they disappear before one can get a handle on them. These pairs are
known as Virtual Pairs. Near a
Black Hole, something interesting can happen. If a member of the pair
wanders too close to the Event Horizon, it may
be trapped inside the Black Hole allowing its twin
to escape and become a real particle
(Bottom left panel)! You cannot get something for nothing, and
so what is going on is that a little bit of the mass of the black hole is
stolen to make the escaping particle real. This causes the
black hole to lose mass and thus to
evaporate.
For certain models of dark energy, the rate
at which the Universe expands increases
leading to an exponentially growing rate for expansion for
the Universe. The resulting rapid expansion is so fast that it
will tear apart all structure in the Universe after a finite amount
of time has elapsed.
For the calculation shown to the left, the
The Big Rip occurs after ~22 billion
years. Although a long time into the future,
The Big Rip happens
on a much shorter timescale than the effects discussed above.
We expect The Big Rip
to take place in models where the
density of dark energy increases
as the Universe
expands.
Hydrogen gas is not an
efficient absorber of light and so the Universe becomes transparent.
To understand look at the figure to the left which shows a
tube that contains hydrogen gas and the spectrum of the tube at the
lower right. An electric
current is run through the tube exciting the hydrogen atoms causing
them to radiate. Notice that the hydrogen atoms produce only certain colors
(wavelengths) of light. Absorption being the inverse of emission means that
the same hydrogen atoms will absorb only the same colors of light which it
emits. Most of the light impinging on hydrogen gas is unaffected by
the presence of the gas!
Before the Era of Recombination
, the Universe was
opaque to radiation, that is, the Universe was "foggy" in that
light could not travel very far which meant
that our view of the Universe was limited. After
Recombination, the Universe cleared up, it became transparent
and our view of the Universe was unimpeded.
What are the consequences of this?
After
Recombination, we can get
unimpeded view of the distant Universe. So, as the Universe gets older,
we can see further and further into the past. However, we cannot see
forever because at some point the Universe becomes opaque; we can only see
up to Recombination, that is, we can see the
Universe as it was 378,000 years after the Big Bang.
This is what we see when we observe
the Cosmic Microwave Background Radiation, the CMB.
This ia analogous to the situation we encounter when we see
the light from a cloud in the sky. We see the clear space between the
cloud and us, but we cannot see into the dense, opaque part of the cloud.
Temperature Variations in the Cosmic Microwave Background
F. 106 → 109 years, Stars, Galaxies,
and Large-Scale Structure Formation
Protogalaxies begin to form. This may be annoying since
the clumps of matter and the
clumps of the clumps of overdense matter will distort the CMB.
The reason is that if these clumps are formed by the attraction of
gravity, then such large overdensities take a fairly long time to form
and will be visible during Recombination, the time of CMB formation.
G. 1 billion years → 3 billion years
H. 3 billion years → 13.7 billion years and Dark
Energy Era
I. 13.7 billion years →
End
The ultimate fate of the Universe falls into three
categories: (i) the Big Rip; (ii) Continual expansion to
Big Freeze; and
(iii) the Big Crunch, depending upon whether
Ω
is > 1 → the Big Crunch, Ω < 1 →
Continual
expansion, or Ω = 1 → stops after an infinite
amount of time has elapsed if Ωm = 1 or continual
expansion if ΩΛ = 1.
After 100 trillion years (1014 years)
or so, star formation ceases, leaving only
the lowest mass stars and the remnants of more massive stars,
black holes,
neutron stars, and white dwarfs. After 1017 years, the stellar
remnants will cool to the point where they become invisible. Also, by this
time normal stars (powered by nuclear fusion) will have burned out.
Furthermore, by this time, stellar evolution will have
turned much of the hydrogen into helium and heavier elements and we will no
longer see the
Big Bang signature chemical composition of 90 % hydrogen and 10 %
helium, the helium abundance may approach 60 % by this time.
