Did a skydiver see a meteoroid in flight?

April 6, 2014

A story surfaced over the last few days about a Norwegian skydiver who captured on film a rock flying by just after he deployed his parachute. Actually this happened about two years ago during which time some experts have been examining the evidence and have concluded that what he recorded was a falling meteoroid (it becomes a meteorite once it hits the ground)  I was very skeptical about this story and was waiting for the bad astronomer to debunk it but he failed to do so, so I will have to do it for myself. Update 7-4-2013: Phil Plait has now posted a more skeptical followup suggesting similar ideas as written here (and by others elsewhere) which I fully agree with.

meteorittfall_2columns_gallery-580x353

The first thing to think about is what are the chances of this happening? Records indicate that only one person has ever been hit by a meteorite. The person was a German boy who was hit by a pea sized meteorite. He was fine. There is also a story of a dog that was hit and vaporized but that story seems to be fake. In addition there are also a handful of cases  where cars have been hit by sizable meteorties. At least one, the Peeksill meteorite was verified.

peekskill_1

It could be that there are more unrecorded cases but we dont need precise numbers so let’s assume that as many as one car a year is actually hit by a sizable meteorite. There are about a billion cars in the world and most of them are outside exposed to the sky for most of the time. That is a lot of target space. On the other hand there are not too many active skydivers, maybe a hundred thousand at most doing on average a few jumps a year. Let’s be very optimistic and call it a million jumps where an event might have been recorded. A typical descent lasts about 5 minutes which is about a hundred thousandths of a year. You can work it our yourself, with these generous figures and the assumption that a car hit has a similar cross-section to the claimed event,  then we expect about one event like this every 100 million years. Yes, coincidences happen, but not something like that.

So what are the alternative theories. One possibility is a planned hoax. It would not be hard to put such a hoax together and there are plenty of hoaxes around, but the motivation to do an elaborate science hoax like this seems lacking. This is much more likely to be a hoax than a meteoroid but I dont think that is the best solution.

The most plausible explanation is the first thing that came into my mind and which others on slashdot have also suggested. It was a small stone that fell out of the parachute. Some people have disputed this because they say it was going too fast. In fact the estimated speed of the falling stone depends on its size or distance from the camera. The meteor experts estimated a large rock of a few kilograms travelling at 300 km/hr, but that is really based on the assumption that it is a meteoroid falling at terminal velocity. If it was a stone that fell out of a parachute it would have to be much smaller and slower and therefore nearer to the camera. This is equally consistent with what was recorded.

The object fell past about four seconds after the canopy opened. This is perfectly in line with expectations if it was on top of the parachute and bounced off. Is it possible for a skydiver to pack a stone in his parachute? Yes certainly as this video shows. A testimony from a skydiver on slashdot confirms that this happens. It is not common but it is common enough to expect someone to capture it on video sooner or later.

Update 8-4-2014: There is now an official explanation along the lines that the stone fell out as the parachute began to open and then passed the camera as the skydiver turned back a few seconds later. This is possible, as is my own similar explanation that it bounced off the top of the parachute and fell back a few seconds later.

Now everybody is saying that it is all fine and they knew all along that it might not have been a meteorite etc. Funny that the video and all the news reports made a very strong case for it being a meteorite and very few of us immediately noticed that this was very unlikely. That is the power of wishful thinking, confirmation bias and the persuasiveness of the authority of a qualified expert . If you were fooled, you will know to be more skeptical next time. In fact if you watch the video again you will see that they considered the possibility that it fell from the chute and that this was the one other possibility that they could not easily dismiss, but they did not say that outright. They just conveniently skipped over it and talked about other theories that could be more easily dismissed. The expert is also reported to have said that the chances of this happening are like winning the lottery three times in a row. That is about right and it is something that never happens but he did not seem to get that. Sometimes people’s desire to be involved in something special are just so strong that their powers of reason are put on hold.

Of course this does not mean that everything a scientist says can be dismissed. It means you have to listen to the reasoning rather than just the claim. It is not always possible to estimate the approximate probability for something being true as it was in this case and not everyone has the right kind of sense to make the right judgement. If that is you then you might want to take note of who quickly saw the truth in this story and who didn’t so you have a better idea of who to trust next time :-)



Abel Prize for Yakov Sinai

March 27, 2014

When a tool maker designs a drill she does not expect it to also be useful as a hammer or a torch. You might be able to use it as either at a pinch but it would not be very effective. If you want a universal tool that can do lots of different things you need to design it with those things in mind from the start and the result may be just as complex as a box of individual tools.

