Lesson 3 – Where did it all begin? An introduction to the beginning of time and the Universe.

Lesson learning outcomes:

By the end of this lesson, you should be able to:

1.   Describe the history of the early Universe and the Big Bang, including the annihilation of matter and antimatter, the creation of particles, rapid inflation, and the period where radiation could freely travel in the Universe.

2.   Outline the evidence that supports the Big Bang theory.

3.   Explain the cosmic microwave background and what it means.

4.   Outline the different possible shapes of the Universe.

3.1 The Big Bang

We begin our journey at the beginning of time.

Figure from the Cosmic Perspective.

Edwin Hubble (who is the namesake for the Hubble Space Telescope) discovered that the Universe was expanding, and like all incredible scientific discoveries, shook the scientific community. Hubble’s discovery suggests that if the Universe is currently expanding, then it must have been smaller in the past. If we trace this logic continuously further in the past, we eventually arrive at the conclusion that the Universe started as a single point, leading to the idea of a Big Bang.

We touched on the concept of the observable Universe previously. We can apply the same thinking here. If we try to look at increasingly distant objects (and therefore further into the past) to “observe” the Big Bang, we quickly realise that we can’t do this for a few reasons. Firstly, we are trying to look into a time before stars and galaxies existed (therefore there is no light leaving any objects!). Secondly, the Universe is filled with radiation that appears to be the remnant heat of the Big Bang that can be traced back to when the Universe was 380,000 years. This is when the Universe first became “transparent to light”, which basically means the time at which light permeated the Universe. So how do we see the Universe before this time? Well, we need computer modelling to help us.

3.2 The Big Bang Theory

The Big Bang theory starts off with the notion that everything that we see today (galaxies, stars, etc.) and the properties of our Universe (temperature, density, etc.) began as a hot, dense collection of matter and radiation. You can think of the expansion of the Universe much in the same way that we would treat a gas that undergoes compression and expansion (as we compress the gas, the temperature increases). To work out how hot the Universe was, we need to model these conditions. The modelling is shown in the figure. These conditions may seem pretty extreme, but a significant range of temperature on the y-axis is not unreasonable in the context of astronomical objects, as you will learn later on in the course.

Figure 22.1 from The Cosmic Perspective. This graph shows how the temperature of the universe has cooled with

time. Notice that both axis scales use powers of 10, so although most of the graph shows temperatures during the first

second of the Big Bang, the far right extends to the present (14 billion years ≈ 4 × 1017 s). (The small kinks in the

graph at temperatures of 1012 K and 109 K correspond to the moments when the universe became too cold to produce

new protons and new electrons, respectively.)

3.3 The creation of particles

We are now going to discuss how particles were actually created.

You might be wondering where all the stuff in the Universe came from? Surely, if temperatures were so ridiculously high during the first few seconds of the Big Bang, there would be no matter like the matter we see today. And yes, that is true. We know that energy cannot be created nor destroyed, but it can change form; matter and energy can transform between these two states according to Einstein’s equation: E = mc^2. We don’t see reactions happening like this in nature very often anymore, but they’re a regular occurrence in particle accelerators (you’ve probably heard of the largest accelerator  in operation,  the  Large  Hadron  Collider  (LHC), operated  by the  European  Organisation for  Nuclear Research, also called  by its  French title  Conseil européen pour la recherche nucléaire, or CERN). Such particle accelerators do as their name suggests: they use electric fields to accelerate and magnetic fields to steer two beams of charged particles, typically protons, to extremely high speeds (near the speed of light) and collide the beams head-on. A lot of interesting things happen when the two beams collide! If you’re interested, you can take a virtual tour of the LHC here.

Notably, when high-energy particles collide, we can produce new particles and annihilate others. A typical example of this is production and annihilation of the electron-antielectron pair (or electron-positron pair). When two gamma ray photons collide (recall that gamma rays have extremely high energies), an electron (matter) and a positron (antimatter) are produced. There are similar reactions for other particle-antiparticle pairs, such as protons and antiprotons, or neutrons and antineutrons.

These reactions were occurring rapidly during the first moments of the Universe, so it might seem an impossible task to actually model the early Universe. Surprisingly, it is not completely impossible given our current technological and engineering capabilities (i.e., to build instrumentation so that we can perform experiments to aid our understanding) and our knowledge of the laws of physics. Even though we can’t observe the earliest moments of the Universe, we can reproduce many of the conditions in laboratories. We have observed the behaviour of matter and energy at temperatures as high as those that existed in the first one ten-billionth (10- 10 ) seconds after the Big Bang. Prior to that, although increasingly more difficult to understand, physicists have ideas about what might have happened. And yes, while the timescales we are talking about are incredibly short, the Universe changed a lot in that first second after the Big Bang!

