Unraveling the Mystery of the Cosmos

Abstract

Cosmology is a discipline that describes the physical nature of the universe. This discipline encompasses the branch of physics, metaphysics, and astronomy. In general, the universe is believed to start from the Big Bang, followed almost instantaneously by the cosmic inflation approximately 13.8 billion years ago, and it is conjectured to end, very likely, with the Big Freeze. Cosmologists are interested on the origin, evolution, and the ultimate fate of the universe. This chapter brings readers to take a glimpse on the mystery of the universe. It gives a succinct elucidation on the growing journey of the universe—from its birth, its advancement to the present and future, to its demise. The technology used to detect cosmic sources, the theories that underpin the universe, and some of the equations used to estimate its properties are also briefly illustrated here.

1. Introduction

Imagine reposing yourself on a couch in your backyard after pandering your appetite with a palatable dinner. You raised your head and looked up onto the sky. It was a clear fine night, with the round white moon and twinkling stars illuminating the sky. A sudden surge of adrenaline rushed through your bloodstream, when the epiphany of how nature itself could create such a breath-taking scenery struck you. You immersed yourself into the beauty of nature. Then, all of a sudden, you realized that something looked different—one of the stars inclined toward the north was behaving in a peculiar fashion. It was glowing in bluish-white and was much brighter than any other stars, and, above all else, you could not recall it being there in the previous nights. You were baffled and started to sink yourself into deep contemplation. You were searching for an answer for its sudden emergence, but as a novice who knows next to nothing about cosmic elements in outer space, you failed to find a convincing one. Coincidentally, an astronomer, an astrophysicist and a cosmologist were staring at the sky, and they too noticed that bluish-white star. As aficionados in their fields, however, they could probably deduce at first sight that it was the remnants of a dying massive star, known as a supernova. Had you been equipped with the knowledge beyond the atmosphere of earth, you too would be able to draw the same conclusion as well.

Now, we know that astronomers, astrophysicists, and cosmologists study astronomy, astrophysics, and cosmology, respectively, and that these three disciplines deal with celestial elements and phenomena, but how different are they from each other? Whilst there are overlapping areas among these three fields and that they symbiotically assist each other, their differences are indeed rather distinctive and should not be confused. In general, both the astrophysics and cosmology disciplines are branches sheltered under the large astronomy umbrella.

Astronomy can be broadly defined as a discipline that studies celestial phenomena and the physical and chemical properties of cosmic bodies [1], such as planets, asteroids, comets, stars, nebulae, supernovae, galaxies, quasars, and black holes, just to name a few. It is considered as one of the oldest fields of natural science, in which its history could be dated back to as early as 5000 years ago, when the ancient Egyptians studied the stars with their naked eyes. One of the oldest artifacts of astronomical observation is the Nebra sky disk, which is believed to have been constructed in the 1600s BC [2]. As can be seen in Figure 1, this 30-cm disk, found in northern Europe, is engraved with images of the sun, crescent moon, and stars. Although the initial intention of this piece of art is still a mystery, there are a few interpretations, with some people believing it to be a sky map. Today, the positions of celestial objects, including those that are invisible to our naked eyes, can be mapped by astronomers using sophisticated instruments and telescopes, and they endeavor to accurately plot the positions and movements of these objects. Observations through modern telescopes and modeling through mathematical techniques also enable astronomers to better understand the celestial objects and their interactions with the surrounding environment. These techniques are the two main types of modern astronomy, that is, observational astronomy and theoretical astronomy. As their names imply, the distinction between these two types of astronomy is obvious—observational astronomy identifies the celestial bodies and phenomena via experimental inspection; whereas theoretical astronomy uses mathematics and physics to describe the observed objects and phenomena to tell their past, present, and future.

Figure 1.

The Nebra sky disk. Credit: Vincentz, 2022 (https://commons.wikimedia.org/wiki/File:Nebra_1b.jpg).

