## Inflation after Planck

Posted in The Universe and Stuff with tags , , on February 18, 2019 by telescoper

Gratuitous Picture of Planck

There’s a paper on the arXiv by Debika Chowdhury et al with the title Inflation after Planck: Judgment Day and abstract:

Inflation is considered as the best theory of the early universe by a very large fraction of cosmologists. However, the validity of a scientific model is not decided by counting the number of its supporters and, therefore, this dominance cannot be taken as a proof of its correctness. Throughout its history, many criticisms have been put forward against inflation. The final publication of the Planck Cosmic Microwave Background data represents a benchmark time to study their relevance and to decide whether inflation really deserves its supremacy. In this paper, we categorize the criticisms against inflation, go through all of them in the light of what is now observationally known about the early universe, and try to infer and assess the scientific status of inflation. Although we find that important questions still remain open, we conclude that the inflationary paradigm is not in trouble but, on the contrary, has rather been strengthened by the Planck data.

This is a pretty good introduction to live issues around the theory of cosmic inflation in the light of the results from the Planck mission. I’ll leave it to you to judge whether or not you agree with the concluding sentence of the abstract!

## The Early Stages of Cosmic Inflation

Posted in The Universe and Stuff with tags , , , , on July 13, 2017 by telescoper

When asked to cite the article that first presented the theory of the inflationary Universe most cosmologists would probably offer the famous paper published by Alan Guth in 1981.

However, I recently stumbled across a paper by Demosthenes Kazanas that was published (in the Astrophysical Journal Letters in 1980. I hadn’t seen this paper before a few days ago, and I don’t think it is very well known. Here is part of the front page:

You can get the full paper here.

I know that there were other papers floating around in 1980 that got part of the way to the theory of inflation, but this one seems very close to the theory, e.g. talking about exponential expansion in the context of the cosmological horizon problem.

Interestingly, while Guth (1981) has garnered many thousands of citations, Kazanas (1980) has been cited fewer than 300 times.

Does anyone know the story of this paper, and why it has largely  been overlooked by the exponentially-expanding literature on cosmic inflation? And,while I’m on the topic, can anyone suggest other early contributions to the theory that have been similarly neglected? Please let me know through the comments box below.

## Planck Update

Posted in The Universe and Stuff with tags , , , , on February 5, 2015 by telescoper

Just time for a very quick post today to pass on thhe news that most of the 2015 crop of papers from the Planck mission have now been released and are available to download here. You can also find some related data products here.

I haven’t had time to look at these in any detail myself, but my attention was drawn (in the light of the recently-released combined analysis of Planck and Bicpe2/Keck data) to the constraints on inflationary cosmological models shown in this figure:

It seems that the once-popular (because it is simple) $m^2 \phi^2$ model of inflation is excluded at greater than 99% confidence…

Feel free to add reactions to any of the papers in the new release via the comments box!

## BICEP2 – The Video!

Posted in The Universe and Stuff with tags , , , on July 23, 2014 by telescoper

A video about cosmic inflation and the BICEP2 results produced by the University of Sussex and PhD Comics!

## Has BICEP2 bitten the dust?

Posted in The Universe and Stuff with tags , , , , , , , , , , on June 5, 2014 by telescoper

Time for yet another update on twists and turns of the ongoing saga of  BICEP2 and in particular the growing suspicion that the measurements could be accounted for by Galactic dust rather than primordial gravitational waves; see various posts on this blog.

First there is a Nature News and Views article by Paul Steinhardt with the title Big Bang blunder bursts the multiverse bubble. As the title suggests, this piece is pretty scathing about the whole affair, for two main reasons. The first is to do with the manner of the release of the result via a press conference before the results had been subjected to peer review. Steinhardt argues that future announcements of “discoveries” in this area

should be made after submission to journals and vetting by expert referees. If there must be a press conference, hopefully the scientific community and the media will demand that it is accompanied by a complete set of documents, including details of the systematic analysis and sufficient data to enable objective verification.

I also have reservations about the way the communication of this result was handled but I wouldn’t go as far as Steinhardt did. I think it’s quite clear that the BICEP2 team have detected something and that they published their findings in good faith. The fact that the media pushed the result as being a definitive detection of primordial gravitational waves wasn’t entirely their fault; most of the hype was probably down to other cosmologists (especially theorists) who got a bit over-excited.

