“Scientific discovery and scientific knowledge have been achieved only by those who have gone in pursuit of it without any practical purpose whatsoever in view.” –Max Planck

Tomorrow morning, at 8 AM my time, the press conference that cosmologists have spent the past decade waiting for will finally happen, and the Planck satellite — the most powerful satellite ever to measure the leftover radiation from the Big Bang — will finally unveil its results about the origin and composition of the Universe.

Image credit: ESA / LFI and HFI Consortia.

Image credit: ESA / LFI and HFI Consortia.

They’ve figured out how to subtract the galactic foreground in all of the seven wavelength-bands where Planck operates to unprecedented sensitivity, and the science is ready to be released! Let’s use this opportunity to take a look back on what we know right now, where-and-what the uncertainties are, and what Planck can (or, at the very least, might) teach us about the Universe!

Image credit: Rhys Taylor, Cardiff University.

Image credit: Rhys Taylor, Cardiff University.

1.) The Big Bang is safe.

The Big Bang is the idea that the Universe was once in a hot, dense, ionized state and expanded to become our star-and-galaxy-rich cosmos that we live in today. There are three separate cornerstones that lead to this picture: the observed Hubble expansion of the galaxies, whose recession rates increase with increasing distance from us, the observed primordial abundances of the light elements, which are predicted by Big Bang Nucleosynthesis to give us a Universe with about 75-76% hydrogen and 24-25% helium (by mass), and the leftover, nearly uniform blackbody (CMB) radiation at just a few degrees above absolute zero, coming from all directions in space, which marks the leftover glow from the Big Bang itself.

Image credit: Whittle Rodman, University of Virginia.

Image credit: Whittle Rodman, University of Virginia.

In the context of General Relativity, our tried-and-true description of gravity in this Universe, only an expanding, cooling Universe in the context of the Big Bang leads to these three predictions simultaneously, and nothing the Planck satellite observes will change that.

Image credit: WMAP Science Team / NASA.

Image credit: WMAP Science Team / NASA.

2.) The Universe is mostly dark energy, followed by dark matter, with normal (baryonic) matter making up just a small fraction.

There are three sets of large-scale observations that simultaneously lead to this picture, again in the context of General Relativity.

Image credit: J. Colberg and the VIRGO Consortium.

Image credit: J. Colberg and the VIRGO Consortium.

The observed patterns of large-scale galaxy clustering, combined with the data from ultra-distant distance indicators (like supernovae), and the already known patterns of fluctuations in the microwave background on both large (from WMAP) and small (from the South Pole Telescope and others) scales, all indicate a Universe that’s made up of about 71-74% dark energy, 20-24% not-too-hot dark matter, with the remaining 4.6% made up of normal, standard model particles. These standard model particles include everything we’ve ever observed directly, including protons, neutrons and electrons, photons and neutrinos, and all the exotic, unstable matter we’ve ever created.

Image credit: Kowalski et al., 2008.

Image credit: Kowalski et al., 2008.

So none of those things will change substantially, although the dark energy/dark matter numbers may shift around a small bit in that range. Although Planck will measure the large scales more accurately and in more wavelengths than WMAP before it, that science has already been done, and Planck will only refine it, not overthrow it. The way it will refine it is extraordinary; while WMAP was limited by the sensitivity of the instruments on it, that’s not the case for Planck, according to the ESA:

Planck will provide even more precise measurements with an accuracy set by fundamental astrophysical limits… In other words, it will be impossible to ever take better images of this radiation than those obtained from Planck.

But there are some things that Planck can shed some light on, which have the potential to be extremely exciting!

Image credit: LSST / AURA.

Image credit: LSST / AURA.

3.) The age, size and expansion rate of the Universe!

Yes, it’s true that we often quote that the age of the Universe is 13.7 billion years old, the diameter of the observable Universe is 93 billion light-years across, and the expansion rate — or the rate that all galaxies (on average) recede away from one another — is about 71 kilometers/second/Megaparsec. But these numbers are all related to one another, with the age-and-size numbers also dependent on the percentages of dark matter and dark energy.