3.4 Fundamental forces

Forces govern everything that occurs in the Universe. These forces are gravity, electromagnetism, the strong force, and the weak force.

Out of these 4 forces, gravity operates over the largest distances. We have looked at the gravitational force previously. The electromagnetic force is actually far stronger than gravity and depends on the charges of particles instead of mass. It is the dominant force between particles in atoms and molecules, governs chemical and biological reactions, but since both  positive  and  negative  charges  exist  (and  most  large  astronomical  objects  are  neutrally  charged),  the electromagnetic force does not play a significant role over large distances. The strong and weak forces, again, only operate over short distances (atomic nuclei). The strong force binds protons and neutrons together in the nucleus, whereas the weak force plays a role in nuclear reactions (fission and fusion).

Models predict that during the earliest moments of the Universe, when temperatures were incredibly high, the forces described above were perhaps not as distinct as they are today. It is here where we start dipping our toes into the waters of grand unified theories (GUTs) force, which is a combination of the strong, weak, and electromagnetic forces. We won’t be touching on this in great detail, but it is also here where theories such as supersymmetry, superstrings, the theory of everything, etc., become relevant.

Figure 22.3 from The Cosmic Perspective.

What is the universe expanding into? Watchthis TED-EdAnimation!

3.5 A timeline for the early life of the Universe

Let’s have a look at what we know about the timeline of the early Universe. As astrophysicists, we can’t help but categorise things, so we have split the history of the Universe up into eras.

The Planck Era – 10-43 s after the Big Bang

This era is named after the physicist Max Planck. Quantum mechanics tells us that there should have been significant energy fluctuations during this period, and since mass and energy are intricately linked, the gravitational field would have also fluctuated significantly. Physicists are yet to fully understand what was happening during this time period , given that we cannot currently link quantum mechanics and general relativity (the small with the very big!). Eventually, temperatures dropped low enough such that gravity became distinct from the other forces.

GUT era – 10-38 s after the Big Bang

This era is named for the grand unified theories and came to an end when the GUT force (a combination of the strong, weak, and electromagnetic forces) split into the strong and electroweak forces. We know a little bit more about this era compared to the Planck era, but not much. It is predicted that the splitting of the GUT force released an enormous amount of energy, causing the sudden expansion of the Universe called inflation.

Electroweak era – 10- 10 s after the Big Bang

During this era, the electromagnetic and weak forces were still combined and called the electroweak force, and this era marks the simultaneous existence of 3 forces: electroweak, strong, and gravitational. The temperature eventually cooled to a mere 1015  K during this era, but this is low enough for the electromagnetic and weak forces to split.

The electroweak force was a theory developed in the 1970’s that predicted the existence of weak bosons (W and Z bosons) at temperatures above 1015 K. The significance of this is that particle accelerators have been able to reproduce the energies equivalent to these high temperatures and confirmed the production of these weak bosons!

Particle era – 1 millisecond after the Big Bang

During the very hot periods of the Universe’s early history, matter and energy were spontaneously exchanging forms and some very exotic particles were constantly being produced and annihilated. Eventually, when temperatures cooled significantly (to 1012 K), this exchange stopped, and photons became the dominant form of energy in the Universe.

Quarks were one of the exotic particles being produced at the beginning of the particle era and prior, but by the end of the particle era, quarks had combined to form protons and neutrons . Despite the fact that both matter and antimatter existed during this time, there must have been more matter than antimatter, as proven by the fact that matter exists in the Universe today! By how much did matter win over antimatter? Well, we can estimate this by comparing the number of protons and photons in the Universe today (there hasn’t been annihilation since the particle era!). Photons outnumber protons by roughly one billion to one, meaning that for every billion annihilations of protons and antiprotons, there was one extra proton.

Era of nucleosynthesis – 5 minutes after the Big Bang

Nucleosynthesis is the technical term for the creation (synthesis) of nuclei. Lots of nuclei (elements) were forming during this time, including heavy nuclei, but high energy gamma rays broke the heavier, more complex nuclei apart. The density in the expanding Universe dropped along with the temperature, and fusion stopped, marking the end of the era of nucleosynthesis. By the end of this era, the mass content of the Universe was 75% hydrogen, 25% helium, and trace amounts of deuterium and lithium. This is essentially the composition of the Universe today (except some more elements exist thanks to stars: we will learn about this later on!).

Era of nuclei – 380,000 years after the Big Bang

Even though fusion had ceased, the hydrogen, helium, and trace elements existed as a plasma (recall the states of matter that we learned about previously). The atoms were fully ionised so there also existed a sea of electrons, along with high energy photons that ensured the atoms remained ionised.

The era of nuclei ended when temperatures reached about half the surface temperature of the Sun today (3,000 K). It was then that hydrogen, helium, and the other elements could capture electrons and retain them. Photons could travel freely across the Universe. The photons that did this make up the cosmic microwave background that we spoke about earlier.

