Is Dark Energy the same as Acceleration?

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!”)


  1. #1 Eric Hill
    April 10, 2008

    You know this very topic has been bothering me for years. A number of years ago my Astronomy professor, Mark Voit at Michigan State University, tried to explain the expansion of the universe to me by comparing it to a loaf of raisin bread baking in the oven. At first that made a lot of sense to me. However after that the more I thought about this topic the less I have understood it. Ive asked a number of different people a number of times for a good explanation to no avail. Finally I get one that clicks again and I didn’t even have to ask you for it. Thanks! I wish that I would have studied Astronomy in college…

  2. #2 ethan
    April 10, 2008

    I don’t like the “baking bread” analogy, although a lot of professors use it. It’s never resonated with me. Expanding balloons are easier on my head, and a new one that I’m working with is a “moving walkway.” But if this worked for you, then that’s great, and I’ll remember to use it!

  3. #3 Kendall
    April 11, 2008

    Great explanation! I can’t tell you how many times I have read that the universal expansion was accelerating and assumed the rate of expansion was speeding up. Thanx again for a great website!

  4. #4 ethan
    April 11, 2008

    “The expansion of the Universe is accelerating.” I’m guilty of saying it myself. But the slightly longer (and much clearer) answer that I gave here is much more correct. Thanks for asking such a good question, and giving me the impetus to make this post!

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