Archive for September, 2009

Philosophy and Cosmology: Slow Live-Blogging

By Sean Carroll | September 20, 2009 1:37 am

Greetings from Oxford, a charming little town across the Atlantic with its very own university. It’s in the United Kingdom, a small island nation recognized for its steak and kidney pie and other contributions to world cuisine. What you may not know is that the UK has also produced quite a few influential philosophers and cosmologists, making it an ideal venue for a small conference that aims to bring these two groups together.

george_ellis The proximate reason for this particular conference is George Ellis’s 70th birthday party. Ellis is of course a well-known general relativist, cosmologist, and author. Although the idea of a birthday conference for respected scientists is quite an established one, Ellis had the idea of a focused and interdisciplinary meeting that might actually be useful, rather than just bringing together all of his friends and collaborators for a big party. It’s to his credit that they invited as many multiverse-boosters as multiverse-skeptics. (I would go for the party, myself.)

George is currently very interested and concerned by the popularity of the multiverse idea in modern cosmology. He’s worried, as many others are (not me, especially), that the idea of a multiverse is intrinsically untestable, and represents a break with the standard idea of what constitutes “science.” So he and the organizing committee have asked a collection of scientists and philosophers with very different perspectives on the idea to come together and hash things out.

It appears as if there is working wireless here in the conference room, so I’ll make some attempt to blog very briefly about what the different speakers are saying. If all goes well, I’ll be updating this post over the next three days. I won’t always agree with everyone, of course, but I’ll try to fairly represent what they are saying.

Saturday night:

Like any good British undertaking, we begin in the pub. I introduce some of the philosophers to Andrei Linde, who entertains us by giving an argument for solipsism based on the Wheeler-deWitt equation. The man can command a room, that’s all I’m saying.

(If you must know the argument: the ordinary Schrodinger equation tells us that the rate of change of the wave function is given by the energy. But for a closed universe in general relativity, the energy is exactly zero — so there is no time evolution, nothing happens. But you can divide the universe into “you” and “the rest.” Your own energy is not zero, so the energy of the rest of the universe is not zero, and therefore it obeys the standard Schrodinger equation with ordinary time evolution. So the only way to make the universe real is to consider yourself separate from it.)

Sunday morning: Cosmology

9:00: Ellis gives the opening remarks. Cosmology is in a fantastic data-rich era, but it is also coming up against the limits of measurement. In the quest for ever deeper explanation, increasingly speculative proposals are being made, which are sometimes untestable even in principle. The multiverse is the most obvious example.

Question: are these proposals science? Or do they attempt to change the definition of what “science” is? Does the search for explanatory power trump testability?

The questions aren’t only relevant to the multiverse. We need to understand the dividing line between science and non-science to properly classify standard cosmology, inflation, natural selection, Intelligent Design, astrology, parapsychology. Which are science?

9:30: Joe Silk gives an introduction to the state of cosmology today. Just to remind us of where we really are, he concentrates on the data-driven parts of the field: dark matter, primordial nucleosynthesis, background radiation, large-scale structure, dark energy, etc.

Silk’s expertise is in galaxy formation, so he naturally spends a good amount of time on that. Theory and numerical simulations are gradually making progress on this tough problem. One outstanding puzzle: why are spiral galaxies so thin? Probably improved simulations will crack this before too long.

10:30: Andrei Linde talks about inflation and the multiverse. The story is laden with irony: inflation was invented to help explain why the universe looks uniform, but taking it seriously leads you to eternal inflation, in which space on extremely large (unobservable) scales is highly non-uniform — the multiverse. The mechanism underlying eternal inflation is just the same quantum fluctuations that give rise to the density fluctuations observed in large-scale structure and the microwave background. The fluctuations we see are small, but at earlier times (and therefore on larger scales) they could easily have been very large — large enough to give rise to different “pocket universes” with different local laws of physics.

Linde represents the strong pro-multiverse view: “An enormously large number of possible types of compactification which exist e.g. in the theory of superstrings should be considered a virtue.” He said that in 1986, and continues to believe it. String theorists were only forced to take all these compactifications seriously by the intervention of a surprising experimental result: the acceleration of the universe, which implied that there was no magic formula that set the vacuum energy exactly to zero. Combining the string theory landscape with eternal inflation gives life to the multiverse, which among other things offers an anthropic solution to the cosmological constant problem.

