Chapter 1: Scientific Theory and the Big Bang PDF

# Chapter 1: Scientific Theory and the Big Bang ## 1.0: Introduction In this first chapter, we will learn that the foundation of science is the scientific *theory*. We will then take a good look at the most profound (and most complex) of all scientific *theories*, the *Big Bang Theory*, which is t...

# Chapter 1: Scientific Theory and the Big Bang ## 1.0: Introduction In this first chapter, we will learn that the foundation of science is the scientific *theory*. We will then take a good look at the most profound (and most complex) of all scientific *theories*, the *Big Bang Theory*, which is the leading theory of how the Universe originated. ## 2.0: The Scientific Method Many times we read the words 'theory', 'law', and 'hypothesis' used almost interchangeably and certainly any distinction between them seems unclear. In science they do not mean anything close to the same thing! Take a look. ### Scientific Method - Make Observation - Invent Hypothesis to Explain Observation - Pass: Test Hypothesis - Fail: Pass Many Hypothesis -> Theory - Pass: Test Theory - Fail: Pass Many Theory -> Law - Fail: **Fig. 1.1: The scientific method** **Image source: USGS** ## 2.1: Hypothesis A *hypothesis* (plural: hypotheses) is an educated guess based upon observation - sometimes, only one observation. Usually, a hypothesis can be supported or rejected through experimentation or more observation. While it can be supported or rejected, it cannot be proven to be true. ## 2.2: Theory A *scientific theory* summarizes a *hypothesis* (or group of hypotheses) that explain a set of observations, supported by repeated testing. A theory is considered valid as long as there is no firm evidence to dispute it. So, like hypotheses, *theories* can also be supported or rejected as we learn more. Basically, if evidence accumulates to support a hypothesis, then the hypothesis becomes accepted as a good explanation of some phenomenon, and becomes a *theory*. The *theory* is not guaranteed to be true, but it is the best we can formulate based on current evidence. Thus, both hypotheses and *theories* attempt to explain the 'why' of some action and *theories* are considered to be much better formulated and tested than hypotheses. ## 2.3: Law A *law* governs a body of observations. At the time it is made, no exceptions will have been found to that law. *Scientific laws* describe certain scientific observations, but they do not explain why the behaviours are observed. A *law* does not say "why" things happen, rather it predicts how they happen. A quick way to tell the difference between a *theory* and a *law* is to ask if the statement explains "why" something happened; if it does, it is a *theory*, not a *law*. If it describes "how" something happens, it is a *law*. Please note that the word "why" is key here - without an explanation of why something happens we do not have a complete *theory*. Typically, *theory* and *law* go hand-in-hand. **Example: Newton developed a Law of Gravity which predicts the behaviour of an object as it falls - but Newton's Law of Gravity does not explain why an object falls. Again, the word "why" is key here – to provide the answer to why objects fall at a constant acceleration we need the Theory of Universal Gravitation, which states that every particle in the Universe attracts every other particle in the Universe. This theory is an attempt to explain many different observations, most importantly that objects on Earth fall with a particular acceleration (one of the Laws of Gravity), and that the Earth's moon and the planets follow elliptical orbits (a Law of planetary Motion). Thus Newton's theory of Universal Gravitation is provides the "why" that lies behind the observations. So the Law of Gravity explains the observations we have of falling objects, but does not say anything about why objects fall. For the "why", we need the Theory of Universal Gravitation.** ## 3.0: The Big Bang ## 3.1: The Theory The *Big Bang Theory* is an effort to explain exactly what happened at the very beginning of the Universe. **Figure 1.2** is a rather crude attempt to illustrate how things progressed from the *Big Bang* to the Universe of today and tomorrow. Before the *Big Bang*, the Universe did not exist. At the birth of the Universe, time and space were created in a gigantic expansion that emanated from a 'singularity.' **Fig. 1.2: The Big Bang** **Image source: NASA** ### What is a singularity? - There is no readily understood definition to offer; however, it can be said that a 'singularity' is an area in space-time where gravitational force is so high that all known laws of physics break down and do not apply. ### What is a gigantic expansion? - Is that the equivalent of a gigantic explosion? Not really. To have an explosion there has to already be space into which the explosion spreads. But the *Big Bang* created space (and time), so perhaps we can think of an infinitesimally small balloon, which in the tiniest fraction of time, suddenly expands - and keeps on expanding. In this tiny instant, time and space had a finite beginning (at least if you are inside the balloon). This is really confusing – and there is no pretense otherwise! If this sequence occurred (a singularity, a *Big Bang*, and creation of time and space), then can it be true that before that singularity there was nothing: no space, no time, no energy? That is what the *Big Bang Theory*, as currently expressed, states. Let