<p>The observable universe:<br /><br />Imagine the beginning of time. If light were around, it would take time to reach you, but this is the beginning of time so no light has had time to move yet. Right at the beginning, your <em>observable</em> universe has no size at all! As time moves forward, any light that exists will move at the speed of light. Suddenly you might see a small distance all around you, as light starts coming in from different directions.<br /><br />After a year, you would be able to see 1 light-year in all directions. After 100 years therefore, your observable universe would be a sphere, 100 light-years in radius. After 13.7 billion years, your observable universe would be 13.7 billion light-years in radius, as you receive light that has been travelling for 13.7 billion years.<br /><br />Hah! If only it were <em>that</em> simple! The problem comes when considering that the universe is <em>expanding</em>. At the start of things our observable part of the universe was very small and the universe was expanding incredibly fast, much faster than light. Also, light could not move freely until around 370,000 years after the Big-Bang. Before that, photons were frequently interacting with other particles and atoms didn't exist as everything was very hot and mixed up!<br /><br />But at 370,000 years in, the universe had been expanding and the temperature had cooled enough for atoms to form in a flash of light (the universe finally became transparent and photons first moved freely throughout it). These photons filled the universe at that time, and we still receive these photons today. They are now stretched into microwaves (by the expansion of the universe) and are known as the Cosmic Microwave Background Radiation (CMBR).<br /><br />As all this was happening, the universe was expanding. When we worked out how much we thought those photons had been stretched by the expansion, it told us how much bigger the universe is today, than it was when those photons were emitted. We estimate that, when the CMBR was emitted, our observable universe was around 42 million light-years in radius, around 1100 times smaller than it is today.<br /><br />Hang on though! Didn't I earlier imply that, 370,000 years after the BB, our observable universe would be 370,000 light-years in radius? Well, that radius, based on the time that light takes to travel, is not actually a useful measure of distance at all! It is a measure of time elapsed only. When astronomers say the universe is 13.7 billion light-years in radius they are not giving you a distance through space, they are giving you a distance through time. Proper distance is a different thing entirely (although at distances closer to today, they are essentially the same). <br /><br />The CMBR photons we receive today have been travelling for 13.7 billion years, but they were emitted at a proper distance of only 42 million light-years away, all that time ago. The reason they have taken so long to reach us is that the universe is expanding, putting more distance in between photons and their eventual "targets".</p><p>At the beginning, imagine a point in space was right next to the point where our galaxy finally formed. The universe is expanding incredibly fast, carrying all neighbouring points directly away from us in all directions. If we move on to only 370,000 years later that (originally neighbouring) point in space was 42 million light-years away - that's how fast the universe was expanding, early on. Then the CMBR photons were emitted.</p><p>When those photons were emitted, the space they were travelling through was receding from this point in space so fast that, from our point of view, it was as if the photons themselves were receding from us too! The gradual deceleration of the expansion allowed those photons to <em>eventually</em> start making actual progress towards us, from our point of view. By the time they found themselves in regions of space where an object was receding from us slower than light, they were 5.7 billion light years away from this point in space, and the universe was around 4.5 billion years old! (This is when those photons crossed into our Hubble Sphere as it was at that time)<br /><br />13.7 billion years after they were originally emitted, 9.1 billion years after they found themselves in space that was receding from us only sub-luminally, we receive those CMBR photons that were only emitted 42 million light-years away. And the real mind-bender is that we think that the original emission point is now over 46 BILLION light-years away. The edge of our observable universe, the most distant point from which we have received CMBR photons, is 46 billion light years away and continues to recede from us. That "edge", known as the surface of last scattering, was receding from this point in space at over 58 times the speed of light when those CMBR photons were emitted, it is still receding at around 3 times the speed of light today and we assume there are galaxies there now, but all we see is the radiation emitted from there, long ago. <br /><br />The other mind-bender is that the whole universe is probably larger than our observable universe. After a fraction of a second, when our observable universe only had a radius of 10cm, there may well have been the same thing happening 20cm away. When the CMBR was emitted, and our observable universe was only 42 million light-years in radius, there might have been CMBR emitted 80 million light-years away, or much further away than that. Today, when we think our observable universe has a radius of 46 billion light-years and we assume, as galaxies formed in <em>these parts</em> that there would be galaxies throughout, there could be galaxies whose own observable part of the whole universe is totally separate from ours, galaxies that are 100s of billions of light-years away, outside of our observable part of the universe. </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>