The material and contents of the universe include about a 100 billion galaxies, that's a hard number to get our heads around. So let's use the Hubble Deep Field to illuminate this. The Hubble Deep Field was a time when the Hubble space telescope stared at a particular patch of sky for several weeks, looking incredibly deep, about 10 billion times fainter than the eye can see. It's worth remembering that the patch of sky observed was tiny, about the size of the head of a pin held at arm's length just a few minutes of arc across. Yet this tiny region of space contain thousands of galaxies at the limit of Hubble's vision. In this animation, we track across the Hubble Deep Field looking at even a tiny sliver of the field as the galaxies slip by each one that you see is billions of light years away. The brightest perhaps a few billion light years away, and the faintest smudge is eight or 10, or even 11 billion light years away. It's hard to imagine as we look at these distant stellar systems that on some of them, someone or some thing isn't doing the same thing and looking back at us. We have a 100 billion galaxies scattered with large spaces in between in a universe that's 46 billion light years in any direction. Those galaxies are made of stars and the Milky Way contains a typical amount of stars. There are galaxies both larger and smaller than the Milky Way. The full stellar census of the universe is 10 to the 23 stars a 100,000 billion billion stars. Since we know that virtually every star is likely to have planetary systems, you can start to imagine why astrobiologists are confident that there's life somewhere else in the universe. The census of atoms in all those stars and in the spaces between them, is 10 to the power 80. This is the largest pure number in science. It's an actual number, the number of atoms in the visible universe, most of them hydrogen and helium, a small fraction of a percent all the other chemical elements. Strikingly, and in a way that can only be explained by modern cosmology and the Big Bang Theory, there are more photons or particles of radiation in the universe than atoms by a factor of nearly a billion. So there are about 10 to the 89 photons in the universe. They are noticeable by us because they have very feeble energy and are microwaves. There's nothing about our situation in the extragalactic universe that's exceptional as far as we can tell. Our sun is indeed an average star located on the periphery but not the very edge of the Milky Way galaxy, that is one of many spiral galaxies we can find with our telescopes. Our large-scale environment of galaxies is not atypical either. We are typical or mediocre on every scale that we can inspect. We're not even made of the stuff that the universe is made of. Strikingly, dark matter and dark energy whose physical nature is not yet understood, compose most of the universe. Normal atoms are tiny fraction. In fact the sum of the atoms in those 100,000 billion billion stars is just a 0.5 percent of what the universe contains. So the visible stuff, all those stars in the galaxies is just like the luminous tip of a pyramid of mostly dark stuff. Mr. [inaudible]. Whenever life gets you down, Mrs. Brown, and things seem hard or tough, and people are stupid, obnoxious or daft. And you feel that you've had quite enough. Just remember that you're standing on a planet that's evolving and revolving at 900 miles an hour. It's orbiting at 19 miles a second, so it's reckoned. The sun that is the source of all our power. The sun and you and me, and all the stars that we can see, are moving at a million miles a day. In an outer spiral arm, at 40,000 miles an hour, of the galaxy we call the Milky Way. Our galaxy itself contains a hundred billion stars. It's a hundred thousand light-years side to side. It bulges in the middle sixteen thousand light-years thick. But out by us it's just three thousand light-years wide. We're thirty thousand light-years from Galactic Central Point. We go around every two hundred million years, and our galaxy is only one of millions of billions in this amazing and expanding universe. The universe itself keeps on expanding and expanding, in all of the directions it can whiz. As fast as it can go, at the speed of light, you know. Twelve million miles a minute and that's the fastest speed there is. So remember, when you're feeling very small and insecure, how amazingly unlikely is your birth. And pray that there's intelligent life somewhere out in space 'cause there's bugger all down here on Earth! Cosmology is the study of the structure and evolution of the universe and its material contents, particles and radiation. Essentially, it's a description of space and time and everything that occurs in our observable universe. It's a subject that's about a 100 years old. There were ideas of the universe of course going back to the ancient Greeks, but they didn't have the means or the observations to distinguish the large nature of space beyond the Earth. Although there was speculation even 2,000 years ago. The revolution starting with Copernicus displaced us in centrality from creation and started to make the universe successively bigger. Newton postulated an infinite universe but he had no real evidence to put on the table as to what the size really was. It was left to the astronomers to start mapping out regions of space far beyond the solar system. William Herschel did a large amount of this work building successively larger telescopes to map out the Milky Way, and used clever distance techniques to decide how far away faint stars were. He deduced that we lived in a system of stars that was tens or perhaps hundreds of thousands of light years across. The real revolution in cosmology happened in the early 20th century when Einstein developed the general theory of relativity which is the mathematical description by which we understand an expanding universe and Hubble detected the expansion through the redshift of galaxies. Somewhat later, the Big Bang model came into play in the middle of the 20th century. Modern cosmology is about 50 or 60 years old. The material contents of the universe are roughly a 100 billion galaxies containing a 100,000 billion billion stars. The sum of all these stars constitutes 10 to the power 80 atoms, the largest pure number in science. Those atoms are mostly hydrogen and helium and they're outnumbered by about a billion to one by photons, feeble microwaves leftover from the Big Bang. This is all in the visible universe. There may be regions beyond this whose mysterial contents we cannot measure. Also, all of these atoms in the stars and galaxies are outnumbered and outweighed by dark matter and dark energy.