It is true that if it turns out that the BICEP2 signal is due to dust rather than primordial gravitational waves then the cosmology community will have a certain amount of egg on its face. On the other hand, this is actually what happens in science all the time. If we scientists want the general public to understand better how science actually works we should not pretend that it is about absolute certainties but that it is a process, and because it is a process operated by human beings it is sometimes rather messy. The lesson to be learned is not about hiding the mess from the public but about communicating the uncertainties more accurately and more honestly.

Steinhardt’s other main point is one with which I disagree very strongly. Here is the core of his argument about inflation:

The common view is that it is a highly predictive theory. If that was the case and the detection of gravitational waves was the ‘smoking gun’ proof of inflation, one would think that non-detection means that the theory fails. Such is the nature of normal science. Yet some proponents of inflation who celebrated the BICEP2 announcement already insist that the theory is equally valid whether or not gravitational waves are detected. How is this possible?

The answer given by proponents is alarming: the inflationary paradigm is so flexible that it is immune to experimental and observational tests.

This is extremely disingenuous. There’s a real difference between a theory that is “immune to experimental and observational tests” and one which is just very difficult to test in that way. For a start, the failure of a given experiment to detect gravitational waves  does not prove that gravitational waves don’t exist at some level; a more sensitive experiment might be needed. More generally, the inflationary paradigm is not completely specified as a theory; it is a complex entity which contains a number of free parameters that can be adjusted in the light of empirical data. The same is also true, for example, of the standard model of particle physics. The presence of these adjustable degrees of freedom makes it much harder to test the hypothesis than would be the case if there were no such wiggle room. Normal science often proceeds via the progressive tightening of the theoretical slack until there is no more room for manoeuvre. This process can take some time.

Inflation will probably be very difficult to test, but then there’s no reason why we should expect a definitive theoretical understanding of the very early Universe to come easily to us. Indeed, there is almost certainly a limit to the extent that we can understand the Universe with “normal science” but I don’t think we’ve reached it yet. We need to be more patient. So what if we can’t test inflation with our current technology? That doesn’t mean that the idea is unscientific. It just means that the Universe is playing hard to get.

Steinhardt continues with an argument about the multiverse. He states that inflation

almost inevitably leads to a multiverse with an infinite number of bubbles, in which the cosmic and physical properties vary from bubble to bubble. The part of the multiverse that we observe corresponds to a piece of just one such bubble. Scanning over all possible bubbles in the multi­verse, every­thing that can physically happen does happen an infinite number of times. No experiment can rule out a theory that allows for all possible outcomes. Hence, the paradigm of inflation is unfalsifiable.

This may seem confusing given the hundreds of theoretical papers on the predictions of this or that inflationary model. What these papers typically fail to acknowledge is that they ignore the multiverse and that, even with this unjustified choice, there exists a spectrum of other models which produce all manner of diverse cosmological outcomes. Taking this into account, it is clear that the inflationary paradigm is fundamentally untestable, and hence scientifically meaningless.

I don’t accept the argument that “inflation almost inevitably leads to a multiverse” but even if you do the rest of the argument is false. Infinitely many outcomes may be possible, but are they equally probable? There is a well-defined Bayesian framework within which one could answer this question, with sufficient understanding of the underlying physics. I don’t think we know how to do this yet but that doesn’t mean that it can’t be done in principle.

For similar discussion of this issue see Ted Bunn’s Blog.

Steinhardt’s diatribe was accompanied  yesterday by a sceptical news piece in the Grauniad entitled Gravitational waves turn to dust after claims of flawed analysis. This piece is basically a rehash of the argument that the BICEP2 results may be accounted for by dust rather than primordial gravitational waves, which definitely a possibility, and that the BICEP2 analysis involved a fairly dubious analysis of the foregrounds. In my opinion it’s an unnecessarily aggressive piece, but mentioning it here gives me the excuse to post the following screen grab from the science section of today’s Guardian website:

Aficionados of Private Eye will probably think of the Just Fancy That section!

Where do I stand? I can hear you all asking that question so I’ll make it clear that my view hasn’t really changed at all since March. I wouldn’t offer any more than even money on a bet that BICEP2 has detected primordial gravitational waves at all and I’d offer good odds that, if the detection does stand, the value of the tensor-to-scalar ratio is significantly lower than the value of 0.2 claimed by BICEP2.  In other words, I don’t know. Sometimes that’s the only really accurate statement a scientist can make.

## Is Inflation Testable?

Posted in The Universe and Stuff with tags , , , , , , , , on March 4, 2014 by telescoper

It seems the little poll about cosmic inflation I posted last week with humorous intent has ruffled a few feathers, but at least it gives me the excuse to wheel out an updated and edited version of an old piece I wrote on the subject.