The strangest thing is that this is not the case with mathematical tools. A concept or method used to solve one mathematical problem often turns out to be just as useful in solving others that are completely different and seemingly unrelated. To give a simple example, the number pi was first defined to quantify the ratio of the circumference to diameter of a circle, yet it appears in a whole host of mathematical and physical equations that have nothing to do with circles. The same is true for a few other special numbers such as e, the base of the natural logarithms. Why do the same few numbers keep coming up in mathematics instead of different ones for every problem? Other examples abound. Special functions, groups, algorithms and many more mathematical structures prove useful over and over again. Why does mathematics have this natural universality?

People who don’t know mathematics well think that mathematicians invent the methods they use in the same way that people invent stories or fashions. Most mathematicians say that this is not the case. Their experience of developing new mathematics feels more like a process of discovery rather than invention. It is as if the mathematical structures were already there before any human was aware of them. You can also invent mathematical structures like the game of chess, but chess is not regarded as important in mathematics because it is not useful for solving unrelated problems. It does not have universality and it is this universality that distinguishes the interesting mathematics from the uninteresting.

Why then is such universality so easy to find in mathematics without trying to look for it? Why does it even extend into physics where deep mathematical ideas originally used to solve problems in pure mathematics turn out to be important for understanding the laws of nature (complex numbers, differential geometry, topology, group theory etc.) ? It is even more surprising to pure mathematicians when theory developed by physicists turns to be useful in mathematics, yet even string theory has already proved useful for solving a whole host of mathematical problems that seemed otherwise intractable. It remains a huge and deep mystery why this happens.

Sinai

Yesterday the Abel committee in Norway announced that it was awarding its annual prize of 6 million Kroner to Yakov Sinai for his fundamental contributions to dynamical systems, ergodic theory, and mathematical physics.  Sinai’s work covers complex dynamical non-linear systems with many variables. Naively our prior expectation for such systems would be that they are going to behave in complex unpredictable ways and the only thing they are likely to have in common is going to be randomness, but that is not what happens. Systems in statistical physics have entropy and temperature. These are macroscopic emergent quantities that follow derived laws which are common to different systems irrespective of the microscopic description of the dynamics. They have phase transitions and near these transitions you get critical phenomena that obey universal laws.

Sinai looked in particular at chaotic systems of non-linear dynamical equations where more universal emergent behavior is found and described in terms of certain Feigenbaum constants. Another area he worked in was algorithmic complexity of binary sequences and dynamical systems. All this work is highly applicable to practical problems but it is also important as a tool for understanding universality and why it arises. Perhaps one day by building on the work of Sinai we will learn much more about the unity of mathematics, the laws of physics and why these things are so beautifully connected by universality.

Congratulations to Yokov Sinai for this well deserved award which will raise the profile of such important work.


Vote for inflation Nobel Prize

March 20, 2014

So who do we think would be the right recipients for any Nobel Prize that might be awarded for inflation assuming the BICEP2 results hold up? (update 28-3-2014: Since people keep commenting that this is premature let me stress again that this is a vote based on the assumption that the experimental results hold up and the theory is agreed to confirm inflation, see my earlier post on this for my somewhat skeptical take given the current standing. Despite the uncertainty it is still of interest to think about who are considered the main discoverers of the inflation theory because there are a lot of news reports that are simplifying the history)

Cast your votes in this poll. You can vote multiple times so you can vote for up to three winners for a theory prize and another three for an experimental prize. (multiple voting  is now closed)

Update (22-Mar-2014): After a few days we can see where this voting is going so thanks to all those who voted so far.

The Guardian has now also discussed the same question and made the point that the Nobel committee will have a hard choice, but you will see that they have not identified all the candidates that we have here. Some people have responded by saying that we should not be thinking instantly about who should win a Nobel yet because it is too soon. I  disagree with that. The story about who are the main people behind this discovery is of immediate interest and by focusing on the possibility of Nobel prizes I think we highlight the human side of the discovery. It is true that we should be cautious about the uncertainty of the discovery until it has been confirmed but that does not stop us talking about the consequences, either scientifically or sociologically. There is a danger of being too negative and missing the opportunity to make some science and worthy scientists known to the wider public while their gaze falls fleetingly on upon physics and cosmology.