Strictly speaking, astrophysics could be perceived as a conglomeration of both observational and theoretical astronomy. This discipline is relatively new—it was only coined as such in the nineteenth century when the German physicist, Joseph Fraunhofer invented and used the spectroscope to study the spectra of cosmic sources. Astrophysicists attempt to learn about the characteristics (such as their chemical properties and energy density) of cosmic objects and phenomena and to study how these objects behave and have evolved over time by means of physics, chemistry, and mathematics. They apply the theories of relativity, quantum mechanics, particle and nuclear physics, thermodynamics, and other fields to better understand what they observe. For example, they might study how a protostar developed into a massive star and finally ended its life as a black hole or pulsar. Given its intricate complexities, nowadays, most works performed in astronomy involve astrophysics; thus, both the terms are used interchangeably today.

Cosmology, on the other hand, differs from astrophysics in that it studies the universe in its entirety rather than individual cosmic sources. Cosmologists endeavor to find answers for the universe’s origin, its expansion to its present state, and its ultimate fate. They also attempt to understand the laws that keep the universe in order and study its properties on a larger scale, including cosmic microwave background radiation (CMBR), dark energy, and dark matter. Cosmologists employ various theories in their work, including the theories of relativity, quantum field theory (QFT), electromagnetic theory, quantum chromodynamics (QCD) theory, quantum electrodynamics (QED) theory, string theory, and the M-theory. Some of the current topics that cosmologists are exploring include the multiverse concept, the theory of everything, the validity of the M-theory, and the possible fates of the universe (the Big Crunch, Big Rip, and Big Freeze). Cosmologists often work closely with astronomers and astrophysicists, as they are required to analyze the data obtained from cosmic observations.

This book focuses on the past, present, and future of the universe and presents some of the latest discoveries that have been puzzling both the public, in general, and scientists, in particular. The first chapter gives a succinct elucidation on the growing journey of the universe—from its violent birth to its current state and future demise. It also provides an overview of the forces that govern nature and the theories that are used to describe them. Additionally, the chapter discusses the technology used to detect cosmic sources that enable us to better understand the universe, including the cosmic microwave background radiation (CMBR) and the relevant theories used to calculate the universe’s properties, such as its size during different phase transitions.

2. The birth of the cosmos: The big bang theory

In cosmology, there are two popular postulates to explain the origin of the universe. They are the steady-state and the Big Bang models. In the steady-state model, cosmologists believe that matter is constantly created as the universe expands, resulting in a constant density of matter over time [3]; the Big Bang model, on the contrary, advocates the expansion of the universe from the singularity, where the space curvature, density, and temperature were infinite at a point in time, and this results in a progressive dilution of matter [4]. There was initially a comparable number of supporters for both models. The Big Bang model, however, has gradually gained more traction, overtaking the steady-state model, as further observational pursuits evince an increasingly number of evidence toward it.

One of the observations, which convinced astrophysicists about the validity of the Big Bang model, is the red shifts of galaxies based on the Doppler effect. According to the Doppler effect, the frequency of a moving object T is perceived to be higher when it is approaching a stationary observer R at velocity + u, and conversely, the observed frequency is lower when T is moving away from R at velocity -u. The Doppler effect can be approximated as [5],

$$f_R\approx f_T\left[1+\frac{\left(\pm u\right)}{c}\mathit{cos}\theta \right]$$E1

where c ≃ 3.0 × 10⁸ m/s is the speed of electromagnetic signal at free space, θ the angle of T relative to the direct line to R (i.e., θ = 0 when both T and R lie on the same plane), and f_R and f_T denote, respectively, the frequency of the signal perceived by R and that transmitted by T. The fact that the observed frequencies of galaxies shifting toward the lower frequency end of the electromagnetic spectrum indicates that they are receding from Earth. By assessing how the spectra of galaxies were being red-shifted in the 1920s, American astronomer, Edwin Hubble found that all galaxies were moving away from Earth and that the farther they were from Earth, the higher were their recession rates [6, 7]. This implies that the universe is indeed expanding, as predicted by Einstein’s original general theory of relativity. The observation leads to Hubble’s law, which concludes that the velocity of a distant galaxy v_R is directly proportional to its distance from earth d_g and is expressed as,

$$v_R=H_0{d}_g$$E2

where H₀ is the Hubble parameter at the present time and is written in the form of H₀ = 100 h kms⁻¹Mpc⁻¹, with the constant h parameterizing the uncertainty. Planck measurements of the cosmic microwave background (CMB) gives h ≃ 0.674 ± 0.005 [8, 9].