Just over thirty  years ago a young physicist came up with what seemed at first to be an absurd idea: that, for a brief moment in the very distant past, just after the Big Bang, something weird happened to gravity that made it push rather than pull.  During this time the Universe went through an ultra-short episode of ultra-fast expansion. The physicist in question, Alan Guth, couldn’t prove that this “inflation” had happened nor could he suggest a compelling physical reason why it should, but the idea seemed nevertheless to solve several major problems in cosmology.

Three decades later, Guth is a professor at MIT and inflation is now well established as an essential component of the standard model of cosmology. But should it be? After all, we still don’t know what caused it and there is little direct evidence that it actually took place. Data from probes of the cosmic microwave background seem to be consistent with the idea that inflation happened, but how confident can we be that it is really a part of the Universe’s history?

According to the Big Bang theory, the Universe was born in a dense fireball which has been expanding and cooling for about 14 billion years. The basic elements of this theory have been in place for over eighty years, but it is only in the last decade or so that a detailed model has been constructed which fits most of the available observations with reasonable precision. The problem is that the Big Bang model is seriously incomplete. The fact that we do not understand the nature of the dark matter and dark energy that appears to fill the Universe is a serious shortcoming. Even worse, we have no way at all of describing the very beginning of the Universe, which appears in the equations used by cosmologists as a “singularity”- a point of infinite density that defies any sensible theoretical calculation. We have no way to define a priori the initial conditions that determine the subsequent evolution of the Big Bang, so we have to try to infer from observations, rather than deduce by theory, the parameters that govern it.

The establishment of the new standard model (known in the trade as the “concordance” cosmology) is now allowing astrophysicists to turn back the clock in order to understand the very early stages of the Universe’s history and hopefully to understand the answer to the ultimate question of what happened at the Big Bang itself and thus answer the question “How did the Universe Begin?”

Paradoxically, it is observations on the largest scales accessible to technology that provide the best clues about the earliest stages of cosmic evolution. In effect, the Universe acts like a microscope: primordial structures smaller than atoms are blown up to astronomical scales by the expansion of the Universe. This also allows particle physicists to use cosmological observations to probe structures too small to be resolved in laboratory experiments.

Our ability to reconstruct the history of our Universe, or at least to attempt this feat, depends on the fact that light travels with a finite speed. The further away we see a light source, the further back in time its light was emitted. We can now observe light from stars in distant galaxies emitted when the Universe was less than one-sixth of its current size. In fact we can see even further back than this using microwave radiation rather than optical light. Our Universe is bathed in a faint glow of microwaves produced when it was about one-thousandth of its current size and had a temperature of thousands of degrees, rather than the chilly three degrees above absolute zero that characterizes the present-day Universe. The existence of this cosmic background radiation is one of the key pieces of evidence in favour of the Big Bang model; it was first detected in 1964 by Arno Penzias and Robert Wilson who subsequently won the Nobel Prize for their discovery.

The process by which the standard cosmological model was assembled has been a gradual one, but the latest step was taken by the European Space Agency’s Planck mission . I’ve blogged about the implications of the Planck results for cosmic inflation in more technical detail here. In a nutshell, for several years this satellite mapped  the properties of the cosmic microwave background and how it varies across the sky. Small variations in the temperature of the sky result from sound waves excited in the hot plasma of the primordial fireball. These have characteristic properties that allow us to probe the early Universe in much the same way that solar astronomers use observations of the surface of the Sun to understand its inner structure,  a technique known as helioseismology. The detection of the primaeval sound waves is one of the triumphs of modern cosmology, not least because their amplitude tells us precisely how loud the Big Bang really was.

The pattern of fluctuations in the cosmic radiation also allows us to probe one of the exciting predictions of Einstein’s general theory of relativity: that space should be curved by the presence of matter or energy. Measurements from Planck and its predecessor WMAP reveal that our Universe is very special: it has very little curvature, and so has a very finely balanced energy budget: the positive energy of the expansion almost exactly cancels the negative energy relating of gravitational attraction. The Universe is (very nearly) flat.

The observed geometry of the Universe provides a strong piece of evidence that there is an mysterious and overwhelming preponderance of dark stuff in our Universe. We can’t see this dark matter and dark energy directly, but we know it must be there because we know the overall budget is balanced. If only economics were as simple as physics.

