Dark energy site open for business!

By Phil Plait | October 17, 2008 9:04 am

I think it’s funny — "both haha" funny and "strange" funny — that we have almost no clue about 72% of the Universe.

That nearly 3/4 chunk of the cosmic budget is dark energy, the mysterious force (or pressure, really) that appears to be making the expansion of the Universe accelerate. We’ve known the Universe is expanding since the early 20th century, but everyone assumed, logically, that the combined gravity of all the objects in space was slowing the expansion, like tension in a rubber band.

Then, in 1998, two independent teams of astronomers and physicists found out that not only is the Universe not slowing its bloating, but in fact is bloating ever-faster. This turned physics on its head and even now, on this 10th anniversary of the discovery, we’re still trying to figure out the basics of the phenomenon. We don’t know what’s causing the acceleration, we don’t know what this energy is, what form it takes, nothing. All we can do is observe its effects and speculate.

If you’re confused about all this, it’s totally OK– we are too. But we do have some clue about what’s going on, and there’s a lot going on. People, smart people, are on the case, and plans are being made. Telescopes targeted to study dark energy’s effects are in the works, for example. Particle accelerators like the LHC may reveal clues. Your tax dollars (and others’) are at work here.

One way scientists are spending money is on making sure you, the public, know as much about this as you can. So NASA created the Joint Dark Energy Mission website, which has info on this matter. JDEM is a space-based astronomical mission that will involve both NASA and the U.S. Department of Energy — because they have a lot of expertise in the field of particle physics and the basic forces ruling the Universe– and will be designed to look to the farthest reaches of space where the effects of dark energy are easiest to see.

The website is still a little sparse, but it does have links to sites with more information about the history of the discovery of dark energy (which is actually pretty interesting) and the science of it as well. It’s Friday, and admit it: you’re bored at work. Why not investigate one of the biggest current mysteries in all of science?

Addendum: I know this’ll sound like I’m making it up, but I’m not: it was only after writing this that I realized I do talk about dark energy in the new book, since it has a huge role in how the Universe evolves over trillions of years, and even longer periods of time. Much longer. I don’t go into huge detail in the book, but I explain how this plays into the eventual death of the cosmos itself… and maybe even its rebirth.


Comments (38)

  1. One thing that I cringe at every time I hear people discussing “dark Energy” and “Dark Matter” is that somehow the uneducated figure that since we have a name for it, it’s understood. They point at the disagreements and confusion in the current understanding of this as some sort of proof that science has their collective craniums rectally inverted… I try to do my best to explain that these are phenomenon that we are just now gathering information on, and diligently trying to figure them out. I explain that this is how science works, and that even though gravity is such a venerable and revered theory, SOMETHING has got to be going on that needs explaining… Why do people insist on being dense?

    As to the chapter on either the “big rip” or “big chill” I look forward to it! The “and maybe even its rebirth” really piqued my interest since that is one of the subjects I really enjoy contemplating.

  2. Greg in Austin

    I love it when you take a complex topic and explain it with a simple analogy.

    “Like a balloon and… something bad happening!” – Fry


  3. Richard Pogge on his astronomy 101 and 162 llecture series (avaiable as podcast) finished his Lecture on Dark Matter and Energy quickly saying:

    “Of course there is also the possibility that Dark energy and Dark Matter does not exist, and we simply got all the math wrong”

    Sometimes I wonder if Dark Matter is not the Aether of the 20 and 21th century, and we are just waiting for some other Einstein reinvent our equations and take care of the numbers. I feel you could address this issue also, and what evidence we have supporting this undectable substance that’s more convincing than “we got the math wrong”

  4. Brian Hamilton

    Alexandre – We’re certainly following up that line of reasoning as well! Right now, the dark matter seems to be fairly well described if you add Einstein’s cosmological constant back into the equation of general relativity. The most reasonable way for a constant to pop into the equation is vacuum energy of the quantum fields, but this usually comes out to be infinite (in the case of the standard model) or many orders of magnitude too large (in the case of Supersymmetric theories). Unfortunately, the largeness of the predicted cosmological constant vs the smallness of the measured value is kind of one of many things that need straightened out in particle physics.

    But, we also have this lingering problem that we’re still not sure what a consistent, precise quantum description of gravity should look like. An active area of exploration is the possibility that there is some larger quantum theory of gravity that in the right limit “looks like” GR with a small cosmological constant, much like how GR “looks like” Newton’s equation in the limit of (comparatively) weak gravitational fields.

