Last November I gave a talk at the Google outpost in Santa Monica, on dark matter and dark energy. I covered a lot of ground pretty quickly, introducing the Standard Model and the basics of the Big Bang as well as some ideas about the dark sector.
This was part of the Authors @ Google series, which features a plethora of great talks. Check out Salman Rushdie, Arianna Huffington, John Hodgman, Tyler Cowen, Anthony Bourdain, Steven Pinker, Lane Montgomery, and dozens more.
I’ve collected various YouTube videos featuring my bad self, but I honestly can’t bear to watch any of them. Can’t stand to see myself speaking (although obviously I have no issues with other people listening raptly). So if any of these are actually Rickrolls, don’t blame me.
February 16th, 2009 at 12:24 pm
Arrgh, the video is no longer available on YouTube, either from yur embedded link or directly from YouTube. How can I listen raptly if it’s not there?
February 16th, 2009 at 12:52 pm
The video is now available. Thanks for posting this.
February 16th, 2009 at 3:47 pm
Physics, Astronomy, Cosmology, etc. are super sweet. I enjoy them very much. I am not a professional physicist but I consider myself generally knowledgeable about these topics, so forgive me if I really miss the mark…
But this whole idea of dark matter and dark energy seems to be just throwing stuff out there to try to explain the apparent lack of matter/energy when the numbers don’t match what is predicted by the theory, namely relativity and the gravitational field theories. Is this not the same concept as the ether which Einstein sought to dispel. Does anyone have a theory about gravity that can explain the cosmos without requiring that some 90% or more of it be unobservable and undetectable. Suppose that the LHC finds the energy signature of the Higgs particle. That would be sweet. maybe the Higgs does turn the energy of particles into mass, that is particles have mass only in a Higgs field. Does this mean that there are sporadic pockets of Higgs particles in and around galaxies and stars to give them mass, but not in the remainder of Universe? To me that seems unlikely. are there any theories that do not require the assumption that this much Universe is made up of this dark ”stuff’? Why don’t we look into that one?
February 16th, 2009 at 6:01 pm
Sean, as I watched this, the one question that kept popping into my head is this: do we at least know what sort of experiement(s)/observation(s) would actually settle these issues once and for all? For instance, would it provide conclusive evidence for quantum gravity if I could provide you a particle accelerator with a diameter from here to, say, the moon? (You know, just say the word.)
February 16th, 2009 at 6:50 pm
First of all sorry for my bad english. Excellent video !!!
I have a doubt (well hundreds, but I’ll ask only one)
You said that the universe is expanding, and the quantity of dark energy is expanding accordingly, while the matter stays the same. The increase of dark energy is not in violation on the laws of thermodynamics? Where is the dark energy coming from?
February 16th, 2009 at 11:18 pm
Philip, if you watched the video, hopefully it became clear that dark matter and dark energy are both hypotheses suggested by data, which are then tested against different kinds of data, and so far they have passed spectacularly well. Of course everyone would love to come up with interesting alternatives, and people (including myself) have certainly tried, but so far the combination of DM and DE is far and away the leading explanation for what’s going on. See also the cosmology primer faq.
dave, sorry to hear that you were beaten up by cosmologists in junior high school.
riemann, it’s not quite that easy, even in principle. There are certainly experiments going on right now that could directly detect dark matter, and that’s a realistic possibility. Dark energy is more difficult; most likely we will just increase our confidence in that idea by process of elimination.
Manuel, those laws are altered in curved spacetime. Energy can be created by the expansion of the universe. See that faq.
February 17th, 2009 at 4:25 am
Dark matter exists:
http://blogs.discovermagazine.com/cosmicvariance/2006/08/21/dark-matter-exists/
As for dark energy, it is the biggest puzzle in modern physics, so it is always possible that it is merely a tag for our ignorance. But one should remember that most of the universe is empty, and if emptiness had non-zero energy, it would be the biggest contribution to the universe’s energy. The actual surprise is not that it is large (70% might seem like a huge fraction), but that it is so small (yet non-zero) that despite the universe’s relative emptiness, it only contributes about 70%. Our theories tell us that it is much more “natural” for dark energy to be about 99.999…..99 % of the total matter/energy content in the universe.
February 17th, 2009 at 7:14 am
@dave tanson:
um… you do understand, do you not, that it has been empirically demonstrated that electromagnetism is far too weak a long-range force to be the over-riding basis of the universe, right? Sure, it rules atomic-to-micro-range interactions (to be superceded by the weak and strong nuclear forces further down the ladder), but long-range it’s next to useless.
And the fact is, DM and DE have a lot of evidence (in the sense that we know enough about what we’re looking at to say that they’re there, even if we don’t know quite what it is.
