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Schwennesen 1 The Earliest Epochs, or the Beginning of Everything Ben Schwennesen Duke University Professor Hubert Bray Schwennesen 2 The Earliest Epochs, or the Beginning of Everything Why is the Big Bang theory so accepted? If one was to approach any given person on the streets and ask them to start listing off all the scientific theories they know of, chances are the Big Bang theory (BBT) would fall early in the list. Indeed, today the vast majority of astronomers and astrophysicists believe that the Big Bang model must reflect how the earliest knowable moments of the universe occurred [0, p. 219], which has led to an assimilation of some of its core concepts into popular culture. Though “in the beginning there was nothing, then it exploded” is a description of BBT that would make plenty of astrophysicists cringe (since, in order for something to explode, there must be space into which it can do so), at least oversimplifications of this sort allow the complexities of the theory to be expressed in a way that lay-folk can recall. Still, though most people will have some conception of what the Big Bang describes, those who know why the theory is so successful and popular are few and far between. There is a wealth of observational evidence that has led the Big Bang model (or more realistically models) of the universe to become so commonplace. Three key aspects of the universe that become much less difficult to explain with BBT are the relative abundances of the light elements in the universe, the Hubble expansion, and the startling uniformity observed in the cosmic microwave background (CMB) radiation that pervades the universe [0, pg. 239]. Though one does not need BBT to explain these phenomena, no alternative explanation with comparable simplicity, generalizability, and elegance has yet emerged [0, pg. 219]. Hubble’s Law, Red Shift, and Accelerating Expansion Edwin P. Hubble (1889 – 1953) is considered one of the great cosmologists of history for his discovery made in 1929 at Mount Wilson (near Los Angeles) using its 100 in. diameter Schwennesen 3 reflecting telescope: not only did Hubble find that light from distant galaxies was systematically shifted towards red wavelengths (as compared to light from closer galaxies), the relationship between magnitude of red shift and the galaxy’s distance was directly proportional [0, pg. 220]: 𝑟𝑒𝑑 𝑠ℎ𝑖𝑓𝑡, 𝑧 = 𝐻𝑑 𝑤ℎ𝑒𝑟𝑒 𝑑 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑜 𝑔𝑎𝑙𝑎𝑥𝑦 & 𝐻 = 𝐻𝑢𝑏𝑏𝑙𝑒 ′ 𝑠 "𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡" = (67 ± 1.2) 𝑘𝑚 𝑠 −1 𝑀𝑝𝑐 −1 (see http://arxiv.org/pdf/1303.5062v2.pdf, pg. 38, for most recent derivation of Hubble constant’s value). (Note: the value of H is not actually constant, but varies with the scale of the universe.) The relationship that Hubble (and perhaps more accurately others) inferred provided an explanation for an issue in general relativity that Einstein struggled to reconcile; when applied to the universe as a whole general relativity seems to imply that the universe cannot be static— there must be either expansion or contraction [0, pg. 225]. Thoroughly-instilled dogma of the time, however, held that the universe was in an overall steady state, and so Einstein added a “cosmological constant” (Λ) to his field equations to accommodate a static universe. With the acceptance of Hubble expansion by the scientific community, Einstein eliminated the term from his equations; much later, the discovery that the universe’s expansion was accelerating due to some “dark energy” led to a reintroduction of the cosmological constant. Thus, even when Einstein was wrong, he was right. That said, credit for proving that general relativity must result in a dynamic universe is owed to Alexander Friedmann, not Einstein himself [0, pg. 225]. Hubble’s law is often explained as a form of the Doppler effect, in which the sound from an object moving away from an observer is shifted downwards in frequency (and vice versa; an example is to imagine the sound of an ambulance as it passes). Such an explanation is confusing, however, for