Blogs / Cosmic Variance

Over the past two years I’ve been working on a ginormous project archiving imaging data of nearby galaxies using the Hubble Space Telescope. We had taken a bunch of wide-field multi-color panoramas of some lovely nearby galaxies before ACS (the spiffy optical imaging camera) failed, and while looking at one of the outer regions of NGC 253, my collaborator Ben Williams noticed this peculiar pair of galaxies lurking in the background. On an image from the ground, the galaxies looked like:

But in the HST image, now released by the Hubble Heritage team, the galaxies looked like:

What’s striking about this pair of galaxies is that one of them is partially occluding the other. Galaxies contain a lot of dust which can block light, particularly at optical and bluer wavelengths. Because the background galaxy is relatively smooth, you get a wonderful view of the location of dust in the foreground galaxy. It’s clear from the image that the dust is found waaaaaay out from the center of the galaxy, in spiral arms that go well past one would call the edge of the galaxy (based on where you see light on the un-occulting side of the foreground galaxy, to the right on the image above). It’s rare that you ever find two galaxies of comparable size occulting one another, and rarer still that the alignment is so perfect for tracing out the dust to such large radii. Previous ISO studies showed that there was likely to be dust at large radii, but its signature could only be detected by stacking images of many different galaxies to build up sufficient signal-to-noise. Here, you actually get to see the presence of dust directly!

Now, you may not be wired to care about dust. But you should. The key to thinking about dust in this context is that dust extinction is an excellent tracer of the presence of molecular gas, as you can see in this beautiful animation from the millimeter wave group at the CfA, which blinks between an optical image of the Milky Way, and a map of the Milky Way’s molecular gas in the same region:

Given the correlation between dust extinction and molecular gas, the HST image shows that molecular gas is probably present at very large radii, and that it’s still in spiral arms, just as you’d expect. Since molecular gas is the immediate precursor to star formation, this potentially helps to understand where the isolated spots of star formation seen in outer disks are coming from.

In addition, collaborators Benne Holwerda (who’s lead author) and Bill Keel have developed all sorts of clever tricks for measuring the reddening law for this dust, which can potentially give constraints on what the dust is made of, and how it might be affecting other Big Important Measurements like high-z supernovae.

All in all, a pretty cool image, with lots of nifty science behind it.

LHC First Beam Day ended with a bang last night in the SF Bay Area - swissnex, an annex of the Consulate General of Switzerland in San Francisco, threw a party! Around 200 folks were there, with physcisists from all around the area. The champagne flowed, the food never ran out (a minor miracle given an entire roomful of physcists) and the consulate even gave out free bags of stuff. What more could you ask for? It was great to celebrate with colleagues and let our hair down.

Wired magazine was there and commemorated the event by interviewing folks about their LHC expectations:

Well, I did say the champagne flowed…. and we were all giddy in our excitement about the LHC! A truly excellent time was had by all.

9:20 am Pacific Time: Let’s be clear. Tonight’s start-up is a symbolic event, not a physics event; as I understand it, the beam will only be circulating in one direction, so there won’t even be any collisions. Still, it’s a very important symbolic event! The first time the beam goes through the entire machine. So, just for fun, here will be a running commentary throughout the day, with links and musings and all that makes the blogosphere special. Co-bloggers are welcome to chime in, and any particle physicists out there who want to say something about the LHC are welcome to comment or email.

9:45 am (Pacific), Sean: Feel free, in the comments, to make predictions about what the LHC will discover (ultimately, not today). Here are mine. Crackpots not welcome. And seriously, folks — black-hole/world-ending jokes are only funny the first million times.

1:14pm (EST), Mark: Here at Cornell there’s going to be a public forum this evening with refreshments, chats with physicists, two talks (by Yuval Grossman and Peter Wittich) and with various instruments and components of the detector on display.