Image credit: Moresco, Michele et al. JCAP 1207 (2012) 053.

Image credit: Moresco, Michele et al. JCAP 1207 (2012) 053.

But the expansion rate has a little bit of uncertainty attached to it. It probably couldn’t be as low as 60 or as high as 80, but no one would be shocked if it turned out to be 68 km/s/Mpc, or maybe as high as 74 km/s/Mpc. This could mean a Universe as old as maybe 14.2 billion years, or as young as 13.3 billion years, depending on how the dark matter and dark energy parameters adjusted. Half-a-billion years may not be a big deal to you, but when you consider that we’ve already got stars that push the 13-and-change billion year limit, it’s pretty important to astrophysicists that the Universe is at least as old as all the stars in it!

Image credit: Prof. Matt Strassler, 2011.

Image credit: Prof. Matt Strassler, 2011.

4.) There are three types of neutrino in the Universe.

We’re pretty sure of this one… aren’t we? I mean, we’ve got these huge particle colliders, we’ve been running them for decades, and we’ve seen how hordes of them decay. The decay of the Z-boson, for instance, tells us that there are 3.003 ± 0.006 neutrinos species whose mass is less than 4.5 × 1010 eV. Considering that the heaviest a neutrino is allowed to be is around 0.08 eV, it makes sense to conclude that there are three.

Image credit: Carlo Giunti, via Luca Merlo of http://neutel11.wordpress.com/.

Image credit: Carlo Giunti, via Luca Merlo of http://neutel11.wordpress.com/.

But the cosmic microwave background should also measure the number of neutrino species in an independent way, and would also be sensitive to a bizarre, hypothetical type of neutrino that particle physics wouldn’t find conventionally: the sterile neutrino! WMAP, with lousy sensitivity, has claimed to have found about 3.6 ± 0.5 neutrino species, and so while not conclusive, it’s suggestive that there could be a new particle (or maybe even 2?) out there! Planck should improve on the WMAP constraints, and this could be interesting.

Image credit: "Cosmic Inflation" by Don Dixon.

Image credit: “Cosmic Inflation” by Don Dixon.

5.) How did the Big Bang get started?

According to both the spectrum of density fluctuations imprinted in the CMB and the large-scale-structure of the Universe, and also the best theoretical solution to many open questions in cosmology, the answer to that is cosmological inflation, or a period where spacetime was expanding exponentially fast. At some point, inflation ended, setting up the Big Bang and creating all the matter and energy known to permeate our observable Universe.

Image credit: Ned Wright (and possibly Will Kinney, too), via http://ned.ipac.caltech.edu/. (Notes by me.)

Image credit: Ned Wright (+ possibly Will Kinney), via http://ned.ipac.caltech.edu/. (Notes by me.)

Of course, we don’t quite understand how all of this happened. As in, there are many models of inflation that could have successfully done this, and we have no way to discriminate between many of them. But the two main classes of models — models of new inflation and models of chaotic inflation — have a major difference between them: chaotic models should produce large amounts of long-wavelength gravitational waves, while new inflation should produce almost none. In optimistic models of chaotic inflation, this would cause a polarization of some of the light from the CMB, something that Planck could — in principle — pick up. So Planck has the dual potential to either detect primordial gravitational waves and verify not only inflation but a particular model of it, or to disfavor the chaotic inflation scenario in favor of new inflation. (Full disclosure: new inflation has long been my preferred model.)

Image credit: Avi Loeb, 2006.

Image credit: Avi Loeb, 2006.

There are other, smaller refinements that could happen, such as a better pinning-down of the epoch of reionization or a more precise measurement of a few cosmological parameters, but these are the five big ones — confirmation of the first two and potential answers to the last three — that I’ll be waiting on. If you want to watch the NASA announcement live online, it’s at 8 AM pacific time on March 21st here, and you can check out the ESA’s page here or call in and listen to the teleconference. WMAP redefined the precision to which we understood the Universe when its first data release happened a decade ago, and now Planck has the potential to take us even further in our understanding of the greatest quest of all: the dream of understanding the entire Universe. I can’t wait to see what they found!


