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Cosmology Explained

Posted in Biographical, The Universe and Stuff with tags , , on September 29, 2008 by telescoper

I’ve always avoided describing myself as an astronomer, because most people seem to think that involves star signs and horoscopes. Anyone can tell I’m not an astrologer anyway, because I’m not rich. Astrophysicist sounds more impressive, but perhaps a bit too scary. That’s why I settled on “Cosmologist”. Grandiose, but at the same time somehow cuddly.

I had an inkling that this choice was going to be a mistake at the start of my first ever visit to the United States, which was to attend a conference in memory of the great physicist Yacov Borisovich Zel’dovich, who died in 1989. The meeting was held in Lawrence, Kansas, home of the University of Kansas, in May 1990. This event was notable for many reasons, including the fact that the effective ban on Russian physicists visiting the USA had been lifted after the arrival of glasnost to the Soviet Union. Many prominent scientists from there were going to be attending. I had also been invited to give a talk, the only connection with Zel’dovich that I could figure out was that the very first paper I wrote was cited in the very last paper to be written by the great man.

I think I flew in to Detroit from London and had to clear customs there in order to transfer to an internal flight to Kansas. On arriving at the customs area in the airport, the guy at the desk peered at my passport and asked me what was the purpose of my visit. I said “I’m attending a Conference”. He eyed me suspiciously and asked me my line of work. “Cosmologist,” I proudly announced. He frowned and asked me to open my bags. He looked in my suitcase, and his frown deepened. He looked at me accusingly and said “Where are your samples?”

I thought about pointing out that there was indeed a sample of the Universe in my bag but that it was way too small to be regarded as representative. Fortunately, I thought better of it. Eventually I realised he thought cosmologist was something to do with cosmetics, and was expecting me to be carrying little bottles of shampoo or make-up to a sales conference or something like that. I explained that I was a scientist, and showed him the poster for the conference I was going to attend. He seemed satisfied. As I gathered up my possessions thinking the formalities were over, he carried on looking through my passport. As I moved off he suddenly spoke again. “Is this your first visit to the States, son?”. My passport had no other entry stamps to the USA in it. “Yes,” I said. He was incredulous. “And you’re going to Kansas?”

This little confrontation turned out to be a forerunner of a more dramatic incident involving the same lexicographical confusion. One evening during the Zel’dovich meeting there was a reception held by the University of Kansas, to which the conference participants, local celebrities (including the famous writer William Burroughs, who lived nearby) and various (small) TV companies were invited. Clearly this meeting was big news for Lawrence. It was all organized by the University of Kansas and there was a charming lady called Eunice largely running the show. I got talking to her near the end of the party. As we chatted, the proceedings were clearly winding down and she suggested we go into Kansas to go dancing. I’ve always been up for a boogie, Lawrence didn’t seem to be offering much in the way of nightlife, and my attempts to talk to William Burroughs were repelled by the bevy of handsome young men who formed his entourage, so off we went in her car.

It takes over an hour to drive into Kansas City from Lawrence but we got there safely enough. We went to several fun places and had a good time until well after midnight. We were about to drive back when Eunice suddenly remembered there was another nightclub she had heard of that had just opened. However, she didn’t really know where it was and we spent quite a while looking for it. We ended up on the State Line, a freeway that separates Kansas City Kansas from Kansas City Missouri, the main downtown area of Kansas City actually being for some reason in the state of Missouri. After only a few moments on the freeway a police car appeared behind us with its lights blazing and siren screeching, and ushered us off the road into a kind of parking lot.

Eunice stopped the car and we waited while a young cop got out of his car and approached us. I was surprised to see he was on his own. I always thought the police always went around in pairs, like low comedians. He asked for Eunice’s driver’s license, which she gave him. He then asked for mine. I don’t drive and don’t have a driver’s license, and explained this to the policeman. He found it difficult to comprehend. I then realised I hadn’t brought my passport along, so I had no ID at all.