So what do the poll results say? The first thing that stands out is that the theorists are getting the most votes, especially Linde, Guth, and Starobinsky. Linde has now rushed ahead of Guth because he suddenly got 50 extra votes. The theoretical bias is perhaps understandable because the media (including me) has said more about the theorists and they have been familiar to us for many years. Indeed Guth and Linde in particular have been tipped for the Nobel long before this discovery. The experimenters are new stars so they have a smaller fan club and get less votes, but the Nobel Committee may see it the other way round. If BICEP2 is confirmed by Planck then it will be clear that a Nobel worthy discovery has been made even if the theory behind it remains uncertain. When the prize was given for accelerating cosmic expansion the committee made it clear that the award was for the observation irrespective of how theorists interpreted it and they are likely to see this discovery the same way until it is clear that inflation is the correct explanation rather than the alternatives.

The Nobel committee could in fact play things in several different ways:

  1. A prize for the experimental side first followed by the theory prize later
  2. A prize for the theory side first followed by the experiment
  3. A combined prize for both
  4. A prize just for the theory
  5. A prize just for the experiment
  6. No prize at all.

I predict option 1 assuming confirmation, but any of the others are quite possible. Choosing the experimental prize is already difficult. There is an interesting story about how it was caltech postdoc Brian Keating who originated the idea for this experiment and then persuaded Jamie Bock to take it on. This would suggest that Keating and Bock are key candidates for the prize but Keating seems to have dropped out of the picture at some point so he does not get many votes. John Kovac has been promoted as the main leader of the experiment but Chao-Lin Kuo led the team that really made the instrument work and Clem Pryke’s team made crucial discoveries for the analysis. I find it painful to think that at least one of these people will have to be left out but that is the way the Nobel works. If I am forced to make a prediction at this stage I would go with the voting so far and say it will be Kovac and Bock who take the honour on behalf of the BICEP2 team while  Uros Seljak would make a fitting third laureate for his seminal work on B-modes that made the experiment possible.

On the theory side that I already covered there are three classes of theoretical work on inflation that could eventually be rewarded. There is the initial realisation that inflation may be a feature of cosmology and could solve certain problems (flatness, horizon, monopole etc) Guth, Starobinsky,  Kazanas and Sato are independently responsible for this idea. Then there are the people who made crucial predictions of gravitational waves and anisotropies in the microwave background. The ones who got there first are Starobinsky, Einhorn and Mukhanov. The committee favours such predictions for obvious good reasons so any of these people could be up for the prize. Finally we have those who have worked on specific models including Linde, Albrecht and Steinhardt. The problem for these people is that no particular model for inflation has been shown to work yet. It is possible that that work has not yet been completed or that a more recent specific model will be shown to be right. However Linde is such a big figure in the field of inflationary cosmology who has been tipped for the Nobel for years already that I think the weight of nominations will be in his favour and if that is the case then he is surely deserving enough. In my opinion the destination of the theory component of the prize is not yet determined even if the experimental discovery is confirmed and will depend on work that is still to come otherwise I would expect it to go to Guth, Linde and Starobinsky as indicated by the voting.

Update 27-03-2014: see the comments for information about Erast Gliner who published an inflation theory in 1965. I have added his name to the poll but too late.


How certain are the BICEP2 findings?

March 20, 2014

Now that the excitement over the BICEP2 results has abated a little it is a good time to look at just how much we can believe the BICEP2 results and the conclusions that could be drawn from them. This actually breaks down into a whole sequence of questions. Has BICEP2 really seen cosmological B-modes? Were these B-modes formed by primordial gravitational waves? If so does this confirm inflation? Does it tell us anything more specific about inflation? Does it support the multiverse?

There have already been some words of caution pushed around. On the blogosphere this is most notable from Matt Strassler, Peter Coles, Neil Turoc and Ted Bunn. I endorse their point of view and I think it is fair to say that every scientist we have heard from has expressed an appropriate degree of caution  but because of their natural excitement it is easy for onlookers to miss this. There is a danger that soon Planck or another experiment will publish a contradictory result that seems to rule out B-modes at the level BICEP2 has claimed and then the news headlines will be that scientists were wrong again. This would be unfair. It is important to understand that the results are understood to be preliminary but there is no need to wait for confirmation before thinking about what the implications will be on the assumption that they are confirmed.