  1. […] 2: offensichtlich letzteres – und warum man nichts Weltbewegendes erwarten sollte. NACHTRAG 3: und noch ein Primer für die […]

  2. #2 Michael Kelsey
    SLAC National Accelerator Laboratory
    March 20, 2013

    “Foreground subtraction.” In my business, the hardest part of any analysis is background subtraction, where the goal is to count a nice chunky peak sitting on top of some more or less uniform (or at least smoothly and slowly varying) baseline.

    I can’t imagine how to subtract away all of the background “signal” which is sitting on top of the subtle density variations in the CMB, and successfully extract all of those details. If you’ve already done a posting on this issue, my apologies; if not, it would be very interesting.

  3. #3 EBRecordings
    March 20, 2013

    What’s the point in finding out what happened before the big bang ? Is humanity being unrealistic about the reach of its knowledge ? Are trying to “run before learning to walk ?”

  4. #4 Michael Wright
    Silicon Valley
    March 20, 2013

    Wow, fascinating article, I somewhat understand some of this (studied Resnick and Halliday years ago). Makes all what most of us earthers argue about (i.e. politics) seem so small in comparison what is happening around us.

  5. #5 Vince
    United States
    March 20, 2013

    Hey EBRecordings,
    I would argue that the study of the nature of reality is probably the most noble of pursuits. Those same pursuits, over the last 120 years, have vaulted our technology from steam engines to electric engines and from telegraphs to cell phones. Why ANYONE would question the pursuit of knowledge is as frightening as it is insulting.

  6. #6 Rocket Ray
    Springfield OR
    March 20, 2013

    Yeah, ya know, that’s preety much how I see it.

  7. #7 Becky Wragg Sykes (@LeMoustier)
    March 21, 2013

    Hi, a great Planck primer, thanks. Only one request: the figures you include are fascinating, & it would be brilliant to include a (brief) description of what they’re showing in the captions.
    Thanks for blogging, I often read your articles but haven’t commented before.

  8. #8 Mark
    United States
    March 21, 2013

    “8pm my time”….slow clap. Honestly? We don’t know where you live, couldn’t you say EST, PST…or something? People live all around the world you know.

  9. #9 Dan
    March 21, 2013

    Vince#5: Agreed. One need only read the Planck quote in the caption to see the truth in your reasoning. I have always felt that knowledge for its own sake is, not only enough, but paramount and absolutely necessary to the advancement of all mankind. I am always wary, and often frightened, of individuals or groups who ask “why?” when it comes to gaining knowledge. “Why should I care?, Why does it matter?” What’s sad is that you cannot win an argument with someone who is ignorant, especially one who chooses that ignorance, with all the facts laid before them. You will never convince them. It is left to the few, the scientists, the entrepreneurs, to DRAG Humanity forward, while the masses benefit from the myriad contributions of science and technology. It is, by and large, a thankless endeavor. The debates go on and on, about how “we” should be spending “our” money and resources. Why build a telescope when we can build more shopping malls? You try to say things like, “where would we be without minds such as Newton and Edison and Curie?” You get a blank stare, or more nonsensical rhetoric. I’ve since given up. I will speak to children, and other enlightened adults only on such “weird” topics as astronomy and cosmology.
    Somewhere, there is an article about yoga-pants being recalled with over 1000 comments. And this article, this type of marvelous, infinitely fascinating topic, has but 6.
    I can’t think of any more excuses. Nearly half of the planet have access to at least rudimentary scientific facts, and yet only a very, very small percentage of people have any idea about the universe around them, nor do they seem to care. Who can blame the scientists for their eccentricity introvert behavior? Look around you. We have mastered the use of technology (see iPhone users) yet fail to understand the centuries of scientific discovery leading up to the creation the very technology we take for granted. I hate to say it, but it appears to be a losing battle. More and more care less and less about how it all works. You see it every day, and it pains my heart to no end.

  10. #10 Ken
    United Kingdom
    March 21, 2013

    Vince, I couldn’t agree more; surely the one thing above all others separating us from other animals is the urge to understand the nature of things. We want to know what is going on, not just for its utility, but for its own sake. As Sheldon Glashow said: we are curious creatures.