though one certainly may observe genuine Doppler effects in light from distant bodies, the Hubble shift is not among them [0, pg. 228]. This is because the Doppler Schwennesen 4 effect applies to objects moving through space, which galaxies are not doing; instead, space itself stretches between the galaxies, in-turn stretching (red shifting) the wavelength of travelling light along with it. Still, it can be fruitful to calculate the apparent velocity of receding galaxies, as such calculations help to approximate the rate of expansion of the universe, as well as its age [0, pg. 229]. Production of the Light Elements Elements heavier than helium are mere trace constituents of the universe as we observe it today; together, hydrogen, deuterium (hydrogen’s heavy isotope), and helium make up nearly 100% of the universe, with approximate relative quantities of 75% hydrogen and deuterium and 25% helium [1, pg. 99]. In 1957, it was demonstrated that atoms heavier than helium were almost certainly built up exclusively by nuclear fusion in stars’ cores and violent supernova explosions [0, pg. 222]. The abundance of helium, however, cannot be explained in this manner since (i) the amount of helium observed in stars does not correlate with their ages and (ii) the conversion of hydrogen into helium releases so much energy (consider the H-bomb) that buildup of 25% helium in the universe would cause stars to shine far brighter than they do in reality [0, pg. 222]. The nucleosynthesis phase of the Big Bang explains helium’s abundance, providing one of the most crucial experimental verifications of BBT to date [0, pg. 237]. The Planck Epoch (𝟎 𝒔 < 𝒕 ≤ 𝟏𝟎−𝟒𝟑 𝒔, 𝑻 ≥ 𝟏𝟎𝟒𝟎 𝑲) Though physicists often describe “extrapolating backwards” to a point of infinite temperature and density when explaining the Big Bang (and this is the reality of the mathematics at work), for the purpose of comprehension working forwards from t → 0 seconds is preferable. The time interval 0 𝑠 < 𝑡 ≤ 10−43 𝑠 is commonly referred to as the Planck epoch, though some (not inaccurately) deem it the “epoch of ignorance” [10; 0, pg. 234]. Using classical general Schwennesen 5 relativity, as t → 0 s, the size of the universe approaches zero, while its temperature and density both approach infinity [1, pg. 99]. That is, backwards extrapolation through time using GR results in an apparent singularity 13.8 billion years ago; though this may seem almost elegant, to physicists it represents a huge flaw: the story of our universe beginning in a singularity causes the laws of physics to break down entirely [10]. There is a good reason that physicists today will not make strong claims about what specifically occurred during the Planck epoch (at least not if they wish to be taken seriously); the two indispensable theories of modern physics, general relativity (GR) and quantum mechanics, could be said to have a deep mutual animosity. In the everyday experience of humanity, such an “animosity” is not important, since GR and quantum physics tend to apply individually to their respective domains; during the Planck epoch, however, one needs both theories, since the universe was simultaneously expanding (requiring GR) and extremely hot and dense (requiring QM) [1, pg. 98]. The union of the two theories is usually called quantum gravity, and one may describe physicists as being not even somewhat close to unraveling its mysteries (though not for lack of effort). Barring the existence of higherdimensional branes (present in string theory and its offshoots) that would circumvent quantum gravity, its effects would dominate particle interactions over a time-scale known as the Planck time (tp). The calculation of tp using dimensional analysis is not exceptionally difficult [10]: Let c be the speed of light (relativistic effects), h be Planck’s constant (quantum effects), and G be the universal gravitation constant, i.e., c = 3 x 1010 cm/s, h = 6.63 x 10-27 gcm2/s, & G = 6.67 x 10-8 cm3/(gs2) Then, to find tp, we must combine the constants to obtain seconds units, i.e., 𝐵 𝐷 𝑐𝑚 𝐴 𝑔 ∙ 𝑐𝑚2 𝑐𝑚3 𝑐 ℎ 𝐺 =𝑠⟹( ) ( ) ( ) = 𝑠. 