What astronomers do

By Phil Plait | September 26, 2006 10:13 pm

I’m working on a huge project for my day job right now, and it involves writing a lot of basic material about dark energy. I’m supposed to be brief, but how do you concisely describe dark energy?

You can’t. Even at 900 words I think I’m giving it too short a treatment.

I’ll forgo the details about dark energy is for now because it’s not directly related to what I want to say here –and anyway, when the project is done I’ll be able to link to it.

In the course of reading up on DE so that I understand it well enough to explain it simply (ha! Sure!), I came across a technical paper called Measuring Cosmology with Supernovae. It’s written by Saul Perlmutter and Brian Schmidt, two astronomers. But not just any two astronomers! They were the heads of their respective teams, both of which independently figured out that the expansion of the Universe is accelerating.

The reason I’m bringing this up is because, for one thing, I wouldn’t have expected them to write a paper together. They don’t hate each other or anything– in fact, they’re both very nice fellows. But in the years leading up to 1998, their two teams were rivals, and the competition was fierce. But in science, even rivals for such a big goal — and figuring out the fate of the Universe is maybe the biggest — can be on friendly terms.

But there’s another reason why I’m writing this.

Back then (and still, today) they were both trying to observe very distant exploding stars, technically called Type Ia supernovae. This particular flavor of star, it so happens, explodes with a total energy that is almost entirely predictable. Well, "predictable" is the wrong word. Really, when you observe how the brightness of the explosion changes over time — days, weeks, and months — you can calculate the total energy released. This in turn tells you how bright the supernova really is. When you compare that to how bright it appears through the telescope, you can determine the distance– and that’s one of the Holy Grails of astronomy. These things can be seen at tremendous distances, billions of light years. This in turn can be used to figure out all sorts of interesting things about the Universe at large, like its overall shape, what it’s made of on the grand scale, and how it’s expanding.

In the late 1990s, both teams were bagging lots of supernovae. When they compared the distance they got for the supernovae versus how bright they were, they got a surprise: the supernovae were all too faint. It was as if they were farther away than expected. As they so succinctly put it in their paper:

Both samples [from the two teams] show that [supernovae] are, on average, fainter than would be expected, even for an empty Universe, indicating that the Universe is accelerating.

Don’t be fooled by the dry declarative nature of that sentence: a huge amount of work went into being able to say it. They had to eliminate a host of other things that could be making the supernovae look fainter, like dust between us and them, possible chemical differences between supernovae we see locally and ones that are far away, and lots of other esoteric causes. After much work, they scratched everything else off the list one by one, until they were left with the weirdest idea of all: the Universe was not just expanding, but accelerating.


But even that is not why I’m writing this entry. In the paper Brian and Saul wrote, there is some math that involves relativity and such. It goes on for a while, relating such things as the expansion rate, the density, and the pressure — which, believe it or not, is fairly standard stuff for cosmologists. Finally, though, the equations are put together in a way that tells you something very important… and in the paper, Saul and Brian make a statement so astonishing that when I read it, I exclaimed out loud (using words I won’t write here). I literally stood up from my desk and had to tell everyone in my office about it. The authors said:

Combining Eqs. (4 – 6) yields solutions to the global evolution of the Universe.

Read that again. "Combining Eqs. (4 – 6) yields solutions to the global evolution of the Universe."

This is a review paper, so they skipped the several hundred steps to get to equations 4, 5, and 6. But still, in the end, this is what they are saying: you can write down some relatively simple equations, and from there determine the ultimate overall state and fate of the Universe.

How frakking cool is that? The whole Universe, from end to end, stem to stern, port to starboard, the whole enchilada, the sum total, the whole nine yards, the complete package, the whole kit and caboodle… and if you’re smart enough, work hard enough, and have enough smart and hard-working people before you lay the groundwork, you can calculate the global evolution of the whole shebang!

That’s I’m writing this entry.


Man oh man. I love this stuff.


Comments (40)

Links to this Post

  1. A Ler…-- Rastos de Luz | September 28, 2006
  1. Seeing the word global used in relation to the universe like that feels a bit awkward and i’m not sure i understand it.

  2. JC

    Just a fun-spirited nitpick, but: “port to bow”?

    And yes, talking about this stuff is fun. I just wish that *doing* it every day were so enjoyable…

  3. TJ McDermott

    The phrase you’re looking for is WETSU:
    We Eat This Stuff Up.

