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Published 22nd December 2007 by

Cosmology is the astrophysical study of the history, structure, and dynamics of the universe. The word cosmology is derived from two Greek words, "kosmos" meaning the world, and "logos" meaning knowledge or science.
Cosmology Series
  1. What is Cosmology?
  2. The Speed of Light
  3. Redshift and Blueshift
  4. What is the Cosmic Microwave Background Radiation?
  5. Cosmic Scale Factor and Expansion of the Universe
  6. The Physics Governing the Universe
  7. The Nature of Light
  8. The Four Forces in the Universe
  9. The Expansion of the Universe

Cosmology as a science has experienced extensive changes over the past centuries, growing from a purely philosophical path to a modern science relying on accurate astronomical measurements. It has evolved to become an extremely complex science, including the study of particles, on scales tinier than an atom, as well as the formation and evolution of stars, galaxies and structures on scales as large as the observable universe.

The first question usually asked in cosmology is this:

When did the Universe start and how will it end?

Only in the last century has it been possible for scientists to establish that the universe is expanding. The remaining challenges for cosmology are to determine:

  1. How fast the universe is expanding?
  2. How long this expansion will last?
  3. Will the expansion will ever stop or even reverse?

How fast the universe is expanding?

To answer the first question, we can measure the average mass density of the universe and find out whether there is sufficient mass for the gravitational force to stop the current expansion, or secondly, by observing the expansion velocities of galaxies at great distances, and therefore earlier times, to measure the rate at which the universe is expanding is changing.

The second method includes a promising approach using observations of supernovae and gravitational lenses. Both methods are being actively pursued, and results are hoped for in the near future.

The expansion of the universe can be interpreted as understanding that at one point the universe must have been enormously dense and hot. So hot that it consisted almost entirely of radiation. As the universe expanded, it cooled. This idea is known as the Hot Big Bang model.

Cosmic Background Radiation map from COBE

Cosmic Background Radiation map from COBE


For decades it remained untested and controversial. Today observational data like the Cosmic Background Radiation (CBR) and the abundance of light elements, such as helium, deuterium and lithium, are in good agreement with the predictions of the Hot Big Bang model.

How long this expansion will last?

The second question is also the subject of extensive research, and involves investigations into what dark matter is and what cosmological role does it play?

Details of the statistical properties of the expected structures (galaxies, clusters, superclusters in the universe) are dependent on the amount of mass present in the universe. There is strong observational evidence that apart from the luminous objects which radiate enough energy to be detected using modern technology, there is a vast amount of invisible matter, the so-called dark matter.

Scientists think that Dark matter may account for more than 80% of the total mass in the universe. The nature and exact distribution of the dark matter are still a mystery, despite a range of proposed models. The most obvious candidate for this dark matter is ordinary matter in the form of old, burned-out stars or stars that are too small to shine. However, the predictions of the abundance of the conventional matter, obtained by the measurement of a number of light elements, shows that it can account for only a small fraction of the dark matter. Something other than ordinary matter must be present - exotic particles.

Exotic particles would be a relic of some process in the very early universe. It is very important to identify this exotic dark matter, by direct search and by accelerator experiments, with particle theory providing guidance to develop the experiments. At present, the best-accepted candidates for the exotic dark matter are weakly interacting particles (WIMPs), axions, and neutrinos with finite mass. Of these, only neutrinos are known to exist, the rest is purely theoretical.

Will the expansion will ever stop or even reverse?

Maps of the distribution of galaxies in the current universe reveal a network of thin, filamentary structures of galaxies separated by quasi-spherical voids. When looking at objects at large distances we are observing the universe's past because of the time taken for light to reach us. The detection of large-scale structures at very large distances provides strong observational constraints on the models of structure formation. The early universe contained small density irregularities, as measured today by fluctuations in the CBR, and the amplitude of these small bumps grew via their self-gravity to make the overall structure seen today. By carefully measuring changes in the overall expansion at ever increasing distances, hence back in time, we can create some mathematical models for the rate of expansion for the universe and propose theories on what will happen in the future.

Recent observations made by the COBE and WMAP satellites observing and accurately measuring the Cosmic Background Radiation have effectively, transformed cosmology from a highly speculative science into a predictive science. This has led many to refer to modern times as the "golden age of cosmology".

In this series, we will cover the above topics in more detail as well as looking at how the large-scale structure of matter formed in the early universe, the nature of zero point energy, the sub-atomic particles, and much more.

Tutorial Series

This post is part of the series Cosmology. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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