What is Cosmology?:
Cosmology is the study of the Universe and its components, how it formed, how it has evolved and what is its future. For the first time in history, technology has allowed cosmologists to make astronomical observations that probe billions of years into the past, to the edge of the knowable universe. This course on 'Observational Cosmology' is meant to be an overview of this new era of discoveries, and the evolution of our understanding of the Universe driven by key astronomical observations through history.
What's all the excitement about?:
For 80 years, astronomers have known that the universe is expanding, that galaxies appear to be flying away from one another. Measurements of the speed and distance of other galaxies show that the more distant a galaxy is from the Milky Way, the faster it recedes. This phenomenon is called the Hubble expansion. Working backwards from their data, astronomers infer that the universe must have been created at a definite time, between 10 billion and 14 billion years ago.
In its early stages, the universe must have been enormously dense and hot -so hot that at one point it consisted mostly of radiation. As the universe expanded, it cooled. This basic idea is behind the so-called hot Big Bang model. This model provides the theoretical framework for our current understanding of the cosmic history from the the early Universe, just a few seconds after the Big Bang, till the present epoch, about 10-15 billion years afterwards.
A dream of cosmologists is to be able to make detailed studies of galaxies at great distances and early times. What did galaxies look like soon after formation? How do they evolve? When did they form? With the Hubble Space Telescope (HST) and giant (8- to 10-meter) new ground-based optical telescopes just commissioned and being built, the astronomical exploration of deep space (astronomers say "high redshift") has already started. The major discoveries in the last decade have left no doubt that a new era is beginning for observational cosmology.
This course aims to provide an introduction to this new era of discoveries. The fundamental questions that we will address in this course, and the key observations on which the current answers are based on, are summarized below.
The cosmic questions:
The most direct cosmological observation you can make is to find some standard candle, an object with a known luminosity, and follow its change in apparent luminosity with distance (like watching the headlights of a distant car). The problem is most objects, like galaxies, change in brightness from the past till now. One object which is constant is the brightness of a supernova, but until recently the technology to capture them at high distances was not avaliable. Below is the results of the high redshift supernova project, a unique combination of space and ground-based telescope work.
This new observation implies that something else is missing from our understanding of the dynamics of the Universe, in math terms this means that additional cosmological constant in Friedmann's equation, L. The implication here is that there is some sort of pressure in the fabric of the Universe that is pushing the expansion faster. A pressure is usually associated with some sort of energy, that has been named dark energy. Like dark matter, we do not know its origin or characteristics.
|Figure 2. Data from surveys of galaxies show the universe to be clumpy and uneven. The voids and "walls" that form the large-scale structure are mapped here by 11,000 galaxies. Our galaxy, the Milky Way, is at the center of the figure. The outer radius is at a distance of approximately 450 million light-years. Obscuration by the plane of the Milky Way is responsible for the missing pie-shaped sectors. North is at the top. (Courtesy of Smithsonian Astrophysical Observatory, 1993. Northern data-Margaret Geller and John Huchra, southern data-Luiz da Costa et al.)|
These questions are at the heart of humankind's quest to understand our place in the cosmos within the current understanding of the universe. They involve some of the most fundamental unanswered questions of physical science. Although there is no direct, practical application for this knowledge, the influence of our research in cosmology is far reaching. Cosmological knowledge affects -and is influenced by- religious beliefs, ethical choices, and human behavior, which in turn have important long-term implications for humanity. Few areas of the human endeavor excite our imagination as much as curiosity about the universe. How did it start? What is our role? How will it end? Throughout human history one finds this desire for knowledge about the heavens and human existence, and virtually all periods of enlightenment and progress have been times of rapid discovery in astronomy and physical science. Future generations will look back and evaluate our era's contributions similarly.