Blogs / Cosmic Variance

While we’re getting the multiverse out of our system, let me point to this interview with Leonard Susskind by Amanda Gefter over at New Scientist (also noted at Not Even Wrong). I’ve talked with Amanda before, about testing general relativity among other things, and she was nice enough to forward the introduction to the interview, which appears in the print edition but was omitted online.

Ever since Albert Einstein wondered whether the world might have been different, physicists have been searching for a “theory of everything” to explain why the universe is exactly the way it is. But one of today’s leading candidates, string theory, is in trouble. A growing number of physicists claim it is ill-defined, based on crude assumptions and hasn’t got us any closer to a theory of everything. Something fundamental is missing, they say (see New Scientist, 10 December, p 5).

The main complaint is that rather than describing one universe, the theory describes some 10⁵⁰⁰, each with different kinds of particles, different constants of nature, even different laws of physics. But physicist Leonard Susskind, who invented string theory, sees this huge “landscape” of universes not as a problem, but as a solution.

If all these universes actually exist, forming a huge “multiverse,” then maybe physicists can explain the way things are after all. According to Susskind, the existence of a multiverse could answer the most perplexing question in physics: why the value of the cosmological constant, which describes how rapidly the expansion of the universe is accelerating, appears improbably fine-tuned to allow life to exist. A little bigger and the universe would have expanded too fast for galaxies to form; a little smaller and it would have collapsed into a black hole. With an infinite number of universes, says Susskind, there is bound to be one with a cosmological constant like ours.

The idea is controversial, because it changes how physics is done, and it means that the basic features of our universe are just a random luck of the draw. He explains to Amanda Gefter why he’s defending it, and why it’s a possibility we simply can’t ignore.

Hey, has anyone heard about this string theory landscape business, and the anthropic principle, and some sort of controversy? Hmm, I guess they have. Perhaps enough that whatever needs to be said has already been thoroughly hashed out.

But, hey! It’s a blog, right? Hashing stuff out is what we like to do. So I’ll modestly point to my own recent contribution to the cacophony: Is Our Universe Natural?, a short review for Nature. To give you an idea of the gist:

If any system should be natural, it’s the universe. Nevertheless, according to the criteria just described, the universe we observe seems dramatically unnatural. The entropy of the universe isn’t nearly as large as it could be, although it is at least increasing; for some reason, the early universe was in a state of incredibly low entropy. And our fundamental theories of physics involve huge hierarchies between the energy scales characteristic of gravitation (the reduced Planck scale, 10²⁷ electron volts), particle physics (the Fermi scale of the weak interactions, 10¹¹ eV, and the scale of quantum chromodynamics, 10⁸ eV), and the recently-discovered vacuum energy (10^-3 eV). Of course, it may simply be that the universe is what it is, and these are brute facts we have to live with. More optimistically, however, these apparently delicately-tuned features of our universe may be clues that can help guide us to a deeper understanding of the laws of nature.

The article is not strictly about the anthropic principle, but about the broader question of what kinds of explanations might account for seemingly “unnatural” features of the universe. The one thing I do that isn’t common in these discussions is to simultaneously contemplate both the dynamical laws that govern the physics we observe, and the specific state in which we find the universe. This lets me tie together the landscape picture with my favorite ideas about spontaneous inflation and the arrow of time. In each case, selection effects within a multiverse dramatically change our naive expectation about what might constitute a natural situation.

About the anthropic principle itself (or, as I much prefer, “environmental selection”), I don’t say much that I haven’t said before. I’m not terribly fond of the idea, but it might be right, and if so we have to deal with it. Or it might not be right. The one thing that I hammer on a little is that we do not already have any sort of “prediction” from the multiverse, even Weinberg’s celebrated calculation of the cosmological constant. These purported successes rely on certain crucial simplifying assumptions that we have every reason to believe are wildly untrue. In particular, if you believe in eternal inflation (which you have to, to get the whole program off the ground), the spacetime volume in any given vacuum state is likely to be either zero or infinite, and typical anthropic predictions implicitly assume that all such volumes are equal. Even if string theorists could straightforwardly catalogue the properties of every possible compactification down to four dimensions, an awful lot of cosmological input would be necessary before we could properly account for the prior distribution contributed by inflation. (If indeed the notion makes any sense at all.)