If anyone was not able to see much of the press conference on Monday due to the web overload, a video recording is available. Here is a direct link . There was also a live streamed colloquium from Stanford yesterday evening that was very watchable and informative. I don’t know if a recording will be made available. Many questions were raised at these events about the reliability of the experiment and its implications. Even the question about the multiverse has come up several times. More about that later.

So let’s look at some of the important questions in a little more detail

Has BICEP2 really seen cosmological B-modes?

Everybody agrees that the BICEP2 team know their stuff and have been very professional in their approach to evaluating their results. They have performed many consistency checks on their data. In particular they have played the game of partitioning their data in two according to a variety of criteria and then examining each half independently. This is a standard technique for identifying systematic errors and every test was passed. They have expressed high confidence that their observation really shows B-mode polarisation in the cosmic microwave background but everyone is subject to the human failings of cognitative bias so this alone is not enough.

Their positive cross-correlations between the BICEP1, BICEP2 and Keck are another good indication that the results are not instrument error but there are common elements of the analysis that could still be wrong. For example their was much made at the colloquium of an numerical analysis method devised by one of the team members that greatly improved there ability to extract the signal. The B-mode data is less than 20% of the strength of the E-mode signal from which it must be separated so you have to be careful that there is no systematic error (leakage) in this process. The BICEP2 team know this as well as anyone and have used data simulations to confirm that the method works. I have no reason to doubt that this is a good check but it would be wise to see if other independent teams can replicate the same B-mode pattern independently. There was also some talk about doing a correlation-check with data from the South Pole Telescope. These correlation checks are valid confirmations even though BICEP1 and SPT do not have sufficient sensitivity on their own but they are not anything like as good as another team independently producing a B-mode signal that shows the same bumps.

fast_hawking

It is worth trying to put a confidence level on how good you think the result is. I don’t do bets but I can consider a thought experiment in which Hawking is forcing me to make a bet under threat of running me down at high speed on his wheelchair. That makes it possible for me to imagine what are the breakeven odds I would accept as a way to evaluate my Bayesian estimate for the probability that BICEP2 have indeed seen cosmological B-modes.  I would put this figure at about 80% for now and I consider that to be a good level of confidence at this early stage. This will change dramatically either up or down when other completely independent results either confirm or refute the BICEP2 results.

There was an interesting exchange during the discussion at the end of yesterdays colloquium when someone asked “How could Planck have missed this?” It was pointed out that Planck has a very broad mission  compared to BICEP2 whose only goal was to find the B-modes. Someone from Planck then stated quite pointedly that they had not yet published anything that would justify the claim that they had missed this signal. Very interesting! There is a further long list of other experiments that have the capability to look for B-modes too so soon we should know the answer to this particular question with a much better level of confidence.

Were these B-modes formed by primordial gravitational waves?

So if we now assume that the observations of B-modes is good we can ask about how certain we can be that these are a signature of primordial gravitational waves.  At the Colloquium we heard from  Uros Seljak who was the first cosmologist to realize in 1996 that B-mode CMB polarisation could be used as a signal for these gravitational waves. He was followed shortly after by  Marc Kamionkowski, Arthur Kosowsky, and Albert Stebbins. See this report from Berkeley for the details of the story. If I were in a position to make nominations for the Nobel prize for the observational side of this discovery I would want to include Seljak along with people from BICEP2 but past awards indicate that the phenomenologists who take these crucial steps usually fail to be recognised at this level because they fall between the two stones marking the original theoretical insight and the final experimental discovery, too bad.

I noticed that several theorists claimed that the B-modes can only be produced by gravitational waves. I think we need to be cautious about this claim. The well known additional source of B-modes is gravitational lensing from background galaxy clusters. Fortunately this effect can be accurately modelled because we can observe the galaxies and we know very well now how much dark matter is clumped around them. The power spectrum plot from BICEP2 compares their signal with the lensing background. Here is the plot with the lensing background filled in yellow to show clearly how the signal stands out above the lensing. Simply put the gravitational wave spectrum is seen a higher angular scales than the lensing so it is clearly separated.

power

Could something other than lensing or gravitational waves produce B-modes. One thought that came to my mind was the magnetic fields can also twist the polarisation of radiation as first shown by Michael Faraday. None of the theorists have discussed this possibility so I was at first willing to accept that you need tensor modes rather than vector fields to produce the B-modes, but then I found this presentation by Levon Pogosian  that seems to say otherwise. Traditionally magnetic fields have been discounted in cosmological models but in recent years some have found reason to be more open to their importance.  I have no idea if this is a viable alternative source for the B-modes but until I hear a plausible direct refutation I am keeping this open in my mind. The good news is that new radio telescopes such as SKA and LOFAR should be able to detect signals from cosmic magnetic fields so I think this is a question that can be settled .

