Astronomy Question: Speed of Celestial Bodies?

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John asks:

According to Hubble’s discovery, the galaxies are moving away from each other at a growing speed, but I read that the speed of matter after the BB was enormous and then with time it has been slowing down. Isn’t this a contradiction?

Great question John!
Let me start by separating your question into two parts, matter and space.

Special Relativity as postulated by Einstein in 1905, states no matter (or information) can travel faster than light.

Hubble’s (The Astronomer, not the space telescope), work in Astronomy provided some of the early foundation for cosmology as we know it. Hubble’s discoveries led to our understanding, as you pointed out, of an “expanding” universe.

What current theories state is that while matter/information/light cannot travel faster than the speed of light, “space” itself is not bound by this restriction. I believe there may also be some theories that state for a brief, brief period just after the big bang, there was matter that did travel faster than light speed, but I’ll be the first to admit that I’m not a Cosmologist.

The analogy used commonly (I’ve heard it on many science shows and read in many articles) is raisin bread. Consider the raisins akin to galaxies and the “dough” space between them. While the bread bakes, the dough rises/expands, so while the raisins themselves aren’t moving, the “space” between them is!

You can actually use the Hubble constant and the speed of light to determine at what distance between two galaxies must have in order for the galaxies to be moving apart faster than the speed of light. Hint: a bit over 4,000 Megaparsecs ( One Parsec is 3.26 light years). To give you an idea of scale, the observable universe has a radius of about 14 Gigaparsecs (about 45 billion light years). So, while our universe is estimated to be just over 13 billion years old, cosmic expansion has put our observable radius to the above-mentioned 45 billion light year range.

With regards to speeding up or slowing down, dark energy is theorized to be what is responsible for the expansion of the universe to be accelerating.

Hope this helps shed some light (pardon the pun) on the physics of the expanding universe.

Ray Sanders is a Sci-Fi geek, astronomer and blogger. Currently researching variable stars at Arizona State University, he writes for Universe Today, The Planetary Society blog, and his own blog, Dear Astronomer