Early Universe Research Paper

Introduction

Before diving into a discussion about the early universe, it is helpful to look at the observable properties of our present universe first. Modern cosmology is based on the cosmological principle, which states that on a sufficiently large scale of about 200 Mpc, the distribution of matter in the universe is homogeneous (the same everywhere) and isotropic (looks the same in all directions). The Universe is considered to be uniform with the same average density and pressure independent of the location of the observer. Edwin Hubble and Milton Humason were the first to discover a relationship between the redshift and distance of galaxies in 1929. Their observations suggested that galaxies, aside from their local peculiar motion with respect to a cluster or supercluster, are moving away from

one another.

In addition, more distant galaxies appear to be receding faster than closer ones. The whole Universe is in a state of expansion, with every galaxy moving away from every other galaxy. This relation is summarized as the Hubble Law: z=H_0/c d. A key point that is arising from this relationship is that as we go back in time, galaxies must have been closer and closer together. If you go back far enough, the Universe must have been concentrated at a single point in space, which leads us to the topic of this paper: the exploration of the early Universe. In order to understand the processes and interactions taking place in the first 400,000 years after the Big Bang, we need the tools and predictions of high energy particle physics and cosmology. Figure 1 provides a timeline of the events and processes taking place in the early Universe and will be used as an orientation for the following discussion of the Standard Model of particle and the observable cosmic microwave

background radiation.

Figure 1: Timeline of the early Universe (Source: http://pariscosmo.in2p3.fr/en/content/early-universe)

The Standard Model of particle physics
Our current understanding of the early universe and its connection to particle physics is shown in figure 1. Even today, we can only probe the last stages of the early universe experimentally by Big Bang nucleosynthesis (BBN) and the observed cosmic microwave background (CMB). Particle accelerators have enabled us to replicate the conditions present when the Universe was about 10-10 seconds old. BBN and the CMB can explain the time between 10-2 < t(s) < 1013, which corresponds to energies between 10-10 and 10-2 GeV (Pralavorio, 2013). Our current best understanding of the interactions present in the microscopic world are summarized in the Standard Model of particle physics. This models provides the basis for exploring the conditions and processes present in the early universe. According to CERN (2016), “the Standard Model explains how the basic building blocks of matter interact, governed by four fundamental forces.” Thus we will discuss these four types of interactions before moving on to the Standard Model.
In nature, four types of interaction can be found. On a microscopic level, in atomic and nuclear physics, matter and radiation can be understood in terms of the strong, electromagnetic and weak force, whereas for more massive objects, the gravitational force is dominating. The four fundamental forces differ in strength and work over different ranges. The gravitational force has an infinite range, but is the weakest of the four. Electromagnetic interactions also have an unlimited range, but are many times stronger than gravity. Over a very short range, on a subatomic level, the weak and strong force are most effective (CERN, 2016). The gravitational force governs, for example, the motion of heavenly bodies, like the motion of the planets around the sun. In atoms, the electromagnetic force causes electrons to bind to nuclei, and light is “a particular vibration of electric and magnetic fields (an electromagnetic wave)” (Pralavorio, 2013). The strong interactions take place on an even smaller scale; they give rise to protons and neutrons being bound together in the nucleus. As the name indicates, the strong force is the strongest of the four, and enables the equally charged protons to overcome their mutual repulsion and stick together in the nucleus. The weak interactions occur in form of a β—decay, where a neutron transforms into a proton.
Three of the fundamental interactions result from the exchange of force-carrier particles, also called exchange particles, which are part of a broader group called “bosons”. They are shown in red in figure 2. Interacting matter particles transfer discrete amounts of energy and momentum by exchanging bosons with each other (CERN, 2016). Out of the five different bosons, the photon carries the electromagnetic force, the W+, W- and Z0 bosons are responsible for the electroweak force, and the gluon gives rise to the strong force. The Standard Model of particle physics explains the operation of electromagnetic, weak and strong interactions, their carrier particles, and how these forces act on all of the matter particles. In figure 2, the six quarks are shown in purple and the six leptons in green. These are the elementary particles that constitute the building blocks of matter. Particles made up of three quarks like the proton (two up quarks and one down) and the neutron (two down quarks and one up) are called baryons (Jones et al., 2015).
Our current understanding of particle physics and the Standard Model enables us to describe the early Universe just 10-44 seconds after the Big Bang. Only three minutes later, after the quarks underwent a phase transition to form hadrons, primordial nucleosynthesis begun. This important era in the cosmological evolution occurred when the expansion of the Universe caused the temperature to drop to a point where protons and neutrons were no longer free in the cosmic plasma, but neutrons and protons combined into slightly heavier nuclei. Within two minutes, besides hydrogen, mostly helium-4 and small amounts of deuterium, helium-3 and lithium were synthesized in the primordial plasma (Gorbunov et al., 2011). Big Bang nucleosynthesis is the earliest directly studied epoch so far. According to Gorbunov et al. (2011) general relativity and known microscopic physics, which is the physics of nuclei and weak interactions, is used to calculate the abundances of light elements. In addition, measurements of the primordial abundances have gotten more and more precise in the past decades. The good agreement between primordial nucleosynthesis and our abundance observations is a cornerstone of the theory of the early Universe. This two-minute era took place a few hundred second after the Big Bang before the expanding universe cooled down for a few hundred thousand of years.
The Cosmic Microwave Background Radiation
After the primordial nucleosynthesis phase, the Universe expanded and cooled for about 300,000 years before entering the post-nucleosynthesis era. The conditions in the Universe were just right for nuclei and electrons to combine to form neutral atoms, and the process of an electron getting bound into an atom, and as a result, the emission of a photon, is called recombination. As the Universe expanded, the temperature dropped to about 4500 K, which enabled neutral atoms to survive.
According to Gorbunov et al. (2011), at higher temperatures photons had high enough energy to overcome the binding energy for keeping electrons in atoms, and thus atoms were likely to be ionized. As the temperature, and therefore energy of the photons dropped, recombination became more favorable. This lead to the production and survival of neutral atoms and photons. The above reaction outlines these two processes. The recombination process decreased the density of free electrons, making interactions between free electrons and photons rare, and the Universe transparent to radiation. This time, approximately 370,000 years after the Big Bang is therefore called the time of last scattering, and marks the beginning of photons travelling freely through space. Figure 3 illustrates this process.