10:26am (PDT), JoAnne: Actually, it is the end of the world as we know it. I will never again have to write a paper detailing the signatures of some crazy new Terrascale theory, wondering if there is any chance of connection to reality. I will never again have to plot a cross section as a function of the Higgs mass. In fact, I will never again have to do a loop over the Higgs mass in a code. I will never again wonder how electroweak symmetry is broken, how the hierarchy between the electroweak and gravity fundamental scales is maintained, whether there is a WIMP dark matter particle, or whether supersymmetry or extra spatial dimensions actually exist. Fundamental questions and roadblocks that have plagued us for literally decades will finally be answered and we will at last be able to move forward instead of spinning our wheels. Yes, indeed, the world will be truly different.

10:47am (Pacific), Sean: Of course we are not the only blog covering this. The US/LHC Blogs have lots of information, and Tommaso Dorigo offers some inside scoop. There is also main CERN page for the event, and one for press releases.

12:02pm (Pacific), Sean: The real excitement of the LHC startup is, of course, that it’s an excuse to party. Mike in comments already mentioned the Fermilab pajama party. Here at Caltech, where it’s not quite so ridiculously late at night, we’re having pizza and beer. And (for the wimps who can’t stay up), a lunch BBQ tomorrow. Everyone should feel free to put together their own party! Suggested soundtrack. (Dammit, I’m violating my own rules.)

12:54pm (Pacific), Sean: I’ve asked some experts to chime in. Here is Gordy Kane, University of Michigan:

The Standard Model(s) of particle physics and cosmology are wonderful established descriptions of the world we see. They leave out a lot we would like to understand, from dark matter and the matter asymmetry of the universe, to WHY the forces and particles (quarks and leptons) are what they are. LHC won’t tell us much more about the world we see and how it is made, but the discoveries there will point the way to “WHY”. It’s a WHY machine.

The discovery that makes sense is supersymmetry, i.e. the superpartners of some of the Standard Model particles. There’s a lot of indirect phenomenological evidence that indeed some superpartners will be seen at LHC, such as the unification of the forces at very short distances, the absence of large new effects at the LEP and Tevatron colliders, and the very good indirect evidence for a light Higgs boson. A supersymmetric world is also one where we can understand how the electroweak symmetry is broken and how the matter asymmetry arises, and it has a dark matter candidate. I estimate ten or twenty gluinos and a lot of Higgs bosons will be produced in October this year (but not seen unless we are very lucky about the decay signatures). IF the LHC indeed establishes the world is supersymmetric, there is a great bonus – we can write string theories at the Planck scale where the laws of nature should be written and calculate predictions for LHC experiments and dark matter from them, and we can extrapolate data from LHC and dark matter experiments to the Planck scale to see what theories are suggested. Without that window we might never learn the underlying theory from which everything emerges.

It’s very lucky that our technologies and our society allowed us to afford and to build the LHC to study nature so deeply (another anthropic idea?). It’s very unlikely (because of technological and financial and cultural limits) that we can ever have a further facility to extend this study, so we’re very lucky that a framework like string theory has emerged, one that addresses all the basic questions, at the same time we may be able to get from LHC the data that can test and establish it.

1:24 pm (PDT), JoAnne: The History Channel (US cable TV) is airing
The Next Big Bang at 8 PM this evening. The show details our expectations for the LHC and features David E. Kaplan of Johns Hopkins as well as many other of your favorite physicists, so don’t forget to tune in!

1:58pm (Pacific), Sean: Ph.D. Comics weighs in.

6:53pm (EST), Mark: BBC World News America, starting in a few minutes on the East coast, and repeated later, will have a piece on the LHC.

4:05pm (Pacific), Sean: Prize for the best paper title goes to Mihoko Nojiri, arXiv:0809.1209.

The Night before the LHC
Authors: Mihoko M. Nojiri

Abstract: I review recent developments on the use of mT2 variables for SUSY parameter study, which might be useful for the data analysis in the early stage of the LHC experiments. I also review some of recent interesting studies. Talk in SUSY08.

