Dark Energy: Hard to Kill (Part 1)

Posted by Ethan on November 4, 2009
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Once you can accept the Universe as being something expanding into an infinite nothing which is something, wearing stripes with plaid is easy. -A. Einstein

But accepting the expansion of the Universe is easy compared to accepting the existence of dark energy. Why — and how — is there some mysterious property inherent to space that prevents the expansion rate from dropping to zero? Why is the expansion rate as large as it is today? Why, of the four options we can reasonably conceive of, is the Universe obeying this “accelerating” picture below?

The why and how are questions that we do not yet have an answer to. Nonetheless, dark energy is practically as universally accepted among cosmologists as evolution is among biologists. In this new series, I’d like to take you through our current understanding of dark energy, and why we can’t just wave our hands and explain it away.

How did this all get started? Well, you look out at things in the Universe, at things far away, and you’re actually looking back in time. You’d think that if you looked at something one million light years away, you’d be looking backwards one million years in time, since light travels at the speed of light.

And you’d actually be wrong. Why? Because the Universe has been expanding during that one million years! Well, if you look at things in the Universe that happen at a whole bunch of different distances, you can figure out how the Universe has expanded over its history.

And if you know how it’s expanded, you can learn what is it made up of. Well, if the Universe were all matter, it would do one of the first three cases in the top image, which I’ll repeat for you here.

But if the Universe has dark energy in it, it should do the “accelerating” case at the far right. How can we tell them apart? Well, Hubble’s law links two special things: distance and velocity. But the way these things are linked depends on what’s in your Universe, like so:

The big thing is that, in a Universe with dark energy, distant objects will appear to be fainter than in a Universe without it. And in 1998, that was exactly what was discovered: Type Ia supernovae, formed the same way at all times in the Universe, were fainter than they should have been at large distances!

This was measured by two independent teams and subsequently confirmed numerous times. In fact, this was my discovery of the decade for the 1990s! And yet, it’s one of the most unsettling parts of our picture of the Universe, that over 70% of the total energy in the Universe is this mysterious dark energy. We’ve been trying to explain it away, do without it, or come up with a reasonable alternative for our observations ever since, and we haven’t been able to do it.

And I think it’s worth telling you about all the different ways we’ve tried, and all about why those ways don’t work. And at the end, you can decide whether it’s hard to kill or not. So come back soon for parts 2, 3, and everything after!

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  1. #1 Reeja
    November 4, 2009

    Hi,
    How you manages such a detailed description with so much images. I love it tough it was a bit difficult for me to grasp.I love to hear your opinion about my article on meteoritics in Bright Hub. http://www.brighthub.com/science/space/articles/53421.aspx

  2. #2 NewEnglandBob
    November 4, 2009

    Ready and eager to hear it.

  3. #3 Max Fagin
    November 4, 2009

    I’ve never understood how the teams who first detected the acceleration of the universal expansion were able to conclusively rule out that the dimness of type Ia supernova was not due to a higher concentration of interstellar dust in the early universe. Could you explain how that was ruled out?

  4. #4 Tim
    November 5, 2009

    Color–interstellar dust scatters short wavelength light more efficiently than long wavelength light. Supernovae obscured by dust would appear redder, so according to your hypothesis the most distant objects would be redder than expected (reddening is already taken into account in the calibration of Type Ia’s as standard candles). There are also more subtle selection effects: for large distances, you will preferentially find those objects least extinguished by dust. There are a ton of issues with the calibration of Type Ia supernovae, but this would be the subject of an entire review article. In any case, these issues don’t seem to affect the basic conclusion (supported by other lines of evidence) of an accelerating universe.

  5. #5 IanW
    November 5, 2009

    I like your new pic better than the old one but you still don’t look like your namesake on the poster…!