Another evidence that further corroborates the Big Bang model is the cosmic microwave background radiation (CMBR). Discovered serendipitously in 1965 by American astronomers, Arno Penzias and Robert Wilson [10], the CMBR is the remnant that survived from the early stage of the universe when it was only 375,000 years old [11]. It conceals a plethora of spatial and spectral information about the infant universe. At 375,000 years after the Big Bang, the CMBR possessed a black body temperature of 3000 K. Owing to the cosmic expansion, however, the temperature has dropped to 2.725 K at the present, and it peaks at the microwave region of the electromagnetic spectrum.

By observing the CMBR, astrophysicists found various puzzling phenomena. Firstly, they learned that the temperature of the CMBR is virtually uniform across the entire sky. Now, other than time dilation and length contraction, we learn from Einstein’s special theory of relativity that no information can travel faster than the speed of light c, as given in (3)–(5),

$$E=\frac{m{c}^2}{\sqrt{1-{\left(\frac{v}{c}\right)}^2}}$$E3

$$p=\frac{\mathit{mv}}{\sqrt{1-{\left(\frac{v}{c}\right)}^2}}$$E4

$$E^2={\left(\mathit{pc}\right)}^2+{\left(m{c}^2\right)}^2$$E5

where E is the energy of an object with mass m, momentum p, and velocity v. This is to say that two causally disconnected regions are unlikely to share the same temperature. Astrophysicists refer to this problem as the horizon problem.

The second puzzling phenomenon is the flatness problem. Measurements of the anisotropies in the CMBR show that the curvature constant k is nearly zero, which implies that the universe is practically flat. To astrophysicists, this finding was somewhat out of their expectation. Since Einstein’s general theory of relativity states that mass warps the fabric of space-time, the universe, which consists of mass, is naturally predicted to have a certain curvature. The curvature constant k is given in (6) as [12].

$$k={\left(\frac{\mathit{HR}}{c}\right)}^2\left(\frac{\epsilon }{{\epsilon }_c}-1\right)$$E6

where H is the Hubble parameter, R the radius of the universe, and ε the energy density. The critical density ε_c represents the energy density for which the geometry of the universe is flat, and it is given by [9].

$${\epsilon }_c=\frac{3H^2}{8\mathit{\pi G}}.$$E7

where G is Newton’s gravitational constant.

To form celestial objects such as stars and galaxies through gravitational effect, anisotropies must exist in the early universe. When CMBR was initially discovered, it appeared to be uniformly distributed in all directions. The launch of the Cosmic Background Explorer (COBE) satellite in 1989 probed more extensively into the CMBR, and it unveiled small random fluctuations of temperature throughout the sky [1]. In 2009, the European Space Agency (ESA) launched the Planck space telescope, which was capable of distinguishing temperature variations in the CMBR in the order of one part in a million [13]. Figure 2 illustrates the tiny fluctuations of the CMBR as seen by Planck. Although the observational results confirmed the prediction of anisotropies in CMBR, astrophysicists were uncertain as to how they could exist, and this becomes the third problem of CMBR.

Figure 2.

Anisotropies in the CMB. Credit: Imagtek, 2019 (https://commons.wikimedia.org/wiki/File:PlanckMCD256H.png).

In 1981, American cosmologist Alan Guth brought forward the idea of cosmic inflation, which can help to solve the problems of CMBR [14]. The theory was later fine-tuned independently by Andrei Linde and by Andreas Albrecht and Paul Steinhardt. According to the cosmic inflation theory, when the nascent universe cooled down, it was trapped in a false vacuum, which constituted high energy density. This positive energy mediated an exponential expansion of space based on Einstein’s general theory of relativity. The accelerating expansion, which was faster than the speed of light, was believed to have started at 10⁻³⁶ s and ended at 10⁻³³ s to 10⁻³² s after the Big Bang. Astrophysicists named the field and quanta, which drove the cosmic inflation, the inflaton. Friedmann modified Einstein’s equation of general relativity and was able to show that the radius of the universe R can be expressed as a function of time t, that is, [12],

$$R=R_0\mathit{exp}\left(\sqrt{\frac{2\Lambda }{3}}t\right)$$E8

where R₀ is the radius of the universe at t = 0, and Λ is the cosmological constant, which is computed to be 2.036 × 10⁻³⁵ s⁻² [15]. It can be easily seen that the size of the universe expanded 10²⁶ times in 10⁻³⁴ s [12].