I forgot to mention that Eunice was black and that her car had Alabama license plates.

I don’t know what particular thing caused this young cop to panic, but he dashed back to his car and got onto his radio to call for backup. Soon, another squad car arrived, drove part way into the entrance of the parking lot and stopped there, presumably so as to block any attempted escape. The doors of the second car opened and two policemen got out, kneeled down and and aimed pump-action shotguns at us as they hid behind the car doors which partly shielded them from view and presumably from gunfire. The rookie who had stopped us did the same thing from his car, but he only had a handgun.

“Put your hands on your heads. Get out of the car. Slowly. No sudden movements.” This was just like the movies.

We did as we were told. Eventually we both ended up with our hands on the roof of Eunice’s car being frisked by a large cop sporting an impressive walrus moustache. He reminded me of one of the Village People, although his uniform was not made of leather. I thought it unwise to point out the resemblance to him. Declaring us “clean”, he signalled to the other policemen to put their guns away. They had been covering him as he searched us.

I suddenly realised how terrified I was. It’s not nice having guns pointed at you.

Mr Walrus had found a packet of French cigarettes (Gauloises) in my coat pocket. I clearly looked scared so he handed them to me and suggested I have a smoke. I lit up, and offered him one (which he declined). Meanwhile the first cop was running the details of Eunice’s car through the vehicle check system, clearly thinking it must have been stolen. As he did this, the moustachioed policeman, who was by now very relaxed about the situation, started a conversation which I’ll never forget.

Policeman: “You’re not from around these parts, are you?” (Honestly, that’s exactly what he said.)

Me: “No, I’m from England.”

Policeman: “I see. What are you doing in Kansas?”

Me: “I’m attending a conference, in Lawrence..”

Policeman: “Oh yes? What kind of Conference?”

Me: “It’s about cosmology”

At this point, Mr Walrus nodded and walked slowly to the first car where the much younger cop was still fiddling with the computer.

“Son,” he said, “there’s no need to call for backup when all you got to deal with is a limey hairdresser.”

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Is there an Elephant in the Room?

Posted in Cosmic Anomalies, The Universe and Stuff with tags , on September 26, 2008 by telescoper

A couple of weeks ago I was in Cambridge giving a talk at a nice cosmology meeting housed in the splendid Centre for Mathematical Sciences. How the other half lives. The building is not only palatial, it is also very well designed for informal interactions and discussions. When I was a student at Cambridge this building didn’t exist and the Department of Applied Mathematics and Theoretical Physics was housed in rather ramshackle but characterful buildings in Silver Street, right in the city centre. I don’t know what department is there now.

I gave a talk with the title “Fishing for Elephants in the CMB”. I always think it’s a good idea not to give too much away in the title, although perhaps in this case I went a bit too far. Quite apart from the mixed metaphor, it doesn’t really give any clue at all as to what I was talking about. Mind you, I’m not sure at the end of the talk the audience was any the wiser either.

The idea for the title came from the phrase “There’s an Elephant in the Room”, a curious expression that even has its own wikipedia entry, as well as being the title of the picture shown here made by the artist Banksy. It refers to something that is large and obvious, but is being ignored for some reason, usually because it is considered impolite to draw attention to it. My talk of course wasn’t about real elephants but the possibility that there may be a metaphorical one in the field of cosmology, something that is consciously ignored by most of the community.

In yesterday’s post, I referred to the importance of the cosmic microwave background in establishing the so-called “concordance model” of cosmology. But as well as providing compelling evidence in support of this theory, the CMB has also thrown up a few bits of evidence that are quite difficult to reconcile with the standard description of the Universe.

Perhaps the most famous of these anomalies is the so-called “Axis of Evil“, which is an unexplained alignment of features in the pattern of temperature fluctuations observed across the sky by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. In the concordance picture, the fluctuations are basically random so there shouldn’t be coherent alignments like this.

