I've been out of college and grad school for a number of years now, but one little question that popped into my head during freshman astronomy still vexes me. If we can use red shift to determine the rate and direction at which objects are moving away from Earth, couldn't we take a sampling of objects and extrapolate from their speed and movement the origin, or point in space, from which they are travelling, ie, the origination point of the Big Bang? I read the other question that explains how all objects are moving away from each other and how space is expanding, but that doesn't account for the fact that the big bang is always described as this tiny point of super condensed matter. I'm thinking that extrapolating redshifts could point us back to that pinpoint of matter. Thanks for your time.
The Big Bang is often described as a tiny bit of matter, but that's an oversimplification. If the Big Bang occurred in a specific point in space, spewing galaxies in all directions, then we would expect our galaxy to be one of many galaxies sitting on an expanding shell of galaxies, with the center of that shell being the point of the "Bang." This, however, is not what we see, and not what the BB predicts.
If we were on a shell of galaxies, we would see many galaxies when we looked in directions along the shell, and few galaxies when we looked perpendicular to (up out of or down into) the shell. Moreover, distances and redshifts in such a scenario would depend on the direction we were looking. As we looked tangent to the shell, we would see many nearby galaxies with small redshifts. As we looked down into the shell, we would see more distant galaxies with higher redshifts. (Up out of the shell we would see only empty space.) This is not what we see. Galaxies, distant and nearby, are evenly distributed all around us. The number of galaxies and their redshifts are completely independant of which direction we look (we say that they are "homogeneous"), and that homogeneous distribution is also "isotropic," meaning that no matter where in the univerese you were, you would see exactly the same average distribution of galaxies and redshifts.
No, that little point of matter that was the Big Bang was not a little point of stuff inside an empty universe. It was, in fact, the entire observable universe. There was no "outside" of that point into which it could explode. In fact, the Big bang was not an explosion at all; it was simply the very hot state of the early universe. Distances between objects were much shorter back then, but the universe was still homogeneous and isotropic. Wherever you were in the early universe, you would see a homogeneous, even, distribution of matter and energy around you. There was no empty "space" outside of this point of matter into which it could expand, for all of space was already there, in that little "point." The expansion of the universe is manifested only in the stretching of space itself, perpetually increasing distances between distant objects, not in some "empty space" gradually getting filled as matter streams into it. These distances expand in all directions equally, and so cannot be traced back to a single point. If you try to do this, you find that the single point is your telescope, no matter where in the universe you observe from. After all, the "point" in question was all there was of space: the entire observable universe. The Big Bang happened everywhere. It happened right where you are sitting, where the Andreomeda galaxy is now, and in the most distant reaches of the universe. It's just that the reaches of the universe were not quite as distant those many billions of years ago.
This page was last updated June 27, 2015.