The expanding universe – Food for thought

As divisive and self-important as humanity sometimes gets, the universe just keeps moving around us. Hate and fear race across the page, and yet LIGO observes gravity waves, Space-X lands rockets, and 3D printers learn to print replacement organs. We are all really very small.

The most widely accepted estimates put the age of the universe at just under 14 billion years. We’ve seen objects out to 13.3 billion light years away, in all directions. That doesn’t mean those most distant objects are 13.3 billion miles away. They were that distance away back when the light we are seeing today was emitted; 13.3 billion years ago.

The universe is expanding, so those objects are now a distance further away, the amount determined based on on the speed at which the universe is expanding. 13.3 billion years ago, the light we are now seeing started off toward us. While the light has been moving from the point of origin toward us at 186,000 miles per second, the objects have been moving away from the same point of origin, in the opposite direction.

The current school of thought on the rate of expansion puts those observed objects at 46 billion light years away, as of this writing. That means that, in the 13.3 billion years since the big-bang, some amount of matter has traveled as far as 46 billion light years. If true, this brings to mind a paradox: if nothing can travel faster than the speed of light, how did those galaxies get further away from us than the speed of light would allow?

The answer is interesting, and non-intuitive. The expansion is equal in all directions and from all points of perspective that we can observe; we can see just as far in any direction. The only way this is possible is if there is not a center of the universe anywhere within the area we can observe, and if “explosion”, in the conventional sense, is really not the right term to use to describe the big bang. Think of “big bang” as a name or label, not a descriptive term.

In an explosion, everything does start in one place and expand out from that point. Heavier parts move slower, but go farther. Lighter parts initially expand out faster, but lose the momentum battle to air resistance and don’t go as far. During the active time of the explosion, it’s denser near the center, and less dense near the outside edge. Following the active time of the explosion, there is no longer any explosive material, so the center area empties out, becoming hollow, with an inside front moving outwards at a speed slower than the outside front.

That’s not what’s happening in our universe. The matter isn’t expanding away from a center point. Space, the carrier, the “fabric” of our universe, is expanding and taking the matter with it.

One way to visualize this is to take a rubber band and poke thumbtacks into it, equal distance apart, all around it. The rubber band represents the fabric of the universe, and the thumbtacks represent galaxies. You can stretch the band in any direction, and all of the tacks will move away from each other equally.

The carrier, the rubber band, is expanding, and the dots are moving with it. The same thing will happen no matter where you pull on the rubber band, so no point can be clearly identified as the center of your rubber band universe. The tacks, however, are not changing in size. In the rubber band model, the tacks are galaxies, which are bound and contained by gravity (or maybe dark matter), and thus not expanding internally with space.

The objects aren’t moving relative to a center. Space is expanding. Just as the thumb tack does not change spots on the rubber band, the distant galaxies don’t change spots in the fabric of space. The light from those galaxies, however, can still only travel at 186,000 miles per second. Any light from galaxies that were outside of our viewing range 13.8 billion years ago will never reach us. In fact, if the rate of expansion of space keeps increasing, earth bound observers will gradually be able to see a smaller and smaller portion of the universe.

Galaxies are moving relative to each other, but not as the primary method of expansion. It’s more difficult to wrap one’s head around this in three dimensions, and this example is quite simplified, but it’s about as representative as we can get without a lot of math that I don’t have in my head.

Back to your regularly scheduled negative news.