The inflation solves the horizon problem because all points were causally in contact prior to it. The flatness problem can also be explained by the theory since the rapid expansion stretches the space curvature of the universe to almost flatness. During inflation, the quantum fluctuations in the density of matter were enlarged to astronomical scale. Hence, the anisotropies in the CMBR are also deemed sensical. Besides solving the three problems observed from the CMBR, the inflation theory also answers the monopole problem. This problem arose because physicists failed to find traces of topological defects such as magnetic monopoles. Based on the grand unified theories (GUTs), however, there should be a myriad of massive particles and topological defects when the symmetry of grand unification was broken. After the inflationary epoch, the density of these monopoles was diluted incredibly, rendering them extremely difficult to be detected.

In 1998, the Supernova Cosmology Project team led by American astrophysicist Saul Permutter [16] and the High-Z Supernova Search team led by American astrophysicist Adam Riess and Australian astrophysicist Brian Schmidt [17] measured the universe expansion using Type Ia supernovas as standard candles. To their astonishment, they found that the high-redshift supernovas were fainter than expected, an indication that the expansion rate has been accelerating. Astrophysicists attribute the acceleration to the repulsive force imposed by dark energy. Although dark energy is still an unknown form of energy to us, we have various explanations for it. For instance, some physicists hypothesized that it is a kind of dynamic field known as quintessence. The simplest possible explanation for dark energy, however, is that it is a quantum vacuum energy of the sort, which could be mathematically associated with the cosmological constant Λ in Einstein’s general theory of relativity in (9)

$$G_{\mathit{\mu \upsilon }}+\Lambda g_{\mathit{\mu \upsilon }}=\frac{8\mathit{\pi G}}{c^4}\left({T_{\mathit{\mu \upsilon }}}^{\mathit{mat}}+{T_{\mathit{\mu \upsilon }}}^{\mathit{vac}}\right)$$E9

where $G_{\mathit{\mu \upsilon }}=R_{\mathit{\mu \upsilon }}-\frac{1}{2}R{g}_{\mathit{\mu \upsilon }}$is the Einstein’s tensor, which depicts the space curvature; $R_{\mathit{\mu \upsilon }}$ the Ricci curvature tensor; R the scale curvature; $g_{\mathit{\mu \upsilon }}$ the metric tensor, which constitutes the properties of every point within the space-time fabric; $\frac{8\mathit{\pi G}}{c^4}$ and G denote, respectively, Einstein’s and Newton’s gravitational constant; and ${T_{\mathit{\mu \upsilon }}}^{\mathit{mat}}$ and ${T_{\mathit{\mu \upsilon }}}^{\mathit{vac}}={\rho }_{\mathit{vac}}{g}_{\mathit{\mu \upsilon }}$ are, respectively, the energy-momentum tensor, which represents the local energy, momentum, and stress within the space-time fabric for ordinary matter and vacuum. The left-hand side (LHS) of (9) describes how matter-energy warps space-time, whereas the right-hand side (RHS) of the equation describes the propagation of matter-energy through space-time. Upon correlating the LHS and RHS of (9), one can find that vacuum energy and Λ are identical when the vacuum energy density ${\rho }_{\mathit{vac}}$ (is)

$${\rho }_{\mathit{vac}}=\frac{\Lambda c^4}{8\mathit{\pi G}}.$$E10

It is worthwhile noting that one primary outstanding issue, which stems from this relationship, is that the theoretical value of Λ computed from QFT mismatches the value obtained from the study of the Type Ia supernovas, with the former deviating from the latter by more than the order of 120 [18]. This issue is commonly known as the cosmological constant problem, and it is still up in the air, awaiting an answer today.

