In a comment on my last post, What is Dark Energy, Kendall asks the following, which is such a good one I think it deserves its own post:
I thought the expansion was accelerating? Aren’t you saying that it is on its way down to 85% of its current rate? Sounds like expansion is slowing, but still leaves us with an open universe…
People do say the expansion of the Universe is accelerating. But that doesn’t mean that the expansion rate is accelerating. It means that if you take a look at any one galaxy that isn’t gravitationally bound to us in the Local Group (that is, any big galaxy that isn’t named Andromeda), it’s going to recede from us at a faster and faster speed. Since the expansion rate doesn’t drop to zero, but rather (according to the best measurements today) will drop to 60 km/s/Mpc, then if a galaxy is 10 Mpc away from us today, it’s moving away from us at 600 km/s. Later on, when that galaxy is 20 Mpc from us, it moves away from us at 1200 km/s. When it’s 100 Mpc away from us, it moves away at 6000 km/s. And when that galaxy finally gets to be 5000 Mpc away from us, it will move away from us at 300,000 km/s, or at the speed of light!
Is that even possible? Yes, but this one is going to be difficult to explain. Because it’s not that “the galaxy is moving faster than the speed of light,” but the space between galaxies is expanding, and since that isn’t made of any matter or energy, it isn’t subject to special relativity: there is no speed limit on the expansion rate of spacetime. So while it appears that this breaks the laws of special relativity, it’s perfect consistent in the framework of general relativity.
In conclusion, what this means is that if you look at the galaxy and measure “what is its motion relative to us,” you will find that it appears to accelerate and eventually move away faster than the speed of light. But if you put it in the context of an expanding Universe with General Relativity and Dark Energy, you get a Universe with a constant expansion rate where galaxies remain roughly stationary relative to space, and the space between them expands at this constant rate forever. Are you confused? Well, if you’re not, you’re doing better than most astrophysicists!
Got a hankering for more Space stuff than my website can provide you with? Check out this week’s Carnival of Space over at Will Gater’s website, where they highlight my post on whether all stars eventually explode. (Quick answer: No! Some just go “poof!”)