    So that’s kind of my grad student’s perspective on the way people are trying to see if there’s a way that it can be explained consistently by a) something we already know about, or b) getting the math “right.” At this point, though, it’s just as likely that it’s something new and exotic as it is something more mundane in the mathematics. Either would be exciting!

    Of course, I am not an expert in this sort of thing, and this is my summary from various seminars I’ve been dropping by in between classes.

  5. Abbie

    Your tax dollars (and others’) are at work here.

    I thought they just went towards overhead projectors.

  6. Tobin

    Maybe one day will understand that darned luminiferous aether as well! 😉

  7. Off topic: Just saw on CNN’s site that Side B start up ran into a snag. Dr Plait, care to enlighten us? (And sorry if you get two posts on this, my intertoobs are acting strange.)

  8. Chris A.

    Personally, I’ve been uncomfortable with Dark Energy ever since it was “discovered.” The recent suggestion (Copernican Principle be damned) that our universe may be an atypically underdense region compared to what’s beyond our horizon sits better with me. I find it less of a stretch to believe that there’s some currently-poorly-understood feature of underdense regions that make them more conducive to the development of intelligent life (anthropic principle) than the idea that there’s a “fifth force” that creates itself in some sort of cosmic inversion of the snake eating its own tail.

    Just my 1/50th of a USD.

  9. Gary Ansorge

    I wonder how long it will take before astrologers invoke “Dark MAtter/Energy” as an explanation for their inaccuracies???

    Alexandre: Phil has some great posts showing the gravitational interaction of dark matter with two galaxies. Search the blog for more data,,,I’m too lazy to do it for you,,,

    Now for a question:
    How would what we see of spatial expansion (presumably dark energy driven) differ from spatial elongation due to the presence of a Great Attractor?
    MAybe what we’re actually seeing is what happens when one (local) universe approaches another, much larger space/time “bubble”. (Just another way of saying “We’re all swirling down the drain,,,”)


    Gary 7

  10. Jose

    At the risk of sounding like an idiot, here’s my modified gravity solution.

    All you need to do is create a law or constant that says the net curvature of space is always equal to zero. Now if this were true, repulsive gravity at a distance becomes a requirement of General Relativity. Any gravitational “dip” now has a corresponding “bulge”.

    Take the classic the bowling ball on a rubber mat analogy often used to describe General relativity, only throw out the rubber mat and replace it with a waterbed. If you drop a marble close to the bowling ball it will roll towards the bowling ball. Now keep dropping the marble at greater and greater distances, and you’ll eventually reach a point where the marble starts rolling away from the bowling ball due to the bulge it creates.

    Now have at it.

  11. Utakata

    Dark Energy = You know Darth Vader was right all the long.

  12. Phil, there’s a balance between plugging your book while attracting readers and plugging it so much that you actually start losing them out of sheer fatigue. We all know your book is being released: you don’t have to mention the fact in every single post you make. That’s counterproductive, believe me.

  13. It has to be considered though — the very assumption of dark energy rests upon type Ia supernovae being effective standard candles. We could still be wrong about that, as some recent work has conjectured.

    Humans are very good at ‘believing it when we see it’ even if we don’t really know what we’re looking at…

  14. Annette

    Chris A: “The recent suggestion (Copernican Principle be damned) that our universe may be an atypically underdense region compared to what’s beyond our horizon sits better with me.”

    I like this idea as well.. but then whats after the dense space? My brain hurts again… its just not cut out for astronomy. 😛

  15. JohnW

    I understand that the Type 1a is a standard candle, and that, at the redshift distance calculated for distant ones, they are too dim. What I don’t understand is how too dim = accelerating expansion.

  16. Quiet Desperation

    I still think the maths are off somewhere.

  17. JohnW — simply put, either they’re giving off less energy and therefore “dimmer”, or they’re further away and so less of that energy reaches us.

    The principle is that a type Ia only ever occurs when a white dwarf reaches it’s chandrasekhar limit (around 1.44 solar masses). Assuming the chandrasekhar limit is constant, you have a constant amount of energy emitted from every type Ia and therefore a very effective standard candle.

    The new hypothesis (conjecture?) is that the chandrasekhar limit is not a constant and was lower in the early universe (therefore giving out less energy, therefore causing “dimmer” supernovae).

    IMHO, using Occam’s Razor; assuming a value to be variable rather than constant seems a lot more straightforward than assuming 72% of the universe is made up of some hitherto unknown phenomenon.