Still, there are issues, especially the measure problem: how do you compare different quantities when they’re all infinitely big? (E.g. number of different kinds of observers in the multiverse.) Linde doesn’t think any of the currently proposed measures are completely satisfactory, including the ones he’s invented. A big problem with Boltzmann brains.

Another problem is what we mean by “us,” when we’re trying to predict “what observers like us are likely to see.” Are we talking about carbon-based life, or information-processing computers? Help, philosophers!

Linde thinks that the multiverse shows tendencies, although not cut-or-dried predictions. It prefers a cosmological constant to quintessence, and increases the probability that axions rather than WIMPs are the dark matter. Findings to the contrary would be blows to the multiverse idea. Most strongly, without extreme fine-tuning, the multiverse would not be able to simultaneously explain large tensor modes in the CMB and low-energy supersymmetry.

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CATEGORIZED UNDER: Philosophy, Science, Travel

Well, That Was Fast!

By Julianne Dalcanton | September 18, 2009 12:14 pm

Last week saw the first public release of data from the refurbished Hubble Space Telescope, and its new imaging camera (Wide Field Camera 3, or “WFC3”).

Over the past week, four papers have shown up on astro-ph using new WFC3 data of the Hubble Ultra Deep Field (see the prescient comment by Brian Mingus in the original blog post):

Bouwens et al
Oesch et al
Bunker et al
McLure et al

All of these papers are based upon data released on September 9th, from a large “Treasury” program to extend the wavelength coverage of the Hubble Deep Field. The first two papers were produced by the team that actually proposed the observations, and the second two were from groups that were sitting around eagerly waiting for the first group’s data to be publicly released1.

All of the papers deal with the statistics and properties of extremely high redshift (i.e. distant and young) galaxies. The dominant technique for finding high redshift galaxies has been looking for “drop out” galaxies. These are galaxies that have essentially zero flux in blue filters, due to absorption from intergalactic gaseous Hydrogen, and significant flux in all redder filters (at wavelengths that are largely unaffected by the same gas). Higher redshift galaxies “drop out” of progressively redder filters, because the rest frame (un-redshifted) wavelength at which the gas absorbs the galaxy’s light appears redshifted to longer and longer (redder) wavelengths for highly redshifted galaxies. This technique was pioneered by Guhathakurta, Tyson, & Majewski in 1990, and put on the map as a technique for galaxy selection by Chuck Steidel throughout the 90’s.

"Drop-outs" from the Hubble Ultra Deep Field

These early works looked for “U-band dropouts”, which turn out to be star forming galaxies at a redshift of 3 (about 2.5 billion years after the big bang). Subsequently, people had the bright idea to just keep pushing the drop out technique to redder wavelengths, to look for ever more distant galaxies. However, this comes at a cost, since more distant galaxies tend to be much fainter, so you need to work harder to have statistically significant detections in the red filters, and strong contraints on the absence of detections in bluer filters. The new WFC3 data pushes this technique out of the optical and into the infrared, selecting galaxies at redshifts from 6 to 9, when the universe was 0.5-1 billion years old. (Note: in the picture above, each successive column shows an image taken at redder and redder wavelengths. The likely distant galaxies are those that show up in the rightmost three columns but none of the leftmost columns)

The new papers all find that at these early times, the star formation rate of the universe is on the rise. This isn’t too surprising, given that you’re getting so close to the beginning of the universe — early on, structure hasn’t really had much time to form, so naturally you should find that fewer galaxies have yet had time to go about their business.

Evolution of the star formation history of the universe

All in all, these are nice results doing just what the new data was designed to do.

1 In Jackson Hole, I once saw a bald eagle sitting high in a tree above a stream. There was an osprey sitting lower down the same tree. The river guide said the eagle waits for the osprey to catch a fish, and then just steals the fish from the osprey.

2 After posting this, I found a nice write-up by Ron Cowan here, as well as discussion of the result from the always lovely Peter Coles here. There’s also a cute discussion of the stress involved in such publications over at andxyl’s place.


