History of the Universe
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).

The Big Bang theory for the Universe is in good shape. The Universe is expanding, is bathed in a low temperature sea of background radiation (the CMB), and has a chemical composition (primrily hydrogen and helium) which can be understood in terms of the Big Bang theory. There are issues for sure, but the basic Big Bang theory is secure.

I. Overview of the History of the Universe

Timeline for the Universe: From Today to the Beginning

Today, 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, there are only four ways (forces) in which objects in the Universe interact:

Massive objects interact though the gravitational force
Charged objects interact through the electromagnetic force
sub-atomic particles interact through the strong (nuclear) force
sub-atomic particles interact through another force known as the weak (nuclear force)

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.

Using what we know, we find that:

at the earliest times in the Universe, the Universe is dominated by radiation.
As the Universe expands and cools, eventually matter takes over and dominates the evolution of the Universe. The crossover takes place around 50,000 years after the Big Bang.
As the Universe continues to expand and cool, Dark Energy takes over and dominates the evolution of the Universe. This crossover occured 5 to 6 billion years ago.
We are still in the Dark Energy era. At early times, Dark Energy is less important than matter and radiation and so may be ignored in the discussion of the very Early Universe. This statement carries a caveat, however, in that some repulsive force did act early on in the evolution of the Universe during what is called the Era of Inflation which took place at the end of the so-called GUT (Grand Unified Theory) era.
In the sections that follow, we discuss the evolution of the Universe in more detail.

II. Timeline of the Universe

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.

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

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.

Pair Production and Annihilation
After inflation, the Universe continues to expand. The Universe is still exceedingly hot, T > 10¹⁵ Kelvin and very energetic. The high-energy radiation is very energetic. This has important consequences. In his Special Theory of Relativity, Einstein showed that there is an equivalence between mass and energy, E = mc² .

Equivalence of Mass and Energy,

and the conservation of mass and energy in the Universe.

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.

Something odd happens during this time, however, because for an unknown reason more matter quarks and leptons are produced than anti-matter quarks and anti-matter leptons, even though they should always be created in matter/anti-matter pairs. The slight excess of matter over anti-matter leads to the outcome that there is matter in our Universe today.

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.

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

Big Bang Nucleosynthesis
The final symmetry breaking has already occured by this time (at time 10^-11seconds); quarks and gluons have combined to form protons; protons and electrons combined to form neutrons; and the stage is set for the formation of the elements in a process referred to as nucleosynthesis. In nucleosynthesis, hydrogen, helium, a little lithium and the hydrogen isotope deuterium are produced in the few minutes after 1 second has elapsed into evolution of the Universe. The top panel shows how helium nuclei are produced from four hydrogen nuclei (protons). The middle panel shows how the larger, more massive element Carbon may be produced. This chain almost never completes in the early Universe because the intermediate step where Beryllium forms leads to a bottleneck. Beryllium decays (breaks apart into helium nuclei) after 6.710^-17 seconds, way too fast for it capture another helium nucleus.

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

Recombination and the CMB
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).

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

Cosmic Microwave Background (CMB)--Planck observations showing temperature fluctuations about the average temperature 2.73 K of about ± 0.0001 K. The above picture was made by subtracting the average temperature 2.73 K, and the dipole temperature anisotropy from the complete image to show the fluctuations.

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.
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.

G. 1 billion years → 3 billion years

Quasars and some radio galaxies become detectable

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 Ω_m = 1 or continual expansion if Ω_Λ = 1.
The Big Rip is the ultimate fate of the Universe if the dark energy density increases as the Universe expands.

Current data suggests that the Universe will expand forever resulting in the Big Freeze or a Big Rip

Today, after 13.7 billion years (1.37x10¹⁰ years), the Universe consists of dark energy, radiation, and matter clustered into stars, groups of stars, galaxies, clusters of galaxies, clusters of clusters of galaxies.

Stars:
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.

Galaxies:

The Milky Way galaxy is currently a member of the Local Group of galaxies; it is expected that within 1 trillion years (10¹² years), collisions between members of the Local Group will cause the formation of a large galaxy and the ejection of other galaxies (see video that shows a simulation of a collission between two galaxies), the Local Group evaporates (see Java applet illustrating 3-body problem). In fact, within only 3-5 billion years, we will interact and merge with our neighbor galaxy, M31 Andromeda. Similar evolution occurs in other large galaxy clusters.
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.

As the Universe continues to expand, galaxies continue to evaporate from clusters with an analogous process where stars evaporate from galaxies occurs. Galaxies are thought to eventually lose 90 % of their mass, the remaining mass settling into massive black holes at their centers. The evaporation is essentially complete after 10¹⁹ years. The stars then eventually evaporate from the former galaxy clusters after 10²³ years leading to the disintegration of galaxy clusters.
At this time the temperature of the Universe is around one-trillionth of a Kelvin (10^-13 Kelvin) certainly making the CMB undetectable.

Comment--In around 10 trillion years (10¹³ years), the increasing expansion rate of the Universe will redshift the light from galaxies just outside the local cluster of cluster of galaxies to about z ~ 10⁵³ and everything simply winks out of sight because of the monstrous redshift; galaxies and all other objects will no longer be visible. Galaxies will no longer be visible within our Horizon.

After the galaxies and clusters of galaxies evaporate, the Universe is a collection of black holes, black dwarfs (formerly white dwarfs), neutron stars, some gas and dust, dark energy, and dark matter

What happens next is a little uncertain. As the Universe continues to expand, matter particles start to decay. Free neutrons decay within 11 minutes and even neutrons in nuclei are known to decay. Furthermore, if GUT theories are correct, then we also expect protons to decay before 10⁴¹ years elapse. Stellar remnants such as neutron stars and white dwarfs are expected to disappear dissolving in a sea of fundamental particles.

So, somewhere after 10⁴¹ years have elapsed, black holes will dominate the Universe with light, leptons and dark energy still in the mix.
Eventually, however, it may even be possible that black holes will evaporate (through a hypothesized process known as Hawking Radiation) . The evaporation process is excruciatingly slow, an estimate for the evaporation time of a black hole is

t_evap = 2.1x10⁶⁷(M_bh/Mass of Sun)³ years
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!

Hawking Radiation
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.

The Universe, now composed of dark energy, leptons, and light, will continue to expand and cool leading to the Big Freeze.

Big Rip
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.

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