I was asked to make the paper speculative and provocative, so hopefully I succeeded. The real problem is that draconian length constraints prevented me from making arguments in any depth — there are a lot of contentious statements that are simply thrown out there without proper amplification. But hopefully the main points come through clearly: calculating probabilities within an ensemble of vacua may some day be an important part of how we explain the state of our observed universe, but we certainly aren’t there yet.

Here’s the conclusion:

The scenarios discussed in this paper involve the invocation of multiple inaccessible domains within an ultra-large-scale multiverse. For good reason, the reliance on the properties of unobservable regions and the difficulty in falsifying such ideas make scientists reluctant to grant them an explanatory role. Of course, the idea that the properties of our observable domain can be uniquely extended beyond the cosmological horizon is an equally untestable assumption. The multiverse is not a theory; it is a consequence of certain theories (of quantum gravity and cosmology), and the hope is that these theories eventually prove to be testable in other ways. Every theory makes untestable predictions, but theories should be judged on the basis of the testable ones. The ultimate goal is undoubtedly ambitious: to construct a theory that has definite consequences for the structure of the multiverse, such that this structure provides an explanation for how the observed features of our local domain can arise naturally, and that the same theory makes predictions that can be directly tested through laboratory experiments and astrophysical observations. Only further investigation will allow us to tell whether such a program represents laudable aspiration or misguided hubris.

It’s that time of year – lists of the best and the worst of 2005 are popping up everywhere. One of my favorites is published by the American Institute of Physics with their annual compilation of the Top Physics Stories of the year. The list contains 25 physics events which occured throughout 2005, ranging from the arrival of Cassini at Saturn to various exciting physics results to the announcement of the Nobel Prize. It’s a great read and a great way to catch up on the hot news from various physics subfields. It’s a standard reference for anyone who is on a departmental colloquium committee.

According to the AIP, this year’s most interesting results ranged from the development of lasing in silicon, the biggest burst ever recorded from a soft-gamma repeater, to the observation of geoneutrinos. The latter is cool: KamLAND (the Kamioka Liquid scintillator AntiNeutrino Detector in Japan) has observed the appearance of electron neutrinos orginating from radioactive decays inside the Earth, presumably from U-238 or Th-232.

The top story of the year was the quest to observe quark-gluon plasma at the Relativistic Heavy Ion Collider at Brookhaven National Lab. Theorists say that quarks and gluons become unconfined at high temperatures and form a plasma-like sea of free particles, consistent with conditions in the early universe. The search for this quark-gluon plasma has been long and frought with peril (CERN miraculously announced its discovery just before the turnon of RHIC – an announcement which later proved to be premature and a stretching of the truth). But now, after years of collecting and analyzing data, all 4 RHIC experiments have formed a concensus on the observation of a quark-gluon liquid. That’s right, not a plasma at all, but a liquid! So much for the theorists! When gold ions collide at RHIC, the detectors see a dense liquid which flows with very little viscosity. In fact, it flows so freely that it approximates a perfect liquid, the kind governed by the standard laws of hydrodynamics. Having discovered the surprising liquid nature of the free quark-gluon sea, the experiments next want to probe its properties, such as its heat capacity and its reaction to shock waves. However, RHIC has been subject to extensive budget cuts this year (run time is reduced by 61% fewer hours) and the collider may be terminated in the near future.

