Cosmology is the study of the structure and evolution of the universe. In this section, we'll study the size of space going out to the most distant objects astronomers can measure with the largest telescopes. With large telescopes, we look out in space and also back in time because light does not travel with infinite speed. Three hundred thousand kilometers per second means the nearest galaxies are seen as they were a few million years ago, the most distant galaxies are seen as they were billions of years ago. We've managed with the largest telescopes to look back within five percent of the age of the universe, now measured to be 13.7 billion years. The contents of the universe are still enigmatic, because 95 percent of the universe is dark energy and dark matter, both still poorly understood in a physical sense. The rest of the matter, normal stars and normal galaxies, is five percent. Cosmology is based on a Big Bang model, the idea that the universe had a hot dense beginning. The Big Bang model is well supported by a web of evidence, primarily the expansion of the universe observed for the recession velocity of galaxies, the microwaves leftover from creation, which is an imprint of the radiation in the universe seen when it was only 400,000 years old. Also we can only explain the large amount of helium in the universe from nucleosynthesis occurring in the first three minutes when the universe itself was as hot as the center of a star. Finally, we have good evidence from large telescopes that the universe was not always the same. Galaxies were smaller and bluer in the past and have been assembled from smaller pieces, and we can observe this process occurring over billions of years. The critical piece of evidence supporting the Big Bang is the microwave background, now observed in exquisite detail with the WMAP satellite. More speculative additions to the Big Bang model such as the inflation scenario are on the edge of being tested by microwave observations. This would take us back to the first tiny fractions of a second after the Big Bang. Current ideas of the future of the universe are based on the omnipresence of dark energy causing an accelerated expansion. In all likelihood, the universe will continue to grow larger and more diffused forever. At the frontiers of the Big Bang theory is the idea of the multiverse. It's a conventional part of the Big Bang theory that physical space, all there is, is larger than the observable space we can see with our telescopes. But the Big Bang theory also involves the idea of quantum genesis or perhaps our other parallel space-times unobservable by us that exists contemporaneously with our universe. This speculative idea may be on the edge of being tested by future observations. The subject of cosmology addresses the biggest questions there are in astronomy: why are we here; how did we get here; and where did it all come from?