Other sources of the B-modes such as galactic dust and synchrotron radiation were considered by the BICEP2 team but were only discounted at 97% confidence. This is not a high confidence level for such an important result. The significance of an observation is only valid when all possible backgrounds are accounted for so the 5 to 7 sigma claims are questionable here. We have seen many observations in particle physics and cosmology at 2 to 3 sigma fade away as new data arrived. As outsiders who could hear such results from hundreds of experiments that are taking place we should not discount the “look elsewhere effect” that this implies.

The only other possibility I can think of as an alternative explanation for B-modes would be if cosmic structure had formed in dark matter before the moment of last scattering when the CMB decoupled from the visible matter. This is not the accepted view in cosmology but since the surprisingly early formation of galaxies after that time has not been explained I will not discount it. Ironically there is an improved chance of something like this happening if the primordial gravitational waves are present and that weakens the doubt that this alternative casts. Finally we should perhaps allow for the more remote possibility that something else we have not thought of can  produce these B-modes.

So  with Hawking on my heals how would I rate my confidence that B-modes are a signature of gravitational waves. Again I think I would have to put it at about the 80% level which is pretty good confidence. This figure is harder to improve than the certainty in the BICEP2 experiment itself  but future measurements of cosmic magnetic fields would make a significance difference.

Assuming primordial gravitational waves have been observed, did inflation happen?

Primordial gravitational waves have been described as a “smoking gun” for inflation. That would of course be the starter gun that set the universe off rather than a murder weapon, I hope. However most versions of inflation theory that had risen in popularity before these results had been announced predicted very small values of r. Very few can come close to accounting for r=0.2 or even r=0.1 if we take the lower side of the error range. There are some models that might, such as the axion monodromy inflation and even Linde’s chaotic inflation that had been all but abandoned until now. Indeed you could make the case that it would have been a better signal for inflation if primordial gravitational waves had been ruled out at this amplitude because most inflation models predict that. In a different sense it is a good thing that so many inflation models would be discounted by BICEP2 if it stands because it concentrates inflation theorists in a new direction, but the harsh fact remains that they do not yet have a fully viable theory for inflation. Scaling invariance in the CMB has already been a good result for inflation but there may be other theories that explain it (e.g. gravitational waves from other phase transitions have been cited as an alternative).

The simple truth is that we know so little about the earliest origins of the universe that we cannot honestly place very high confidence on the claim that primordial gravitational waves are a sure fire signature of inflation. Nevertherless I will once again place an my confidence level at 80% in inflation if the primordial gravitational waves are well confirmed.

What would it take to improve this figure? I would like to see a fully viable theory of the inflation mechanism with an uncontrived prediction (or retrodiction) of the power spectrum of gravitational waves. When this power spectrum is known to better precision (and that should now converge rapidly over the next few years) and it agrees nicely with the model , then we can be very happy with the result.

So has inflation been confirmed as a Nobel worthy theory?

Not yet. The problem is that there are at least these three major levels of uncertainty. For each one I give a good confidence level of 80%, but 80% cubed is only about 50%

When Sean Carroll asked us three years ago if we thought inflation happened I put my assessment at only 40% likely. This had probably increased to about 60% in the intervening years because of Planck results, but I am still open minded either way. I should make it clear that I do have a high level of confidence in the big bang theory itself but the inflation part cannot be considered settled until we have both a good theory and a solid observational confirmation. I do appreciate its strength in resolving cosmological problems and its successful prediction of CMB fluctuations but I want to see more.

The BICEP2 result does not yet change my mind very much, but the good news is that there is now hope that further confirmation will make a huge difference. More detailed measurements of the B-modes have the potential to tell us in great detail how the process of inflation started, progressed and ended. Just when we thought our understanding of physics was hitting a brick wall we get this gift of an observation that promises to completely revolutionize our understanding once again.

