Module2 - Part 2: Account of the science of the Universe (2008)

The beginning. The ultimate point of unification of all physical laws and yet also that point where every thing we know about those laws breaks down and leaves us with only more questions.  However, in spite of the limitations to our knowledge, science has built a remarkably robust consensus on the story of the universe from very early times to the present. Through what has been pieced together from astronomy and particle physics within the framework of General Relativity (GR) there is broad agreement about the history of the universe from about the first 1 billionth of a second after the beginning.

Physics looks back through time through three 'windows' on the early universe (see Table 1). The primary way of perceiving is through our theories, extrapolating known laws back to the beginning then directs and organizes the quest for understanding (see for e.g. Gell-Mann,2007). The second window is the direct observation the universe around us; we observe the past as we look at light from distant events. The final window is our experiments, where comparable conditions to those found in the Big Bang or astronomical objects are simulated and our theories tested.

	Developmental stage of universe	Windows on the universe
Time relative to Big Bang   (seconds)		Theoretical	Observational	Experimental
Before?	Augustinian Era	Theology
0	Singularity	Classical GR predicts infinities;                       – the laws break down.
0 > 10-43	The Planck Era	Heuristic quantum theory of gravity in place of future unified theory of GR and QM – supergravity or String Theory	Is our universe filled with  gravitational radiation from the big bang? Can this tell us about quantum gravity and GR?
10-43 > 10-36	The grand unification Era	GUT
10-35
	Baryogenesis	Why does the observable universe have more matter than antimatter?	Matter still in existence	No observed preference for energy to create matter over antimatter
10-32	Inflationary Era
10-23		Standard Model of particle physics – Does the Higgs Boson give rise to mass?	Particles have rest mass	Large Hadron Collider (LHC) - particle accelerator at CERN, will look for Higgs. (May 2008 - now found)
10-12	Electroweak Era – the weak force and electromagnetism are unified at this stage.	Quantum ElectroDynamics, Standard Model of particle physics valid to these energies.	Neutrino background – will be very hard to observe as they seldom interact	Accelerators explore here, e.g. Relativistic Heavy Ion Collider: created quark-gluon plasma
1013	Recombination- the universe becomes transparent to light.	Black body thermal radiation	CMB radiation starts its journey Temperature ~ 3000K
5´1017 (Now)		Anthropic principle to explain  “Goldilocks” conditions	We exist, & observe stars, galaxies, CMB, the universe...

Table 1. Windows on the universe.

When Einstein formulated general relativity he recognised that its equations implied that the universe must be evolving in time. Like Newton, he preferred the idea of an eternal universe and in order to maintain the static solution he introduced the cosmological constant in order to balance gravity. It was Friedman and then Lemaître who first proposed the concept of a dynamic universe with a beginning, and the expansion of the universe was then confirmed observationally by Hubble. By identifying the standard candle of Cepheid variable stars, in 1923 Hubble first proved that some nebula were in fact galaxies far beyond our own. Further observations showed that these galaxies were moving away from us, as indicated by the Doppler shift of the light we receive from them. When the distance was plotted against the speed of the galaxy it could be seen that the further away from us a galaxy was, the faster it was receding. In other words it looked like at some point in the past all the matter in the galaxies would have been compressed together. The linear relationship between recessional velocities of galaxies and their distance from us, known as Hubble's law, is still one of the main evidences for the Big Bang.

Figure 1. Summary of the Stages of Evolution in the

              Standard Cosmological Model of the Universe, the Big Bang.

This gave rise to the framework of the Standard Model of cosmology (see Fig 1). All solutions of GR originate at a point when the density is infinite and the scale factor (the distance light could have traveled from the beginning; the universe could still be infinite in extent at the beginning) is zero. This point of origin is the Big Bang singularity, at this point the laws of physics break down and it is not possible to predict what the emerging universe will be like. This was an apparent dead end with the need for an external reason to set the initial conditions. Fred Hoyle, Hermann Bondi and Thomas Gold, among others, saw it as a philosophically ugly concept. This led them to recover the notion of an eternal universe by proposing the Steady State model where matter is continually created in the expanding space and on average the universe is unchanging.

Early Big Bang theorists George Gamow and Ralph Alpher built on work by Hans Bethe who first provided those nuclear mechanisms by which hydrogen is fused into helium (see Era of nucleosynthesis in Fig 1). The primordial nucleons (neutrons and protons) were formed from the quark-gluon plasma of the Big Bang as it cooled below ten million degrees. A few minutes afterwards, starting with only these neutrons and protons, nuclei up to lithium and beryllium (both with mass number 7) were formed but only in relatively small amounts. In the Big Bang model this process of primordial nucleosynthesis predicts the observed ratio of Hydrogen to Helium (~ 25% He). Ironically further work on nucleosynthesis of heavier elements in stars by Hoyle gave further support to the Big Bang theory. Hoyle's work explained how the abundances of the heavier elements increased with time as the galaxy aged and filled the gap in the Big Bang nucleosynthesis explanation. His theoretical work required the existence of an excited state of the carbon nucleus of 7.65 MeV that had not been observed before. At Hoyle's pressing nuclear physicist Willy Fowler and his team looked for and found (in 1953) the previously overlooked state (Singh, 2005:395).

Figure 2. It is thought that at high enough energies the Strong force, and

       ultimately gravity, will be unified with the other forces.

This particle physics based avenue of inquiry into the beginning of the universe has been extended much since Hoyles' time. Experiments which recreate high energy conditions similar to those a couple of seconds after the big bang can now take us back to where electromagnetic force is unified with the weak nuclear force, Fig 2.