Computer Simulation of the Cosmic Web

The concordance cosmology has been constructed not only from observations of the cosmic microwave background, but also using hints supplied by observations of distant supernovae and by the so-called “cosmic web” – the pattern seen in the large-scale distribution of galaxies which appears to match the properties calculated from computer simulations like the one shown above, courtesy of Volker Springel. The picture that has emerged to account for these disparate clues is consistent with the idea that the Universe is dominated by a blend of dark energy and dark matter, and in which the early stages of cosmic evolution involved an episode of accelerated expansion called inflation.

A quarter of a century ago, our understanding of the state of the Universe was much less precise than today’s concordance cosmology. In those days it was a domain in which theoretical speculation dominated over measurement and observation. Available technology simply wasn’t up to the task of performing large-scale galaxy surveys or detecting slight ripples in the cosmic microwave background. The lack of stringent experimental constraints made cosmology a theorists’ paradise in which many imaginative and esoteric ideas blossomed. Not all of these survived to be included in the concordance model, but inflation proved to be one of the hardiest (and indeed most beautiful) flowers in the cosmological garden.

Although some of the concepts involved had been formulated in the 1970s by Alexei Starobinsky, it was Alan Guth who in 1981 produced the paper in which the inflationary Universe picture first crystallized. At this time cosmologists didn’t know that the Universe was as flat as we now think it to be, but it was still a puzzle to understand why it was even anywhere near flat. There was no particular reason why the Universe should not be extremely curved. After all, the great theoretical breakthrough of Einstein’s general theory of relativity was the realization that space could be curved. Wasn’t it a bit strange that after all the effort needed to establish the connection between energy and curvature, our Universe decided to be flat? Of all the possible initial conditions for the Universe, isn’t this very improbable? As well as being nearly flat, our Universe is also astonishingly smooth. Although it contains galaxies that cluster into immense chains over a hundred million light years long, on scales of billions of light years it is almost featureless. This also seems surprising. Why is the celestial tablecloth so immaculately ironed?

Guth grappled with these questions and realized that they could be resolved rather elegantly if only the force of gravity could be persuaded to change its sign for a very short time just after the Big Bang. If gravity could push rather than pull, then the expansion of the Universe could speed up rather than slow down. Then the Universe could inflate by an enormous factor (1060 or more) in next to no time and, even if it were initially curved and wrinkled, all memory of this messy starting configuration would be lost. Our present-day Universe would be very flat and very smooth no matter how it had started out.

But how could this bizarre period of anti-gravity be realized? Guth hit upon a simple physical mechanism by which inflation might just work in practice. It relied on the fact that in the extreme conditions pertaining just after the Big Bang, matter does not behave according to the classical laws describing gases and liquids but instead must be described by quantum field theory. The simplest type of quantum field is called a scalar field; such objects are associated with particles that have no spin. Modern particle theory involves many scalar fields which are not observed in low-energy interactions, but which may well dominate affairs at the extreme energies of the primordial fireball.

Classical fluids can undergo what is called a phase transition if they are heated or cooled. Water for example, exists in the form of steam at high temperature but it condenses into a liquid as it cools. A similar thing happens with scalar fields: their configuration is expected to change as the Universe expands and cools. Phase transitions do not happen instantaneously, however, and sometimes the substance involved gets trapped in an uncomfortable state in between where it was and where it wants to be. Guth realized that if a scalar field got stuck in such a “false” state, energy – in a form known as vacuum energy – could become available to drive the Universe into accelerated expansion.We don’t know which scalar field of the many that may exist theoretically is responsible for generating inflation, but whatever it is, it is now dubbed the inflaton.

This mechanism is an echo of a much earlier idea introduced to the world of cosmology by Albert Einstein in 1916. He didn’t use the term vacuum energy; he called it a cosmological constant. He also didn’t imagine that it arose from quantum fields but considered it to be a modification of the law of gravity. Nevertheless, Einstein’s cosmological constant idea was incorporated by Willem de Sitter into a theoretical model of an accelerating Universe. This is essentially the same mathematics that is used in modern inflationary cosmology.  The connection between scalar fields and the cosmological constant may also eventually explain why our Universe seems to be accelerating now, but that would require a scalar field with a much lower effective energy scale than that required to drive inflation. Perhaps dark energy is some kind of shadow of the inflaton