But the Axle of Elvis isn’t the only curiosity in the cosmology shop. There is also a significant asymmetry between North and South on the sky (with respect to the ecliptic plane) when the two celestial hemispheres should be statistically indistinguishable if the standard model is correct.

There also exists a peculiar cold spot. Of course a fluctuating temperature pattern must contain places colder than average and places hotter than average. However, the standard model assumes these are drawn from a Gaussian (or “normal“) distribution, in which large fluctuations are extremely rare. The cold spot we see in the WMAP is colder than the coldest cold spot expected if the standard model is right, with odds of greater than 1000:1 against.

And there’s more. Statistical measures of the fluctuation pattern, such as the correlation function, pixel variance and quadupole moment, all give results for the real sky that are discordant with theory, although admittedly some are more significant than others. There are others too but I have no time to go into them, except to say that they may be related to the ones I’ve already mentioned, or at least share a common cause.

So what’s going on? The most conservative view is that there is nothing in the data that can’t be explained by the standard model and what we are seeing is a consequence of over-interpreting one or two chance coincidences. In the words of Fred Menger

“If tortured sufficiently, data will confess to anything.“

There may indeed be some truth to this, but serious attempts have been made to assess the statistical significance of the various results and my personal reaction is that, while coincidences do happen, it is unwise to dismiss 1000:1 results as mere flukes. On the other hand, these assessments are difficult and the significance may have been miscalculated.

More likely is the presence of some slight unidentified systematic artefact in the data. Not being an experimentalist it’s unfair to cast doubts on the brilliant work of the WMAP team, but one should keep an open mind about this possibility.

But as a theorist I have to admit that the most exciting possibility is that, lurking out there somewhere, are clues to a radical departure from orthodox theory. Many suggestions have been made, and no doubt most of them will be shown to be wrong. But the most dramatic thing that can happen in science is when the only game in town is “none of the above” and we are forced to think outside the box altogether.

I’m certainly not going to argue that we need to ditch the standard model or that cosmologists should all become obsessed with these tantalising conundrums. But in focussing exclusively on questions related to the standard model and its parameters, we may be throwing away a great deal of potentially exciting information. Every now and again, it’s worth checking your waste basket in case there’s something in it that you really shouldn’t have binned.

I realise that there are probably too many mixed metaphors in this piece. They’re a habit of mine and when you get to my age it’s difficult to change. After all, you can’t teach an old leopard to change its spots in midstream.

 

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Mesmeric Universes

Posted in The Universe and Stuff with tags , , on September 25, 2008 by telescoper

It’s probably going to be difficult to describe what these images really are without going into enormous amounts of technical detail, but I think they are fun so I thought I’d put the pictures up with only a brief description. The remind me a little bit of the sort of hypnotic swirl sometimes used to put people under, although there’s a bit more to them than that.

According to our the standard “Big Bang” model, our Universe satisfies the Cosmological Principle which is that it is both homogeneous and isotropic, i.e. that it is the same in every location and looks the same in all directions. Of course we know our Universe isn’t exactly like that because it contains lumps of stuff called galaxies that correspond to variations in its density, but if look at sufficiently large scales it begins to look smooth. Sand is lumpy if you look close at it, but if you look at it from a long way away it looks smooth. The universe is supposed to be similarly smooth if you take a coarse-grained view.

The primary reason for incorporating the Cosmological Principle into models of the Universe is to make the mathematics simple. Einstein’s General Theory of Relativity is such a difficult theory that there are very few situations where the equations can actually be solved. One case where exact solution is relatively easy to achieve is that of homogeneous and isotropic space, which is such a symmetric state of affairs that much of the complexity of the Einstein equations disappears. Cosmological models based on this solution are generally called the Friedman models, after Alexander Friedman who first derived the solutions in the 1920s.

Despite their simplicity, the Friedman models turned out to be surprisingly accurate at describing our actual Universe which we now know to be very close to homogeneous and isotropic. Evidence for this comes from the Cosmic Microwave Background (CMB) which is astonishingly smooth across the sky. Variations in the sky temperature of the CMB are about one part in a hundred thousand of the mean temperature, which is smoother than the surface of a billiard ball.