Does BICEP2 support the multiverse?

This question was asked twice at the press conference (44 minutes and 53 minutes into the video recording) John Novak was quick to interject that as an experimentalist he is firmly opposed to theories that have no observational consequences. The theoriests in the room countered that by pointing out that people doubted inflation initially because they did not believe it could make testable predictions (this was all said with a lot of laughter). Linde and Guth were then consulted and they said that it is hard to develop models of inflation that do not lead to the multiverse (such as eternal inflation) There are certainly many other theorists who would have provided a very different view if they had been there.

My personal view does not count for much but I have always discounted eternal inflation because it is a conclusion that is situated at the end of a long combination of speculative and unconfirmed ideas. It also does not fit with my favourite philosophical position but nobody else should care about that and anyone needs to be ready to update their philosophy if experiment requires. On the other hand I do find that the multiverse is a fitting explanation for small levels of fine-tuning and it follows naturally from what quantum gravity seems to be trying to tell us about the landscape of the vaccum (in string theory and alternatives) I just see the multiverse as the range of logically possible solutions of the physics rather than something that is actually realised.

Now if the results from BICEP2 are confirmed and improved leading to a solid theory of inflation and that theory tells us in a convincing way that the multiverse must be out there, then things will be very different. Most of the levels of speculation I was concerned about would then have turned into solidly confirmed science.  It is hard even then to see how a directly observable prediction could be made using the multiverse although that is not something we can completely discount and if the theory is complete and convincing enough it may even not be necessary.

I am sure there will be more opinions than physicists with an opinion on this subject, but that for what it is worth is mine.


“first direct evidence of cosmic inflation” BICEP2 results

March 17, 2014

At a presentation from the Harvard-Smithsonian the BICEP2 team have announced that they have the “first direct evidence of cosmic inflation”. As rimoured they have detected what they believe to be primordial gravitational waves with a ratio or tensor to scalar modes of r=0.2 (+0.07 -0.05) which is 5 sigma over the null hypothesis. This is a game-changing result for inflationary cosmology and possibly for quantum gravity research because the result indicates that the scale of the inflation is only about a factor of 100 below the Planck scale. These results and the future followups that will no doubt be carried out could be the experimental test-bed for the leading edge of theoretical physics including string-theory.

The papers are now online but everything is down at the monet due to heavy load. I just managed to get this snapshot of the abstract and a few pictures. More when I have it.

bicep2abstract

These images show the actual signal from a small patch of the sky (on left) compared to a simulation based on predictions from inflation and cold dark matter (on right)

BICEP2signal

The signal is stronger than many theories predicted so it will have an immediate effect on the direction of theoretical research in quantum gravity and the first moments of the universe

http://www.youtube.com/watch?v=jq-OvV-XHdc

Full paper is at http://bicepkeck.org/b2_respap_arxiv_v1.pdf

inflation

This graph is the money plot showing where we now stand in observational inflationary cosmology. Blue is the new result using BICEP2 compared to previous results from Planck and other sources. Note that Planck should release more polarisation measurements soon.

https://www.youtube.com/watch?v=ZlfIVEy_YOA

Should have spotted this video yesterday, very moving.

The most interesting thing now is going to see how theorists react to these results. They will have implications for inflationary cosmology (obviously), galaxy formation and quantum gravity. To get the ball rolling theorist Liam McAllister has a guest post on Lubos Motl’s Blog with the quote “The tensor fluctuations write quantum gravity on the sky” exciting stuff!


Who should get the Nobel Prize for cosmic inflation?

March 16, 2014

Tomorrow we might hear some good news about a discovery of primordial gravitational waves and within a few more weeks that could be confirmed in more detail by Planck. If this happens the observational status of the theory of cosmic inflation will change dramatically because primordial gravitational waves have been described as a smoking gun for the theory. Well that may be an exaggeration but  the observed scale invariance of the CMB anisotropy spectrum is already a good pointer towards inflation so could the combination be enough to sway the notoriously cautious Nobel committee towards awarding a prize for the theory?