𝑠 𝑠 𝑔 ∙ 𝑠2 𝐴 𝐵 𝐷 Schwennesen 6 The result is a system of linear equations that may be solved using Gaussian elimination to find that 5 1 1 5 1 1 ℎ𝐺 𝐴 = − ,𝐵 = ,𝐷 = ⟹ 𝑡𝑝 = 𝑐 −2 ℎ2 𝐺 2 = √ 5 ≈ 10−43 𝑠𝑒𝑐𝑜𝑛𝑑𝑠. 2 2 2 𝑐 Aside on Useful Quantities There are a number of physical quantities which change monotonically (i.e. increase OR decrease; no points of inflection) throughout the history of the universe, making such quantities useful in tracking its state throughout the various epochs. These quantities are: the age of the universe t, scale factor (dimensionless function of time representing the state of the universe’s expansion) a, redshift (as measured today) z, and the temperature of the CMB radiation Tγ (today approximately 2.7 Kelvin) [10]. These quantities are strongly inter-related by the formulas [10] 𝑎(𝑧) = 1 1 ⟺ 𝑧(𝑎) = − 1, 1+𝑧 𝑎 𝑇𝛾 (𝑎) = 2.7𝑎−1 ⟺ 𝑇𝛾 (𝑧) = 2.7(𝑧 + 1). The general relationships of the quantities to the age of the universe t involves far more rigorous calculations, but it is generally sufficient to understand that in a radiation-dominated universe, 𝑎 ∝ 𝑡1/2 (i.e., age scales as the square of scale factor a), while in a matter-dominated universe 𝑎 ∝ 𝑡 2/3 [0, pg. 233]. For our purposes, the most important factors are t and Tγ (which we will now simply denote by T); the universe’s age is relevant for obvious reasons, and temperature is a highly useful quantity for various reasons. Here, we will primarily use the temperature of the universe to know when the threshold temperature of various particle species are exceeded, i.e., the temperature at which a given particle may exist independently without rapidly decaying out of existence. Such measures are only meaningful because the particles within the universe remain in thermodynamic equilibrium, otherwise their average kinetic energy would not be constant Schwennesen 7 (meaning T would be indeterminate) [0, pg. 231]. Temperatures in the Planck epoch were (at least) a staggering 1040 K [10]. The Grand Unification Epoch (𝟏𝟎−𝟒𝟑 𝒔 ≤ 𝒕 ≤ 𝟏𝟎−𝟑𝟔 𝒔, 𝟏𝟎𝟑𝟔 𝑲 ≤ 𝑻 ≤ 𝟏𝟎𝟒𝟎 𝑲) If supersymmetry turned out to be correct, during the Planck epoch all four fundamental forces were unified under the regime of quantum gravity [10]. At the end of this era, however, the universe had cooled and expanded enough for gravity to decouple from the other three fundamental forces, the strong and weak nuclear forces and electromagnetism [0, pg. 234]. According to Grand Unification Theories (GUTs, of which there exist many in competition), these three forces remained unified as a single force, sometimes referred to as the electronuclear force (or the GUT force) [10; 0, pg. 234]. Temperatures at the time exceeded the threshold for the hypothetical X & Y bosons (and their antiparticles, 𝑋 & 𝑌), which are the would-be forcecarriers for the electronuclear force. As a result, the universe contains quarks, leptons, and all force-carrying particles. Throughout the epoch, the GUT interaction is suspected to maintain equal amounts of quarks, antiquarks, leptons, and antileptons; once T drops below the X & Y particles’ threshold, however, annihilations and decays quickly cause these particles to vanish from the Retrieved from: http://www.hep.ucl.ac.uk/theory//unification.png universe. A crucial prediction of GUTs is that their decay results in a small excess of quarks over antiquarks, to the tune of 109+1 quarks for every 109 antiquarks [0, pg. 234]. Though this Schwennesen 8 excess may seem negligible, were it (or a similar excess) not there, matter and antimatter would have annihilated completely and the universe would have been an expanding void of radiation. During this era, physical characteristics like mass and charge were effectively meaningless. Towards the end of the epoch, the strong interaction “freezes out” of the electronuclear force, leaving what is known as the electroweak force [10]. Inflationary Epoch (𝟏𝟎−𝟑𝟔 𝒔 ≤ 𝒕 ≤ 𝟏𝟎−𝟑𝟐 𝒔, 𝟏𝟎𝟑𝟑 𝑲 ≤ 𝑻 ≤ 𝟏𝟎𝟑𝟔 𝑲) As successful as the Big Bang theory has been, a number of very serious issues have been raised which it alone could not explain. These issues include, interestingly enough, one of the three essential pieces of