What I Did on My Summer Vacation – Part 2

By John Conway | September 18, 2009 11:47 am

Travel is broadening, and in particle physics we get to do a lot of it. In July, having temporarily settled my father into a nursing home after being hospitalized (the subject of my last post, Part 1), I was able to meet my commitment to travel to Krakow, Poland, to give a plenary talk on the search for the Higgs boson at the annual Europhysics conferenceheld at the Jagiellonian University there (where Copernicus studied for four years, 1491-1495).

Central Krakow emerged from World War II, which began nearly exactly 70 years ago, nearly unscathed. The central square is one of the more beautiful in Europe, similar in a way to that of Prague. But it was hard to avoid waling there without imagining what it must have looked like during the war, occupied by German soldiers who had made Krakow the center of their regional government during the war.

From the square one can take tours in little golf-cart-like jitneys, and see some of the interesting historical sites, including the Jewish Quarter (Kazimierz) and Schindler’s famous enamelware factory. Some of the apartment buildings in Kazimierz are still in the state they were at the end of the war, a rather grim reminder of the central role Krakow played in the Holocaust.


From Krakow one can take day trips to a number of interesting places, and we visited the spectacular salt mines of Wielicka, a UNESCO World Heritage site, which have amazing, huge rooms carved out of the rock.

But there was another interesting place to tour that we were hesitant about – Auschwitz. Others who took the tour came back saying that it was well worth the journey, over an hour by bus each way, but tended not to say much more about it…hmmm.

So on our last free day we took the plunge, signed up for the tour, and went. The bus traveled through quite rural countryside on two-lane roads, past farms and villages, roughly following the Vistula river, until reaching the town of Oswiecim, which the Germans called Auschwitz.

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CATEGORIZED UNDER: Politics, Travel, World

The Threat to Mt. Wilson

By Julianne Dalcanton | September 18, 2009 12:18 am

Some of you may have followed the threat to the historic Mt. Wilson observatory from the fires in Los Angeles earlier this month. Below is a fantastic time lapse video shot from one of the facilities on the mountain. You can see how close the fire came (though thankfully, the firefighters did a superb job in protecting observatory with targeted back burns to create firebreaks around the site).

As this video shows, astronomical observatories are frequently at risk from wildfires, since both tend to occupy dry remote mountaintops. Indeed, close to seven years ago, one of Australia’s major observatories on Mt. Stromlo was nearly obliterated by the fires that raced through the area:

Thankfully, Mt. Wilson survived this round.

PS. You can find a bit more about some of the ground breaking work that was done at Mt. Wilson along with some terrific old Life magazine photos here.

CATEGORIZED UNDER: Space, Technology

Planck First Light

By Sean Carroll | September 17, 2009 12:59 pm

If you haven’t heard that Planck has seen first light, you haven’t been reading the right cosmology blogs: see Andrew Jaffe, Peter Coles, and Planck’s own Twitter feed. Planck is of course the European Space Agency’s microwave background satellite experiment, which was launched back in May. Since then it’s been tumbling in space about once every minute, doing a leisurely scan of the sky. The survey is not nearly completed, but all systems seem to be running smoothly.

Here’s the region it’s looked at so far, superimposed over a visual-light map of the Milky Way:


And here’s a zoom in on one region, as seen in two different wavelengths:


So far the scientists are playing with the data to learn about the instrument, not so much about the microwave background. Andrew predicts a big splash of papers from Planck in August 2012. We’ll be looking for a bunch of things: Are the overall features of the CMB consistent with predictions from inflation? Are there “non-Gaussian” features indicating extra power in some regions? Is the strength of the perturbations equal on all scales, or does it gradually diminish at smaller distances? Did we learn anything surprising from the polarization, such as tensor modes that could come from inflation or an overall rotation that could come from quintessence? Does the universe have a preferred direction?

I’m sure it will be front-page news, whatever that news turns out to be. Stay tuned.


Where We Are on the Laffer Curve

By Sean Carroll | September 16, 2009 9:55 am

The Laffer Curve is a simple idea: a government can’t raise taxes forever and expect to increase revenue along the way. Eventually you’re taking so much in taxes that people don’t have any reason to earn income. The argument is simple (and correct): if you have zero tax rate you get zero tax revenue. If you raise taxes just a bit, nobody will be discouraged from working, and you will collect some amount of revenue; therefore, the curve of revenue versus tax rate starts at zero and initially rises. But if the tax rate is 100%, nobody has any reason to work, and your total revenues will be back at zero. By the wonders of math, there must therefore be a maximum of the curve somewhere in between 0% and 100% tax rate.