Guth wasn’t the sole creator of inflation. Andy Albrecht and Paul Steinhardt, Andrei Linde, Alexei Starobinsky, and many others, produced different and, in some cases, more compelling variations on the basic theme. It was almost as if it was an idea whose time had come. Suddenly inflation was an indispensable part of cosmological theory. Literally hundreds of versions of it appeared in the leading scientific journals: old inflation, new inflation, chaotic inflation, extended inflation, and so on. Out of this activity came the realization that a phase transition as such wasn’t really necessary, all that mattered was that the field should find itself in a configuration where the vacuum energy dominated. It was also realized that other theories not involving scalar fields could behave as if they did. Modified gravity theories or theories with extra space-time dimensions provide ways of mimicking scalar fields with rather different physics. And if inflation could work with one scalar field, why not have inflation with two or more? The only problem was that there wasn’t a shred of evidence that inflation had actually happened.

This episode provides a fascinating glimpse into the historical and sociological development of cosmology in the eighties and nineties. Inflation is undoubtedly a beautiful idea. But the problems it solves were theoretical problems, not observational ones. For example, the apparent fine-tuning of the flatness of the Universe can be traced back to the absence of a theory of initial conditions for the Universe. Inflation turns an initially curved universe into a flat one, but the fact that the Universe appears to be flat doesn’t prove that inflation happened. There are initial conditions that lead to present-day flatness even without the intervention of an inflationary epoch. One might argue that these are special and therefore “improbable”, and consequently that it is more probable that inflation happened than that it didn’t. But on the other hand, without a proper theory of the initial conditions, how can we say which are more probable? Based on this kind of argument alone, we would probably never really know whether we live in an inflationary Universe or not.

But there is another thread in the story of inflation that makes it much more compelling as a scientific theory because it makes direct contact with observations. Although it was not the original motivation for the idea, Guth and others realized very early on that if a scalar field were responsible for inflation then it should be governed by the usual rules governing quantum fields. One of the things that quantum physics tells us is that nothing evolves entirely smoothly. Heisenberg’s famous Uncertainty Principle imposes a degree of unpredictability of the behaviour of the inflaton. The most important ramification of this is that although inflation smooths away any primordial wrinkles in the fabric of space-time, in the process it lays down others of its own. The inflationary wrinkles are really ripples, and are caused by wave-like fluctuations in the density of matter travelling through the Universe like sound waves travelling through air. Without these fluctuations the cosmos would be smooth and featureless, containing no variations in density or pressure and therefore no sound waves. Even if it began in a fireball, such a Universe would be silent. Inflation puts the Bang in Big Bang.

The acoustic oscillations generated by inflation have a broad spectrum (they comprise oscillations with a wide range of wavelengths), they are of small amplitude (about one hundred thousandth of the background); they are spatially random and have Gaussian statistics (like waves on the surface of the sea; this is the most disordered state); they are adiabatic (matter and radiation fluctuate together) and they are formed coherently.  This last point is perhaps the most important. Because inflation happens so rapidly all of the acoustic “modes” are excited at the same time. Hitting a metal pipe with a hammer generates a wide range of sound frequencies, but all the different modes of the start their oscillations at the same time. The result is not just random noise but something moderately tuneful. The Big Bang wasn’t exactly melodic, but there is a discernible relic of the coherent nature of the sound waves in the pattern of cosmic microwave temperature fluctuations seen in the Cosmic Microwave Background. The acoustic peaks seen in the  Planck  angular spectrum  provide compelling evidence that whatever generated the pattern did so coherently.

There are very few alternative theories on the table that are capable of reproducing these results, but does this mean that inflation really happened? Do they “prove” inflation is correct? More generally, is the idea of inflation even testable?

So did inflation really happen? Does Planck prove it? Will we ever know?

It is difficult to talk sensibly about scientific proof of phenomena that are so far removed from everyday experience. At what level can we prove anything in astronomy, even on the relatively small scale of the Solar System? We all accept that the Earth goes around the Sun, but do we really even know for sure that the Universe is expanding? I would say that the latter hypothesis has survived so many tests and is consistent with so many other aspects of cosmology that it has become, for pragmatic reasons, an indispensable part our world view. I would hesitate, though, to say that it was proven beyond all reasonable doubt. The same goes for inflation. It is a beautiful idea that fits snugly within the standard cosmological and binds many parts of it together. But that doesn’t necessarily make it true. Many theories are beautiful, but that is not sufficient to prove them right.