  11. #11 Drew
    March 21, 2013

    How do we know the background radiation detected is not from previous lSupernovas in this part of the Milky Way?

  12. #12 SCHWAR_A
    March 21, 2013

    Hi Ethan:
    “…there are 3.003 ± 0.006 neutrinos species whose mass is less than 4.5 × 10^10 eV.”

    This exponent seems much too large to me…

  13. #13 Wow
    March 21, 2013

    “This exponent seems much too large to me…”

    You are obviously no astrophysicist.

    Indeed, an ERROR BAR of that magnitude isn’t, in astrophysics, necessarily a bar to the conclusion being used.

  14. #14 Eric Lund
    March 21, 2013

    @SCHWAR_A: That mass would be 45 GeV. What Ethan is saying is that if there were a fourth decay channel for the Z boson that involved neutrinos of less than that mass, our particle accelerators should have seen it. We can’t rule out a heavier neutrino (not that there is any compelling argument for a fourth neutrino flavor at least 11 orders of magnitude more massive than the three known). We also can’t rule out a neutrino that does not participate in Z boson decay (I’m not sure how that works, either, but IANA particle physicist). The WMAP results suggest that at least one of these two types of neutrinos may exist.

    The Planck results apparently leaked out. I have already seen a story from AP that they have measured the age of the Universe at 13.81 billion years. It’s not quite 0700 PDT (which, to answer an earlier question, is Ethan’s time zone) as I write this.

  15. #15 CB
    March 21, 2013


    Didn’t make it to the end of the article before deciding to complain in the comments section, I see.

  16. #16 Bob McElrath
    March 21, 2013

    The number on the neutrino mass should be about 2 MeV from the Z decay kinematics. What you wrote was half the Z mass, but if the neutrino was very heavy, we could see it in the photon spectrum. (That experiment looks at e+ e- -> Z gamma).

  17. #17 Julie Reed
    La Jolla, CA USA
    March 21, 2013

    Is there a link to the announcement mentioned to have taken place this morning?

  18. #18 CB
    March 21, 2013

    Results are out!

    – Universe is 13.8 billion years old.

    – DM is 26.8% and DE is 68.3% of the universe, more and less respectively. Normal matter slightly increased at 4.9%.

    – Most exciting: Significant asymmetry in temperature across the sky, and a large particularly cold patch. Theorists, start your engines!

  19. #19 CB
    March 21, 2013

    *13.82 billion years old

  20. #20 jschmidt
    March 21, 2013

    If we didn’t encourage exploration at all levels and disciplines, we be stuck in the dull world of thinking the earth is flat and the solar system revolves around the earth. Great job to the satellite builders and scientists, and of course to Max as well.

  21. #21 jschmidt
    March 21, 2013

    CB -universal cooling? or universal change? Which is it? Stake out your claim now before Gore gets to it.

  22. #22 Wow
    March 21, 2013

    “Stake out your claim now before Gore gets to it.”

    Yeah, the continual whine of the under-achiever in the face of someone who has gotten somewhere and NOT had to prostitute their principles.

  23. #23 CB
    March 21, 2013

    Also, Hubble Constant is 67.3 km/s/MPc

    I wish the press statements I was reading (from the ESA website, and Nature) included error bars. 🙁

  24. #24 jschmidt
    March 21, 2013

    Wow- good comment on Gore- he;s been wrong a lot.

  25. #25 Wow
    March 21, 2013

    Ah, you are not only ill-bred, you’re an idiot too.

    If you want to talk bullshit, do so elsewhere.

  26. #26 jschmidt
    March 21, 2013

    Wow- typical insulting liberal. You don’t pick on sarcasm to well do you. I didn’t know you were the moderator of opinions. Why are liberals always angry when someone has a different opinion? Not very progressive of you.

  27. #27 Wow
    March 21, 2013

    Typical rightwingnut primadonna.

    You insult someone then when someone insults you, you go all prissy and butthurt.

    Get over yourself, pal.

