emh@bonnie.UUCP (Edward M. Hummel) (12/17/84)
<> I haven't been following the latest trends in cosmology very closely, but I recall hearing something about parts of the universe being "unobservable" that seems a bit puzzling. I'm very fuzzy on the details and would appreciate it if someone could correctly state the idea. Here goes.... In the "big bang" theory (e.g. new inflationary universe version) regions of the universe are speeding away from other regions at very high speeds. As per Hubbles law the parts 'farthest' from us are travelling away from us at very high velocities. The puzzling thing is that there are parts which are so far away that light from them has not had enough time to reach us (the universe being only 12 billion years old). This implies that they are outside of a 'light cone' centered at our present. If this is now true, then it must have always have been true. I.e. matter can not outrun the light cone. How is this reconciled with the very small size of the "universe" at t=0 ? What does it imply about the structure within the initial singularity? Comments, answers, further questions appreciated. Thanks, Ed Hummel ..ihnp4!clyde!bonnie!emh
ethan@utastro.UUCP (12/19/84)
[] > In the "big bang" theory (e.g. new inflationary universe version) >regions of the universe are speeding away from other regions at very >high speeds. As per Hubbles law the parts 'farthest' from us are >travelling away from us at very high velocities. The puzzling >thing is that there are parts which are so far away that light from them >has not had enough time to reach us (the universe being only >12 billion years old). This implies that they are outside of a 'light >cone' centered at our present. If this is now true, then it must have >always have been true. I.e. matter can not outrun the light cone. > How is this reconciled with the very small size of the >"universe" at t=0 ? > What does it imply about the structure within the initial singularity? This problem is referred to as the "horizon" problem and can be summarized as follows: When we look in opposite directions in the universe the most distant regions we see have never exchanged signals with one another. How is it that they look the same? In particular, the microwave background temprature is identical in both directions to a part in 10^3 (even that is due to a dipole moment which can be ascribed to our local velocity). There are two answers to this. The first (and least satisfactory) is that the initial conditions of the universe require a homogeneous universe. It is left to one's imagination to decide whether this is due to theology or quantum gravity (or whether the two are the same :-) ). The second is the "inflationary" universe. This is the proposal that the visible universe started out so small that signals could be exchanged across it in less than an expansion time. If the equation of state of the contents of the universe tells one that the pressure is postive (or at least not less than minus 1/3 of the energy density) then any region that one can exchange signals across at early times becomes a smaller and smaller fraction of the visible universe and the horizon problem is unsolved. On the other hand, if the universe has a very negative pressure then the universe expands so that it becomes *harder* to exchange signals at later times (and eventually impossible). In such schemes the distant parts of the universe that we see as just being able to exchange signals were in close contact at early times and are now seeing each other for the second time (not the first). If the vacuum can carry an energy density that affects gravity (not the case now to high precision) then it has a pressure equal to minus the energy density (from Lorentz invariance). This causes the universe to expand exponentially (i.e. with a constant rate of expansion). The appropriate buzzword here is DeSitter space. In this case the apparent size of the universe is a gross underestimate of the size of the region which was in causal contact at early times. Why should the universe have a vacuum energy density at early times? It appears to be a natural consequence of the idea that we live in a spacetime whose vacuum is not truly symmetric but instead is filled with a field that causes the different fundamental forces to act differently. If this state of the vacuum has zero energy density, then it follows that the symmetric state expected at *very* high temperatures has a large energy density. None of the above is truly an inescapable conclusion from particle physics. In fact, right now it looks more like a wish list that cosmologists have presented to particle physics. Whether or not it is correct is something that will become clear when particle physicists succeed in constructing a reasonable theory that unifies the forces of nature. Somehow I have the feeling I've answered your question at too great a length. The above will not be the official opinion of the University of Texas until such time as it can be reliably ascertained that hell has frozen over to a depth of at least 10 meters. "I can't help it if my Ethan Vishniac knee jerks" {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712