Appendix: Timeline of Clues
The next three pages contain a timeline of the Cosmological Clues from 1900 to 2020 that are discussed in this book. There are a few points of interest I would like to highlight.
Although the period from 1900 to 1920 does not have many Cosmological Clues it was a very active period for laying down the fundamental theories of modern physics that cosmology relies upon today. The laws of gravity were revolutionised by Einstein’s theory of relativity (which appears on the timeline) and also quantum theory was born during this period (also developed by Einstein and for which he won the Nobel prize in 1921). With theories of inflation and the need to solve the Dark Matter Problem and the Antimatter Problem, quantum theory has become as important to cosmology as it is to all other physics disciplines. The clues for quantum theory are not included in this timeline.
It is noticeable that during and after the Second World War there was not much astronomy activity and the new clues that were developed during that time were to do with nuclear processes, the theory of which was advanced significantly due to the interest in developing a nuclear bomb. Fred Hoyle and George Gamow were important players in applying the new nuclear physics to cosmology.
The last 40 years has been a very active one for cosmology and many new clues have been found. The more recent period may seem less active but this is a reflection that often time is needed to determine if the clue is real and the impact it has on cosmology. There will be many clues being worked on today that are likely to appear on a future timeline.
Atomic matter Also called baryonic matter, visible matter, ordinary matter and known matter. It is the matter in the universe that we can see in the stars, galaxies, gas and dust. It makes up about 15% of all the matter in the universe.
Baryonic Acoustic Oscillations (BAO and standard ruler) Oscillations of matter in the very early universe that created the temperature fluctuations in the CMB and a distinct distance that galaxies prefer to be apart from each other which can be used as a ‘standard ruler’ to measure expansion of the universe. Today this BAO distance is 490 million light-years.
Big Bang Nucleosynthesis (BBN) The creation of the first chemical elements, mostly hydrogen and helium, within the first few minutes after the Big Bang.
Cepheid variable stars A type of pulsating star that has a well defined relationship between timings of the pulses and brightness. It is used to determine the distance to galaxies and is an important rung on the cos mo log)' distance ladder.
Cosmic Microwave Background (CMB) Uniform background light seen across the whole universe that was produced when atoms first formed 380,000 years after the Big Bang. We see it today as microwave radiation at a temperature of 2.7K and wavelength of 2 millimetres.
Cosmic Web The large scale structure of the universe observed by the distribution of galaxies. It looks like a three-dimensional spiders web and consists of galaxy clusters (nodes), walls, sheets, filaments, and voids (holes) where there are very few galaxies.
Dark matter halos The dark matter that exists in a uniform sphere around galaxies. It generally goes out to much larger diameter than the visible galaxy.
General theory of relativity Einstein’s theory of gravity states that gravity is the curvature of spacetime. Matter curves space-time and the curvature of spacetime makes matter move. It is a fundamental theory used in cosmology.
Gravitational lensing Light is deflected by the gravitational attraction of very large masses such as galaxies and galaxy clusters, which act as gravitational lenses. This effect is also seen in the CMB.
Redshift Galactic redshift (Doppler shift): an object moving away from us emits light that has a longer wavelength (is redder) than if it was stationary. Cosmological redshift: the increase in wavelength of light due to the expansion of the universe.
Standard candle An object in the sky, such as a supernova or Cepheid star, that has a known brightness. The distant to these objects can be calculated and are an important part of the cosmological distance ladder.
Standard Model of Cosmology (ACDM) Also called the Lambda Cold Dark Matter model. The Standard Model of Cosmology provides an explanation of the evolution of the universe. It is based on the Big Bang, the expansion of the universe, cold dark matter, and dark energy.
Standard Model of Particle Physics The theory that describes the behaviour of fundamental particles and three of the fundamental forces (electromagnetism, weak nuclear force, and strong nuclear force). It is a very accurate model based on quantum field theory. It includes baryons (protons and neutrons) and leptons(electrons and neutrinos).
Supernova A massive explosion of a dying star that produces the heavy elements in the universe. Type la are a particular type of supernova that are used as a standard candle to measure distance to galaxies.