  6. #6 jon radin
    November 5, 2009

    the dark matter dark energy explainations for both the perceived expansion velocity/acceleration of galaxies and the structure – rotation – of galaxies seemseveral logical steps beyond the facts as we know them. Because we see light fainter than it is calculated to be accelerating expansion, the expansion mustbe caused by energy, the energy is not detectable as types of energy in our current hypothesis, therefore a huge percentage of the universe is unknown. Dark matter, well the red shift we see implies a velocity which would require a gravitational field to hold the moving bodies in their positions which could only exist if there were l ot more matter than we can percieve with our instruments and that matter must be of an unknown type. A lot of inferential steps to decide the universe is 95% unknown. An awful lot

  7. #7 Tim
    November 5, 2009

    Jon, to what inferential step do you object? There is abundant evidence for dark matter from rotation curves, mass to light ratios, and separation of dark and baryonic matter in galaxy collisions. If this matter interacted with light at all we would be able to detect it. There is also plenty of evidence for an accelerating universe, and while dark energy is not the only explanation, it appears (presently) to be the simplest. There are plenty of people with their own pet theories though, and some of these will be tough to rule out.

  8. #8 David
    November 5, 2009

    I’m looking forward to this. Your “dark matter” series was fabulously well done.

    Perhaps you can explain why Einstein choose to put the lambda term on the left (the G side) instead of adding into the T part.

  9. #9 Sili
    November 5, 2009

    If by “a lot” you mean “the most parsimonious model”, then you’re right, yes.

    The observations stand by themselves. It is then for us to fit our models to them and then scour the models for predictions. I have no doubt that Ethan will get to those in the coming parts.

    Some people keep whining about how DM and DE makes them uncomfortable – as if that was an argument. Claiming that it’s all epicycles*, yet they don’t bother coming up with ‘Dark Ellipses’ to solve their perceived deviations. Whine whine whine!!

    *Incidentally, I’m trying to relearn some history of mathematics and the purely geometrical constructions and arguments to determine the epicycles and deferents needed for just the first iteration of modelling done by Ptolemy is wayyyyyy beyond anything I’m capable of doing. This is not easy stuff – pretty damn hardhitting work, actually. (And just today I heard on Are We Alone that apparently the Antokythera Mechanism took epicycles into account.)

  10. #10 Nathan Myers
    November 5, 2009

    I’ll go out on a limb and predict that systematic measurement error will not be one of the explanations evaluated.

  11. #11 Hannes
    November 5, 2009

    How can you be sure that the faintness of type Ia supernovae on large distances is not due to the curvature of the universe itself?

  12. #12 Mena
    November 5, 2009

    An aside, sorry, but Craig Hogan will be giving a lecture at Fermilab next Friday:
    “The Sounds of Spacetime: Black Holes, Early Universe, Cosmic Strings, and Holographic Noise”:
    http://www.fnal.gov/culture/NewArts/Lectures/09-10/hogan.shtml

  13. #13 Detlef
    November 5, 2009

    OK, I’m one of the guys who feels rather uncomfortable with this DE story. I’m a fan of energy conservation.
    Coming back to Nathan Myers’ comment. May be someone can clarify the following: My understanding of the presented conclusions is that they depend strongly on the evaluation of the SNI distances. To evaluate these distances the result of a chain of distance measurements is assumed to be valid: in minimum the conclusions are based on 1. paralax measurements (accuracy 0,1% or less), 2. cepheides standard (accuracy 5-10%), 3. SNI standard candle (accuracy seems to depend on content of exploding star). How does these inaccuracies influence your m/z-graph and the conclusions?
    I don’t want to touch the wide field of light/radiation power measurement accuracies.

  14. #14 DaveH
    November 5, 2009

    “I’m a fan of energy conservation.”
    That’s probably a good thing in itself, but in the context I think it’s a non-sequitur.

    It’s not just type 1 supernovae – e.g CMBR is part of the evidence too, but I’m sure Ethan will go through all the different strands of evidence.

    Hubble’s data back in 1929 was very inaccurate, as it happens, but it’s a bit late to pin hopes of a 100% baryonic matter universe on inaccurate measurement *now*. I think that would require the universe to weigh 1/20th of what has been measured, wouldn’t it?

  15. #15 JakeR
    November 5, 2009

    Please put up an index of URLs where readers can find all the articles on dark matter and dark energy. A friend is absolutely convinced that the science on dark matter is flawed, and I’d like to be able to point him to your rational discussions.