us go over the observations that suggest that the Universe really was produced by a *Big Bang* event (Please review the appendix provided at the end of this chapter, entitled “What Banged?”). ## 3.2: The Observations (the Evidence) There are several lines of observational evidence that support the theory, but we will consider the three main ones, together called the "Three Pillars of Proof". We will consider each individually (below), but here is a list of them: - Recession of stars/galaxies (as described by Hubble's Law) - The characteristics of cosmic microwave background radiation - The abundance of light elements. - ## 4.0: Hubble's Law First, let us check out who Hubble was. Edwin Hubble was one amazing man. He lived from 1889 to 1953, and during that time - He demonstrated that there were many galaxies in the Universe - not just the one we are in (the Milky Way Galaxy) - He proved that the Universe is expanding - He showed us how to measure distances in space. Hubble fought to have astronomy recognized as belonging to the subject of physics. After his death, the Nobel Prize Committee officially made this recognition (unfortunately, he was never awarded a Nobel Prize, as they are not awarded posthumously). **Fig. 1.3: Edwin Hubble: Dude with a pipe (Image source: AIP)** In 1990, however, scientists at NASA installed a huge optical telescope into Earth's orbit, naming it the *Hubble Space Telescope* in honour of Edwin Hubble (many of the observations that appear throughout this course came from evidence collected by the Hubble telescope). ## 4.1: Light's Redshift and Hubble's Law In order to understand the observations that led to Hubble's Law, we have to make a comparison (as Edwin Hubble did) between a property of sound and a property of light. You all know that if you stand by the side of a railroad and a train passes blowing its horn, the sound pitch is very different when the train is approaching (the wavelengths of sound are compressed and shortened) than when it is moving away (the wavelengths of sound are stretched and lengthened; Fig. 1.4a). Of course, a person inside the train hears the same sound all the time. This apparent change in sound is called the *Doppler Shift*, or the *Doppler Effect*. All it means is that when an object coming toward you makes a sound, the sound waves are compressed by the motion of the noisy object and sounds differently to you than when the same sound waves are being carried off away from you. **Fig 1.4: Doppler shift (Image source: Pearson Pub.)** Hubble knew that waves of light would behave somewhat like waves of sound when the light source was moving toward or away from the observer (as had been proven in a science lab around 1918 by a scientist called Keeler) (Fig. 1.4b). If the light source is moving toward the observer the light wavelength appears to shorten (i.e., to move into the blue spectrum, becoming "blue-shifted”), and if the light source is moving away from the observer the light wavelength appears to lengthen (i.e., to move into the red spectrum, becoming red-shifted) (Fig. 1.4b). In fact, Hubble realized that the faster the light-emitting object was moving, the greater the shift. The speed of light is fixed and cannot change, so when Hubble observed apparent changes in speed of light (from a star), it meant the stars had to be moving away from Earth. In fact, applied to everything he could see, the whole Universe had to be expanding, and with it the light waves moving through it. Edwin Hubble observed that the more distant a galaxy is from us, the more 'red-shifted' is the light we receive from that galaxy, and hence the faster that galaxy is moving away from us. (This is a characteristic of an expanding Universe). So, the amount of redshift can be used as a measure of a star or galaxy's distance from Earth. ## THE ELECTROMAGNETIC SPECTRUM **Fig 1.5: The electromagnetic spectrum (Image source: NASA)** From all these observations, Hubble formulated an equation, known as the Hubble Law: $ v = Hod $ Where v is the speed expressed in kilometres per second, d is the distance of the star/galaxy away from Earth in parsecs (1 parsec = 3.26 times the distance light travels in one year), and Ho is the Hubble constant. That makes Ho the speed of expansion of the Universe; Hubble assumed it was a constant (that has turned out to be somewhat wrong). In simplest terms, you can calculate how far from Earth an object is by dividing its velocity (obtained from its amount of redshift) by the rate of expansion factor. Always remember your assumption: your calculation assumes that Ho (the expansion factor) is a constant; if the expansion of the Universe has changed over time, your calculation will be inaccurate (more about that later). ## 5.0: Cosmic Microwave Background Radiation It is estimated that it was extremely hot in the first seconds of the Universe and as it expanded, it cooled. The hot light photons, produced in the early period, have since lost energy and dropped from the visible light energy range into the microwave energy range (Fig. 1.5) - and that constitutes the cosmic microwave background (CMB) that we can still see today. Scientists figure that CMB can be seen from anywhere in the Universe because it comes from all directions, and with nearly the same intensity. This CMB was first discovered in 1965, as 'noise' in a very sensitive microwave radio receiver - and the researchers first thought it was the result of pigeons nesting in the antenna. When they