    But then, I’m an astrochemist, not a cosmologist. My brain doesn’t work quite so well over such large scales. 😉

  18. (By the way, if anyone spots any glaring errors in what I just wrote, please do tell me!)

  19. Gary Ansorge

    Early to me implies 10^-43 seconds after the big bang, not 5 billion years later, which is when we’re seeing that implies another expansion phase. Regular galaxies were already around when these distant type 1As were shining and since they follow our current rules, I expect so do the type 1As,,,

    GAry 7

  20. Torbjörn Larsson, OM

    No, the comedy was when we discovered that we previously had no idea that such a big piece of the puzzle was missing. I find it rather encouraging that we now have a consistent model. And that the parsimonious explanation involves vacuum energy, a rather bigger (initially infinite) piece that we pretended wasn’t there.

    I find it less of a stretch to believe that there’s some currently-poorly-understood feature of underdense regions that make them more conducive to the development of intelligent life (anthropic principle) than the idea that there’s a “fifth force”

    Um, AFAIU the weak anthropic principle has been used to propose explanations for the unnatural small magnitude of the dark energy as well. (Small compared to vacuum energy.) So it seems to me that it can’t be used to choose between the described possibilities.

    Btw, does anybody know if underdense regions can somehow predict all current observations of dark energy, such as, I believe, the ISW of superclusters and supervoids, or is it restricted to predicting the overall expansion acceleration?

  21. Torbjörn Larsson, OM

    IMHO, using Occam’s Razor; assuming a value to be variable rather than constant seems a lot more straightforward than assuming 72% of the universe is made up of some hitherto unknown phenomenon.

    Seems one can have long discussions on various measures to value equally predictive theories before the fat lady sings. Parsimony is IMHO the most useful measure, on account of suggesting simpler so more likely correct theories, giving less reversals if suggesting the wrong theory, and last but not least aligning itself with general concepts of ‘beauty’.

    But it can’t be used to value known or unknown mechanisms as far as I can see; it is concerned with the number of parameters used in a model. A variable function has at least one more parameter than the baseline constant, so it is less parsimonious in comparison, AFAIU regardless of the nature of the mechanism.

    Btw, if theories are simple, which seems to be the case, it follows that applications have to be messy. Um, like Navier-Stokes and turbulence, for example. (Oh, I guess you can suggest some type of idealized turbulence as a primitive emergent object, but if it works then how do you connect with the underlying dynamics?) So, depending on what you mean with “phenomenon”, it isn’t entirely surprising that we find lots and lots of unknown ones out there, hopefully a potentially infinite number (to make life interesting).

    But now we are moving fast into philosophistry. We observe that nature is bountiful and we therefore suspect, I believe, that we will experience it thus (e.g. standard particle theory!) until we reach a possibly fundamental and potentially simple theory. So unless we somehow know that dark energy is fundamental I don’t think we should be concerned.

    [OTOH, 72 % sounds a lot doesn’t it? But how about the volume of unknown parts of the universe when the ancients thought the universe was just about the solar system! It’s not like we haven’t been here before…]

  22. Wildride

    OK, I can finally reveal the truth. Dark energy is, in fact, the strongest force in the universe, aka love. And, it turns out, dark matter is actually a product of the force known as sleepy.

    It’s good to get the off my chest.

  23. I don’t know what dark matter is but I think I will like it because I like dark chocolate. Too much of a stretch…?

  24. BigBangSkeptic

    I have a problem with the Big Bang theory if our Universe is accelerating. Although I believe dark energy could very well exist, part of me thinks that it also might be an explanation for a potentially flawed argument on how our Universe was “born” in the first place. To say that everything “existed after it didn’t exist”, doesn’t that violate the law that energy cannot be created nor destroyed? I understand this comment may be controversial because most of the scientific community embraces the Big Bang theory, but I feel that there’s gotta be another explanation. I’m interested to see how the research on dark energy goes, I’m sure there’s going to be a lot of surprises along the way.

  25. Todd W.


    IIUC, the current thinking as regards the Big Bang is not that there was no matter before it and then suddenly matter existed after it. Rather, matter and the four forces were all bound together. Some unknown mechanism then caused them to split. Whatever it was required an enormous amount of energy to tear the strong and weak nuclear forces, gravity and magnetism apart from one another. That energy also started a chain reaction in matter, slamming particles into one another, releaing even more energy and causing new elements to form.

    Someone please correct me if I’m wrong. That’s just what I remember from a recent program on the BB on either Discovery Channel or History Channel.