4:25pm (Pacific), Sean: There will be a live webcast from CERN beginning at 11pm Pacific, with the actual beam scheduled for half an hour later. But right now you can click the link, and listen to a pre-packaged CERN video. You can also watch the startup on EVO, if you know what that means (or care to learn).

4:50 pm (PDT), JoAnne: Yours truly has just been recruited for a 5 minute live radio interview on KCSB (the station is on the UC Santa Barbara campus) at 7:30 tomorrow morning. I guess David Gross has the good sense to be asleep at that hour! In any case, I’ll be sure to drink some coffee first, lest I spew some gibberish on blackholes.

6:55pm (Pacific), Sean: Sorry, the “live” blogging took a hiatus while I was talking to Hal Eisner, a TV reporter (”extraordinaire,” he asks me to add) from the local Fox affiliate. He, quite rightly, was hectoring me mercilessly in an attempt to explain the purpose of the LHC at a level accessible to six-year-olds. (He also tried very hard to get me to say “God particle,” which I mostly resisted.)

What is the purpose? It’s to discover the laws of nature, of course, or at least extend our knowledge of them. But that doesn’t always quite cut it for people. I think it would suffice to the aformentioned six-year-old; kids are naturally curious, but adults have it beaten out of them by a relentlessly pragmatic world. Among other things, the LHC represents a tremendous triumph of the basic inquisitiveness of the human species.

7:20 pm (PDT), JoAnne: There’s a host of First Beam Day activities planned for tomorrow across the US. Check the listings for an event near you. Here in SF Bay area, swissnex, the annex of the Consulate General of Switzerland in San Francisco, is throwing a party tomorrow night in coordination with SLAC and LBNL. Much fun will be had by all!

7:53pm (Pacific), Sean: If you’re wondering whether the Large Hadron Collider has destroyed the world yet, see here.

If you’re wondering whether physics is more or less tawdry than politics, see here.

8:17pm (Pacific), Sean: The right response to end-of-the-world chatter is to change the subject — it’s just crackpottery, not a legitimate scientific debate. But damn, you have to be impressed with the vigor of the meme. Far and away the first thing that comes to mind when a person on the street hears “giant atom-smasher in Switzerland” is “might destroy the world.” How do we combat that? What is the one idea we would like to pop into people’s minds when they hear that phrase, and how do we get it there?

11:52pm (EST), Mark: Gotta sleep, but will try to tune into BBC Radio 4’s Big Bang Day when I wake up!

9:26pm (Pacific), Sean: Reporting now from the High Energy Physics conference room here at Caltech. In an hour and a half we’ll open a live feed to our colleagues at CERN, who will be updating us on what happens. Of course, the best answer is simply “all systems nominal.” The only way a detector will actually see anything (as I understand it) is if the beam is not focused perfectly from the start, which is perfectly possible. If the beam is well-behaved, it will just zip through.

But of course, there are many steps along the way, and “first protons circumnavigating the accelerator” is as good a “turn on” event as any. Folks in the know have assured me that CERN will not be hosting multiple “trust us, this is the real start” events — this is it.

9:48 pm (PDT), JoAnne:
From looking at our comments, it’s clear that some folks are still genuinely frightened by the LHC. This should not have happened. The LHC is one of the most exciting scientific journeys in our lifetimes! We should all watch it in wonder and be amazed at its discoveries.

Many a thoughtful, carefully analyzed and written scientific treatise has appeared which thoroughly disproves the claim that the LHC will destroy the Earth. But these aren’t published or mentioned or taken seriously by the press…. (HELP – I’m sounding like a Republican!)

So, let me present a different, non-scientific, but emotional argument. We physicists are human beings too. We have children, parents, siblings, friends, etc, that we care deeply about. We care about this planet and its future and the future of our families. There are literally thousands of physicists, worldwide, involved in the LHC. If there was a serious concern, the scientists themselves would have stepped forward.