  16. #16 edrowland
    November 5, 2009

    An ongoing complaint with this blog: a recurring habit of claiming convincing support without really presenting actual evidence.

    In this case, there’s no discussion of the actual data, which appears less than impressive at first glance (whatever it is). Or whether the dataset really is that impressive given that there are two two very noticeable outliers in the right-most cluster of a tiny dataset that don’t seem to fit any of the three curves. Oh, and a reference to another column which makes the same claim without an equal absence of data, argument, or references.

    It would be a pleasant change to be swayed by actual data and reasoning, rather than wooo-woo. Which is what this column so often feels like. An occasional reference or two would be nice too.

  17. #17 Katkinkate
    November 6, 2009

    What I would like to know: is the dark energy and dark matter linked in some way? Could the dark energy be some sort of radiation we can’t detect yet, that comes from the dark matter we can’t see?

  18. #18 jon radin
    November 6, 2009

    I should have first agreed with the prior comments that the presentation by Dr. Siegel was excellent. Secondly my caution is about upending our entire concept of the physical universe because of a few observations which logical, unsupported step by logical unsupported step lead to a radical conclusions. Observation is outrunning theory. We seem to be scuttling around trying to backfill with a theory to meet observation. Remember Relativity proposed a universe radically different than the concept of that time. But it predicted the result of the 1919 eclipse and it was that observation which validated the theory. It may well be that the dark matter and dark energy concepts are accurate. But without a theoretical scaffold, which will predict future observations and explain the why and how, dark energy and dark matter are speculative ideas. The apparent emotion of some of the responses should have no place here and strikes me as emotionally defensive. For the rest, take a piece of paper and start with the observation, star redshifts in a particular galaxy are XYZ. that is the observable. From that we infer relative speed to us, rotational speed, we calculate mass, we estimate distance, etc, etc. and decide there must be more gravity than the mass we estimate can account for, in order for this galaxy to hold together. Lee Smollin made, I think, a similar point in his latest book, that the community seems to move in concert to some fashionable theory. Little room or energy is allowed for investigation of other ideas. Scepticism should reside in all of us. Passionate advocacy seems out of place

  19. #19 Bill G.
    November 6, 2009

    Not being a physicist, I have long thought, that the background radiation and the hubble constant,although correct, are evidence that could be interpreted another way.
    especially after reading Paul Davies book “god and the new physics”, around 1984,if I remember correctly.
    The expansion is caused by matter spontaineously appearing in the vacuum of intersteller space and “pushing” space outward, so to speak, or galaxies are pulling space apart, and has been going on forever.
    the backgroud radiation could be caused by something else not yet known, and just mimiking the so called big bang residual radiation.
    I realize this is Hoyle’s steady state theory, but has anyone tried working this theory using these assumptions and comparing it the bang theory which has problems.

  20. #20 Bii G.
    November 6, 2009

    Addendum: what I meant was, if the dark energy and dark matter could be plugged into equations of the steady state kind, and see what happens.

  21. #21 Ethan Siegel
    November 6, 2009

    Bill, the cosmic microwave background makes no sense in the steady state model. For 39 years after its discovery (until his death), Hoyle simply stated, “We live in a fog.” He had no explanation for it, for its isotropy, and especially for the predictions of the peaks in the cosmic microwave background.

    It would also ruin galaxy formation, as we’d constantly be forming new galaxies throughout the Universe as it expands, which we don’t see. The last attempt I saw made at using a steady-state cosmology to explain these observations was in 2003, and it gave predictions which conflicted with the data significantly enough to rule it out.

  22. #22 Detlef
    November 6, 2009

    Sorry, for the potentially misleading introduction. I was interested in a potential conflict of the DE story with accuracies to be considered (from my understanding). I asked for DE not for DM. For DM I agree: there is not much place for skepticism.
    My questions concerning the CMB I shall place if Ethan will adress it.

  23. #23 Thomas Neil Neubert
    November 7, 2009

    Some brave natural philosophers speculated that the Earth was a sphere, not flat Euclidean plane. It took nearly two thousand years until the flat-Earth theory finally was put to rest.

