Wednesday, March 10, 2010

Are Prejudices Against Dark Energy Unfounded?

I want to examine several points by a recent paper Why all these prejudices against a constant? It isn't that this is the best paper on dark energy in the world, just that it highlights several issues relating to dark energy.

This first post is just to whet your appetite.  I start off quoting the paper:
“Arguably the greatest mystery of humanity today is the prospect that 75% of the universe is made up of a substance known as ‘dark energy’ about which we have almost no knowledge at all.”

This is the opening sentence of a (good) popularization article on dark energy. It is just an example, out of very many that can be found in the popular-science and in the technical literature, of how the ‘dark-energy’ issue is perceived by many scientists, and presented to the large public.
You know it is true, people make a big deal about how "mysterious" dark energy is.
We argue here that there is something scientifically very wrong in this presentation. There is no “great mystery”... This is a phenomenon which is clearly predicted and simply described by well-understood current physical theory. It is well understood in the context of general relativity, which naturally includes a cosmological constant. We argue below that the common theoretical objections against this interpretation of the acceleration are either weak, or ill-founded.
I agree with this assessment. As we will discuss in a future post, if you accept GR, it should be straight forward to accept a cosmological constant.  And a small positive cosmological constant would produce exactly something exactly akin to to the dark energy we observe!  So, I also think portraying dark energy as something incredibly bizarre is to be over the top.
It is misleading to talk about “a mystery” (not to mention “the greatest mystery of humanity”), for a phenomenon that has a current simple and well-understood explanation within current physical theories.

It is especially wrong to talk about a mysterious “substance” to denote dark energy. The expression “substance” is inappropriate and misleading. It is like saying that the centrifugal force that pushes out from a merry-go-round is the “effect of a mysterious substance”.
Again, interesting point.

So we will see.  I will post various points the author raises and let you decide.  Personally, I agree people put far too much mystery into dark energy.  In some sense, it is a prediction of GR. (As I will explain this claim in my next post).

We all accept GR right?  So, why do we treat dark energy as such a mysterious thing?

Posts in the series:
1. Dark Energy As A Prediction Of General Relativity.
2. Coincidence Problems and Dark Energy.
3. The 120 Order Of Magnitude Problem.


  1. "...Prejudices Against Dark Energy..."

    I get this mental picture of Martin Luther Kind Jr. giving a speech about racial inequality among energy.

  2. "It's not that most of the matter and energy in the universe is dark, but that most cosmologists are totally in the dark about the real nature of the universe." -- Wallace Thornhill, physicist, October 2006

  3. I agree that from a mathematical standpoint dark energy isn't all that mysterious - it's just an additional constant times the metric in the field equations - however from a physical point of view, there is zero understanding of where that additional constant comes from or why it should have the value it does. Can you give me a single reason why that constant should be constant? or why it's not negative? Adding a term to an equation to make the mathematics work out without so much as a hint to the physics makes physicists nervous.

    When you say that ~70% of the universe is made up of something that we have no physical understanding about, that sounds like quite a mystery to me. Add in the fact that the only plausible source for this term that anyone has come up with so far gives you an answer that's off by 120 orders of magnitude and you have a real puzzler.

    I do however agree that we should stop calling it a substance. It's not some intergalactic goo with negative pressure, which is what some people wind up picturing.

  4. Nick,

    "Adding a term to an equation to make the mathematics work out without so much as a hint to the physics makes physicists nervous."

    Next post is for you. I already promised I will explain why we should expect this term to be there, *a priori*.

    "I get this mental picture of Martin Luther Kind Jr. "

    That's funny.

    I'm glad you are sharing your concerns. These are the very concerns this paper, and these posts, will address.

    Does this mean you will all be convinced dark energy isn't mysterious? No, but at least you will be given intelligent reasons why the above concerns may be over-reacting. We'll see.

    "gives you an answer that's off by 120 orders of magnitude and you have a real puzzler."

    Again, this will be addressed in the future. Let's just say, the point of this paper is to present decent reasons why people are way over reacting and being really unfair when they make statements like this.

  5. Does anyone else have concerns about dark energy they would like to share. I just need to make sure all the bases are covered.

    I'm not promising everyone will be convinced, but I do promise intelligent responses.

  6. OilIsMastery,

    Nice quote. Though, I agree somewhat, I will dissagree that we are *totally* in the dark.

    For instance, we know enough about dark energy and matter to make real predictions that have been verified very well.

    Obviously, though we don't know all the details, we know enough to do real science with this stuff.