  26. Jose

    The Big Bang model doesn’t quite say that universe was created from nothing. It just takes us back to a time when everything we see was much closer together than it is now. Eventually we reach a point where our current models of physics break down, and we can’t really say what happened before that point.

  27. BigBangSkeptic

    Well, I’ll elaborate a little more. My issue with it lies in the contradicting information about whether the Universe is “slowing down” or “speeding up”. I believe there is evidence out there that supports both. In this sense, I just can’t seem to find it plausible that the Universe came from a singular point of dense forces, an unknown reaction happened, and now it is both accelerating and slowing down from an unknown reaction – it seems contradictory. Maybe the Universe just “is” and always “will be”, but not in the sense that it is static, but ever-changing much like Earth. Now, I would favor a birth-rebirth cycle if more evidence came to light, however, as we are discussing the acceleration of the Universe we really need to understand exactly what the parameters of the Universe are and if it does have the possibility to implode back on itself. And then you can get really picky and ask what’s outside of the Universe if it is closed, and so on, and so forth… it tends to get complex easily. This is why I favor the Universe not subsequently being a cycle, but always existing the way we perceive energy to “always exist”.

    I’m actually very interested in alternate theories dealing with other dimensions as a catalyst for the Grand Unified Theory. Just because we are 3d entities doesn’t mean there aren’t any other planes of existence out there that are suitable for the threads that weave our reality… It may be a stretch of the imagination, but different dimensions have the potential to explain big questions like why gravity behaves the way it does and why on a quantum scale particles seem to be jumping “in and out” of existence.

    All in all, I know these questions won’t be anywhere close to be answered in my lifetime, but any new revelations will capture my attention for sure.

  28. jess tauber

    We SEE and ‘expanding’ universe, which is equivalent to ‘shrinking’ matter (which somehow only a small handful of folks have noticed).

    Either way the cosmos is rescaling- possibly both things are actually happening in an absolute rather than a relative sense, but its hard to discern from our perspective.

    However, we are familiar with the concept of gravitational attractors, which move large masses in a linear fashion. Given Hubble expansion, the universe gets bigger, faster, for every segment of the journey taken.

    This is the same as the mass shrinking as it approaches the attractor, ever more quickly.

    If one thinks of a point with radii emanating from it, and some body approaching between pairs of radii of fixed angle, if the body continues to maintain the same angular separation during the approach, then it must get smaller. At the same time the reduced distance to the attractor will increase the attraction felt, for some forces, and the approach will accelerate.

    Enter the Higgs boson, spin 0/2. IF spin has some relation to spatiotemporal dimensionality (I speculate it does), then such bosons will have zero dimension including in time- it exists everywhere and nowhere, everywhen and nowhen- but has rest mass. The universal attractor? The larger the chunk of universe one examines, the larger the net contribution from Higgs, and the more obvious the effect.

    From the POV, fermionic matter (spin 1/2) is ‘falling’ towards the Higgs which gives it mass in the first place through differential coupling, accelerating as it goes. But because H is a ‘point’ in ST, it can never ‘get’ there no matter how fast it tries.

    At the same time, the true (spin 4/2) gravity field would be repulsive (from our perspective), but would be interacting with spin 3/2 materials, which would be speeding up and expanding to try to catch up with G. The 3/2 stuff might contribute to ‘dark matter’, ‘dark energy’ or both, but would be awfully strange, again if spin equates to spatiotemporal dimensionality, because it wouldn’t consist of point particles as we understand them. Both 3/2 and 4/2 effects would be traveling >>C, G immeasurably so.

    Only the normal spin 2/2 bosons have a sort of neutral position within this scheme- with H and G hyperdecelerating and accelerating respectively relative to C.

    WE see all this from a skewed position, and so have great difficulty working things out- much as the ancients had problems differentiating the relative motions of the different bodies within the solar system and larger sky. How many thousands of years did that take?

  29. Jose

    To my knowledge, type 1A supernova are the only way we have to get an accurate measure of the distance of far away galaxies. So there’s not a contradiction in the data. It’s just that the redshift of galaxies very far away from us is a little more than it should be if the expansion of the universe was decelerating. It was a completely unexpected result, but neither an accelerating nor decelerating universe says too much about the big bang. Both scenarios agree that our universe is currently expanding. That fact, as well as other completely independent lines of evidence, point to a big bang.