However, it remains possible that our Universe may be slightly asymmetric and it is interesting to know what the CMB would look like if this were the case. Unfortunately there is no general cosmological solution available, so we have to tread slowly. One approach is to look at Universes which are homogeneous (the same in every place) but not isotropic (they look different in different directions). This might be describe the situation if the Universe were expanding more quickly in one direction that the others, or if it were rotating.

Actually the theory of homogeneous anisotropic universe models is quite well established and there is a full classification of all the possibilities, into the nine so-called Bianchi types. This is mathematically very complicated, so I won’t give details. However, my PhD student Rockhee has been calculating what the CMB pattern would look like in these models and the results are very pretty so I’ve included a few examples here. The little animated gifs show what the sky looks like as the Universe evolves in such cases. In all cases it starts as a pure quadrupole, i.e. a 90 degree variation across the sky. You might have to click on the image to see the animation.

The first one is Bianchi Type V. This is an example of a model in which the space is curved, so that as time goes on the initial quadrupole is focussed by gravitational effects into a smaller and smaller region of the sky. The preferred direction in this (and the other models) is picked to be in the centre of the image and the projection shows the whole sky. Hot spots are blue and cold spots are red, which is the way a physicist should plot temperature.

The next example is Bianchi Type VII_0 which is a flat Universe with rotation. What happens is that the initial quadrupole in this case gets twisted by the rotating space-time into a sort of spiral pattern. Late on in the evolution of such a Universe, an observer would see an interesting swirly structure in the cosmic microwave background.

The final example is my favourite, Bianchi Type VII_h. This one is a sort of combination of the two above examples. It has both rotation and curvature, so there is a swirly pattern which also gets focussed into a small bit of the sky. An observer living in such a Universe would see a prominent spot on the sky lying in the direction of the axis, which in this case is chosen to be in the centre of the diagram.

We’ve also been working out what the sky would look like in polarized light for these, but that’s even more complicated. If you’re really interested, I’ll post a link to the paper when it’s done…

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The MacGuffin Factor

Posted in Science Politics, The Universe and Stuff with tags , , , on September 22, 2008 by telescoper

Unpick the plot of any thriller or suspense movie and the chances are that somewhere within it you will find lurking at least one MacGuffin. This might be a tangible thing, such the eponymous sculpture of a Falcon in the archetypal noir classic The Maltese Falcon or it may be rather nebulous, like the “top secret plans” in Hitchcock’s The Thirty Nine Steps. Its true character may be never fully revealed, such as in the case of the glowing contents of the briefcase in Pulp Fiction , which is a classic example of the “undisclosed object” type of MacGuffin. Or it may be scarily obvious, like a doomsday machine or some other “Big Dumb Object” you might find in a science fiction thriller. It may even not be a real thing at all. It could be an event or an idea or even something that doesn’t exist in any real sense at all, such the fictitious decoy character George Kaplan in North by Northwest.

Whatever it is or is not, the MacGuffin is responsible for kick-starting the plot. It makes the characters embark upon the course of action they take as the tale begins to unfold. This plot device was particularly beloved by Alfred Hitchcock (who was responsible for introducing the word to the film industry). Hitchcock was however always at pains to ensure that the MacGuffin never played as an important a role in the mind of the audience as it did for the protagonists. As the plot twists and turns – as it usually does in such films – and its own momentum carries the story forward, the importance of the MacGuffin tends to fade, and by the end we have often forgotten all about it. Hitchcock’s movies rarely bother to explain their MacGuffin(s) in much detail and they often confuse the issue even further by mixing genuine MacGuffins with mere red herrings.