    You’re a waste of carbon and you’re merely pissed off that Gore managed to do something with his life and still has his integrity, whereas you’ve whored yourself out for decades and got nowhere.

    And so you slag off others. But they’d better not be mean and slag you off, because that’s what liberals do.

    Oh, I guess that must mean you, right?

    Now, this is a science site, not a rightwingnut gathering place, so piss off back to your sister’s house and play that banjo.

  28. #28 Wow
    March 21, 2013

    ” Not very progressive of you.”

    Hang on, you earlier said:

    ” typical insulting liberal”

    It appears not even YOU believe the bullshit that you spout.

  29. #29 Wow
    March 21, 2013

    “Why are liberals always angry when someone has a different opinion? ”

    Yet YOU got angry when I had a different opinion about you than you hold yourself.

    I guess rightwingnuts are always unable to follow a thought through more than two words.

    Now, if you want to go piss off to your rightwingnut troll feasting ground, put a link and I’ll follow you there, but respect the property of Ethan and stop posting rubbish on his site.

    Rightwingers like you never respect other people’s private property, though, do you.

    Not very capitalist of you.

  30. #30 HenryC
    Birmingham, Al. USA
    March 21, 2013

    I have always wondered if virtual particles from Zero point energy theory could be part or most of the Dark Matter. I am sure it has been investigated, but I have never seen the results. I assume the current testing will illuminate the dark matter question and I will be watching.

  31. #31 Wow
    March 21, 2013

    They can’t, Henry: they (by dfinition) don’t exist long enough for anything else to find out.

    Well, not entirely true: there’s a version of the maths but that would lead to (IIRC) a difference between measured value and calculated from that their of a factor of 10^100.

    Even for astrophysicists, that’s a bit high…

  32. #32 AlanF
    United States
    March 21, 2013

    Never been to this site before. Enjoyed the article and enjoyed the comments . . . until I got to the intellectual discourse between the rightwingnuts and leftyloonies. Get a life, you guys!

  33. #33 Wow
    March 21, 2013

    Hey, if I’d turned up here and said:

    You know that George Bush Jr fella? He’s a fooking idiot, inne?

    Then maybe you’d have a point.

    As it is, I think an xkcd is appropriate:


  34. #34 Dan
    March 21, 2013

    Want to get to reach people better? Re do all this as a video.

  35. […] Well, it’s not a perfect universe after all. Cosmic background radiation suggests an imperfect universe, which is 100M years older than previously thought. New physics may arise. You may want a good backgrounder on the background. […]

  36. #36 Nubbygee
    March 22, 2013

    This may seem like a “elementary” question … but, doesn’t it look like the expansion of the “universe” is the diametric-opposite of a black hole? … that the “Big Bang” is the expression or effusion of a “Black Hole” that has ripped through a weak point in the membrane of a”space/time/energy” barrier?

  37. […] e, quando finalmente eu acessei o blog dele, me deparei com o post “O que todos deveriam saber sobre o universo, na véspera [da divulgação dos dados] do [Te…. Bem… se Ethan esperava alguma novidade importante dos dados do Planck, eu também […]

  38. #38 Mark Robson
    United Kingdom
    March 22, 2013

    It’s good to know that ”The Big Bang is safe” but I am curious about the ”the observed Hubble expansion of the galaxies”. Can you tell me which actual observation(s) or measurement(s) demonstrates that the distance between all galaxies increases over time to verify that the universe is expanding?
    And I understood that the CMB contains no emission or absorption lines, so how is its redshift calculated ?

  39. #39 Wow
    March 22, 2013

    Come back in 100 million years time and we’ll have the data.

  40. #40 Wow
    March 22, 2013

    Or should that be
    “Seven and a half …”
    “What? Not till next week?”
    “Million. Million years. I told you it would be a long time”.

  41. […] Starts with a Bang blog gave a very good primer on what was expected from the data on the eve of the release, which I recommend reading before […]

  42. […] What Everyone Should Know about the Universe on the eve of Planck [Starts With A Bang] […]

  43. […] What Everyone Should Know about the Universe on the eve of Planck [Starts With A Bang] […]

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