The Physics Of The Universe
The story of the Universe, if considered chronologically from its birth to possible fate is an often confusing one which starts and ends with complex, unfamiliar physics, abstract thinking and the sorts of questions which have taxed the finest minds in science. Perhaps a more logical approach is to build a picture of the universe as it has been built throughout human history, also taking in some of the advances in scientific thinking and technology that have allowed us to develop our understanding of the universe.
The Earth as a body in space, the Solar System.
It was in ancient Greece that the followers of Pythagoras first conceived of the earth as a spheroid body in space, Aristotle later giving the proof that the earth always casts a circular shadow on the moon during eclipses (Hoskin, 1997). Various Geocentric (earth-centred) models of the Universe existed around this time (Hoskin, 1997) most notably Aristotle’s (Figure 1) which would influence much of western thinking about the Universe for the next two millennia (Fox, 2002). The idea of what would come to be known as the Solar System as a distinct region in space was created in the sense that celestial bodies were separated into near (planets and sun) and distant (stars).
Figure 1: Aristotle’s Geocentric Model of the Universe (Zabludoff, 2003)
The model of the solar system was subject to many revisions as astronomers sought to map what appeared to be extremely complex paths of the planets around the earth.
Heliocentric Solar System, gravity, the nature of stars.
In 1543 Copernicus published a Heliocentric (sun-centred) model of the solar system (Figure 2), which explained the complicated paths of the planets ‘around’ the Earth (Hoskin & Gingerich, 1997). This view of the Universe was not widely adopted until popularized by Galileo a century later. Descartes first developed the idea that the stars seen in the night sky were bodies like our own Sun. It was around this time that Kepler began to develop the idea that massive bodies exert a force on one another, although he thought of it as a sort of magnetism (Hoskin, 1997). Newtonlater formalized these ideas, believing that the universe was static and infinite, the gravity of all the bodies balancing perfectly to keep them in place.
Figure 2: Copernicus’ Heliocentric Model of the Universe. (Morris)
Galileo used then newly emerging telescopes, providing further evidence for the Heliocentric model of the solar system and also observing satellites orbiting the planet Jupiter. (Hoskin, 1997). During the 18th and 19th centuries, advances in telescopes allowed astronomers to study the light emitted from distant stars which lead to the discovery that they are very similar to our own Sun.
Analysis of the light emitted by stars gives information about the chemical composition of the star. Astronomers in the mid-nineteenth century began observing the light refracted through prisms: the angle of refraction is linked to the wavelength of the light which is in turn linked to the composition of the fuel which is producing that light.
Figure 3: The life cycle of sun-sized and massive stars. (Knight, 1998)
Take the Sun as an example: The Sun was ‘born’ about five billion years ago. The birth process took about five million years (Pople, 2001). A cloud of gas (mainly hydrogen) known as a nebula formed and began to condense under its own gravity into a body known as a protostar. The condensing led to a rise in temperature as gravitational potential energy of gas atoms was transferred to kinetic (particles accelerate towards the gravitational centre of the protostar) and particles collided with each other creating a heating effect. The core became hot enough for nuclear fusion to begin and the Sun entered its Main Sequence phase (which it is still currently in – about halfway through). The energy released through fusion prevents further gravitational collapse. The planets in the solar system formed from outlying nebula matter which was not drawn in to form the protostar.
Figure 4: Nuclear Fusion in the Sun. (goalfinder.com, 2006)
When all the hydrogen present in a star has been consumed, nuclear fusion ceases and the core of the star collapses. The temperature rises and energy is released by Helium fusing into Carbon. The outer layers expand and cool as the Sun becomes a red giant. In the case of The Sun, this will destroy the inner planets: Mercury, Venus, Earth and Mars. After further changes, the outer layers drift away into space and the core and inner layers become a White Dwarf which is so dense that normal atomic structure breaks down and electrons form a degenerate electron gas whose pressure prevents further collapse. After fusion stops, the White Dwarf cools and fades to red and then black.
Stars smaller than the sun become white dwarfs without going through the giant stages. Massive stars (eight or more solar masses) may become supernovae – where a catastrophic collapse of the core blows away the outer layers in a huge explosion millions of times brighter than a star. The cores of super massive stars may form Neutron stars, where the gravitational collapse is so catastrophic that electrons and protons are forced together into neutrons, forming a body which is essentially a giant nucleus. If the core is even bigger, the collapse will be yet more catastrophic and even neutrons formed will collapse under gravity, forming an ultra-dense body called a black hole from which even radiation cannot escape.
Cosmology
The idea that celestial bodies were not stationary in a static, infinite universe but part of an expanding universe was first conceived by Albert Einstein. According to his Theory of Relativity, gravity is due to matter curving space in the same way that a person standing on a trampoline creates a curved ‘well’ in the canvas of the trampoline. This means that the Universe is finite but unbounded. Put simply, it curves in on itself and creates an endless universe where a body travelling in a straight line will eventually return to its starting point. Einstein’s model also predicted an expanding universe, although he initially rejected the idea. (Fox, 2002).
