After Budapest, London. I had a short meeting at the Royal Astronomical society in Burlington house, certainly a hallowed hall of learning. There was some time before and during the meeting to walk around town and take pictures. It’s challenging taking pictures at night on film, but I think I am learning…
The lights of Leicester Square at night,
Polite people queuing for a party,
A business dinner at night, yes these are serious people,
Having a drink at night, of course,
And now in Burlington house, a great seat of learning in Picadilly,
But see how serious everyone is ! It looks like being in church.
And that is it!
21st-century Insoluble Pancakes: dark matter, dark energy and how we know what we know
These days, we know far more about the origin, nature and fate of the Universe than at any time in history. Justification? Well, any good description of the Universe has to be able both to provide a framework for understanding what has happened in the past and provide predictions for what will happen in the future. It should, more than anything, be consistent with observations. During the past few centuries, the quantity and quality of observations we have made have vastly increased. Applying our new-found knowledge of physics we’ve constructed new instruments and these have allowed us to probe the contents of the Universe right back to the “last scattering surface”, the brick wall beyond which no photon can penetrate.
But there is a problem…
But there is a problem. Our current best cosmological model, the one which matches most observations, happens to contain two substances whose precise nature is still somewhat, shall we say, uncertain. This model is called “Lambda CDM” which means that it has Lambda, “dark energy”, and CDM, which stands for “cold dark matter”. Perhaps that should be with a comma, as in cold, dark, matter? In any case, these two substances, dark matter and dark energy, according to this “standard model”, account for most of the energy content of the Universe. Ordinary material is just the few percent left over. Needless to say, intellectually, this is not a satisfactory state of affairs.
Worse yet, this “standard model” has proved surprisingly robust. Data from the last big cosmology mission, Planck, analysed in part by my colleagues at the IAP, provided a final data set which seems to be in almost perfect agreement with the predictions of the standard model. There is just a hint of a discrepancy with a few measurements at lower redshifts from separate experiments which could be very well explained by imperfect astrophysical and not cosmological knowledge. At the same time, many teams have been trying for the best part of the last few decades directly detect dark matter particles. Other than the mysterious DAMA/LIBRA result, which shows an oscillating signal of who-knows-what there has been no hint of a dark matter particle. The range of particle masses excluded by other experiments are getting smaller and smaller. In particle accelerators like the Large Hadron Collider, no evidence has been found for kinds of particles that dark matter is supposed to be made of (although their mass limits have been narrowed).
Certainly this situation has penetrated the popular consciousness. Many people aware that there is some “dark stuff” which nobody knows what it is. But you see, this is only half true. In fact, the characteristics of dark matter are known very well because it must have those properties for the fabulously successful standard model to match most of the observations.
Now, notice that I said most of the data. There are a selection of problematic data which may or may not be in agreement with our cosmological theory. Here’s the thing though. Any theory which purports to explain any discrepant observations has to only explain the discrepant observations, but everything else as well. That’s hard. Maybe there is no dark matter at all? Maybe it’s like Ptolemy’s “epicycles”, a complicated construct masking an underlying simpler truth? Or maybe gravity works differently on large scales?
The answer is…
So a few days ago on Facebook, buried amongst the cat videos, I came across this article by David Merritt which promised to be a philosophical attack on Lambda-CDM. I had high hopes, but reading the paper in more detail, it doesn’t seem to deliver a knockout blow to Lambda-CDM. Merritt characterises dark matter and dark energy as “conventionalist strategies”, a term borrowed from the great philosopher of science Karl Popper. This is bad: Popper explains that hypotheses which are added to a theory which do not increase its degree of falsifiability are conventionalist. I have to say, I adhere strongly to Popper’s ideas: if you cannot prove a theory wrong by observations, then it is not a real scientific theory. These “conventionalist strategies” are “sticking plasters” added to an existing theory when it should in fact be discarded.
Merritt also argues that a large number of the difficult and as yet unresolved problems in the standard model (many dynamical in nature) have been ignored by textbook writers. He provides an extensive hit-list of cosmology textbooks and whether or not his three named problems are discussed or not. They mostly are not. But is this a problem?
It seems to me that Lambda-CDM has been very successful given our ignorance of its constituents. The weight of observations consistent with theory is large. No other explanation has been proposed which agrees just as well with all this data, and I suspect many of the problems on Merritt’s list may simply be resolved by a better understanding of how normal matter interacts with dark matter. This is a very complicated process, and probably can only be solved numerically using very large computer simulations.
I am not saying that the current situation is satisfactory. I think simply that the hypothesis of dark matter and dark energy is more palatable, for instance, than arbitrary modifications of general relativity. Should we really discard Lambda-CDM for such a theory? Merritt argues that dark matter and dark energy are unverifiable hypotheses, but surely a modification to general relativity without any theoretical motivation is worse? That said, there are theories of modified gravity which have more robust origins. But as I said, we must not forget that that theories must also match all the existing observations, including the discrepant ones!