3. The evolution of the cosmos

The current standard model used to explain the universe is known as the Lambda Cold Dark Matter or, in short, Lambda-CDM or ΛCDM model. As implied by its name, the universe is regarded as comprising three major components – viz., the dark energy (denoted by the cosmological constant Λ), the cold dark matter, and the ordinary baryonic matter. The cold dark matter is an invisible matter that interacts weakly with the ordinary baryonic matter. Astronomical observations and cosmological theory suggest that the universe is made up of 69% of dark energy, 25% of dark matter, 5% of “ordinary” atomic matter, and 1% of other observable components. Among the remaining 1% components, neutrinos, CMBR, and black holes constitute, respectively, 0.1%, 0.01%, 0.005% [19]. The breakdown of the composition of the universe is graphically summarized in Figure 3.

Figure 3.

The components that compose the universe.

According to the ΛCDM model, the Big Bang commenced when the universe expanded from the singularity. Based on the timeline after the Big Bang, the evolution of the universe can be broadly classified into the Planck (0 to 10⁻⁴³ s), grand unification (10⁻⁴³ to 10⁻³⁶ s), inflationary (10⁻³⁶ to 10⁻³² s), quark (10⁻¹² to 10⁻⁵ s), hadron (10⁻⁵ to 1 s), lepton (1 to 10 s), nucleosynthesis (10 to 1000 s), recombination (375,000 years), dark age (375,000 to 200 million years), star and galaxy formation (200 million to 1 billion years), solar system formation (9.2 billion to 9.3 billion years), Stelliferous (10⁶ to 10¹⁴ years), degenerate (10¹⁵ to 10³⁹ years), black hole (10⁴⁰ to 10¹⁰⁰ years, assuming that proton decay occurs), and dark (10¹⁰⁰ years onwards, assuming that proton decay occurs) eras [20]. A summary of the evolution of the universe from the Big Bang until the present is given in Figure 4.

Figure 4.

Timeline of the universe. Credit: TheAstronomyBum, 2014. (https://commons.wikimedia.org/wiki/File:The_History_of_the_Universe.jpg).

In the Planck era, the size of the universe is merely about 10⁻³⁵ m, and it is dominated by the quantum effect of gravity. When it entered the grand unification era, gravitational force started to separate from the other three forces. During the inflationary epoch, the size of the universe underwent an exceedingly rapid expansion in a very brief period of time. Since then, both the temperature and density of the universe have been dropping. The strong nuclear force separated out from the electroweak force during the cosmic inflation. The weak nuclear force subsequently separated from the electromagnetic force in the quark era, resulting in the presence of the four fundamental forces of nature today. Baryons such as protons and neutrons and their anti-particles were formed from quarks and anti-quarks in the hadron era. Leptons, including electrons and their corresponding neutrinos started to dominate in the lepton era. During the nucleosynthesis period, protons and neutrons combined to form nuclei of simple atoms, that is, hydrogen, helium, and lithium, via nuclear fusion. In the recombination era, electrons were attracted into the orbits of hydrogen and helium nuclei, forming neutral atoms. It was also in this era that the photons from the cosmic microwave background became disentangled from the charged particles, thereby, ensuing a transparent universe. The period after the recombination era and before the formation of quasars, stars, and galaxies is better known as the dark age because the universe is literally dark in this window. About 200 million years after the Big Bang, the gravitational attraction due to the irregularities in the universe cause molecular gas of hydrogen to coalesce to form protostars and then stars. This was followed by the attraction of stars into clusters, mediating the formation of galaxies about 1 billion years after the Big Bang. Our solar system was formed relatively late, roughly about 9.2 to 9.3 billion years after the Big Bang. Today, we are living in the Stelliferous era, where the universe has existed for about 13.8 billion years. This era is predicted to last for a maximum of 10¹⁴ years after the Big Bang. Subsequently after that, in the degenerate era, all stars have used up their hydrogen fuel. In this period, the universe is dominated by degenerate remnants made up of brown dwarfs, white dwarfs, neutron stars, and black holes. Brown dwarfs, white dwarfs, and neutron stars eventually will be gone too, as a result of proton decay. What will be left in the universe are only black holes. The universe therefore enters the black hole era. But when all the black holes evaporate via Hawking radiation, the universe becomes virtually empty. This is the dark era where the universe is merely dominated by electrons, positrons, and dark matter at this point. In this one possible fate, the universe will face its Big Freeze where it finally rests peacefully at an extremely low energy level, with its temperature settled slightly above absolute zero.