In between the classical singularity of GR and the experimentally probed electro-weak unification, there are an infinite number of potential string theories (VanProeyen, 2004) and hundreds of inflationary models and untested grand unification theories all looking for astronomical observations, or for the new results from the Large Hadron Collider (LHC), in order to confirm or eliminate theorys on the bases of their predictions. The LHC will recreate the conditions existing a billionth of a second after the beginning of the Big Bang. The re-creation of these conditions should provide us with information about energy levels beyond those for which the Standard Model of particle physics gives intelligible answers. The hope is that we will gain insight into a new theory of particle physics and the conditions of the very early universe.

The Steady State model survived for some time, but this model could not account for the observations of radio sources from 1955 on, which showed that there were many more radio sources further away from us than there were close by. This meant that either we are situated in a region of the universe in which radio strong sources are less frequent, or that there were more radio strong sources at some time in the past than when the radiation was emitted. Neither possibility supports the steady state prediction that the distribution of sources should be constant in space and time, although questions about discrepancies in the observations remained for a while.

The prediction and observation of the exact temperature of the cosmic microwave background radiation (CMB) in 1964 finally confirmed the Big Bang over the Steady State model. The Big Bang model predicted that there must have been a time in the past when the universe was hot and dense, with radiation and matter in equilibrium and we should be able to observe the left over radiation. Using this 'light' the earliest we can reach is the point the universe became transparent when it was about 300,000 years old. Before this time matter was fully ionised and the charged particles scattered the light, Fig 3. When matter had cooled enough the electrons combined with the baryonic matter to give neutral atoms. At this point radiation de-coupled from matter and free streamed through the universe (Fig 4). As the space it was traveling through expanded it's wavelength was red-shifted so that it's temperature has reduced from about 3000K to 2.7K today.
Figure 3. Observations take us back through time as we look at more distant objects.

We can only see back in time to the point where the universe became transparent.

 Figure 4. Before and after recombination.

Before the observations of the Hubble expansion and the CMB the most cosmologically significant observation was that the sky at night is dark. Olbers' paradox states that in an infinitely large universe there will always be a star on any given line of sight, and so the night sky should actually be as bright as the surface of the sun. This paradox is only solved by the facts that the universe is finite in time (so that light has not yet reached us from very distant stars) and the initial radiation of the Big Bang has been red shifted and cooled.

Figure 5. Stellar evolution

After recombination the universe expanded getting cooler and darker until gravitational attraction amplified the initial fluctuations in the smooth distribution of matter and clouds of gas and dust formed (Fig 5). The more compact matter becomes, the stronger the force of gravity; so the process of collapse accelerates and becomes more violent as particles collide and compress to form a sphere at the nebula's heart. The Protostar heats as it collapses until it is hot enough for nuclear reactions to start. The star ignites, and starts burning Hydrogen. At this point it has joined the 'Main Sequence' of stars on the Hertzsprung-Russell plot (Fig 6) where it remains for some time as the radiation pressure (from the heat of collapse and nuclear reactions) maintains the star from further collapse.

The future evolution and 'death' of a star depends on its mass. Higher mass stars evolve more quickly with higher temperatures which enable then to fuse heavier elements up to iron. As time passes the star builds up a central core of elements for which the temperature is not high enough to fuse. For stars over 0.4 solar masses the outer layers of the star will expand forming a red giant while the core continues to contract. In addition to the thermal pressure from the kinetic energy of collapse or nuclear reactions there are two further barriers to the continuing gravitational collapse of the star core. The first is electron degeneracy which arises from the Pauli exclusion principle which limits the number of electrons that can exist in a given energy level. For main-sequence stars with a mass below approximately 8 solar masses the mass of the core remains below this limit, called the Chandrasekhar limit. The core stops collapsing and the outer layers will be blown off leaving a white dwarf. Stars with higher mass will develop a degenerate core whose mass will grow until it exceeds the limit. Inverse beta decay is then energetically favored and the electrons are captured by protons in the core. At this point the star will explode in a core-collapse supernova, leaving behind a neutron star or, if the mass of the remnant of exceeds ~3-4 solar masses (either because the original star was very heavy or because of accretion of additional matter) even the final barrier of neutron degeneracy pressure is insufficient to stop the collapse. After this there is no known mechanism which is powerful enough to stop total collapse to a point or singularity. The strength of gravity up to the “event horizon” is now so strong that light can not escape and the star is a black hole.

Figure 6. Hertzsprung-Russell diagram showing an Example of Stellar Evolution.

         (The diagonal lines indicate the radius of the star relative to the sun.)

Just as there is a balance between the thermal pressure and the gravitational force for a star of a given mass, temperature and density (described by Jeans equation) that determines if collapse will happen, there is a balance between the kinetic energy of the Hubble expansion and the gravitational potential energy of the universe. Balancing these two terms leads to the definition of the critical density:

At this critical density the cosmological density parameter Ω_m is defined as being exactly equal to 1.

This is described as being a flat universe which will continue to expand asymptotically to zero speed at infinity.

If there is less matter than this and Ω_m<1 the Universe will continue to expand indefinitely. The result of continuous expansion would be, according to the 1st Law of Thermodynamics, a Universe which would gradually cool down until the temperature became 0 Kelvin. If the density is larger than the critical value Ω_m>1 the Universe will expand to a point, and then collapse on itself. It has been postulated that if the Universe were to carry on this course, all matter would re-condense into a singularity, and maybe recreate another big bang. Current observations suggest that the expansion of the universe is accelerating, theory can account for this by adding a “dark energy” term to the equations of GR (Ω_ν>0). The red curve on Fig 7 shows the evolution taking in to account this accelerated expansion due to dark energy.

Figure 7. The end of the universe?