Eras of atoms and galaxies – to present

Thankfully, the density in the Universe was not uniform (equal in all places), and there were places where the density of matter was slightly higher than surrounding areas. Gravity meant that matter could continuously clump in the areas of higher density, and this was the start of protogalactic clouds (clouds of matter than eventually form galaxies). Stars form in the clouds first, which make up the galaxies they become.

Around 1 billion years after the Big Bang marks the formation of galaxies, and the generations of stars and galaxies to come. We will learn more about this in lessons to come.

3.6 Evidence for the Big Bang

The Big Bang theory made two major (and testable) predictions:

1)   Radiation produced at the end of the era of nuclei should be observed across the entirety of the Universe, and

2)   Around 25% of the initial hydrogen created in the Big Bang should have formed into helium by the end of the era of nucleosynthesis.

Let’s look at the first prediction: the presence of a cosmic microwave background radiation.

“Boys, we’ve been scooped. ”

The story of the discovery of the cosmic microwave background is really quite hilarious . The discovery, announced in 1965, was officially done by two physicists working at Bell Laboratories in New Jersey: Arno Penzias and Robert Wilson. At the time, they were actually using a radio telescope originally build for satellite communication to look for neutral hydrogen (recall where radio waves fall in the electromagnetic spectrum!), however they kept detecting unexpected noise in their measurements. They suspected most things, including poorly insulated wiring and bird droppings on the antenna, but they did not expect to have stumbled upon one of the most significant pieces of evidence for the Big Bang. They contacted another colleague in Princeton, Robert Dicke, who was also searching for the permeating radiation. Upon hearing the news of Penzias’ and Wilson’s discovery, apparently Dicke turned to his fellow colleagues and said, “Well boys, we’ve been scooped.” Penzias and Wilson received the Nobel Prize in Physics in 1978.

What should the cosmic microwave background look like? Recall from our discussions earlier that heat radiated from a single source should do so according to the thermal radiation spectrum. Therefore, it was predicted that the cosmic microwave background should also fit the thermal radiation spectrum given it is heat radiated from the Universe. It was predicted that the radiation could travel freely in the Universe when the temperature was around 3,000 K, meaning a peak in the intensity of a thermal spectrum at around 1,000 nanometres (nm). The Universe has expanded by a factor of around 1,000 since then, stretching the photons by a factor of around 1,000 also. The 1,000 nm then becomes 1,000,000 nm, or 1 mm. This corresponds to a temperature that is a few degrees K above absolute zero.

NASA’s satellite the Cosmic Background Explorer (COBE) was launched in the early 1990’s and was designed to gather more information about the cosmic microwave background. The results: the cosmic microwave background does indeed fit perfectly to a thermal radiation spectrum.

Figure 22.8 from The Cosmic Perspective. This graph shows the spectrum of the cosmic microwave background

recorded by NASA’s COBE satellite. A theoretically calculated thermal radiation spectrum (smooth curve) for a

temperature of 2.73 K perfectly fits the data (dots).

You might be wondering where the speckled maps of the Universe come from if the cosmic microwave background fits a thermal spectrum so perfectly. Well, COBE, the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite have mapped the temperature of the Universe in all directions to incredible precision and accuracy. The temperature is essentially uniform, but there are tiny fluctuations that equate to a few parts in 100,000. You might remember before that fluctuations in density were needed to be the seeds of galaxy and star formation, so these

temperature fluctuations also prove that there were indeed density fluctuations in the early Universe! Let’s now move on to the next prediction: the abundances of elements.

You will learn later on that stars are comprised predominantly of hydrogen, and fuse hydrogen atoms together to create heavier elements, such as helium. This continues, with heavier elements fusing to produce even heavier elements, and this is how elements heavier than hydrogen are produced in the Universe. We predict then that the early Universe was predominantly hydrogen and helium, and as the Universe ages and stars live and die, the amounts of heavier elements present increases. As a reference point, we can take the Milky Way Galaxy, which has a helium content of around 28%. If we calculate the total amount of hydrogen that would have converted into helium in the cores of stars, we realise that this only accounts for a tiny fraction of the total observed helium. Therefore, the majority must have come from the Big Bang. In fact, no galaxy observed has a helium content less than 25%.

Scientists use the temperature of the Universe now and the number of protons in the Universe to calculate the predicted amount of helium at the end of the era of nucleosynthesis, and the magic number is 25%.

Where does this magic number of 25% come from? It all comes down to the number of neutrons and protons in the Universe.