Lastly, one of the top 25 is from my homeground, SLAC! It is billed as the best measurement yet of the weak nuclear force. The experiment, known as E-158 (we sometimes have the custom in high energy physics of naming our experiments simply by the numbers, i.e., this is the 158th experiment to be conducted in the End Station at SLAC), carried out the most precise measurement of the weak mixing angle at low momentum transfer. The weak mixing angle relates the stength of the electromagnetic coupling to that of the weak coupling, and provides a sensitive test of our Standard Model of particle physics. This mixing angle has been measured extremely precisely at energies corresponding to the Z-boson resonance, but a full test of our Standard Model requires precision measurements of this quantity at different energies as well. Using the polarized 90 GeV electron beam from the SLAC linac, E-158 measured Moller scattering, which is electrons scattering off of electrons to form electrons + more electrons. It may sound a bit silly, but it is a very clean process from which we can precisely determine the properties of the particles being exchanged in this scattering (the photon, Z boson, and possible new heavy bosons). The accuracy of E-158’s measurement is roughly 0.6% and reaching this level of precision was an experimental tour de force. The End Station at SLAC has since been shut-down for HEP experiment due to budget constraints.

The bottom line from this “best of” list is that there is lots of exciting physics being produced!

Just because you can’t see the dark matter doesn’t mean you can’t take a picture of it. Via Universe Today, here’s a press release from Johns Hopkins announcing a beautiful new image of the reconstructed dark matter density in cluster CL 0152-1357 by Jee et al. (I couldn’t find the paper online, but you can get a higher-resolution version of the picture at Myungkook Jee’s home page.) The dark matter is in purple, the galaxies are in yellow.

How do you do that? It’s not because we’ve detected some form of light coming from the dark matter. Rather, we’ve detected (once again) its gravitational field — this time, via the tiny distortions in the shapes and positions of background galaxies (weak lensing). This is a form of gravitational lensing that is so subtle you could never detect it happening to a single galaxy — it would be impossible to distinguish between lensing and the intrinsic shape of the galaxy. But if you have a large number of background galaxies (which the universe is kind enough to provide us), you can use statistics to reconstruct the gravitational field through which the light travels, and hence figure out where the dark matter must be.

Of course we’re still trying to detect the dark matter, both directly (in ground-based experiments) and indirectly (looking for high-energy radiation produced by annihilating dark matter particles), not to mention using particle accelerators to actually produce candidate dark matter particles. Over the next ten or twenty years, probing the properties of dark matter is going to be one of the top priorities at the particle/astrophysics interface.

While I’m shirking my blogging responsibilities by linking to series of posts elsewhere, there’s an interesting discussion about hostility to atheists at the Volokh Conspiracy: see here, here, here, and here. You’d be unsurprised (I suspect) to learn that Americans find atheists to be one of the most untrustworthy brands of people around. Just to get an idea, here are the answers from a 2005 poll that asked whether “your overall opinion of [the group] is very favorable, mostly favorable, mostly unfavorable, or very unfavorable?”

Well, I suppose it’s understandable, since atheists are constantly killing innocent members of other sects in the name of their belief system. Oh wait, no they’re not. Must be the War On Xmas that is hurting our ratings.

Hopefully Mark’s post explains why there hasn’t been much content from this occasional blog lately — at least three of us are distracted by the New Views symposium (about which I also hope to say something substantive soon). While you’re all waiting for our ungrounded speculations about the universe to return, why not cleanse the palate with some real experimental physics? Chad Orzel at Uncertain Principles has just completed a week’s worth of blogging about the work in his lab. Check out the entries to see the unpredictable hazards of hands-on research. (For a theorist like me, a typical unpredictable hazard is when the barista uses 2% instead of whole milk in my latte.)