Hubble first proved, surprisingly as late as 1925, that there were galaxies outside our own – clusters of stars similar to our own Milky Way, many thousands of light years away (Fox, 2002). Hubble was able to compare the known brightness of a coruscating type of star called a Cepheid with the brightness as seen through his telescope to calculate their distance from earth.
Observations of light emitted from distant stars and reflected by other bodies revealed a startling phenomenon – all the visible light emitted from the bodies was dominated by light from the red end of the spectrum and indeed all the radiation emitted from the stars was skewed towards longer wavelengths. This phenomenon became known as Red Shift.
The Doppler effect is the phenomenon where waves emitted from a receding body appear ‘stretched out’ to a stationary observer as successive wave peaks take longer to reach the observer. Waves emitted from an approaching body appear ‘squashed up’ as successive wave peaks reach the observer quicker. This is the cause of the apparent rise and fall in pitch from a siren when a police car or fire engine races first towards and then away from us, but it is also the cause of as Red Shift – that is to say, everything we see in the universe is red-shifted because everything in the universe is moving away from us (except for the Andromeda galaxy which is moving towards us and so appears blue-shifted). Furthermore, the more distant a body, the more red-shifted it appears and by inference the faster it is moving away. If everything in the universe is moving away from us and the furthest things are moving fastest then the universe must be expanding!
Figure 5: Doppler Effect and Red Shift. (Addison Wesley, from Mendez, 2002)
In fact, the recessional speed of celestial bodies (v) is proportional to their distance from the Earth (d)
v = Hd
Where H is the Hubble Constant. This allows a crude estimate to be made of the age of the Universe (Pople, 2001).
The discovery that the Universe is expanding posed two fundamental questions: How did it start?and Will it stop?
If the universe is expanding, the more time elapses, the bigger the universe becomes and the further apart the bodies occupying the universe. But what if we do a thought experiment where we imagine that the clock is running backwards and the universe is shrinking? If the trend continues indefinitely, once we have rewound the universe as far as it will go and the clock reads zero, all the contents of the universe have converged on a single point of zero breadth, width and height – the Singularity (Or primeval atom as George Le Maitre who first conceived of the notion, before Hubble had provided the evidence, would have it). Now if the universe is run forwards from this point, all the contents of the universe explode out from the Singularity to fill an ever increasing volume. This is the Big Bang.
The idea that the entire Universe once occupied an infinitesimal point seems frankly ludicrous, so it is worth looking both at the evidence that supports the Big Bang Theory and some of the Physics that explains how such extreme conditions could be possible.
The expanding Universe must have started in a small confined volume and then exploded outwards. Beyond this, the evidence for the Big Bang theory is that radiation from space (Cosmic Microwave Background Radiation) is at the same intensity in any part of the universe which means that all the universe was once at the same temperature, strongly inferring that it all came from the same place!
If the idea that all of the universe could fit into a speck of zero size seems impossible to accept, think of it this way: a vast amount of energy contained in a zero-sized Singularity – so potent that the fabric of space itself is curled up within it. The Big Bang becomes then, not the expansion of matter into empty space, but the creation of something in nothingness as the energy expands, driven by a single ‘superforce’, creating physical space in its wake. The energy begins to subdivide into different packets (bosons (quanta of energy) and fundamental particles of matter such as quarks and leptons) and so the superforce begins to separate into the four fundamental forces of the modern universe: gravity, electromagnetic, weak and strong nuclear. The universe then undergoes a period of rapid inflation, the fundamental particles group together into larger and larger particles and the four forces continue to separate as the energy which hasn’t formed into particles is demarcated into different types of boson. The Universe continues to expand, the particles group into atoms, atoms cluster together under gravity and many, many years later (as the universe continues to expand) the galaxies form. The Big Bang is not a explosion of matter into existing space, but the expansion of space itself. (Pople, 2001)
Figure 6: A timeline of the Universe. (Zeng, 2007)
As to the second question, Will the Universe stop expanding? The answer is ‘it depends’. It depends whether the universe is dense enough for gravity to stop or even reverse the expansion of space.
Calculations compare the average density of the universe ρ with the critical density ρ0 needed for gravity to gradually slow down the expansion (over infinite time). If ρ < ρ0 the expansion continues forever (and the universe cools down to a dark, frozen, mostly empty place). If ρ = ρ0expansion continues, but slows down, approaching zero as time tends to infinity. If ρ > ρ0 the expansion slows down, stops and reverses (and the universe collapses in on itself, possible returning to a singularity – The Big Crunch). The density of the universe is believed to be close to ρ0 (Pople, 2001).
The shape of the universe also depends on its density (since matter curves space according to Einstein’s Theory of Relativity)
Figure 7: Shape of the universe dependent on density. (Bunch & Wiitke, 2004)
The density of the universe depends not only on the amount of conventional matter present, but also on dark matter, a substance with the gravitational properties of conventional matter, but invisible to detection, predicted from the motion of the galaxies to surround them (Pople, 2001) and also on the mysterious Dark Energy, a substance with ‘repulsive gravity’ believed to explain the recent discovery that the expansion of the universe is speeding up (Fox, 2002).
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