An important question is, where are we on the curve? The notion of the Laffer curve has been used to justify all sorts of tax cuts, under the assumption/claim that we are to the right of the maximum, so that cutting taxes will actually increase revenues. Serious economists generally don’t believe this holds true in the U.S. right now, but the lure of the idea is undeniable: lose weight by eating more ice cream!

Via Marginal Revolution, here’s a study by Mathias Trabandt and Harald Uhlig that tries to get it right. Obviously they have models that make various assumptions, and I have no idea how realistic those assumptions are. They study the U.S. and several European countries, and find that Denmark and Sweden are just a bit on the wrong side of the curve for the specific case of capital income taxation. For the most part, however, tax rates lie to the left of the maximum. In the U.S., especially, we are significantly on the left. Here is the graph for labor taxes:


The vertical line is our average tax rate; the curves represent different model assumptions. They estimate the U.S. could increase revenues by about 36% by raising taxes. That obviously doesn’t necessarily imply that we should — but we could.


If Science Knew All the Answers, It Would Stop

By Sean Carroll | September 14, 2009 6:45 pm

I have no idea why Kieran thinks that this Dara Ó Briain video would be my cup of tea. We all know that I am devoted to the ideal of communicative reason between respectful parties speaking in good faith. None of that tawdry mockery and whacking at people with sticks for me, no sir.

Nevertheless, it’s quite charming; perhaps it’s the Irish accent. Ó Briain studied math and theoretical physics at University College Dublin, where he was an officer of the Literary and Historical Society, where I spoke not too long ago. I cannot speculate where the fashion sense came from.


Whither NASA: the Moon? Mars? Science?

By Daniel Holz | September 13, 2009 10:09 pm

The Review of U.S. Human Space Flight Plans Committee has just released a summary of their report. This “Augustine” report (named after chairman Norm Augustine, former CEO of Lockheed, not St. Augustine, which might have made for more entertaining reading) discusses the future of US manned space exploration. The full report should arrive within the month.

The summary makes one critical point: NASA is woefully underfunded to accomplish its stated goals (Let’s go to the Moon and Mars and beyond!).mars geologist studying a rock The committee’s basic message is that, under the current funding profile, NASA can barely retire the space shuttles and the International Space Station. Any ambitious manned space exploration plans will have to be delayed by a minimum of 15-20 years (Bush wanted us to be playing soccer on the Moon by 2020). The committee says an additional $3 billion/year for ten years is required to have a viable manned exploration program, on top of the roughly $10 billion/year currently being spent (for reference, NASA science programs weigh in at under $5 billion/year). As far as space exploration is concerned, the current trajectory isn’t going to get us anywhere.

One interesting aspect of this report is the absence of science. Out of 12 pages, science is mentioned twice. In the third line of the report, we are advised that spaceflight “really is rocket science”. Cute. Towards the end of the introduction, we are told “Human exploration can contribute appropriately to the expansion of scientific knowledge”. The emphasis is theirs, not mine. Perhaps they’re feeling a little defensive? As well they should. From what I can tell, nobody has articulated a compelling scientific case for human beings to go beyond low-Earth orbit. Or even leave Earth, for that matter. From a scientific perspective, the International Space Station has been an unbelievably colossal waste of money. As the Economist tells us, “the useful science that has been done on board could be written up on the back of a postage stamp.” (Sam Ting’s AMS would be an exception. But this is unlikely to have been the most cost-effective way to go about this experiment.) The space shuttle program, on the other hand, has been instrumental in producing amazing science, epitomized by the launching and servicing of the Hubble Space Telescope. Given the immense cost of the shuttle program, however, the science return on investment remains fairly slim. How many space telescopes could have been built and launched by conventional rockets, for the cost of all that shuttle development?

It could be argued that the manned space program is not about science at all. It’s about slipping the surly bonds of Earth and fulfilling our “natural destiny”. There is certainly something compelling about this, although I would argue that the current plans are ludicrously expensive and overly ambitious. My main concern is that the public misses the distinction: NASA sometimes appears to conflate human exploration and basic research. When we talk about sending humans to the Moon or Mars, we’re not talking about scientific exploration. If science is your goal, you send unmanned probes and satellites, at a tiny fraction of the cost. These missions carry no risk to human life, and considerably larger scientific payoff.