North by North West is a fine example of a multi-MacGuffin movie. The centre of its convoluted plot involves espionage and the smuggling of what is only cursorily described as “government secrets”. But although this is behind the whole story, it is the emerging romance, accidental betrayal and frantic rescue involving the lead characters played by Cary Grant and Eve Marie Saint that really engages the characters and the audience as the film gathers pace. The MacGuffin is a trigger, but it soon fades into the background as other factors take over.

There’s nothing particular new about the idea of a MacGuffin. I suppose the ultimate example is the Holy Grail in the tales of King Arthur and the Knights of the Round Table and, much more recently, the Da Vinci Code. The original Grail itself is basically a peg on which to hang a series of otherwise disconnected stories. It is barely mentioned once each individual story has started and, of course, is never found.

Physicists are fond of describing things as “The Holy Grail” of their subject, such as the Higgs Boson or gravitational waves. This always seemed to me to be an unfortunate description, as the Grail quest consumed a huge amount of resources in a predictably fruitless hunt for something whose significance could be seen to be dubious at the outset.The MacGuffin Effect nevertheless continues to reveal itself in science, although in different forms to those found in Hollywood.

The Large Hadron Collider (LHC), switched on to the accompaniment of great fanfares last week, provides a nice example of how the MacGuffin actually works pretty much backwards in the world of Big Science. To the public, the LHC was built to detect the Higgs Boson, a hypothetical beastie introduced to account for the masses of other particles. If it exists the high-energy collisions engineered by LHC should reveal its presence. The Higgs Boson is thus the LHC’s own MacGuffin. Or at least it would be if it were really the reason why LHC has been built. In fact there are dozens of experiments at CERN and many of them have very different motivations from the quest for the Higgs.

Particle physicists are not daft, however, and they have realised that the public and, perhaps more importantly government funding agencies, need to have a really big hook to hang such a big bag of money on. Hence the emergence of the Higgs as a sort of master MacGuffin, concocted specifically for public consumption, which is much more effective politically than the plethora of mini-MacGuffins which, to be honest, would be a fairer description of the real state of affairs.

Even this MacGuffin has its problems, though. The Higgs mechanism is notoriously difficult to explain to the public, so some have resorted to a less specific but more misleading version: “The Big Bang”. As I’ve already griped, the LHC will never generate energies anything like the Big Bang did, so I don’t have any time for the language of the “Big Bang Machine”, even as a MacGuffin.

While particle physicists might pretend to be doing cosmology, we astrophysicists have to contend with MacGuffins of our own. One of the most important discoveries we have made about the Universe in the last decade is that its expansion seems to be accelerating. Since gravity usually tugs on things and makes them slow down, the only explanation that we’ve thought of for this perverse situation is that there is something out there in empty space that pushes rather than pulls. This has various possible names, but Dark Energy is probably the most popular, adding an appropriately noirish edge to this particular MacGuffin. It has even taken over in prominence from its much older relative, Dark Matter, although that one is still very much around.

We have very little idea what Dark Energy is, where it comes from, or how it relates to other forms of energy we are more familiar with, so observational astronomers have jumped in with various grandiose strategies to find out more about it. This has spawned a booming industry in survey of the distant Universe (such as the Dark Energy Survey) all aimed ostensibly at unravelling the mystery of the Dark Energy. It seems that to get any funding at all for cosmology these days you have to sprinkle the phrase “Dark Energy” liberally throughout your grant applications.

The old-fashioned “observational” way of doing astronomy – by looking at things hard enough until something exciting appears (which it does with surprising regularity) – has been replaced by a more “experimental” approach, more like that of the LHC. We can no longer do deep surveys of galaxies to find out what’s out there. We have to do it “to constrain models of Dark Energy”. This is just one example of the not necessarily positive influence that particle physics has had on astronomy in recent times and it has been criticised very forcefully by Simon White.

Whatever the motivation for doing these projects now, they will undoubtedly lead to new discoveries. But my own view is that there will never be a solution of the Dark Energy problem until it is understood much better at a conceptual level, and that will probably mean major revisions of our theories of both gravity and matter. I venture to speculate that in twenty years or so people will look back on the obsession with Dark Energy with some amusement, as our theoretical language will have moved on sufficiently to make it seem irrelevant.