As for me, one of my best arguments is that my bottle of 1990 LaTour remains in my cellar. I’m going to pull it out when we achieve collisions at the next accelerator after the LHC! Oh – and the fact that I’ve just spent the last 8 months undergoing intensive, arduous treatment for cancer so that I too can have a future and be a part of the LHC.

10:00 pm (PDT), John:

B minus two hours. Oh yeah! We’ve waited a long time for this.

11:03pm (Pacific), Sean: Action is heating up, although the pizza has yet to arrive. So I’m going to start paying attention to the “real world.” I’ll come back if any disasters occur.

11:30 pm (PDT), John:

Looks like CERN has stuck a PR video in place of the live webcast…not too surprising…maybe the site got hammered, or they have that up until it starts.

The SPS is cycling nicely. That’s what they’ll use to inject the beam in 30 minutes.

11:37 pm (PDT), JoAnne: This is the error message I’m getting:

Due to a huge interest for this live video feed of the LHC First Beam day, you may not be able to see the live video stream and we apologise for this.
Please try reloading the page, come back later, or check the other connection options available on this page.
Many thanks for your interest in CERN and the LHC!

The folks at CERN should have planned for heavy traffic - I’ve waited 25 years for this and I’m disappointed.

11:48 pm (Pacific), Sean: Getting updates from CERN. No disasters, but there was apparently a tiny glitch with one of the collimating magnets, which has now been fixed.

The current beams are low energy (450 GeV, lower than the Tevatron at Fermilab). They want to ramp up to 5,000 GeV (5 TeV) by the end of October — on October 21st, there is a get-together featuring heads of state, and they would love to have actual high-energy collisions by then.

They will be circulating the beam in both directions — just not at the same time, at least today.

The computing system involves about a hundred thousand processors — soon to be upgraded to a few hundred thousand. Data flies from CERN to Caltech at about 40 GB per second, which they also want to upgrade by a factor of ten.

11:58 am (Pacific), Sean: The webcast is limited to 2000 connections! Who’s the rocket scientist behind that?

Midnight (Pacific), Sean: First beam! Or so they say. (See below.)

12:03 am (PDT), John:

Woo hoo! Did it work?

I think it actually starts in a few minutes. The press kit says

9:00 Live satellite broadcast and webcast begin with an introduction from the commentators in the CERN Control Centre, an animation showing the passage of a beam through the LHC, and highlights of the LHC operators’ daily meeting where they lay out the procedure for getting the first beam circulating in the LHC.

9:06 Coverage begins of the first attempt to circulate a beam in the LHC. Lyn Evans, LHC project leader, will narrate the proceedings from the CERN Control Centre. Video of accelerator operators at work in the CCC will alternate with views of the LHC apparatus in its tunnel 100 meters underground.

12:08 (Pacific), Sean: Well, there was a video countdown. No human being has actually confirmed yet…

12:11 am (PDT), JoAnne: Only 2000 connections? No wonder nobody can get on! With all the hype they should have planned better than this….

12:22am (Pacific), Sean: Robert Aymar, CERN Director General … is speaking in French. Translation: in a few minutes we will let the beam zip through the LHC, sector by sector. (They stick absorbers in the way of the beam at certain points, just to check things in each sector before letting it go.) Sounds like the whole thing will take some time.

Liveblogging closer to the source from Adam Yurkewicz, and from David Harris.

I can’t update our blog because too many people are trying to read it!

12:33am (Pacific), Sean: First beam for real! We saw it! Not yet all the way around, as per previous update.

12:36am (Pacific), Sean: BBC reporter: “Ooh! This is exciting!”

12:38am (Pacific), Sean: Okay, I think the beam they had was … actually still in the injector, not the LHC. Because now there is really beam in the LHC! Still not all the way around.

