Shortcomings of the Standard Cosmology
Despite the self-consistency and remarkable success of the standard Hot Big Bang model in describing the evolution of the universe back to only one hundreth of a second, a number of unanswered questions remain regarding the initial state of the universe.
The flatness problem
Why is the matter density of the universe so close to the unstable critical value between perpetual expansion and recollapse into a Big Crunch?
The horizon problem
Why does the universe look the same in all directions when it arises out of causally disconnected regions? This problem is most acute for the very smooth cosmic microwave background radiation.
The density fluctuation problem
The perturbations which gravitationally collapsed to form galaxies must have been primordial in origin; from whence did they arise?
The dark matter problem
Of what stuff is the Universe predominantly made? Nucleosynthesis calculations suggest that the darrk matter of the Universe does not consist of ordinary matter - neutrons and protons?
The exotic relics problem
Phase transitions in the early universe inevitably give rise to topological defects, such as monopoles, and exotic particles. Why don't we see them today?
The thermal state problem
Why should the universe begin in thermal equilibrium when there is no mechanism by which it can be maintained at very high temperatures.
The cosmological constant problem
Why is the cosmological constant 120 orders of magnitude smaller than naively expected from quantum gravity?
The singularity problem
The cosmological singularity at t=0 is an infinite energy density state, so general relativity predicts its own breakdown.
The timescale problem
Are independent measurements of the age of the Universe consistent using Hubble's constant and stellar lifetimes?
A Brief History of the Universe
The history of the Universe divides roughly into three regimes which reflect the status of our current understanding:
The
standard cosmology is the most reliably elucidated epoch spanning the epoch from about one hundredth of a second after the Big Bang through to the present day. The standard model for the evolution of the Universe in this epoch have faced many stringent observational tests.
Particle cosmology builds a picture of the universe prior to this at temperature regimes which still lie within known physics. For example, high energy particle acclerators at CERN and Fermilab allow us to test physical models for processes which would occur only 0.00000000001 seconds after the Big Bang. This area of cosmology is more speculative, as it involves at least some extrapolation, and often faces intractable calculational difficulties. Many cosmologists argue that reasonable extrapolations can be made to times as early as a grand unification phase transition.
Quantum cosmology considers questions about the origin of the Universe itself. This endeavours to describe quantum processes at the earliest times that we can conceive of a classical space-time, that is, the Planck epoch at 0.0000000000000000000000000000000000000000001 seconds. Given that we as yet do not have a fully self-consistent theory of quantum gravity, this area of cosmology is more speculative.
Chronology of the Universe
The following diagram illustrates the main events occurring in the history of our Universe. The vertical time axis is not linear in order to show early events on a reasonable scale. The temperature rises as we go backwards in time towards the Big Bang and physical processes happen more rapidly. Many of the transitions and events may be unfamiliar to newcomers; we shall explain these in subsequent pages.
Orders of magnitude
The timescales and temperatures indicated on this diagram span an enormous range. A cosmologist has first to get the order of magnitude (or the power of ten) correct. Quantities which are given as 10 to some power 6 (say) are simply 1 followed by 6 zeros, that is, in this case 1,000,000 (one million). Quantities which are given as 10 to some minus power -6 (say) have 1 in the 6th place after the decimal point, that is, 0.000001 (one millionth). At extremely high temperatures we tend to use gigaelectron volts (GeV) instead of degrees Kelvin. One GeV is equivalent to about 10,000,000,000,000K.