This century’s insoluble pancake…
If Flann O’ Brien was around today I am sure he would have had a lot of fun with these ideas. After all, O’Brien’s philosopher-scientist De Selby claimed that night was an accumulation of black air … but did he mean dark matter?
O’Brien was writing at a time when the strange ideas of quantum mechanics were slowly becoming common currency. Schrodinger was lived in Dublin at the same time as O’Brien. O’Brien was keen to show how our modern conception of the Universe could sometimes lead to troubling conclusions. The “spooky action at a distance”, Einstein’s description of quantum mechanics, led to O’Brien’s rural police station with a direct link to eternity. And today, with dark matter and dark energy?
The Observatoire de Haute-Provence (OHP, as we call it to save words) is remote. To get there, you take the TGV to Aix-en-Provence, and then rent a car and drive north for around an hour. The nearest town is Manosque and the observatory is in the mountains. Each year I go there with two other astronomers to teach observational astronomy to a group of Masters-level students. The course is organised by Herve Dole who meticulously sorts everything out, helped out a lot of course by people at OHP and the Universities. We usually have two or three nights on the 120cm and 80cm telescopes, and usually it is clear for around two or two of those nights. There is a lot of work involved: for each group of students, there is a full scientific project, starting from planning the observations and finishing with a report and presentation. This year was particularly interesting: we successfully managed to observe an a transiting exoplanet, a planet which passes in front of its star and causes its light to dim.
The story of OHP is a long one. Observations started here at around the same time the IAP (Institut d’Astrophysique de Paris) was created a story which you can read about here. At the start, the OHP was, more or less, the “observing station” of the IAP, a place where the latest instruments could be tested out and where the still-new field of astrophysics could get the data it needed. I won’t go into the whole history of OHP here, but a succession of larger telescopes were built there, culminating with the mighty 193cm telescope in 1958. It was with this telescope that astronomers made the discovery of the first planet outside our solar system, 51 Peg, in 1995. The funny story here is that around the time I was studying for my Masters’ degree in Victoria, astronomers there were using an almost identical telescope at the Dominion Astrophysical Observatory for the same scientific project (with different instruments). Alas, their control of systematic errors was not quite good enough; they missed out.
The situation at OHP has evolved. The number of astronomers using the site has greatly decreased in the last few decades with the arrival of other astronomical observatories situated in locations like Hawaii and Chile. Investments in the site have declined, reflecting a policy of concentrating resources in larger telescopes further away. But there is a lot to be said for smaller telescopes with modern instrumentation nearby, as I saw when I visited the JPASS observatory in Spain in 2015. There they have constructed a new small modern telescope on a site like OHP but with modern state-of-the-art detectors. Despite spending a lot of my time working on larger projects, I think there is a lot to be said for keeping sites like OHP open providing a compelling niche can be found.
Today, there are only a few people around after the sun sets. Although there are many telescopes, only one or two are regularly used. Some have been converted to remote, robotic operation. In the past, a chef cooked legendary evening meals each night, but today we re-heat meals made earlier in the day in the microwave (which never get things quite hot enough.) Still, the observatory has decided that educating students in the techniques of observational astronomy is a priority, and each year many nights of telescope time are dedicated to student projects. There is accommodation on-site which is very comfortable. The course that I teach in is part of the Master 2 program for astronomy in the Paris region, and we have tried very hard to keep an empirical, practical approach to our course. You wouldn’t expect it, but fewer and fewer astronomers actually travel to telescopes these days, and I personally appreciate seeing in person the telescope and dectectors even if they are not at the forefront of technology.
This year is my second trip to OHP with film cameras. I wanted to capture something of the feeling of being there in the middle of winter. Each morning one could see wonderful things like this:
Here are a a few photographs I took around the observatory during the day.
The swimming pool
The 193 at sunset
And as night falls, time to observe, or eat, depending if your observations are finished or not.
Didier G. at the controls
OHP at night
As each year, we stayed a week, arriving on Saturday afternoon and leaving early on the following Saturday. It was kind of nice to be slightly abstracted of the concerns of everyday life. When I wasn’t working, I worked on my article for Emulsive. I learned things. I learned that there is such a thing as the “Qatari Exoplanet survey” and our students successfully confirmed one of their transiting exoplanets (and I actually did a fair amount of work aligning their hundreds of images for them). We ate galette, which is a long tradition in France and in Provence it’s even better. And I took a lot of photographs of trees and domes. And group photographs. The best ones are always the unplanned ones. Like this one, taken at sunset. Almost everyone is there, except Herve, but here it is anyway:
I am already looking forward to returning in 2018!