12:40am (Pacific), Sean: Carlo Rubbia seen wandering around the LHC control room.

12:46am (Pacific), Sean: They removed another absorber, and now the beam has reached CMS! I think that’s 3 octants from the beginning.

1:02am (Pacific), Sean: They’ve made it about half way around, and are preparing a beam dump. Sadly, our reserved time on the videoconference has run out, as has my stamina, so I’m heading home. They’re predicting that a full circle will be achieved in the next half-hour or hour.

See you tomorrow!

1:12 am (PDT), JoAnne: The beam is at Point 8, which is 3/4 of the way around! Thanks to SkyNews for the feed!

1:18 am (PDT), JoAnne: Now the beam is at ATLAS, 7/8 of the way through. They are giving ATLAS some events (not collisions, but beam halo and beam gas). Lyn Evans, LHC project manager, was heard to say that he’s going to win his bet, whatever that is.

1:23 am (PDT), JoAnne: BEAM! We have BEAM! All the way round! Now they’re doing it again.

1:43 am (PDT), JoAnne: SkyNews has just interviewed folks in the control rooms for each of the 4 experiments. All of the detectors turned on without trouble and are excited to be getting beam halo and beam gas events. LHCb and ATLAS saw the muons from the beam dump!

Now that the beam has safely travelled through the full accelerator, it’s time for some shut-eye.

7:38 am (PDT), JoAnne: Turns out that the live radio interview was with KCBS here in the Bay Area (which makes much more sense than KCSB in Santa Barbara - our communications department got that wrong!) and just finished. They mainly asked questions about the operation of the accelerator, what comes next, etc. They did ask if the research was open and if all the results would be public or if some of it would be kept secret. And, yes, the subject of those pesky blackholes came up…

9:34 am (Pacific), Sean: As commenters have noted, Google has caught the fever:

But here is something better: the signal from ATLAS when beam first went through.

Click for the full glory!

Women are from Maserati, men are from Lamborghini. At least, that’s what science tells us:

To test the theory that high-performance cars get people hot, Moxon had 40 men and women listen to recordings of the three Italian exotics and a Volkswagen Polo. Everyone had significantly more testosterone after hearing the exotics, and all of the women were turned on by the Maserati. The guys, on the other hand, were drawn to the Lamborghini…

As for the Polo? Everyone had less testosterone after listening to it.

The effect could simply be related to Italian cars vs. German cars, of course, rather than the high-performance engines. No word on how Porsches would stack up against Grecavs. Clearly more research needs to be done.

Note that the Wired blog post is entitled “Science Proves Exotic Cars Turn Women On,” but the study indicates that men are turned on as well. A fast car is equal-opportunity sexy.

I just got back from the Cosmo-08 conference in Madison, which was great fun (and I’m sorry I had to miss the last couple of days). But just because I’m traveling, doesn’t mean that science stops happening. It just means I might be late in blogging about it, if I were moved to do so, which in this case I am.

The big news is that the Gamma-Ray Large Area Space Telescope, a satellite observatory launched back in June, reached two milestones: (1) it got a name change, from GLAST to the Fermi Gamma-Ray Space Telescope (on the theory that not enough things are named after Fermi), and (2) they released the first new picture of the gamma-ray sky! And here it is; click for higher resolution.

You can clearly see the Galactic plane, of course, as well as a few objects that shine brightly in gamma-rays — a handful of pulsars, and one distant blazar. GLAST Fermi will be cranking out the science over the next several years, from down-and-dirty astrophysics to searches for annihilating dark matter. See Andrew Jaffe, Phil Plait, and symmetry breaking.

Meanwhile, some of the folks who brought you the Bullet Cluster have now come up with MACSJ0025.4-1222 (or MAC-Daddy-J, as pundits have suggested).