    At the beginning of the 20th century both Edwin Hubble and Albert Einstein asserted that the visible universe was not flat 3-dimensional Euclidean space but rather a curved 3-dimensional spherical space known mathematically as a 3-sphere. 100 years later cosmologist refuse to believe that our visible universe is a 3-sphere; and until they do they will continue to predict such extraordinary things as the hypothetical dark energy.

    I will read carefully the upcoming series. But I will note that cosmology is a subspecialty of astronomy and physics and though the peer reviewing fellow cosmologist may “self” review and mostly agree that dark energy exists, few physicists and astronomers agree. John Barrow points out that “In 1952, the Vatican embraced the picture of the expanding Big Bang universe.” And as I pointed out in a previous comment physicist Roger Penrose is still skeptical. and without the foundation of the Big Bang, dark enery is left without any theoretical foundation.

    Needless, to say all of the observations and data that Ethan discusses are valid; I only disagree with the theoretical interpretation with which they are wrapped.

  24. #24 Ian
    November 8, 2009

    Questions:
    “Nonetheless, dark energy is practically as universally accepted among cosmologists as evolution is among biologists.”
    Is it? This feels like rhetoric. Evolution now has a long history of observation and demonstration and forms the core for most current biological thought. Dark energy’s history is short and no one seems to know what it really is. As I understand it, observation implies dark energy, but explanation and demonstration are still lacking. Or is there now more solid experience with it?

    “But if the Universe has dark energy in it, it should do the “accelerating” case at the far right.”
    I would think: since the universe is accelerating, physicists have theorized dark energy as the cause. I bring up this point in conjunction with the last one because I am confused about the state of dark energy in science — is it that dark energy has theoretical foundations elsewhere and, since hypothesized, has been strengthened by the accelerating universe? Or is it that dark energy is a response to the observation of that our universe is accelerating, and is thus simply one possible hypothesis (the strongest) among many? How solid a ground is the dark energy hypothesis standing on?

    I know a lot of writers, including yourself, give it solid ground, but I don’t know what that ground is and it seems to me the ground is not as solid as writers say. Can you point me to other writings on the matter?

  25. #25 Thomas Neil Neubert
    November 11, 2009

    Regarding the comment, “dark energy is practically as universally accepted among cosmologists as evolution is among biologists.”

    Evolution is a descriptive theory. No biologist that I am aware has ever written a book title The First Three Minutes, meaning the first three minutes after the origin of life. Rather, biologist accept that they cannot explain the origin of life and are content to describe the processes of evolution as they pertain to The Origin of Species, as observed now and in the recent past. The theory of evolution has evolved to use populations of species and other statistics and DNA,etc but it does not predict the future of life and it simply describes the processes of evolution.

    On the other hand, the Theory of the Big Bang (upon which) the Theory of Dark Energy rests) casts itself as The Origin of the Universe and as a predictive and a historically exact physical theory right down not only to The First Three Minutes but right down to the first 10^-50 seconds, 10^-100 seconds; however small you like. Meanwhile, the smallest time interval ever physically measured is about 10^-25 seconds.

    If we are to stay in the realm of physical science; then we must stay approximately within the realm of physical measurements. If we stray outside of the realm of physical measurements by maybe 10^25 orders of magnitude; then we have definitely entered the realm of very exact mathematically fantasy. (pg 136-137)

    The theory of evolution never enters the realm of fantasy science; because it sticks to the data and it sticks to the realm of description and does not go into the predictive realm or the historic realm of mathematical fantasy.

  26. #26 Rob Knop
    November 11, 2009

    In fact, we’ve even found the binary companion that gave rise to the 1572 supernova!

    I know I’m coming way too late to this comment game, but— I would still leave that companion as very much in the “maybe, candidate” category. I was working on an observational proposal my last year before I scattered out of the field that would have either added weight for or against this candidate. (It was depressing. The telescope was not oversubscribed, *and* the proposal was ranked in the top 25% of all proposals to that telescope. But we were given no time. Sigh. There are so many ways to get screwed.) It may well be that this star is just a random thick disk star that happens to be tooling through the neighborhood.