We can rule out “electric universe cosmology” pretty effectively, and even if it were a possibility, it makes no predictions that match what we see in a DE/DM universe. In fact, those predictions completely rule those “alternate cosmologies” out altogether.
February 17th, 2009 at 8:02 am
[...] The Dark Sector @ Google | Cosmic Variance | Discover Magazine (tags: astronomy videos) [...]
February 17th, 2009 at 8:10 am
[...] Via Cosmic Variance. [...]
February 17th, 2009 at 8:52 am
Regarding the advances in the last ten years that allow brane-world scenarios without losing gravitons, are we talking Randall-Sundrum models?
February 17th, 2009 at 9:15 am
Sean,
How can you know that the universe is not expanding in something? Like a brane scenario or something similar. Like the FRW metric in a R^5 space or something like that.
thanks
February 17th, 2009 at 10:40 am
Jeff, we’re talking about both Randall-Sundrum and Arkani-Hamed-Dimopoulos-Dvali models; anything with “large” extra dimensions.
who, we are not expanding into something in the conventional sense that people think of a balloon expanding into ordinary space. Sure, there are brane-world models where the definitions become subtle, but the basic thing to get across is the idea that an “expanding universe” does not require an external space.
February 17th, 2009 at 10:46 am
Excellent!
I confess to being an ignoramus … but a very interested ignoramus.
Sean, that is the best description of our current knowledge I’ve ever seen. I’d love to share this with my friends, but I don’t have any friends sufficiently knowledgeable to be interested.
My teenaged desire to be a theoretical physicist has caused me to follow the field over the years. A high school math teacher made me believe that I didn’t have the proper turn of mind pursue that goal … if I ever meet that man, I’ll give him a piece of my mind.
Thanks. I may be ignorant (from your perspective), but you fed my fascination. Thanks.
February 17th, 2009 at 2:04 pm
You got a video on http://online.kitp.ucsb.edu/online/lens06/carroll/ also. Video link: http://online.kitp.ucsb.edu/download/lens06/carroll/snd/Carroll_KITP.m4b I stumbled upon it after googling around and it led me to this blog.
February 17th, 2009 at 10:07 pm
Sean,
I watched your video until I got stuck. The place I got stuck on was where you said
1. Dark energy is evenly distributed and
2. Dark energy has constant density.
What makes you say that? It seems to me you’re just making a guess and really have no idea. For one thing, if these were both true conservation of energy and information and the first law of thermodynamics would not apply to the universe as a whole. Are you really willing to throw out the most valuable concept in all of nature the minute you see something you don’t understand? That’s sort of like Bush throwing out habeus corpus because he has no fundamental values. That’s how I would view a physicist willing to throw out those physical laws. Has it occurred to you that quantum physics and its concept of vacuum energy has some problems with it and IT might have to be altered instead?
February 17th, 2009 at 10:23 pm
Eric, conservation of energy works a bit differently in an expanding universe than you seem to think it does. It’s not “throwing out” anything, it’s just taking into account the effects of general relativity. (And the fact that total energy is not conserved is in excellent agreement with the data, e.g. from primordial nucleosynthesis.) See the discussion above.
The idea that dark energy is evenly distributed and has constant density is obviously only a first hypothesis, which should then be tested. It has been tested, and so far it’s done very well. Better observations could always discover some small variations, of course.
February 17th, 2009 at 11:00 pm
Sean,
I’m not saying I have the answers either but I think you may be basing some of these ideas (tell me if I’m wrong) on work done in the 60s based on virtual particle pair production. My understanding, and I think I’m correct, is that it was a stopgap mathematical invention. What I don’t understand, and maybe you can tell me, is why so many physicists refuse to think of gravity itself as being a change in the vacuum energy density surrounding a massive body? Is it just because the Plank length sum of quantum oscillator energy always adds up to the same amount?
Has it occurred to you that just as temperature has lowered from the early universe till now so has wavelength measurements increased(as in cmb radiation). And just as those wavelength measurements have stretched out so has the Planck length stretched out. You are assuming measurement of time, which also infers constant measurement of distance and volume, is the same in todays universe as it was in the early universe. But there is good reason to believe that length and temperature are all tied to rates of passage of time and that time passed much more quickly in the early universe. This would suggest the same amount of energy is within our universe as it was in the early universe an the only reason it doesn’t seem to be so is because our time measurement standard has changed.
I just think you are dead wrong on many of your assumptions. I suppose its not your fault because you were taught these things but I think you should start thinking for yourself. I really have a problem when wrong info just continues to get propagated just because its convenient not to ask questions. Don’t throw out the baby with the bath water just because someone tells you to!