figured out that it was not pigeons but this phenomenally old signal, they won a Nobel Prize for Physics in 1978. **Fig 1.6: Cosmic microwave background (Image source: NASA)** As you can see from the map made of CMB (Fig. 1.6; signal converted to temperature), it shows the same pattern of distribution throughout all parts of the Universe. The only explanation that makes any sense is that this CMB represents the very last remnants of the light/heat energy of the Big Bang's initial expansion. This is supported by the fact that while the general temperature of space should be 0 on the Kelvin scale (which is equal to -273° on the Celsius scale), the actual average temperature is 2.726 K (which would be -270.424 °C). You can see inhomogeneity in Fig.1.6; this amounts to temperature variation on the order of 1 degree in 100,000 degrees – so it is greatly exaggerated in the figure and in reality is not much at all. ## 6.0: Abundance of Light Elements The third 'pillar of proof has to do with the ratio of all the various atoms of the three lightest elements: hydrogen (75%), helium (25%) and lithium (trace). The observed abundance of all the different atoms of those elements can be explained only if they originated from one single ratio of the first subatomic particles of matter that can be formed from a super-hot environment. The only way to get that one critical ratio is through a unique event like a *Big Bang*. Neat! Those are the primary points that tell us the *Big Bang* is a fact, not a hoax. Now let us see what was produced. ## 7.0: Shape of the Universe Knowing the shape of the Universe is important for many reasons but perhaps the most important is the question: How will the Universe end? There are three unique shapes (and some mixes, but let us just look at the unique ones). **Fig 1.7: Shape of the Universe (Image source: NASA)** 1. First, like the hypothetical balloon used to illustrate the *Big Bang*, the Universe might have what we call positive curvature, like a sphere (Fig. 1.7, top). In this case, which we call a "closed" universe, the Universe would be finite in size but without a boundary, just like the balloon. In a closed *universe*, you could, in principle, fly a spaceship in one direction and eventually arrive back where you started from. Closed *universes* are also closed in time: they eventually stop expanding, and then contract in a "Big Crunch". [Sort of like tossing a ball into the air; for a while, the energy of your throw keeps the distance between the ball and you expanding but, eventually, the energy fades and the ball gets pulled back by gravity]. So that is a model that depends greatly upon there being sufficient matter in the Universe that gravity can eventually pull things back together. The good thing about this model, of course, is that the production of universes is more likely to be a cyclical event: Bang...a universe, then a crunch collapse; Bang...a universe, then a crunch collapse; and on and on. 2. Another possibility is that the Universe might be "open," or have negative curvature. Such universes are sort of saddle-shaped (Fig. 1.7; middle). They are also infinite and unbounded. Parallel lines eventually diverge. Open universes expand forever, with the expansion rate never approaching zero (so the ball you tossed up in the air just kept going and going). 3. The final possibility is that the Universe is flat (Fig. 1.7; bottom). You can imagine this kind of universe by cutting out a piece of your balloon material and stretching it with your hands. The surface of the material is flat, not curved, but you can expand and contract it by tugging on either end. Flat universes are infinite in spatial extent and have no boundaries. Parallel lines are always parallel. Like saddle-shaped universes, flat universes also expand forever, but the expansion rate approaches zero (the ball goes and goes, but eventually just appears to hang there, the movement outward is so slow!). Some others have hypothesized other potential shapes (trumpets, crosses, slinky-toy-like spirals), but we will ignore them. To all three of our models, a density parameter is critical. If space has a negative curvature, there is insufficient matter around to allow gravity to act and stop the expansion; let us say the density parameter is less than 1 (in Fig. 1.7 that is signified by Ωο<1). If space has a positive curvature, there is more than enough matter around to allow gravity to pull everything back together (we can express that as 2o>1). If space is exactly flat, we can say there is exactly the 'critical' value of matter around that will prevent the Universe from pulling back together or from expanding indefinitely to oblivion (Ωo=1). So what is out there that affects gravity? Certainly, there is conventional matter: stars, planets, asteroids, comets, etc. Unfortunately, that seems to be a bit less than 5% of the Universe (Fig. 1.8). If that is all there is, then we might as well just turn out the lights; pretty soon, the distances between stars will be so great, we will not be able to see any others than the Sun. Some years ago something else was hypothesized: *dark matter*. This is matter we have never seen because it gives off no (electromagnetic) energy, but we know it exists because we can detect its gravitational attraction to conventional matter. **Fig 1.8: Cosmological composition (Image source: Wikipedia commons)** In fact, it constitutes about 27% of the matter of the Universe. If dark matter alone is all