  30. Gary Ansorge

    I still prefer the “Big Condensation” idea. As though we had dropped a rock in a super cooled Arctic lake,,,

    It’s so cool to have anomalies that imply there’s more to know than we have ever dreamed of,,,

    One thing we can say for sure,,,this universe is NOT a boring place.

    GAry 7

  31. Tom Marking

    “If one thinks of a point with radii emanating from it, and some body approaching between pairs of radii of fixed angle, if the body continues to maintain the same angular separation during the approach, then it must get smaller. At the same time the reduced distance to the attractor will increase the attraction felt, for some forces, and the approach will accelerate.”

    Yes, but if we are point A and point B is some distant galaxy, then in that scheme whatever ruler we use to measure the distance to B must also be getting smaller because it is composed of matter. Therefore according to the ruler the distance to B remains constant.

    It’s like the ol’ philosophical conundrum:
    Q: How could you tell if everything in the universe suddenly doubled in size?
    A: You couldn’t because all of your physical measuring instruments have also doubled in size.

  32. Maura

    Thank you for the JDEM link.

    Although I was not a science major I was interested enough in astronomy and cosmology in college to take some intro classes and audit a graduate course for fun. Didn’t think much about it after I graduated in ’94.

    Then one day a few years later I went to the library and got a book (don’t even remember the title) about cosmology. In school I had heard from professors that we were probably close to figuring out just how much the expansion of the universe was slowing down over time, so I flipped to the part of the book that covered that topic. And my mind got blown like never before or since. It was my “remember where you were when…” moment. I remeber with crystal clarity where I was when I found out about Dark Energy.

    In that moment I totally understood what some scientists say about being satisfied when they prove a theory but being elated when the expected proof yields more questions. To this day I wish I had about a spare $100,000 to pay off the old student loans and go back to study cosmology. Maybe when I retire…

    All this by way of saying thanks for the excellent blog and the many, many links to incredibly interesting stuff.

  33. William

    @Jose: I too had that concept of gravity as a wave structure. IMy ocncept was of very, very long wave structure (galactic in dimensions), which became repulsive at great distance as the first sinusoidal peak became manifest [your “bulge”], and then continued to oscillate sinusoidally with decreasing amplitude (but continued great wavelength) as the wave propagated to infinity.

    For indivdual particles the effect in immeasurably tiny. But for galaxies the effect would be huge.

    The bias of the wave structure (position on the magnitude axis) would determine if it was net repulsive or attractive on intergalactic scales.

    If it was net repulsive, it would act in a similar fashion as dark energy.

  34. Would you believe it, anyone, if someone had a simple idea for what dark energy is, that the professionals wouldn’t even discuss it?!

    Even if it gave a good match to supernovae and WMAP data, with a predicted matter density of 0.25?

    4 Journals have rejected it, although its being considered again.

    Truth is, its in their interest for the mystery to continue…

  35. Gus

    John — Yes I believe it!

    Based on laboratory simulation I believe I have identified dark energy as the energy contained in traveling shock waves.

    For details : ISBN 978-1-4343-0661-6 (sc)

  36. Starman

    Dark Matter…..When one considers Human Knowlege is about eqivalent to the size of our species in comparison to the size of the Universe, its no wonder that our Text Books all reflect this understanding. Knowlege driven by eons of Pre-concieved ideals and opinions of how it should be, amount to slamming ones head against a wall, when reality raises its head, we really become confused. The Reality that the Universe as decribed within Text Books, is not reflective of Reality, should be a clue, that our understanding of Physics and the Mechanics of the Universe often do not match observations, then age old dogmas kick in, Dark Matter is one of these concepts, we cant see it, we cant measure it, but its there, and it formed possibly at the same time the Universe did, many hundreds of Billions of years ago. Time is another misconception, as we try to set a date to anything within the universe, just like the Big Bang, there wasnt just one but multiple Bangs, yet as usual, here we are in our infinite Speculation, and they call it mainstream Science….

  37. stephen

    A blind motorist who never left his car his entire life is driving through an unknown forest was asked about why the road was there. The motorist says: The road goes this way because it passes through where there are no trees. I would ask the blind motorist what a tree is and get the response trees are the things not in the path of the road. I would ask the motorist, have you ever seen a tree? He’d say no, someone told me about them. I’d say how do you know the trees are there and he’d respond I believe they are there because someone told me they are there. So it’s only by faith that you stay on the road because you believe in something you’ve never seen.

    Dark Matter is something that is not seen but known is there, much like God. And God did say He created the dark!


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