## Lecture notes:

Day #5, Tuesday, January 28, 1997: The goal today is to explore the question "From where did we come?" The answer is that the universe was born out of "PRIMORDIAL CHAOS."
• The start of our quest for our origins is in the discovery in 1929 by Edwin Hubble that galaxies are moving away from us. Except for members of our "local group" of galaxies, lying within a few million light years from us, every galaxy in the universe….and we mean every one…is moving away from us as though we suffered from a severe case of cosmic body odor. The further a galaxy is from us, the faster is it moving away. One of Hubble's startling discoveries was that the speed of recession increases directly proportional to distance. A galaxy that is twice as far as another is traveling twice as fast; one that is 100 time more distant is traveling 100 time faster! Very strange! This proportionality of velocity and distance is known as the Hubble relationship and can be written V=Hxd, where V is velocity, H is the so-called Hubble constant, and d is distance.
• An even stranger feature of this relationship is that at a sufficiently great distance from the earth, galaxies should be moving at the speed of light. IMPOSSIBLE As far as we know, nothing in our universe can travel faster than light. The distance at which galaxies would be traveling at light speed is about 15 billion light years. No galaxy can exist beyond it for it would be traveling faster than light, and could not be inside our universe.
• Space ends thus at a distance of 15 billion light years. So too does time end, 15 billion years in the past.
• Since we travel back in time as we move out in space, the edge of space is also the edge of time. Our universe can not be older than 15 billion years; we and everything within our universe were born 15 billion years ago.
• What was the universe like at that time? IT WAS SMALL. AND HOT We can imagine traveling backward in time like running a movie backward. In the smaller universe, the same amount of matter and energy was concentrated in smaller amounts of space, and therefore the energy density would have been larger and the temperature would have been higher. Smaller and smaller as we go into the past: hotter, hotter, and hotter. Our average temperature of the universe is now 2.7 degrees above absolute zero. At the earliest time we can imagine, the temperature was 1032 K; that time corresponds to the end of Primordial Chaos, which was 10-43 seconds after creation. At that time, the average density of tèe universe was astonishingly high, 1092 grams per cubic centimeter.
• At earlier times, the universe was totally chaotic. Space and time were so chaotic, that there was no sequence of time or organization of space. The universe at that time was too young for messages traveling at the speed of light to move from one particle to another, and all was therefore in darkness and ignorance: no gravity, There was soo much chaos that there was NO PHYSICS, and that is chaotic, believe me. Because of this layer of primordial chaos (extending from time=0 to 10-43 seconds) around the actual moment of creation, we can never explore the actual moment of creation with our physics. It will remain forever outside of our ability to explore, The instant of creation is like what ever lies beyond the edge of space: FOREVER UNKNOWN.
• This very early universe, which was incredibly small and incredibly hot, is what we mean by the BIG BANG: immense temperatures and densities that could not be contained and exploded. In that explosion chaos was converted to order.