evidence for why BBT has become so accepted: the overall uniformity and isotropy (uniformity in all direction) observed in the cosmic background radiation. To solve the issue (sometimes called the horizon problem), one requires an extension of BBT known as inflation. The horizon problem was so vexing to physicists because the intensity of the CMB (when corrected for the motion of the Earth) is the same in all directions to the startling precision of “one part in 100,000” [5, pg. 2]. The CMB was released about 380,000 years after the Big Bang, and its uniformity suggests that the universe’s temperature had become uniform, too, at that time. In order for such a uniformity in temperature to occur in that time span, in preinflationary Big Bang models, information would need to propagate at 100 times the speed of light, a clear contradiction of the established laws of nature. In the context of inflation, however, there needs not be faster-than-light diffusion of particles: the uniformity is established at microscopic scales by standard thermal equilibrium processes, then being scaled to a much larger region of near-perfect uniformity by the rapid acceleration of expansion [5, pg. 3]. A second issue resolved by inflation, considered by some to be even more impressive than the first due to the numbers involved, is called the flatness problem. The problem concerns Schwennesen 9 the ratio of the universe’s average total mass density to the critical mass density that would make the universe spatially flat (exactly 3H2/8πG). If this ratio (which we’ll denote as Ω) was exactly one, it would remain so forever [5, pg. 3]. Though the current value of Ω is one to within only a few percent (Ω = 1.012+.018 −.022 ), initial conditions for the universe where Ω deviates from one by any amount at all (positive or Retrieved from: https://apatruno.files.wordpress.com/2014/03/f1-large1.jpg negative) results in amplification of that deviation over time, so we can infer that the value of Ω in our universe’s very early history must have been dramatically close to one (at tp = 10-43s, Ω must have been equal to one to 59 decimal places) [5, pg. 4]. While such incredible flatness has no explanation in pre-inflationary Big Bang models, inflation naturally predicts it. During the epoch of inflation, Ω is actually driven closer to one with exponential swiftness: Ω − l ∝ 𝑒 −2𝐻𝑖𝑛𝑓 𝑡 , where Hinf is the value(s) of the Hubble parameter (“constant”) during inflation. So, inflation implies that the initial value of Ω is essentially irrelevant; exponential expansion will always drive it to approach unity [5, pg. 4]. There are a number of other issues that inflation addresses, including the absence of magnetic monopoles from the universe and the slight anisotropy of the CMB. The latter issue refers to the fact that though the CMB is incredibly uniform on large-scales, there remains enough variation in different directions (anisotropy) to seed the formation of structures like galaxy clusters. Though the exponential expansion during inflation indeed smoothens the universe to almost perfect uniformity, the necessary density fluctuations are produced towards Schwennesen 10 the end of inflation by quantum fluctuations in the “inflaton field” [5, pg. 4]. The inflaton is a scalar field that is theorized to be a product of a phase transition (of somewhat ambiguous nature) that occurred at the end of the Grand Unification epoch [10]. As the inflaton progressed into its lowest energy state, it created a massive repulsive force that streched the fabric of spacetime with exponential rapidity. Eventually, the inflaton field could no longer remain stable, at which point its huge potential energy was released as a hot, dense sea of quarks, antiquarks, and gluons, known as the quark-gluon plasma (QGP). Since the expansion of the inflaton field red shifted all previously-existing matter to extremely low densities, without this conversion of the inflaton’s energy (a process known as reheating), the universe would be devoid of matter entirely [10]. The exact cause of the exponential expansion described by inflation remains