SONY DSC

Rumors say that Alan Guth and Andrei Linde have been invited to tomorrow’s meeting where the team of astronomers who work with the BICEP2 observatory in Antartica will announce a “major discovery” about B-modes in the cosmic microwave background. E-mode polarisation in the cosmic radiation was produced at the time of last scattering when it decoupled from atomic gas in the early universe. These E-modes could then have been distorted by the tensor modes of the primordial gravitational waves permeating space, twisting the polarisation field of the microwave background into the (hopefully) observed B-modes. So the B-modes are a signature of the gravitational waves that are themselves a remnant of the much earlier inflationary epoch of the universe.

The presence of Guth and Linde at this meeting echos the presence of Higgs and Englert at the announcement of the discovery of the Higgs Boson in 2012, and that is probably no coincidence. Just as Higgs and Englert were awarded the Nobel Prize last year for the theory behind the Higgs discovery, Guth and Linde will be prime candidates for any Nobel Prize awarded for the theory of inflation. However, there was much discussion about who else might have deserved the Higgs prize and the similar decision for inflation could be equally awkward and controversial.

Guth and Linde have already been jointly awarded several honours for their work on inflation theory including the Gruber Prize and Milner’s Fundamental Physics Prize, but the Swedish committee places a higher bar for empirical verification. The general idea of the inflationary universe may pass with the new evidence giving Guth his ticket, but Linde has worked on more specific models of inflation such as slow-roll and chaotic inflation. Brilliant and important though his work is, I am not convinced that he is destined for the Nobel yet. Argue with me if you disagree. Update: after the first 24 hours of analysis Linde’s model of chaotic inflation with quadratic potential appears to be in particularly good shape so perhaps I was being premature. In any case his widely seen status as one of the “principle architects of inflation theory” along with Guth is sure to win him many nominations.

SONY DSC

On the other hand Guth is not the only one with a claim to the original idea of inflation. It has been recorded that he first had the breakthrough idea on 6th December 1979, gave a seminar on the theory at SLAC on 23rd January 1980 and his paper was received on 11th August 1980. At around the same time  Katsuhiko Sato in Japan had written a paper proposing inflation by 21st February 1980 which was received for publication on 9th September 1980, and another similar paper by Demosthenes Kazanas had already been received by 5th May 1980. All three contributions seem to have been independent and similar. The only thing that may have singled out the work of Guth was that his term “inflation” stuck and he was part of a more influential circuit of physicists. Closer examination of dates and the points they made in their papers may separate them, but I think it would be hard to be truly objective about what really counts.

MukhanovStarobinsky

But then all three were preempted by Soviet physicist  Alexei Starobinsky who had already worked out the ideas behind inflation in 1979. Wikipedia describes his contribution like this

“Although Alexei Starobinsky of the L.D. Landau Institute of Theoretical Physics in Moscow developed the first realistic inflation theory in 1979 he failed to articulate its relevance to modern cosmological problems. Due to political difficulties in the former Soviet Union, regarding the free exchange of scientific knowledge, most scientists outside the USSR remained ignorant about Starobinsky’s work until years later. Starobinsky’s model was relatively complicated, however, and said little about how the inflation process could start.”

I think this is an overly negative view of his contribution and I suspect that it owes more to a bias that tries to rationalize the fact that we do not recognize his work as well as we recognize Guth’s. It is notable that he had already predicted the primordial gravitational waves in 1979 before anyone else had even started thinking about inflation. How the Nobel committee will see it I can only guess.  Starobinsky did also win the Gruber prize independently of the prize given earlier to Guth and Linde. He was recognised along with Viatcheslav Mukhanov who, in colaboration with Chibisov (deceased), first calculated the spectrum of anisotropies from quantum fluctuations during inflation and who could therefore be yet another candidate for the Nobel. Once again the Nobel committee will again be inflicted with the headache that strikes them when more than three people deserve their recognition for the same discovery.

Update 26/03/2014: Since the first version of this post I learnt about another thread of discoveries concerning inflationary cosmology that preceded even the work of Starobinsky. In 1965 Soviet physicist Erast Gliner published the earliest known proposal for inflationary cosmology. Andrei Sakharov built on the idea looking at its cosmological consequences including implications for the Horizon problem. A number of papers were written that are now hard to access but a history by Chris Smeenk available online gives a detailed account of their ideas. Another paper by L. E. Gurevich in 1975 is also accessible and shows just how advanced this work had become by that time, five years before the burst of interest in cosmic inflation. Gurevich considered the horizon and flatness problems, primordial inhomogenieties that could lead to galaxy formation and he even speculated about a version of perpetual inflation with multiple universes of “metaglaxies” as he called them.