  4. A concise description of dark energy? Let’s see:

    In Einstein’s general theory of relativity, mass (or the corresponding energy) is not the only thing that gravitates. Pressure also produces gravity, and it’s backwards from what you might expect: matter under pressure gravitates a little more, and matter under tension (negative pressure) gravitates a little less. Enough tension, and the resulting gravity would actually be a repulsive stretching force!

    Now when you stretch something that is under tension (think of a rubber band), you’re putting energy into it. This opens up a very strange possibility. Imagine a substance that is under such tremendous tension that the resulting gravitational stretching puts enough energy in to make more of the same stuff! If the universe were filled with such a substance, it would just stretch and stretch and stretch forever, and the stuff would never run out.

    That’s dark energy.

  5. SMEaton

    Wow…. Wow…. Wow….

    Holy exlpoding raisin bread scenario, BAUTman!
    I was aware of the acceleration scenario…
    I can’t wait to read through that paper and comprehend what I can.

  6. Port to bow? What was I thinking? I fixed it.

  7. Chip

    “Combining Eqs. (4–6) yields solutions to the global evolution of the Universe.”

    This is exciting stuff. Please forgive me pointing out that “global” here means “all” or “comprehensive history” and not simply a globe as in spherical or ball shaped.

    People sometimes get confused by terms as when the otherwise good balloon analogy misled some past posters into thinking the universe was expanding into a preexisting space.

  8. Navneeth

    So the universe is a ship, that measures nine yards from end to end, loaded with Mexican delicacy. You need relativity to figure that out?!


  9. Concerning the global evolution of the universe, didn’t a guy named Laplace also state something similar over a couple of centuries ago? :)

  10. DrFlimmer

    I want to copy a statement from Richard Feynman:
    “Science is like sex, sometimes something useful comes out but that´s not the reason we are doing it!”
    We just do it because it is SOOOOOOOOO great and cool (I´m about to become such a “weird person” 😉 I study physics)

  11. George

    It is nice to see them together. [The reason Schmidt’s team one the accelration race is also worth noting. His team had fewer sn but their data on them was more accurate, due to filters, giving them a sigma advantage which allowed them to declare the acceleration result first.]

    I shouldn’t gripe about their grip, but do we really know enough about the dark to be able to claim we have determined the evolution of the universe?

  12. The Ghost Of Tookie

    >> but how do you concisely describe dark energy

    A fudge factor to balance out the math?

    Ha haaa, I tease the cosmologists. :)

  13. Hi Phil,

    funny, my current blog entry is on the same subject

  14. If you need a short explanation, why not just copy equations 4-6?

  15. Grand Lunar

    For a moment, I thought that when you might write that astronomers “astronom”! Like you did in your book.

    Yeah, that IS cool. Goes to show that math can be a very powerful tool as well.

    The more you know…..

  16. Nigel Depledge

    And this, Phil, is why I love your blog. You write with such enthusaism and passion about a topic that could all too easily become dry and lifeless (and, if cosmology papers are anything like biochemistry papers, your primary sources were probably very dry reading). Thanks for giving it life.

  17. Nigel Depledge

    Incidentally, Navneeth, you may be interested to know that the term “the whole nine yards” comes from fighter pilots in WWII. The machine-guns in their planes were loaded with belts of ammunition, and the belts were nine yards long. If a pilot fired all of this in one burst, he was said to have given it the whole nine yards.

    Isn’t language fun?

  18. Navneeth

    Hi Nigel,
    Thanks for the info! :)

  19. Phil, you big tease. What is it that equations 4-6 actually say? What is the fate of the universe? Big Crunch? Big Whimper?

  20. Rob

    Whole nine yards – while the machine gun explanation is certainly compelling, it seems to be a myth. The actual origin is unknown.

  21. Guillermo

    Phil, the link to the pdf seems not to work. Here’s the link to the abstract, from where a postscript can be downloaded. Nice blog, Phil.

  22. Michael

    You neglected to mention the supernovae observations of Adam Riess’ group in discovering the accelerated expansion of the universe (see http://arxiv.org/abs/astro-ph/9805201).

  23. Jim Reilly

    Never forget–astronomers make stuff up to explain what they do not understand. Interesting stuff, but completeley “made up” none the less.

    Also remember–Science is all about posing an explanation for a physical occurance / phenomenon, and describing a physical test an “experiment”) to test the hypothesis.

    Without that “try-fail-try-fail again-try-AHAH! ” approach, we’d still be living in caves.
    Dr..P Reilly

  24. Michael, I didn’t mention that I played a part in that as well, and was sitting next to Adam at a table with another grad student when the first glimmers on how to normalize Ia light curves were first born…

    But that’s another story.