This weekend I saw Christopher Nolan’s new film, Interstellar. If you know something about movies, then perhaps the best way to understand this film is to say that it is based on a script that Nolan’s brother had originally written for Spielberg. There is plenty of easy and unambiguous sentiment, more so than in any other film he has made until now. Like this: if you leave someone you love, then should try to come back. You had better, you hear! Having seen all of Nolan’s previous films I feel disappointed. I thought he would make a great modern science fiction story which would show us really what it was like to travel great distances and stand on alien worlds that no human has visited before. It does … but not quite.
Toto, I don’t think it’s 1930 any more
Rewind. Here’s the story: Cooper, a drawling midwesterner and ex-fighter-pilot-turned farmer receives some mysterious “instructions” from a restless ghost, telling him to visit a certain location near his farm. There he finds … NASA engineers secretly working to prepare manned missions to a wormhole that’s conveniently opened up near the orbit of Saturn (got all that?). This is good news, because Saturn is of course the most photogenic of all the planets. Bad news, because that’s even further away than the monolith in 2001 which was in orbit around Jupiter, and it will take them two years to get there. Luckily, we at least have that other SF standby, cryosleep, so no sitting around playing cards in space. Whew!The planet Earth of Interstellar is a dried-up dusty place. People live on farms and drive pickup trucks. I almost expected to hear Woody Guthrie singing his Dust Bowl Blues: instead, we have Hans Zimmer and his sepulchral full-on organ tones (and not a church in sight). At the start of the film, we are astonished to see a cast member flip open a laptop. They have computers here? So yeah, it looks like the kind of place you would want to get out of. Weirdly, later on, even in the scenes in outer space, everything looks retro, there is not a touchscreen or hologram in sight. Lots of knobs and buttons and dials and low-def video (quite different from how recent films like Prometheus and District 9 imagine modern space travel).
What can we do with some faster computers?
Anyway, arriving at Saturn we see the wormhole, which nicely distorts the stars behind it. This wormhole leads not only to another solar system, but to another galaxy, and so yes the film should be really called Intergalactic. It’s at this point the film’s big advance from The Black Hole and 2001 become clear: tons more computing power means that we can do a much better job ray-tracing the passage of light around black holes. This, incidentally, is something one of my colleagues at IAP, Alain Riazuelo knows a lot about, having made a series of short films showing how background stars are lensed by massive objects. My friend Mr. Seagull tells me that that Kip Thorne had suggested that he help out, but it turned out that a lot of special effects people are actually recovering from PhDs in astrophysics. So hey!
On the other side of the looking glass
On the other side of the wormhole, our intrepid heroes find themselves with some choices to make: there are three potentially habitable worlds nearby and visiting all of them isn’t going to be easy, not the least because this system contains a nasty large black hole, hence the need for all that ray-tracing. It goes without saying that things don’t work out as expected. One of the most memorable scenes of the film is our explorer Cooper duking it out in a snowy wasteland with the planet’s sole inhabitant, a supposedly idealistic scientist, Dr. Mann, played cooly by Matt Damon. But human beings will be human beings after all! It turns out that Dr. Mann, like everyone else (despite affirmations to the contrary), just wants to go home too. But, this being Hollywood, it all works out fine in the end for the rest of the cast (sorry for the spoilers), thanks to some black-hole strength bending of the rules of physics and causality.
How to get to the next planet in time for tea?
Here’s the astronomer’s polemic: without any additional physics, exploring the Universe is a drag. Voyaging even to nearby stars involves decades-long travel. Nothing says interstellar travel is impossible — it just takes a very, very long time. So, to be truthful, a lot of screen time would be devoted to gliding silently between the stars. For things to happen in a reasonable duration (under three hours, yes) a shortcut needs to be found. Bending space-time with massive objects is probably the least incredible of large number of largely fantastical options. For me, the most realistic description of what the Universe might be really can be found in David Brin’s Existence. Here, the Universe is vast and violent, and all of the travelling is done by machines, in some cases carrying fragments of their creators’ consciousness. That, however, is a lot less fun than boldly going. I sympathise with the movie-director’s predicament: how can you make a good movie about interstellar travel without breaking a few laws of physics?
Going backwards to go forwards
Nolan’s first big hit, Memento was famous for out-of-order story-telling, so you might think that throwing causality out of the window might work out okay. In fact that’s not what’s wrong with the film. The problem is that it is just too much like a big-budget blockbuster movie. Hey, you might say, it is a big-budget blockbuster movie! That’s just it: Nolan was our best hope to make intelligent movies with a wide appeal where things might not work out in the end. He leans too heavily on the films he admires from cinema history, and the plot in some ways is too comforting to be credible. Yes, the alien landscapes are beautiful. However, after going through the worm-hole and travelling to the farthest reaches of the cosmos, we will not be elevated to a higher level of consciousness and become new human beings, and neither will we meet creatures from another dimension. In fact, we will just find…ourselves. The Universe might indeed be empty of life, a terrifying idea, but one could at least hope that we would be changed the journey. So yes, let’s explore. But we need to go further next time.