It’s a similar story — two giant clusters of galaxies smacked into each other, allowing their dark matter to separate from the hot ordinary matter in between. Gravitational lensing lets you figure out where the dark matter is, while X-ray observations reveal the ordinary matter. The Bullet Cluster was pretty darn convincing, but it’s a scientific truism that nothing ever happens just once, so it’s nice to see that magic repeated.

Finally, I wanted to mention something that is somewhat old news, but that somehow had escaped my attention until Dan Hooper’s nice talk at Cosmo-08. That is the WMAP Haze, a phenomenon originally noticed by Douglas Finkbeiner. The WMAP satellite, trying to observe primordial temperature perturbations in the cosmic microwave background, measures several different frequencies, to help correct for foregrounds. The CMB isn’t the only thing that emits microwaves, but nearby dusty astrophysical sources generally depend on frequency in different ways, so one can try to remove their effects by seeing how the maps at different frequencies compare with each other. Some people, apparently, are actually interested in those dusty foregrounds, so they try not only to remove them, but to understand them. And Finkbeiner claims that when we remove all of the foregrounds that we know how to explain, and mask out the parts of the galaxy that are simply too bright to deal with, we are left with this:

There is some mysterious emission near the center of the galaxy, dubbed the “WMAP haze.” There is an explanation on the market, which is what Hooper’s talk was about — the haze could come indirectly from dark matter! Dark matter particles annihilate, so this story goes, giving off a bunch of lighter particles, including electrons and positrons. These electrons and positrons swirl around in the galactic magnetic field, giving off synchrotron radiation, which is what we see as the haze.

True? Plausible? Crazy? I don’t know. The good news is that the dark matter model required to make it work is not thrown together just to fit this result — it’s a fairly vanilla model of weakly-interacting massive particles. The bad news is that it’s hard to understand these dusty foregrounds, and difficult to be sure that you’ve accounted for all of the mundane ones.

The great news is that this is exactly the kind of thing that GLAST Fermi will test, by looking for the high-energy gamma rays that should also be emitted by annihilating dark matter. So stay tuned for some possibly exciting dark matter news, right around the corner.

Physicists often simplify or idealize phenomena to make them more amenable to an initial mathematical treatment. We jokingly refer to this as considering a “spherical cow”. Sometimes one can understand even very subtle phenomena using this technique. However, there are always important effects that one needs the full, non-symmetric nature of the situation to understand.

Here, from The Telegraph, is an example of experimental data illustrating just this point (emphasis mine)!

Dr Sabine Begall and colleagues from the University of Duisburg-Essen looked at thousands of images of cattle on Google Earth in Britain, Ireland, India and the USA. They also studied 3,000 deer in the Czech Republic. The deer tended to face north when resting or grazing.

Although, in many cases, the images were not clear enough to determine which way the cattle were facing they were aligned on a north/south axis.

The scientists concluded that they were behaving in the same way as the deer.

Huge variations in the wind direction and sunlight in the areas where the beasts were found meant that the scientists were able to rule out those factors as being responsible for the direction they were facing.

“We conclude that the magnetic field is the only common and most likely factor responsible for the observed alignment,” the scientists wrote in an article published in the Proceedings of the National Academy of Sciences journal.

All joking aside, I found this fascinating. It is hard to see why this feature would be useful to cows these days, but if you accept the evil theory of evolution, things become a lot clearer.

Their innate ability to find north is believed to be a relic from the days when their wild ancestors needed an accurate sense of direction to migrate across the plains of Africa, Asia and Europe.

I recently returned from a couple of heavy travel weeks, which were exhausting, but well worth it from a physics standpoint. It all started when I left to spend six days in Philadelphia at the 34th International Conference on High Energy Physics (ICHEP08), which was coincidentally hosted by the University of Pennsylvania and Princeton University. ICHEP is one of the two largest particle physics conferences in the world (the other being the Lepton-Photon conference that alternates annually with ICHEP) and I was there to get a fire hose full of what’s going on in the field these days, and to deliver a featured talk on Approaches to Cosmic Acceleration.