  7. Ewwwww. showdown between the optimists versus the cynics. Fight!

  8. Johny,

    If you are referring to me as an optimist be careful. I'm only optimistic about science that I feel has good intelligent motivation for being real. I do think dark energy, and the idea about it being the cosmological constant does, so I support it and agree many people treat it unfairly. (As well shall see.)

    To prove I can be pessimistic: I am really pessimistic for example against quintessence. Quintessence leads to many papers each year, and yet year by year, experiment after experiment sees no sign at all of quintessence. The error bars are getting small enough that a ton of research in this area to me seems silly.

    So I can be pessimistic about serious science. But it is true I am often optimistic about science, like dark energy, that successfully makes predictions and fits into theories in a very straight forward way.

  9. Hey I was just reading about this last night from the book you recommended... "From Eternity to Here" I love serendipity! =:)

    So my question is this, the cosmological constant describes the effect of dark energy given a certain range of values and it describes the effect of gravity given another range of values. Why is it so skitsofrinic? Why is it that the value of a constant can on one hand describe something familiar and comfortable and on the other hand something almost unknown and bizarre? Is this perhaps why the concept of dark energy is disconcerting to some?

  10. Stan,

    I'm glad to see you are reading that book.

    I believe you are referring to the idea that the cosmological constant has to be a special small positive value to be the dark energy? Is this what you mean by "certain range of values"?

    It's true lambda can take on just about any value (see later posts for this), but only a special value explains dark energy.

    To first order I would say: Yes, it can take on a wide range of values, but only certain values make sense given experiment.

    To me it is like Newton's gravitational constant. Technically, nothing in E&M tells us what value it has to be. But experiments show there is only one value that makes sense. If it was another value it would not be related to the E&M interactions we observe just as lambda would not be related to dark energy if it was not small and positive.

    But I will think of a better response. I'm not sure I answered the question you were driving at.

    Thanks for all your comments. :)

  11. As for the value ranges, I was thinking of the value Einstein put there originally to hold the Universe still as apposed to the value(s) that would have the Universe eventually collapse on itself as opposed to the value(s) that have the expansion of the Universe accelerating. Is this a correct understanding of the cosmological constant? A number that affects the rate of expansion or contraction of the Universe?

  12. Stan,

    Yes, the cosmological constant does play a direct role in the expansion or contraction of the universe. (Depending on whether it is positive or negative respectively.)

    But since we know the universe is accelerating in an expanding phase, we would suspect lambda to be small but positive. (If it is this dark energy, which personally believe fits the data so well we should believe it.)

    As for Einstein, the paper mentioned actually talks about this. The authors of the paper claim he did realize a cosmological constant is a prediction of his theory, so he set it to a value to keep the universe from expanding. As much as he was willing to work with crazy theories, to him the universe not being static was too crazy.

    The funny thing is, the value he chose to keep the universe static is unstable! If the universe is perturbed at all it would no longer remain static.

    This just means that Einstein's solution to create a static universe wouldn't actually work. Pretty much the universe can't be static in GR though such a simple trick.

  13. Ok, this is keeping me up tonight and you guys have the answers. Wow, my own personal physics consultants! =:)

    Reading my book tonight, it said that the amount of dark energy per unit of space remains constant even as the Universe expands. Doesn't that violate conservation of energy? Where does the extra energy come from to fill the new space?

  14. Stan,

    The problem is you are thinking logically. :)

    Okay, this is yet another reason why we think the dark energy is the cosmological constant. Mathematically, a cosmological constant seems equivalent to the vacuum energy.

    It turns out the vacuum energy is infinite do to quantum fluctuations. We can only measure energy differences so we never observe quantities in experiments as infinity.

    The energy at a point X is not E but infinity_V + E where infinity_V is the infinite amount of zero point energy in empty space. Something showing up in our experiments as having "twice as much energy" really has energy infinity_V + 2E.

    You can read more about this Zero point energy section of the wikipedia. Look at the subsection on cosmology.

    So, since it is infinite, in some sense there is no violation of the conservation of energy.

    The cosmological constant representing vacuum energy has the same effect as what we see with dark energy.

  15. "It turns out the vacuum energy is infinite do to quantum fluctuations."

    Dang those quantum fluctuations to heck! So why can't we build a quantum fluxtuation capacitor and capture this infinite energy to power my Prius?

  16. Oh wait, somebody already has...

  17. Stan, there are science fiction novels where this happens.

  18. Sean Carroll on the Colbert Report

  19. Stan,

    Thanks for the ink. Notice he put another plug in for the multiverse. :)


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