February 18th, 2009 at 7:43 pm
Sean:
Great lecture, it does a fantastic work at explaining in very clear and easy terms those topics that interest me and thousands of people all around the world. Kudos!
I have one question though. It seems from your talk that the main explanation for Dark Energy is that of virtual particles annihilating each other in “empty” space, right? One of the things that has always plagued my mind (not being a physicist but having some notions of topology and physics) is why couldn’t you explain the accelerating universe by means of the “shape” of the universe itself. If the curvature, for some reason, had the shape of a “heap” then objects far away from the place where the Big Bang occurred would accelerate faster than those close to the place where it happened. In other words, it would be like dropping marbles at the top of a heap, at the beginning most of their movement is explained by the original explosion on a rather flat universe, and as they move away, more and more of their movement is explained by the shape of space itself. Does this make any significant sense at all?
Thanks a lot,
February 18th, 2009 at 9:26 pm
Edgar, it’s not quite right to say that dark energy is virtual particles — they are one contribution, but not the whole thing.
For your other question, it’s a matter of general relativity. We can’t just propose some shape to the universe; we have to find a solution to Einstein’s equation that has the desired properties. The model that seems to work is one where space is pretty close to homogeneous on large scales, although people have tried other possibilities. Once you have that, the equations tell you unambiguously what kinds of matter and energy you need to fit the data, which is how we deduce the need for dark energy.
February 19th, 2009 at 5:12 am
You were at Google and you failed to point out that the discrepancy between theoretical and actual dark energy is a factor of more than a googol?
February 24th, 2009 at 7:46 pm
It’s okay Sean, I can’t stand to watch videos of you speaking either.
March 7th, 2009 at 5:06 pm
Finally found time to watch this. At around the 40 minute mark you mention that the way to limit the influence of the vacuum energy is to take into account the Planck lenght/energy/&c. And yet that’s still a factor 10^20 wrong (which I guess *is* infitely better than infinity).
Anyway – what would the Planck scale have to be in order to take care of that last 20 orders of magnitude?
March 7th, 2009 at 5:29 pm
Sili– it’s 10^120, not 10^20. To match observations, the vacuum energy scale would have to be about one milli-electron volt.
March 8th, 2009 at 6:13 am
Ooops – Sorry.
Thanks – We’d have noticed it then, yes. Pity.
August 5th, 2009 at 8:36 am
On The Origin Of Origins
Dark Matter-Energy And “Higgs”?
Energy-Mass Superposition
The Fractal Oneness Of The Universe
All Earth Life Creates and Maintains Genes
A. On Energy, Mass, Gravity, Galaxies Clusters AND Life, A Commonsensible Recapitulation
http://www.the-scientist.com/community/posts/list/184.page#2125
The universe is the archetype of quantum within classical physics, which is the fractal oneness of the universe.
Astronomically there are two physics. A classical physics behaviour of and between galactic clusters, and a quantum physics behaviour WITHIN the galactic clusters.
The onset of big-bang’s inflation, the cataclysmic resolution of the Original Superposition, started gravity, with formation – BY DISPERSION – of galactic clusters that behave as classical Newtonian bodies and continuously reconvert their original pre-inflation masses back to energy, thus fueling the galactic clusters expansion, and with endless quantum-within-classical intertwined evolutions WITHIN the clusters in attempt to delay-resist this reconversion.
B. Updated Life’s Manifest May 2009
http://www.physforum.com/index.php?showtopic=14988&st=480&#entry412704
http://www.the-scientist.com/community/posts/list/140/122.page#2321
All Earth life creates and maintains Genes. Genes, genomes, cellular organisms – All create and maintain genes.
For Nature, Earth’s biosphere is one of the many ways of temporarily constraining an amount of ENERGY within a galaxy within a galactic cluster, for thus avoiding, as long as possible, spending this particularly constrained amount as part of the fuel that maintains the clusters expansion.
Genes are THE Earth’s organisms and ALL other organisms are their temporary take-offs.
For Nature genes are genes are genes. None are more or less important than the others. Genes and their take-offs, all Earth organisms, are temporary energy packages and the more of them there are the more enhanced is the biosphere, Earth’s life, Earth’s temporary storage of constrained energy. This is the origin, the archetype, of selected modes of survival.
The early genes came into being by solar energy and lived a very long period solely on direct solar energy. Metabolic energy, the indirect exploitation of solar energy, evolved at a much later phase in the evolution of Earth’s biosphere.
However, essentially it is indeed so. All Earth life, all organisms, create and maintain the genes. Genes, genomes, cellular organisms – all create and maintain genes.
Dov Henis
(Comments from 22nd century)
http://profiles.yahoo.com/blog/2SF3CJJM5OU6T27OC4MFQSDYEU