The great thing about a conference like ICHEP is that there are hundreds and hundreds of people to hear speak and discuss physics with, in a fun city that you can enjoy with many of your friends from the field when the sessions are over. The down side is that there are hundreds and hundreds of people to hear speak and discuss physics with, and there is just no way that you can see everyone you’d like to. And there’s no way you can spend enough time with all your friends. Overall, I had a wonderful time though. I attended parallel sessions on particle cosmology, supersymmetry, beyond the standard model physics, formal theory, and so many more that I can’t list them all, plus plenary presentations on almost everything.
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Many of you scoffed last week when I mentioned that Lucretius had been a pioneer in statistical mechanics. (Not out loud, but inwardly, there was scoffing.) But it’s true. Check out this passage from De Rerum Natura, in which Lucretius proposes that the universe arises as a quantum fluctuation:

For surely the atoms did not hold council, assigning order to each, flexing their keen minds with questions of place and motion and who goes where.

But shuffled and jumbled in many ways, in the course of endless time they are buffeted, driven along, chancing upon all motions, combinations.

At last they fall into such an arrangement as would create this universe…

Lucretius, along with Democritus and Epicurus, was an early champion of atomism — the idea that the tremendous variety of substances we see around us arise from different combinations of a few kinds of underlying particles. He was also a materialist, believing that the atoms obeyed laws, not that they received external guidance. So a problem arose: how could all of that regular atomic motion give rise to the complexity we see around us? In response, Lucretius (actually Epicurus — see below) invented the “swerve” — an occasional, unpredictable deviation from regular atomic behavior. And then, he points out, if you wait long enough you will swerve your way into the universe.

It’s a good idea, and one that has been re-invented since then. Boltzmann, another famous atomist, hit upon the same basic scenario. Here is Boltzmann in 1897:

There must then be in the universe, which is in thermal equilibrium as a whole and therefore dead, here and there relatively small regions of the size of our galaxy (which we call worlds), which during the relatively short time of eons deviate significantly from thermal equilibrium. Among these worlds the state probability increases as often as it decreases. For the universe as a whole the two directions of time are indistinguishable, just as in space there is no up or down.

However, just as at a certain place on the earth’s surface we can call “down” the direction toward the centre of the earth, so a living being that finds itself in such a world at a certain period of time can define the time direction as going from less probable to more probable states (the former will be the “past” and the latter the “future”) and by virtue of this definition he will find that this small region, isolated from the rest of the universe, is “initially” always in an improbable state.

Boltzmann imagines the universe as a whole (or what we would call the “multiverse”) is in thermal equilibrium, about which he knew a lot more than Lucretius. But he also understood that the Second Law was only statistical, not absolute. Eventually there would be statistical fluctuations that took the thermal gas and turned them into something that looks like our universe (which, as far as Boltzmann knew, was just the galaxy).

We are now smart enough to know that this kind of scenario doesn’t work, at least in its unmodified form. The problem is that fluctuations are rare, and large fluctuations are much more rare; a universe-size fluctuation would be rare indeed. Who needs 100 billion galaxies when one will do? Or even just one observer? This objection was forcefully put forward by none other than Sir Arthur Eddington in 1931:

A universe containing mathematical physicists [which is obviously the correct anthropic criterion — ed.] will at any assigned date be in the state of maximum disorganization which is not inconsistent with the existence of such creatures.

These days, we throw away the rest of the mathematical physicist and focus exclusively on the cognitive capacities thereof, and call the resulting thermodynamic monstrosity a Boltzmann Brain. The conclusion of this argument is: the universe we see around us is not eternal in time and bounded in phase space. Because if it is, over the long term we really would just see statistical fluctuations, and we would most likely be lonely brains. So either the universe is not eternal — so that it doesn’t have time to fluctuate ergodically throughout phase space — or its set of states is not bounded — so that it evolves forever, but doesn’t sample every possible configuration.