4. The fate of the cosmos

The universe will ultimately reach an end at some point. Nevertheless, there is no one common agreement as to how it will end. Before the discovery of the dark energy in 1998, many believed that there may be a point in time where the density of the universe exceeds the critical density. If this occurred, the universe would cease from expanding, and instead, it would start collapsing back into the infinitely dense singularity. This phenomenon is coined the Big Crunch. A new Big Bang may occur again after the Big Crunch. Nevertheless, most cosmologists decided to ditch this idea now, realizing that dark energy is accelerating the expansion rate of the universe. Hence, it is more likely that the universe will encounter either the Big Rip or the Big Freeze. In the Big Rip scenario, dark energy will eventually outvie gravitational force, and it becomes incredibly strong, so much so that the universe is torn apart. A more plausible and popular prediction, however, is the Big Freeze theory (also known as the Big Chill and the heat death). According to this theory, the universe will ultimately reach thermodynamic equilibrium (i.e., maximum entropy), where heat is fairly distributed. Since energy distribution is homogeneous, no further work is possible. The universe eventually comes to its quiet and lonely death [21].

5. The forces of nature

Four fundamental forces are known to exist in nature. They are the strong nuclear force, weak nuclear force, electromagnetic force, and the gravitational force [12, 22].

1. The strong nuclear force is the strongest among the four forces. It is known to bind hadron particles – viz. protons and neutrons, within the nucleus; it also confines quarks together to form protons and neutrons. The strong nuclear force is the energy source released from nuclear fission and fusion. The strong interaction has a range of 10⁻¹⁵ m. According to the quantum chromodynamics (QCD) theory, the strong force is caused by the exchange of the gluon.

2. The weak nuclear force is in the order of 10⁻¹⁴ that of the strong force. The weak force acts in each individual nucleon (which is the collection of protons and neutrons). This force is responsible for the radioactive beta decay of the atomic nucleus, that is, in beta-minus decay, a neutron turns into a proton, an electron, and an electron antineutrino; whereas in beta-plus decay, a proton is turned into a neutron, a positron, and an electron neutrino. The weak interaction has a range of 10⁻¹⁷ m. The weak force is given by the exchange of the W⁺, W⁻, and Z gauge bosons.

3. The electromagnetic force is in the order of 10⁻² that of the strong force, and it is responsible in binding atoms together. This force also governs the interaction among electrically charged particles—when particles with identical charges (i.e., both of them are either positively or negatively charged) approach each other, they tend to repel, while those with opposite charges attract each other. The electromagnetic field consists of both electric and magnetic fields. During static condition, the fields exist independently. When the fields vary with time, however, they have to exist simultaneously [22]. The electric force F_e exerted on a charge q = 1.6 × 10⁻¹⁹ C is given by

   $${\mathit{F}}_{\mathit{e}}=q\mathit{E}$$E11

   where E is the intensity of the electric field experienced by q. Likewise, the magnetic force F_m is given by

   $${\mathit{F}}_{\mathit{m}}=q\mathit{u}\mathit{\times }\mu \mathit{H}$$E12

   where × is the cross product, u the velocity vector, μ the permeability of the material, and H is the magnetic field intensity. The range of the electromagnetic interaction is infinity. The magnetic field is especially important when studying the characteristics of cosmic sources. According to the quantum electrodynamics (QED) theory, photons carry the electromagnetic force.

4. The gravitational force is the weakest among all forces. It is in the order of 10⁻³⁹ of the electromagnetic force. The gravitational force attracts any objects with mass. Hence, when the object comprises a large mass (for e.g., a star), gravitational force outweighs other forces. Based on Newton’s law, gravitational force F_g can be expressed as

   $$F_e=\frac{G{m}_1{m}_2}{r^2}$$E13

   where G = 6.674 × 10⁻¹¹ m³kg⁻¹ s⁻² denotes Newton’s gravitational constant and r the distance between two objects with masses m₁ and m₂. The range of gravitational interaction is infinity. The gravitational force is caused by the exchange of a quantum, known as the graviton.