that is out there, there probably would be enough total matter to slow the expansion of the Universe and pull things back into a *Big Crunch*. Alright... but, scientists have recently detected that expansion is increasing, not decreasing; so what's happening? Well, it turns out there is some truly mysterious force out there - possibly constituting 70% of the Universe that acts in opposition to gravity: it repels matter. We call this *dark energy*. This is the force that seems to control the expansion of space; when it exactly counterbalances the kinetic energy of the *Big Bang* expansion, we are at the 'critical' value of 1 for a density parameter (i.e. a flat Universe). Right now, with an expansion rate apparently increasing, we seem to be accepting that the Universe is almost perfectly flat – but has just the slightest negative curvature. For the purpose of this course, we will call it 'flat'. ## 8.0: Age of the Universe There are several lines of investigation we can use to determine the age of things - even the Universe. ## 8.1: Radioactivity You are all aware that certain elements have components that are radioactive – they breakdown (at fixed rates) to form other components and give off energy in the process (that is the basis of nuclear reactors, after all). When we look at Chapter 7, we will discuss *radioactivity* - because we use this to interpret the age of rocks on Earth. We can figure out the age of the Universe by (a) observing the compositions of gases around old stars, (b) knowing the exact radioactive processes required to produce these gas compositions from the very first elements created in the *Big Bang*, and (c) knowing all the time factors involved in breaking down one component to yield others. This information gives us Universe age estimates ranging between 11.5 and 17.5 billion years. That is a pretty broad range and the search has been on to find a star - one of the most primitive from really early days - that could be radioactively dated. In 2007 one star was found and dated at 13.2 billion years. So the Universe has to be older than that. ## 8.2: Hubble's Expansion Constant In order for Hubble to develop his equation that connected redshift with distance/velocity of light sources in the Universe, he had to propose a 'constant' for the expansion rate (Ho). Since he produced that rather simple and elegant equation, there have been many refinements to Ho (based primarily on the shape/density matters we discussed previously). However, in any 'rate' expression, there's a time factor (e.g. speed of your car: km/hour), so it certainly is possible to use Hubble's equation to determine the age of the most distant light sources we can find. In 2002, the Hubble Space Telescope found some white dwarf stars - these are the remnants of stars that have consumed all their 'fuel' and are sitting around cooling off. To have gone though the whole life cycle of a star means that a white dwarf is very, very old - thus a prime candidate for dating. All the white dwarfs found in this cluster gave dates between 12 and 13 billion years. Considering that it would have taken something less than 1 billion years for the cosmos to cool sufficiently (from the *Big Bang* event) to form a star, the age of the Universe had to fall between 13 and 14 billion years. ## 8.3: Cosmic Microwave Background Radiation The cosmic microwave background signals offer the most accurate view to date of conditions in the early Universe. Based upon all the best physics available, a sophisticated model of the Universe (from the time represented by that CMB signal map back to the time when the *Big Bang* produced those first photons) has been produced. Many assumptions have been made in order to get a coherent model. Assuming the right model has been developed (another assumption!), the Universe is 13.80 ± 0.04 billion years, according to data published in 2013. ## Appendix: What Banged? To review, the accepted origin of the Universe begins with a *Big Bang* from a singularity, but just how that initial singularity came to be is never explained. There are very few people in doubt of the existence of the *Big Bang*; however, there is an increasing number of skeptics who think there must be more to the story. According to a mathematical physicist, Neil Turok (at the Perimeter Institute of University of Waterloo), the *Big Bang* certainly was big, but not unique; he claims there have been many *Big Bangs* (and universes), and that there will be many more. He suggests the *Big Bang* represents just one stage in an infinitely repeated cycle of expansion and contraction (sort of like clapping your hands in very slow motion?), and that neither time nor the Universe has a beginning or end. How does it all work? Basically Turok proposes a universe consisting of two infinitely extensive sheets, separated by a very thin layer of energy (you could call it dark energy, it does not really matter). This is a 'sandwich' model. Every once in a while, the intermediate layer becomes unstable at some point, gravity starts pulling things together, the layers bounce together, and - from the point of the contact – sufficient energy is generated to produce another *Big Bang*. From then, until the next period of instability (perhaps some trillion years in the future), the Universe continues its life of expansion. Of course, there is much more to it than that but the theory is still under construction, so we will leave the topic at this point. # End of Chapter 1