unknown; some physicists believe, however, that it may be explained by eternal inflation, in which it is hypothesized that speaking of a time “after inflation” may be unwarranted, since (in at least some parts of the universe/multiverse) there is no necessary reason to assume inflation ever halts. Their reasoning explains the process of (Retrieved from: [6]) inflation as a result of a “false vacuum,” a metastable state that appears to be a true vacuum (of the lowest possible energy state), but in reality has a lower energy state that may be reached through quantum tunneling, likely initiated by quantum fluctuations [5, pg. 6; Figure 3 retrieved from this page]. When the false vacuum eventually decays into its lower energy state, a bubble of true vacuum rapidly expands to Schwennesen 11 encompass the entire universe. Any successful theory of inflation with the false vacuum describes that its rate of expansion is much faster than its rate of decay, and therefore even though the false vacuum continually decays, it may never shrink in volume. The implication of this is that the false vacuum continually births local universes, while still expanding itself ad infinitum [5, pg. 6]. That is, the process repeats literally forever, producing an infinite number of non-interacting universes (a multiverse), typically hypothesized to form an elegant fractal structure of pocket universes [5, pg. 7]. Should this theory of inflation prove correct, the implications for our universe could be troubling. This is because the mass of the Higgs boson measured by CERN, about 125 GeV, implies that the universe remains in a state of metastability, and so, “without warning, a bubble of true vacuum could nucleate somewhere in the universe and move outwards at the speed of light,” the end result of which would be the end of the universe as we know it [2]. Though researchers have calculated that the time before such a decay would occur in our observable universe would be far longer than its current age, it turns out that intense gravitational fields, like those found around black holes, could serve as seeds for nucleation of the true vacuum [6]. Luckily, tunneling into a true vacuum bubble is only likely around black holes with very large curvature at the horizon, i.e., around very small black holes. Since the black holes discovered thus far tend to be massive and growing via accretion of mass, it is unlikely that such a small Schwennesen 12 black hole will cause the erasure of the universe anytime soon (barring the existence of primordial black holes) [6]. Though eternal inflation could prophesize the end of our universe, it would be at least tens of billions of years before humans need to worry about the approaching bubble of true vacuum [2]. Furthermore, other researches have cast doubts on the mathematical validity of eternal inflation [7], and alternative theories to explain the accelerated era of expansion are proposed frequently [see 8, for example]. Regardless, today most physicists believe that the evidence for inflation is so overwhelming that something like it must have happened. Overall, calculations indicate that the linear dimensions of the universe must have increased by at least a factor of 1026 (potentially much larger), and in-turn must have increased in volume by at least a factor of 1078; this is very difficult to explain without some form of inflation [10]. Electroweak Epoch (𝟏𝟎−𝟑𝟐 𝒔 ≤ 𝒕 ≤ 𝟏𝟎−𝟏𝟐 𝒔, 𝟏𝟎𝟐𝟎 𝑲 ≤ 𝑻 ≤ 𝟏𝟎𝟑𝟑 𝑲) By the end of inflation, the strong nuclear force had separated completely from the electroweak force (the merger of electromagnetism and the weak force). This electroweak force is far less speculative than the electronuclear force of GUTs, since the temperatures at which it exists (≈ 100 𝐺𝑒𝑉 ≈ 1015 𝐾) may be reproduced in particle accelerators [10]. The exact temperature reflects the threshold of the W and Z bosons that mediate the weak interaction [0, pg. 234]. During this era, quarks and antiquarks existed at such close proximities that the strong force (which, we recall, is stronger over longer ranges) was unable to bind them into hadrons, at least not