Neutrons are slightly heavier (more massive) than protons, meaning that energy is required to convert protons to neutrons (think of E = mc^2). During the early Universe when temperatures were high (> 10 11  K) this extra energy demand was not an issue, and the proton to neutron ratio was roughly 1:1. As the temperature dropped, the conversion started to favour protons. During the era of nucleosynthesis, protons and neutrons could form deuterium (“heavy” hydrogen, or hydrogen-2, has one neutron and one proton in the nucleus). Deuterium undergoes a two-step reaction process to form helium-4. These helium atoms were still destroyed when temperatures were sufficiently high for gamma rays to exist, but eventually temperatures dropped, and helium was not as readily destroyed. Calculations show that the proton:neutron should have been around 7:1. This gives the 25% value as shown in the figure below. A similar reaction process forms the traces of other heavier elements (e.g., lithium).

Figure 22.11 from The Cosmic Perspective. Calculations show that protons outnumbered neutrons 7 to 1, which is the

same as 14 to 2, during the era of nucleosynthesis. The result was 12 hydrogen nuclei (individual protons) for each

helium nucleus. Therefore, the predicted hydrogen-to-helium mass ratio is 12 to 4, which is the same as 75% to 25%,

in agreement with the observed abundance of helium.

3.7 Inflation

Recall from before that we mentioned that the Universe underwent a rapid period of expansion called inflation. Does the concept of inflation fit what we observe in the Universe? (Inflation of a factor of 10^30 is expected to have occurred in 10^-36 s .) Interestingly, it does! We will now look at them in more detail.

Before we do though, are you wondering if this violates the theory that nothing can travel faster than the speed of light? Well, if matter were travelling across space at the rate of inflation, then it would violate the theory. However, remember that inflation is the expansion of space itself, and that matter did not move through space as a result of inflation: the space between objects grew.

We are going to look at the 3 main observations of the current Universe that inflation seems to explain.

1)   We know that density fluctuations lead to the formation of stars and galaxies, but where did these density fluctuations come from?

2)   Why is the large-scale Universe nearly uniform?

3)   Observations of the large-scale Universe show that its geometry appears flat, but why is this the case?

Density fluctuations

The leading theory (that is supported by very complicated, extensive calculations!) is that energy fields (such as the electric and magnetic, described earlier) on the quantum scale fluctuate randomly. This is also supported by a set of principles  in  quantum  mechanics  called  Heisenberg’s  Uncertainty  Principles .  The  Uncertainty  Principle  we  are considering here is the energy-time principle, which states that we cannot accurately measure the energy of a particle and when it had that energy. When the Universe expanded, these quantum fluctuations also expanded to very large scales and became the seeds of irregularities in density that were needed to start galaxy and star formation.

The large-scale Universe

How do two completely separate regions in space (millions of light years apart) have exactly the same properties (e.g., temperature and density)? This is the equivalent of two people on opposite sides of the Earth wearing the *exact* same outfit, down to the brand, colour choice, at the same time without coordinating with each other first.

If the Universe went through a period of inflation, this would solve the issue because the two regions would have been at the same temperature and density, and in the same region of space, before inflation occurred.

A flat Universe

The presence of matter curves spacetime. Einstein’s general theory of relativity tells us this. You’ve probably seen the experiments where an object is placed on stretched material. We can’t visualise spacetime in all of its dimensions, but we know it is there due to the way that light interacts with it.

Figure: Massive objects cause space-time to curve, much like a heavy ball will create a well in a stretched-out piece of

fabric. (Credit: NASA/GSFC/J.Friedlander).

Spacetime varies locally around objects with mass, but the Universe itself has an overall shape. All of the possibilities for the shape of the Universe fall into 3 main categories: flat (critical), spherical (closed), or saddle (open).

Figure 22.14 from The Cosmic Perspective. The three possible categories of overall geometry for the universe. Keep

in mind that the real universe has these “shapes” in more dimensions than we can see.

The shape ultimately depends on the average density of matter and energy in the Universe. The Universe will be flat if the density of matter and energy is equal to the critical density (an arbitrary value that marks the divide between an open and closed Universe). If the Universe’s average density is less than the critical value, the shape will be open. If the average density is more than the critical value, the shape will be closed.

Since the Universe is so large, even if the overall shape is curved, the observable Universe will appear flat. Consider the following analogy: the surface of the Earth appears flat to us in our immediate vicinity (and in fact, as far as we can see) because we are so much smaller than the total size of the Earth. Note that there are ways to see the curvature of Earth, including in airplanes and where a large portion of the horizon is visible (such as the ocean).

Figure: Physics students from the University of Leicester have captured breathtaking images of the Earth's stratosphere using a high-altitude weather balloon. Credit: University of Leicester.

Thanks to surveys such as WMAP and Planck, physicists have been able to measure the temperature fluctuations and plot their results as a function of angular separation between two points in the sky. The physics behind the calculations are very complex, however the results match predictions remarkably well!