  25. The NOVA series on PBS has had several programs over the last several years about the Perlmutter/Schmidt rivalry and the unexpected result. A good reason to support PBS–you would have never heard about this on regular TV.

  26. icemith

    I like these discussions when the ‘seeing’ is good. I only wish my ‘eyes’ were able to see as well.

    Ivan. …(No relation to Blind Freddy).

  27. tacitus

    I’ve long marveled at the idea that someone sitting behind a dusty desk in some dingy university faculty office staring at a page full of mathematical equations can actually be working on resolving the origin, nature, shape, and future of the whole Universe (and beyond, if string theory starts to pan out).

    Even as an atheist, one begins to wonder how all this is even remotely possible (invoking God simply pushes the problem back one future step and so is irrelevant). The mind truly does boggle at the astounding facts and theories we have already uncovered and the prospect of discovering more in the future is an exciting one.

    Now, if only we could find a practical way to circumvent that pesky speed-of-light issue, we could really start to explore the Universe! Until then, astronomers and cosmologists will have to do the heavy lifting for us.

  28. Sir:
    Try my website for anything interesting, new logical relativity, new geometry, new laws, new idea for the great red shifts, link to my 2nd MS Theory of Life, which gives a new equation, and much to show who created the universe and all in it.
    Best wishes,
    Ed. Robinson.

  29. Steve Ziolkowski

    I’m not even as smart as most of you (I was sort of lost on what Eqs. (4-6) meant until Phil explained it…) but boy oh boy, that’s exciting stuff. That the universe in its entirety can be boiled down to a few explanatory equations is mind boggling.

  30. Sergio

    So what IS the solution? Is the universe flat, open or closed? I have to know for my future plans!

  31. Buzz Parsec
  32. 42 enchiladas -YUU-UUU-UMM! 😉

  33. VinNay

    Here is something I’ve been slightly confused about for a while (from a scientific american article):

    “By comparing the brightness of distant, far-away supernovae with the brightness of nearby supernovae,” Botner continued, “the scientists discovered that the far-away supernovae were about 25 percent too faint. They were too far away. The universe was accelerating. And so this discovery is fundamental and a milestone for cosmology. And a challenge for generations of scientists to come.”

    If dark energy is driving the expansion to accelerate over time, shouldn’t closer supernovae (as opposed to further) be fainter than expected because they happened more recently than far away supernovae (expansion was slower in past)? Or are the “close” ones in the local group, and is the local group gravitationally bound such that the accelerating expansion isn’t yet strong enough to overcome the gravity?

    If that is the case, do we see “normal” close supernovae, “very faint” far supernovae, and “slightly faint” super-far supernovae? Does this question make sense?

  34. CJSF


    I thought you worked from home. What “office” did you stand up and exclaim in, then?

    I love reading about this stuff. While I may never truly grasp the maths involved, the concepts and progress of understanding leaves me awestruck and proud, nonetheless. Proud that we can describe and model the Universe and still be in love with it.

    And also, I have a similar question to @VinNay. Shouldn’t closer supernovae seem farther away than expected compared to more distant ones?


  35. VinNay

    Hmmm, strike the last part of my question. The photons from “super-far away” supernovae would still be subject to expansion redshifting because they have been traveling through the accelerating expanding space to us even thought they originated earlier in the acceleration process.

    So assuming I am right about supernovae in the local group, what does the expansion rate over time curve look like? Is it linear?

    Also I just realized this post is from 2006, so I’m not holding my breath on getting an answer.

  36. VinNay

    Thinking along the lines of my previous realization – Do we know if the distribution of dark matter is fairly uniform across the universe? If it isn’t, could just our “local area” in the universe be expanding in an accelerated way? How could we tell the difference?

  37. CJSF

    Yes, I also just noted that this is an old entry – never mind the question about the office, Phil. And, VinNay, yes I think you’re right, come to think of it.


  38. Wzrd1

    @37, VinNay, there are some maps of dark matter now, reflecting galactic cluster structures. From what I’ve viewed, it appears rather like a web in three dimensions.

    Phil, did the red shift data match the acceleration in a fairly linear and proportional manner? Luminosity can be impacted by many factors, though with many different supernovae, the number drop, but the different rates of acceleration would have different red shift amounts. I’ve not heard of any paper that offers the view of reconciling the two observations, only the lower magnitude.


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


See More

Collapse bottom bar