without being instantly blasted apart by high energy photon collisions. Quark Epoch (𝟏𝟎−𝟏𝟐 𝒔 ≤ 𝒕 ≤ 𝟏𝟎−𝟔 𝒔, 𝟏𝟎𝟏𝟔 𝑲 ≤ 𝑻 ≤ 𝟏𝟎𝟐𝟎 𝑲) At the end of the electroweak era, the fundamental interactions had finally all taken on their present forms, but the temperature of the universe remained too high to allow quarks to Schwennesen 13 bind into mesons or baryons; the universe remained filled with a quark-gluon plasma, now containing quarks, gluons, and leptons. Approaching the end of this era (about 10-6 seconds after t = 0), the average energy of interactions between particles fell below the the binding energy of hadrons, and so quarks became permanently confined within protons, neutrons, and other hadrons, in a process known as baryogenesis [10; 1, pg. 99]. The small excess of quarks over antiquarks left behind by Grand Unification is now manifest in a small excess of protons and neutrons over their antimatter foils [0, pg. 234]. Hadron Epoch (𝟏𝟎−𝟔 𝒔 ≤ 𝒕 ≤ 𝟏 𝒔, 𝟏𝟎𝟏𝟐 𝑲 ≤ 𝑻 ≤ 𝟏𝟎𝟏𝟔 𝑲) In the era known as the Hadron epoch, the mass of universe was dominated by hadrons; initially, the temperature in this interval was high enough to allow the formation of hadron/ antihadron pairs [10], which kept matter and antimatter in thermal equilibrium via the following reactions (where p = proton, n = neutron, = photon, and bars ind. antiparticles) [0, pg. 235]: 𝑝 + 𝑝 ⇋ 𝛾 + 𝛾, 𝑛 + 𝑛 ⇋ 𝛾 + 𝛾. However, once the universe dropped below about 1013 K (~ t = 7 x 10-7 s), temperatures were below the threshold of protons and neutrons, and so these particles ceased to be a major independent constituent of the universe. Generally, protons and neutrons annihilated with antiprotons and antineutrons, leaving only a single proton for every 109 photons in the universe, a number which agrees with current measurements and predictions from GUTs [0, pg. 235]. The remaining protons and neutrons were kept in thermal equilibrium with the universe on the whole by the neutrino reactions [0, pg. 235]: 𝑝 + 𝜐𝑒 ⇋ 𝑛 + 𝑒 + , 𝑛 + 𝜐𝑒 ⇋ 𝑝 + 𝑒 − . These reactions occurred in such a way that (due to the slight mass difference between protons and neutrons) neutrons were gradually converted into protons as the universe aged. At 10-5 Schwennesen 14 seconds, the temperature drops below the threshold of pions and muons, and so these more exotic particles cease being a significant presence in the universe [0, pg. 235]. Lepton Epoch (𝟏 𝒔 ≤ 𝒕 ≤ 𝟑 𝒎𝒊𝒏, 𝟏𝟎𝟏𝟏 𝑲 ≤ 𝑻 ≤ 𝟏𝟎𝟏𝟐 𝑲) This seems an appropriate time to contemplate the fact the only one second has passed since the beginning of everything. At 1.09 seconds, the temperature and density of the universe have dropped low enough that neutrinos no longer have enough energy to interact with matter regularly; from this moment on they decouple from the universe (they DO NOT disappear, but interact so infrequently as to be a hardly detectable background actor of the universe) [0, pg. 236]. Without neutrino interactions, there is no process to keep protons and neutrons in thermal equilibrium; the ratio of protons and neutrons (p/n) at the moment of decoupling was about 82/18. This ratio would increase with time, due to the decay of free neutrons [0, pg. 236]. At approximately 100 seconds (~ T = 1010 K), electrons and positrons annihilate [10], leaving a small excess of e- due to the inequality established in the GUT epoch. The remaining electrons maintain thermal equilibrium with photons since they are free charges (and hence readily partake in interactions with electromagnetic radiation) [0, pg. 236]. Epoch of Nucleosynthesis (𝟑 𝒎𝒊𝒏 ≤ 𝒕 ≤ 𝟏𝟑 𝒎𝒊𝒏, 𝟏𝟎𝟗 𝑲 ≤ 𝑻 ≤ 𝟏𝟎𝟏𝟏 𝑲) About 3.2 minutes after the Big Bang, the temperature is low enough for the first deuterium to form without being blasted apart by photons. This marks the start of the process of nucleosynthesis, wherein helium is formed from deuterium by nuclear reactions [0, pg. 236]: 𝑑 + 𝑑 → 3𝐻𝑒 + 𝑛 → 3𝐻 + 𝑝 & 3 𝐻 + 𝑑 → 4𝐻𝑒 + 𝑛. Since 4He is the most stable of all nuclei, the process ceases here. The production of helium via nucleosynthesis bounds all the free neutrons in the universe into nuclei, allowing the strong interaction to cease further