• A Week in the Lab: Slow-Motion Experimental Physics Live-Blogging
• The Big Picture
• Lasers, Eight O’Clock, Day One
• Lock and Load
• Cleanliness Is Next to Somethingorother
• It’s Not Science Without Graphs
• Just-So Stories
• What Now?
• The Final Chapter

I arrived yesterday at New Views of the Universe, the Inaugural Symposium of the Kavli Institute for Cosmological Physics, held at the Hyatt Regency Chicago. I’ve been having a wonderful time, and have been hanging out a bit with co-bloggers Sean and Risa. I will have time to post properly about this trip when I get back, on Wednesday.

However, I wanted to mention one thing immediately. In the last two days I’ve come across a lot of people who I know, who are regular Cosmic Variance readers and who, because they haven’t commented, have remained lurkers.

Now there’s no pressure, but I thought it might be fun to see whether, if you are at this conference, and are a reader, you’d like to identify yourself in the comments.

Anyone?

When The Guardian gets hold of research from your university – you know it’s a big deal. Here’s a little excerpt

Male bats with larger testicles but smaller brains stand a greater chance of having offspring than their smaller testicled, bigger brained rivals.

But if you want a real audience, you want it to appear in The Mirror

Scientists at Syracuse University, New York, found the link after studying 334 bat species.

They wrote: “Because relatively large brains are metabolically costly to develop and maintain, changes in brain size may be accompanied by compensatory changes in other tissues.

Or maybe The Times

In the study, a team led by Scott Pitnick of Syracuse University, in New York State, looked at testicle and brain size in 334 different species of bat. They found that testicle size increased markedly in species with particularly promiscuous females, and that the animals’ brains were smaller to match.

I have some very talented colleagues here at Syracuse, and Scott is certainly one if them. However, it is equally fascinating how interested my home country’s media is in any story involving testicles.

By Jorie Graham. (More here and here.)

The slow overture of rain,
each drop breaking
without breaking into
the next, describes
the unrelenting, syncopated
mind. Not unlike
the hummingbirds
imagining their wings
to be their heart, and swallows
believing the horizon
to be a line they lift
and drop. What is it
they cast for? The poplars,
advancing or retreating,
lose their stature
equally, and yet stand firm,
making arrangements
in order to become
imaginary. The city
draws the mind in streets,
and streets compel it
from their intersections
where a little
belongs to no one. It is
what is driven through
all stationary portions
of the world, gravity’s
stake in things, the leaves,
pressed against the dank
window of November
soil, remain unwelcome
till transformed, parts
of a puzzle unsolvable
till the edges give a bit
and soften. See how
then the picture becomes clear,
the mind entering the ground
more easily in pieces,
and all the richer for it.

I’m in the middle of a particularly hectic week that, nevertheless, is wonderful physics fun.

It started on Sunday evening, when my long-time collaborator, Tanmay Vachaspati, and four other members of the Case Western Reserve Particle Astrophysics group arrived at my house. The cosmology group from Syracuse joined us, making eleven people in total, including faculty, postdocs and students. We all ordered in Chinese food, had a few drinks, and talked about physics and life until, at a fairly late hour, we broke up and distributed our guests among our houses to spend the night. It’s always great to hang out with our group, but this was even more enjoyable. Since I used to be a postdoc at Case Western, two of our guests were people I’ve known for a long time and the rest were people I already knew from other physics meetings.

These people drove for six hours to attend the 4th Syracuse-Cornell Joint Theory Meeting, which was held at Syracuse on Monday. Our visitors from Case and also a couple of students who drove from Buffalo, made this meeting the largest one we’ve held so far. The master organizer was our cosmology postdoc, Levon Pogosian, whose attention to detail minimized both the organizational demands on me and the need for any last minute work. Nevertheless, Monday was a very hectic day.

By 10:15am, all our visitors had arrived – about 50 of us in total – people had had a little breakfast, refilled their coffees and we were ready to begin. The structure of these meetings is that graduate students and postdocs give almost all the talks, sometimes with a few faculty talks, and that we allow plenty of time for the discussion of ideas that are presented. Most people talk about work in progress, which makes these meetings a particularly efficient way to get feedback about new ideas, as well as a great chance for students to get experience giving talks.