Sorry about that, Lucretius. You’ll be happy to know that we’re still struggling with these same issues. Except that you’re dead and famously railed against the irrationality of belief in life after death. So probably you don’t care.

I’ve been blogging the last few weeks about the question of the baryon asymmetry of the universe – the measured excess of matter over antimatter in the universe. Having already discussed electroweak baryogenesis, I’d now like to turn to another possible way that this asymmetry may have come about - leptogenesis.

In order to explain this, I’ll switch gears for a moment to a seemingly unrelated issue in contemporary particle physics, that of neutrino masses. Neutrinos are electrically neutral particles that, until the last decade, were thought to be exactly massless (and indeed are precisely so in the standard model of particle physics). There exists one neutrino particle associated with each electron-like particle (one for the electron, one for the muon, and one for the tau lepton, going by the imaginative names electron neutrino, etc.).

However, careful experiments on neutrinos created both in the Sun and in the upper atmosphere have conclusively demonstrated that neutrinos have an extremely small but non-zero mass. Fortunately for me, and for you, I don’t need to go into my own explanation of why this is so because Heather Ray did a fantastic job of it in her guest post on the MiniBooNE results from April of last year.
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And another paper! Will the science never end?

Superhorizon Perturbations and the Cosmic Microwave Background
Adrienne L. Erickcek, Sean M. Carroll, Marc Kamionkowski (Caltech)

Abstract: Superhorizon perturbations induce large-scale temperature anisotropies in the cosmic microwave background (CMB) via the Grishchuk-Zel’dovich effect. We analyze the CMB temperature anisotropies generated by a single-mode adiabatic superhorizon perturbation. We show that an adiabatic superhorizon perturbation in a LCDM universe does not generate a CMB temperature dipole, and we derive constraints to the amplitude and wavelength of a superhorizon potential perturbation from measurements of the CMB quadrupole and octupole. We also consider constraints to a superhorizon fluctuation in the curvaton field, which was recently proposed as a source of the hemispherical power asymmetry in the CMB.

This is a followup to our paper on the lopsided universe, although the question we’re tackling is a little different. Remember that the point there was that we imagined some sort of ultra-long-wavelength perturbation, much larger than the size of the visible universe, and we asked how that would change the amplitude of small-scale perturbations in one direction of the sky as compared to the other.

In the new paper, we actually address a more basic question: what about the induced temperature anisotropy itself? So instead of looking at the power asymmetry (how does the amplitude of fluctuations in one direction compare to that in the opposite direction), we’re looking at the temperature asymmetry (how does the temperature in one direction compare to the temperature in the other). In fact, we’re looking at the “dipole” asymmetry — not small-scale fluctuations, but the large-scale hemispherical pattern.

Ordinarily, we simply ignore the dipole asymmetry, for a good reason: you get a dipole just from the ordinary Doppler effect, even if there are no intrinsic fluctuations in the CMB. If you have both, it’s hard to disentangle one from the other. But we were considering a supermode that was pretty substantial, and it became an issue — if the predicted dipole was much larger than what we actually observe, it would be hard to wriggle out of.

Except — it exactly cancels. That’s what the new paper shows. (And another paper the next day, by Zibin and Scott, comes to the same conclusion.) We were surprised by the result. There are clearly competing effects: we do have a peculiar velocity, so there is a Doppler effect, and there is an intrinsic anisotropy from the primordial density perturbation (the Sachs-Wolfe effect), and there is also something called the “integrated Sachs-Wolfe effect” from the evolution of the gravitational field between us and the CMB. And they all delicately cancel. We came up with a plausible hand-waving explanation after the fact, but it was the grungy calculations that were more convincing.

Nevertheless, the supermode idea is still constrained — the dipole cancels, but there are higher-order effects (quadrupole and octupole) that are observable. Karl Popper would be proud.