decay of neutrons. By this time, the p/n ratio has been shifted to Schwennesen 15 about 87/13. Thus, since for every 200 particles, 26 neutrons and 26 protons will combine into 13 helium nuclei, we are left with 148 protons to form other elements, implying the mass ratio of nuclei produced to be 4(13)⁄ 200 = 26%, which accounts almost perfectly for the observed proportion of helium in the universe [0, pg. 237]. As asserted earlier, this is among the key experimental confirmations of Big Bang theory. Nucleosynthesis ceases at about t = 13 minutes, when temperatures become too cool to sustain nuclear reactions. Epoch of Recombination (𝒕 ≈ 𝟑𝟖𝟎, 𝟎𝟎𝟎 𝒚𝒆𝒂𝒓𝒔, 𝑻 ≈ 𝟑𝟎𝟎𝟎 𝑲) Relatively little of interest happens between the era of nucleosynthesis and the next crucial epoch of the early universe. That next (and final, for our purposes) epoch, that of recombination, occurred simultaneously with the density of matter first exceeding the density of radiation [0, pg. 237]. Prior to recombination, the energy of photons in the universe was high enough to ionize any hydrogen atoms that managed to form, such that free electrons were prolific. Recombination occurred when photons’ energies dropped low enough for electrons to readily combine with protons into neutral hydrogen atoms. Since free electrical charges are now rare, photons have nothing left to interact with and decouple from the matter of the universe. At this point, the universe can be said to become transparent: prior to recombination, Thomson scattering of photons by electrons occurred so frequently that the universe was practically opaque [0, pg. 237]. The moment of recombination is fascinating for many reasons, most prominently because photons are allowed to travel freely after it (albeit travel while being drastically red shifted); therefore, the earliest light observed in the universe by humans, the cosmic microwave background, was released precisely at the moment of recombination. Schwennesen 16 Conclusion: Pragmatism and the Big Bang theory The works of many great artists, perhaps most memorable those of Vincent van Gogh, are often overlooked (or even downright mocked) during the artist’s life. This phenomenon can serve as an analogy to the research physicists perform into very early phases of the universe: the scientists at CERN may not know exactly what practical applications will emerge from their work on high-energy particle collisions, but the proposition that nothing useful will come of it is naïve. Throughout history, ideas in science and mathematics have tended to emerge prior to humanity’s understanding of how to utilize them fully (for example, quantum mechanics may have seemed too “spooky” to be useful when it was discovered, but nowadays quantum computation could allow humans to solve optimization problems that would take classical computers longer than the life of the universe to perform). Some may say particle physicists are bogged down in a realm of abstraction that has no guarantee of aiding humanity in a tangible way; perhaps such criticism is not unfounded, but even if no practical applications emerged, knowledge of how the universe emerged would seem indispensable to eventually addressing why our universe exists in the way it does, and how it might one day come to a close. Schwennesen 17 References [0] Allday, J. (1998). Quarks, Leptons, and the Big Bang. Bristol: Institute of Physics Pub. [1] Bojowald, M. (2011). Quantum gravity in the very early universe. Nuclear Physics A, 862863, 98-103. [2] Boyle, A. (n.d.). Will our universe end in a 'big slurp'? Higgs-like particle suggests it might. Retrieved from http://cosmiclog.nbcnews.com/_news/2013/02/18/17006552-will-ouruniverse-end-in-a-big-slurp-higgs-like-particle-suggests-it-might [3] Eisenstein, D. (n.d.). Baryon Acoustic Oscillations. 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Detection of Anisotropy in the Cosmic Blackbody Radiation. Phys. Rev. Lett. Physical Review Letters, 39(14), 898-901. [10] Terzić, B. (Spring 2008). History of the Very Early Universe. Lecture. Retrieved from http://www.nicadd.niu.edu/~bterzic/PHYS652/Lecture_13.pdf