6. The theory of everything

Physicists have attempted to explain the forces using quantum field theory (QFT). The first force successfully described by QFT was the electromagnetic force. Known as the quantum electrodynamics or QED, this theory suggests that photons act as the carriers of the electromagnetic force and that the force is transmitted by the exchange of these gauge bosons. This is to say that the electromagnetic energy is transferred when a photon is emitted from an electrically charged particle and absorbed by the other. As can be seen from the Feynman diagram in Figure 5, when an electron (e⁻) hits another, a photon (γ) is exchanged. The electron, which emits the photon, recoils in space, while the electron, which absorbs the photon’s energy and momentum, deflects correspondingly.

Figure 5.

A Feynman diagram showing two electrons exchanging a photon, when one collides with the other.

Physicists found that the electromagnetic force can be consolidated with the weak force. This unified force is named the electroweak force. The theory of the electroweak force introduces the W⁺, W⁻, and Z gauge bosons as the quanta of the weak field. The strong force, on the other hand, can be explained using quantum chromodynamics (QCD). According to the QCD theory, baryons (i.e., proton, neutron, delta, lambda, sigma, xi, and omega) and mesons (i.e., pion, eta, kaon, phi, psi, rho, upsilon, d, and b) are composed of quarks. The exchange of the quantum gluon mediates the strong interaction between two quarks.

The successful establishment of a single theory, which combines both the weak and electromagnetic forces, has motivated physicists to explore feverishly for a single theoretical framework of physics capable of explaining all physical phenomena in the universe. Being coined the theory of everything (TOE), this single theory should be able to effectively relate the strong interaction, weak interaction, electromagnetism, and gravitation. The dream of realizing the TOE, however, has not been smooth sailing. Physicists soon learned that such dream is but tantalizing.

In their subsequent step toward the TOE, physicists endeavored to describe the electroweak force and the strong force as a single force. Albeit their persistent effort in materializing this grand unified theory (GUT), luck has not been on their side. Merging the electroweak theory and the QCD theory has not been successful hitherto. Alas, physicists decided to resort to an ad hoc model, which constitutes both the electroweak and QCD theories. More colloquially referred to as the standard model today, this model has been found to produce results, which agree closely with the observational evidence. Even so, however, physicists are not quite contented with the fact that they have to treat the electroweak and strong forces separately and not as a single unified force.

Perhaps the problem that frustrates physicists the most is not their failure in establishing the GUT but their futile efforts in accounting for the gravitational force. As compared to the latter, the former is nothing more than a mere minor setback. The gravitational force is depicted by Einstein’s general theory of relativity—a theory that describes how gravity mediates the curvature of space and time on a larger scale— that is essentially different from the quantum theory. Owing to the Heisenberg uncertainty principle and the probabilistic nature of the quantum theory, physicists find it particularly difficult to quantize gravity. Without the quantum theory of gravity, gravitational force could not be fitted into the standard model, let alone the theory of everything. Above all else, the infinitesimal world of particles could not be reconciled with the infinitely large scale of the universe.

It was not until the development of the supergravity theories that the hope for the realization of the TOE was rekindled. Being a type of quantum field theory, these theories relate general relativity with supersymmetry at the subatomic level. Supergravity predicts gravitational force to be carried by a gauge boson, known as the graviton, and based on the concept of supersymmetry, it is hypothesized to have a superpartner named the gravitino. Supergravity is regarded as the low-energy limit of the higher dimensional string theory. The string theory offers a paradigm shift in the structure of the fundamental constituents of nature. As its name indicates, string theory depicts particles in the form of tiny pieces of vibrating strings, instead of points. Another bizarre feature of the string theory is that it predicts the existence of 10 dimensions, that is, nine spatial and one temporal dimensions. Many believe that six of the dimensions are being curled up or “compactified” into the internal spaces, which are of sub microscopic scales, such that only four dimensions are left noticeable (viz the height, width, length, and time dimensions that one is familiar with). The extra dimensions pose the advantage of allowing supergravity to meld gravity with the other three forces—namely, the electromagnetic and the strong and weak nuclear forces.