But that’s how it goes with MacGuffins. Even the Maltese Falcon turned out to be a fake in the end.

p.s. I heard on Saturday that the LHC is having some problems with its magnets and will actually be off-line for a few months. Last week I heard a particle physicist describing the great switch-on as like “Christmas”. This turns out to have been truer than he can have imagined. Only a week has passed and his most expensive toy is already broken…

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The Last Experiment

Posted in Science Politics, The Universe and Stuff with tags , , , on September 16, 2008 by telescoper

I’ve launched myself into the blogosphere just a bit too late for the feeding frenzy surrounding the switching on of the Large Hadron Collider at CERN last week. Obviously the event itself was a bit of a non-event as it will take years for anything interesting to come out the other end of its multi-billion-dollar tunnel. There are a couple of things worth saying in retrospect, though, now that the dust has settled.

The first is about all this nonsense concerning the creation of black holes that could destroy the Earth. If it were possible to create black holes in the LHC then they would be very puny ones, not capable of destroying anything very much at all. The phrase “black hole” conjures up Hollywood-style images of dead stars rampaging through the Galaxy devouring planets and costing a fortune in special effects. But not all black holes are massive enough to be stars in movies. If the LHC could make black holes it would only make very titchy ones. Since the gravitational effect of a black hole depends on its mass – and these little ones have very little of that – any that did pop out of an event in the LHC would be more of a pin-prick than a hole…

Moreover, energetic particles in the form of cosmic rays are constantly raining down on the Earth’s atmosphere, colliding with hadrons as they do so. The most extreme cosmic rays have energies far in excess of the limit that can be reached by the LHC. If an energetic hadron collision were going to produce a black hole that could destroy the planet, it would have happened a very long time ago and we wouldn’t be around to discuss the possibility.

So how did this daft idea come to dominate the news coverage surrounding the switch-on of the LHC? The press are never reluctant to peddle the doomsday scenario whenever they can as it appears to sell newspapers. But there is probably a bit more to it than that. I think part of it is a side-effect of the exaggerated language used by particle physicists in their attempt to use the LHC to capture the public imagination. “The Big Bang Machine” is just one example. If the experiment were really attempting to recreate the Big Bang, then there would indeed be much to be scared about. But the fact of the matter is that the LHC doesn’t reach energies anything like those reached in the Big Bang (nor even in the many smaller bangs that our Universe indulges in from time to time, such as supernova explosions).

The maximum energy reached by the LHC is going to be about 7 TeV (roughly equivalent to the energy of a bumble bee in flight). Although the very earliest stages of the Big Bang itself are not well understood, we are pretty sure that the primordial fireball started off with energies at least a million million (i.e. 1,000,000,000,000) times larger than this. It is doubtful (to say the least) that we’ll ever be able to build a device capable of reaching such energies, so the only “Big Bang Machine” there will ever be is the one we happen to be living in.

This is perhaps the reason why particle physicists are so desperate to glean maximum publicity for the LHC. It’s cost – though not extreme when compared to, for example, military spending – far exceeds that of any other scientific experiment. When it is over, will it be possible to build an even bigger experiment to probe even deeper into the subatomic world? Funding of such experiments generally comes from the public purse and it seems more than likely that the taxpayer will draw the line very soon. Although it won’t destroy the world, perhaps the LHC is nevertheless the end of the line for experimental physics of that kind.

So by all means let’s celebrate the LHC. It’s a wonderful demonstration of what international cooperation can achieve. It is also a response to the need all humans have to ask questions about our Universe. But let us not forget that our ability to probe the inner space of particles with experiments will always be limited, while the outer space beyond the stars offers much wider horizons.

PS. I can’t resist adding this link, as the best example of the worst of the hysteria about the LHC.

PPS. And this one, which explains why the LHC really is safe.

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