In the foreground are Csaba Csaki, Levon Pogosian and Eanna Flanagan discussing during one of the coffee breaks.

Our talks were fantastic! The students had worked extremely hard and, whether they used whiteboard, slides or a computer as their medium of choice, they managed their time wonderfully (15 minute talks, strictly enforced) and got a lot of information across. Levon had put together an interesting program – here’s the list of talks

• Simon Catterall (Syracuse) “Twisted lattice supersymmetry”
• Babar Qureshi (Syracuse) “Twist deformed supersymmetry”
• Andrew Noble (Cornell) “Electroweak Precision Constraints on the Littlest Higgs Model with T Parity”
• Seong Lee (Cornell) “The flavor of a little Higgs with T-parity”
• Renata Jora (Syracuse) “Linear sigma model with two chiral nonets”
• Matthew Reece (Cornell) “Top partners at the LHC: mass and spin measurement”
• Seong Chan Park (Cornell) “Fully radiative electroweak symmetry breaking”
• Ali Vanderveld (Cornell) “Observational constraints on the matter couplings of K-essence theories”
• Alessandra Silvestri (Syracuse) “Large Scale Structure formation in Modified-Source Gravity”
• Dejan Stojkovic (Case) “Can black hole events from cosmic rays be observed at the Auger Observatory?”
• Tanja Hinderer (Cornell) “Gravitational waves”
• Brian Powell (Buffalo) “The Lyth Bound and Inflation as an Effective Field Theory”
• Sourish Dutta (Case) “A Classical Treatment of Island Cosmology”
• Sarah Shandera (Cornell) “Brane inflation update”
• Mark Wyman (Cornell) “Modeling (p,q) strings with a gauge theory”
• Francesc Ferrer (Case) “511 KeV photons from superconducting cosmic strings”

Great stuff! Afterwards, a remarkable 28 of us went out for, as Levon put it at the end of the program

• 5:45 – to the pub and out to dinner

It was a relaxing and fun way to end a busy and productive day, as you can see. In the picture on the right is Sarah Shandera, a Cornell graduate student, wondering how she will ever eat this huge fish sandwich, while Henry Tye looks on.

The Syracuse crowd was having a blast also; in the left picture is my student Alessandra Silvestri with Syracuse graduate student Anosh Joseph (left) and Levon Pogosian again.

So this was my Monday. Yesterday I taught my last class of the semester and then, in the evening, we had the 5th installment of Café Scientifique Syracuse. I’ll post about that separately, but may not find time for the rest of the week because there’s so much going on.

I finished writing my final exam today and sent it to my students. I wanted to get this finished because late tonight my co-blogger JoAnne Hewett is arriving. JoAnne is giving our departmental colloquium tomorrow, and then a theory seminar on Friday. We’ll be working pretty hard, as well as finding time to relax and perhaps even to discuss things bloggy. I want to make sure I leave enough free time to squeeze all the physics I can out of JoAnne, including some help I need with an extra dimensions project I’m working on with three postdocs here.

JoAnne leaves on Friday evening, and on Saturday morning I fly to Chicago to take part in New Views of the Universe, The Inaugural Symposium of the Kavli Institute for Cosmological Physics, held in honor of David Schramm by the University of Chicago, at the Hyatt Regency Chicago.

This will be an opportunity to see some excellent talks and to discuss cosmology with a large group of my fellow cosmologists. Sean and I will be trying to steal some time to work on a couple of projects that are nearing completion, and I hope to meet Risa at long last. If I could only find a way to see Clifford in the same time frame, that would be something.

As you can see, this is a pretty crazy week. But it is also very exciting to have so many things going on centered around cosmology and particle physics, and science in general, here at Syracuse. There’ll be a little time to relax now that the break is almost here (this is the last week of classes for us), but until then I’m going full tilt.