A predicament that physicists faced was that they found five consistent, but seemingly different, string theories could be derived to explain how the extra spaces could be compactified. The five theories are the Type I, Type IIA, Type IIB, Heterotic SO32, and Heterotic E8 theories. Physicists had difficulty identifying which version of the theory was the genuine one and which four were phony. Further investigations on the theories, nevertheless, suggested that these five theories could be the same after all—that they end up describing the same phenomena in different manners. Today, physicists believe that the string theories (including supergravity) are mere subsets of the more rudimentary M-theory. This theory predicts that 11, instead of 10, dimensions exist and that particles are made not just from strings but objects of different dimensions. The term p-branes is used to define these objects, with the letter p representing integers ranging from zero to nine. It is worthwhile noting that M-theory inspires the multiverse concept—an idea that suggests that different parallel universes are present, with each one of them governed by different laws of physics. Many, however, still cast doubt on the validity of the M-theory, and not to mention the string theory and supergravity as well, since evidence are yet to be found to support them.

7. Cosmic detection

As mentioned in the preceding section, data collection on cosmic sources is crucial for the study of cosmology. The data are obtained through the use of telescopes, which are specifically designed to detect and collect electromagnetic signals emanating from cosmic sources. Due to the cosmic expansion, the cosmic sources in the early universe had been stretched into the radio region of the electromagnetic spectrum. Thus, radio telescopes play a critical role in the study of cosmology, providing cosmologists with the data necessary to understand the universe and its evolution. As illustrated in Figure 6, a typical modern radio telescope, such as those used in the Atacama Large Millimeter/sub-millimeter Array (ALMA) interferometer, constitutes the following components [23, 24].

1. A dish antenna, which consists of a main and a sub-reflector. Dish antennas are used in this case because they have the highest aperture efficiencies when compared with the other types of antenna configurations. The function of the antenna is to detect electromagnetic signal radiated from cosmic sources. The most common types of dish antennas used in radio astronomy are the Cassegrain and Gregorian offset configurations [25, 26, 27].

2. A receiver optics, which is usually constructed from either a pair of mirrors (in some rare occasions, a lens is used, instead) and horns or a horn and an orthomode transducer (OMT). The purpose of the receiver optics is to focus the signal to the feed horn. An example of receiver optics for the ALMA telescope is shown in Figure 7 [24].

3. A waveguide, which is used to channel the signal from the feed horn to the mixer circuit [28, 29, 30]. Waveguide can either be circular or rectangular.

4. A receiver circuit, which is used to feed the signal to the electronics. In radio astronomy, coherent receivers are used to detect the phase and intensity of the signal. Incoherent receivers, on the other hand, only accounts for the intensity of the signal.

5. A detector circuit, that is, a non-linear square-law detector is used in a radio telescope to demodulate the down-converted modulating signal from its carrier.

6. A spectrometer, which allows the spectrum of the signal to be viewed and analyzed.

Figure 6.

Block diagram of a radio telescope.

Figure 7.

The band 8 receiver optics fitted into a self-contained cartridge for the ALMA telescope.

To understand the operation of a radio telescope, one can imagine the beam from a cosmic source to be collected by the dish antenna. The main reflector scatters the beam to the sub-reflector. The sub-reflector, in turn, illuminates the first and then the second mirrors. The signal is subsequently focused onto the feed horn. The waveguide connected to the horn channels the signal to the receiver circuit. The mixer circuit at the receiver modulates the signal with another lower frequency signal generated from a local oscillator (LO). The process of modulation down converts the signal to a lower intermediate frequency (IF) signal. This IF signal is then fed to a detector circuit after it goes through amplification. The detector circuit rectifies the signal before sending it to a spectrometer for analysis. By analyzing the electromagnetic signal emitted by cosmic sources, cosmologists can gain insight into their properties and behavior. This allows us to study a wide range of cosmic phenomena, from distant galaxies and quasars to the CMBR.

8. Conclusion

A brief overview of cosmology is given in this chapter. In a nutshell, cosmology is a branch of astronomy thar studies the evolution process of the universe, from its violent birth to its death. The universe is believed to have expanded from an extremely hot and dense singularity about 13.8 billion years ago—a process that cosmologists name the Big Bang. During the early phase of its expansion, the universe underwent a cosmic inflation, which allowed it to expand to an incredibly large size within split second. The sudden fast expansion of the universe created the primordial seeds for large-scale structures. Gravitational attraction at certain regions with higher density collapses molecular clouds to form stars and galaxies. The universe is predicted to end with the Big Freeze, where it reaches the state of thermodynamic equilibrium.