Primordial Gravitational Waves?

March 15, 2014

The rumor mill has once again turned its wheels a few cogs to throw out some new grist for physicists and cosmologists. This is following the announcement last Wednesday of an announcement this Monday in which the Harvard-Smithsonian Center for Astrophysics will reveal to the world a “Major Discovery”. The best available rumours now say that astrophysicsts working with the BICEP observatory in Antartica will reveal the discovery of primordial gravitational waves in the cosmic microwave background. If true this would be a very big deal indeed because it could be a direct experimental hook into the physics of inflation and even quantum gravity. These are of course the least well understood and most exciting unchartered waters of fundamental physics. Any observation that could provide phenomenology for these areas would be the greatest empirical discovery for the foundations of our universe for decades.

Before going any further it is worth recalling that we have tuned into some webcast announcements recently only to be disappointed that the expected discovery comes in the form of a negative result setting new limits on what we wanted (e.g. LUX, AMS-2 etc) This could turn out to be another case of the same but if so the “Major Discovery” tag will be pointed out as a major piece of over-hype. It is also possible that the announcement has nothing to do with what the rumors say. but that looks increasingly unlikely at this point. So let’s try to understand a little better what it might be about.

The Cosmic Microwave Background has been mapped out in exquisite detail by a series of space and Earth-based observatories including the European Planck mission which provided the best resolution all-sky survey of the CMB. So far Planck has only shown us the fluctuations of the scalar modes but it also looked at the polarisation of the background. Although it stopped working back in 2012 we are still waiting for those maps. Meanwhile some smaller scale results for the polarisation have already come in from land based observatories.

BICEP2SPT

Microwave polarisation can be broken down into two modes using a Helmholtz decomposition which splits a vector field into a sum of two parts: The E-mode whose vector curl is zero and the B-mode whose divergence is zero. The E-mode in the CMB was first observed in 2002 by the DASI interferometer, but it is not particularly interesting. E-mode polarisation is generated by scattering from atoms before the radiation decoupled from matter but long after the period of inflation. Last summer the South Pole Telescope (SPT) found B-modes in the CMB for the first time, but these were known to be due to gravitational lensing of the radiation around massive galactic clusters. These can twist the E-mode polarisation to form B-modes so they are only slightly more interesting than the E-modes themselves. Really these lensing B-modes are not much better than a background that needs to be subtracted to see the more interesting B-modes that may be the signature of primordial gravitational waves.

The B-modes will have an anisotropy spectrum just as the scalar modes do and Planck may eventually provide us with a plot of this spectrum but as an initial result we are interested in the peak ratio of the tensor modes to the scalar modes which is given by a parameter known simply as r. The latest rumor say that a value for r has been measured by the BICEP2 observatory in Antarctica which is a smaller rival to the SPT, both housed at the Dark Sector Lab (pictured) Some more precise and less reliable versions of the rumor say that the answer is r=0.2. This is somewhat bigger than expected and could be as good as a 3 or 4-sigma signal because the sensitivity of BICEP2 was estimated at r=0.06. If this is true it has immediate implications for inflationary models and quantum gravity. It would rule out quite a lot of theories while giving hope to others. For example you may hear a lot about  axion monodromy inflation if this rumor is confirmed, but there will be many other ideas that could explain the result and it will be impossible to separate them at least until a detailed spectrum is available rather than a single data point. Another implication of such a high value for r might be that primordial gravitational waves could have a bigger impact on galaxy formation than previously envisioned. This could help explain why galaxies formed so quickly and why there is more large scale structure than expected in galaxy distribution (see my previous spectulations on this point)

The most important thing about a high signal of primordial gravitational waves for now would be that it would show that there is something there that can be measured so more efforts and funding are likely to be turned in that direction. But first the new result (if it is what the rumors say) will be scrutinised, not least by rival astronomers from the SPT and Polarbear observatories who only managed to detect lensing B-modes. Why would BICEP2 succeed where they failed? Can they be sure that they correctly subtracted the background? These questions are premature and even immature before we hear the announcement, but it is good to go along prepared for the kind of questions that may need to be asked.

For more see TRF, RESONAANCESTToD, Excursionset, Bruce and The Guardian Update: Blank on the Map, Prep. Uni., In the Dark


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