Figure 3: Illustration showing the active scattering of photons before recombination, and how the neutral gas that was formed during recombination is transparent to photons (Source: http://ircamera.as.arizona.edu/NatSci102/NatSci/lectures/eranuclei.htm).

Over the billions of intervening years, the Universe continued to expand and cool down. Due to the expansion of space, the wavelengths of the last-scattering photons have increased to approximately 1 mm, which corresponds to a temperature of about 2.7 K, or -270°C. The redshifted photons fill the Universe today, creating “a background glow that can be detected by far-infrared and radio telescopes” (ESA, 2016). So today, as we observe the cosmic microwave background radiation (CMB), we see the red-shifted photons at the time of last scattering about 400,000 years from the moment of the birth of the Universe.
In 1964, Arno Penzias and Robert Wilson were the first to discover the CMB. While calibrating a radio antenna used for telecommunications, they detected an unexplainable noise within the system that was the same anywhere on the sky. The two scientists were able to deduce that the signal was “as would be expected if the whole sky were a black-body source at a temperature of 3 K” (Jones et al., 2015). This was the first observational evidence for a hot big bang. Following the discovery, the Wilkinson Microwave Anisotropy Probe (WMAP) and the Plank satellite have created an all-sky map showing the temperature fluctuations in the CMB. The results of the Planck mission are shown in figure 4. On a large scale, the cosmic microwave background appears, just like the cosmological principle predicts, homogeneous and isotropic. Gorbunov et al. (2011) state that the high degree of CMB isotropy shows that the Universe highly homogeneous at recombination. On a smaller scale, however, the CMB is not perfectly smooth. Figure 4

Figure 4: Microwave background fluctuations as seen by the Plank satellite.
illustrates the intrinsic anisotropies, small scale variations of temperature, in different colors. In the early Universe, the relative density perturbations Δρ/ρ, also called primordial fluctuations, were on the order of 10-5, comparable to the fluctuations in the temperature. These perturbations are believed to have given rise to structures like stars galaxies and clusters of galaxies (Gorbunov et al., 2011).
Conclusion
In 2016 we have benefited greatly from the huge technological developments during the last century. We have giant particle accelerators and spacecrafts that probe everyday the two extreme realizations of physics: cosmology and particle physics. We have seen that the nucleosynthesis era is the earliest directly studied epoch so far. Precise abundance observations have made that possible. Before that era, cosmologists need the help of high energy particle physics to describe the conditions and processes present in the early Universe. The cosmic microwave background is another direct observation of the early Universe. When we observe the microwave background photons, we look back in time and see the red-shifted photons at the time of last scattering about 400,000 years from the moment of the birth of the Universe.
Our current models enable us to deduce consequences of particle physics in cosmology and of cosmology in particle physics, but there are still three major unanswered questions that have to be answered to improve and complete our current understanding of the early Universe. We still do not know what dark matter and dark energy are, and the presence of matter but not antimatter raises questions. Particle physicists are working hard to resolve these questions by building better and more advanced experiments. An example is the Large Hadron Collider at CERN. With this particle accelerator, scientists can experimentally produce energies up to 8 TeV, which is several orders of magnitude beyond what BBN can probe. Developments in particle physics will therefore directly benefit cosmology, and particle accelerators provide a unique setting for more systematics studies in the future.