Like any science fiction fan, I’m intrigued by the idea of human colonies on the Moon and Mars and beyond. But if these long-term aspirations suck the oxygen out of the room for basic science, humanity on the whole loses out.

Dark Atoms

By Sean Carroll | September 11, 2009 10:04 am

Almost a year ago we talked about dark photons — the idea that there was a new force, almost exactly like ordinary electromagnetism, except that it coupled only to dark matter and not to ordinary matter. It turns out to be surprisingly hard to rule such a proposal out on the basis of known astrophysical data, although I suspect that it could be tightly constrained if people did high-precision simulations of the evolution of structure in such a model.

In fact our original idea wasn’t merely the idea of dark photons, it was dark atoms — having dark matter bear a close family resemblance to ordinary matter, all the way to having most of its mass be in the form of composite objects consisting of one positively-charged dark particle (a “dark proton”) and one negatively-charged dark particle (a “dark electron”). We thought about it a very tiny bit, but didn’t pursue the idea and only mentioned it in passing at the very end of our paper. There is an informal rule in theoretical physics that you should only invoke the tooth fairy (propose an extremely speculative idea or hope for some possible but unprovable result) once per paper, so we stuck with only a single kind of charged dark particle.

But once someone invokes the tooth fairy in their paper, anyone who writes another paper gets to invoke the tooth fairy for themselves. (That’s just how the rule works.) And the good news is that it’s now been done:

Atomic Dark Matter
Authors: David E. Kaplan, Gordan Z. Krnjaic, Keith R. Rehermann, Christopher M. Wells

Abstract: We propose that dark matter is dominantly comprised of atomic bound states. We build a simple model and map the parameter space that results in the early universe formation of hydrogen-like dark atoms. We find that atomic dark matter has interesting implications for cosmology as well as direct detection: Protohalo formation can be suppressed below $M_{proto} sim 10^3 – 10^6 M_{odot}$ for weak scale dark matter due to Ion-Radiation interactions in the dark sector. Moreover, weak-scale dark atoms can accommodate hyperfine splittings of order $100 kev$, consistent with the inelastic dark matter interpretation of the DAMA data while naturally evading direct detection bounds.

(Note that one of the authors has been a guest-blogger here at CV.) It looks like a great paper, and they seem to have done a careful job at chasing down some of the interesting implications of dark atoms. In fact the idea might be more robust than that of the one in our paper; the fact that dark atoms are neutral lets you slip loose of some of the more inconvenient observational bounds. And the last sentence of the abstract points to an intriguing consequence: by giving the dark matter particles some structure, you might be able to explain the intriguing DAMA results while remaining consistent with other (thus far negative) direct searches for dark matter. Stay tuned; that dark sector may turn out to be a pretty exciting place after all.


Attack of the Boltzmann Brains!

By Sean Carroll | September 10, 2009 8:56 am

It is a truth universally acknowledged that a provocative scientific idea will, before too long, end up in the hands of villains that must be fought by superheroes. Witness Boltzmann brains. Sure, they’ve already made a cameo in Dilbert, but the stakes were pretty low. Now Jim Kakalios (author of the excellent The Physics of Superheroes) sends along sends along a couple of snippets from The Incredible Hercules #133 — in which our intrepid protagonists are attacked by freak observers fluctuated out of thermal equilibrium!

Boltzmann Brains in The Incredible Hercules

Actually here they are described as “freaky observers,” rather than the more conventional “freak observers.” That description brings to mind Smoove B rather than Ludwig Boltzmann, but who knows? Maybe unlikely thermal fluctuations tend to be pretty kinky.

Boltzmann Brains in The Incredible Hercules

And yes, before you all start in: we know that Boltzmann Brains don’t really make for a credible alien menace, if you insist on being persnickety about what they supposedly really represent. It’s not that they “perceive” a universe more chaotic than ours — it’s that they would dominate the total number of observers if the universe really were more chaotic than ours. (Which it isn’t!) Also, they would tend to dissolve back into the chaos from which they came, rather than staging a coordinated attack on our homeland. Still! What a novel challenge for the Allies’ greatest hero.

CATEGORIZED UNDER: Humor, Science and the Media, Time

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