When generating theoretical ideas scientists should be fearlessly radical, but when it comes to interpreting evidence we should all be unflinchingly conservative. The Planck measurements have also provided a tantalizing glimpse into the future of cosmology, and yet more stringent tests of the standard framework that currently underpins it. Primordial fluctuations produce not only a pattern of temperature variations over the sky, but also a corresponding pattern of polarization. This is fiendishly difficult to measure, partly because it is such a weak signal (only a few percent of the temperature signal) and partly because the primordial microwaves are heavily polluted by polarized radiation from our own Galaxy. Polarization data from Planck are yet to be released; the fiendish data analysis challenge involved is the reason for the delay.  But there is a crucial target that justifies these endeavours. Inflation does not just produce acoustic waves, it also generates different modes of fluctuation, called gravitational waves, that involve twisting deformations of space-time. Inflationary models connect the properties of acoustic and gravitational fluctuations so if the latter can be detected the implications for the theory are profound. Gravitational waves produce very particular form of polarization pattern (called the B-mode) which can’t be generated by acoustic waves so this seems a promising way to test inflation. Unfortunately the B-mode signal is expected to be very weak and the experience of WMAP suggests it might be swamped by foregrounds. But it is definitely worth a go, because it would add considerably to the evidence in favour of inflation as an element of physical reality.

But would even detection of primordial gravitational waves really test inflation? Not really. The problem with inflation is that it is a name given to a very general idea, and there are many (perhaps infinitely many) different ways of implementing the details, so one can devise versions of the inflationary scenario that produce a wide range of outcomes. It is therefore unlikely that there will be a magic bullet that will kill inflation dead. What is more likely is a gradual process of reducing the theoretical slack as much as possible with observational data, such as is happening in particle physics. For example, we have not yet identified the inflaton field (nor indeed any reasonable candidate for it) but we are gradually improving constraints on the allowed parameter space. Progress in this mode of science is evolutionary not revolutionary.

Many critics of inflation argue that it is not a scientific theory because it is not falsifiable. I don’t think falsifiability is a useful concept in this context; see my many posts relating to Karl Popper. Testability is a more appropriate criterion. What matters is that we have a systematic way of deciding which of a set of competing models is the best when it comes to confrontation with data. In the case of inflation we simply don’t have a compelling model to test it against. For the time being therefore, like it or not, cosmic inflation is clearly the best model we have. Maybe someday a worthy challenger will enter the arena, but this has not happened yet.

Most working cosmologists are as aware of the difficulty of testing inflation as they are of its elegance. There are also those  who talk as if inflation were an absolute truth, and those who assert that it is not a proper scientific theory (because it isn’t falsifiable). I can’t agree with either of these factions. The truth is that we don’t know how the Universe really began; we just work on the best ideas available and try to reduce our level of ignorance in any way we can. We can hardly expect  the secrets of the Universe to be so easily accessible to our little monkey brains.

## Inflationary Opinion Poll

Posted in The Universe and Stuff with tags , , , , , on February 28, 2014 by telescoper

Compare and contrast this abstract of a paper on the arXiv from Guth et al. from last year:

Models of cosmic inflation posit an early phase of accelerated expansion of the universe, driven by the dynamics of one or more scalar fields in curved spacetime. Though detailed assumptions about fields and couplings vary across models, inflation makes specific, quantitative predictions for several observable quantities, such as the flatness parameter (Ωk=1−Ω) and the spectral tilt of primordial curvature perturbations (ns−1=dlnPR/dlnk), among others—predictions that match the latest observations from the Planck satellite to very good precision. In the light of data from Planck  as well as recent theoretical developments in the study of eternal inflation and the multiverse, we address recent criticisms of inflation by Ijjas, Steinhardt, and Loeb. We argue that their conclusions rest on several problematic assumptions, and we conclude that cosmic inflation is on a stronger footing than ever before.

and this one, just out,  by Ijjas et al.:

Classic inflation, the theory described in textbooks, is based on the idea that, beginning from typical initial conditions and assuming a simple inflaton potential with a minimum of fine-tuning, inflation can create exponentially large volumes of space that are generically homogeneous, isotropic and flat, with nearly scale-invariant spectra of density and gravitational wave fluctuations that are adiabatic, Gaussian and have generic predictable properties. In a recent paper, we showed that, in addition to having certain conceptual problems known for decades, classic inflation is for the first time also disfavored by data, specifically the most recent data from WMAP, ACT and Planck2013. Guth, Kaiser and Nomura and Linde have each recently published critiques of our paper, but, as made clear here, we all agree about one thing: the problematic state of classic inflation. Instead, they describe an alternative inflationary paradigm that revises the assumptions and goals of inflation, and perhaps of science generally.

I’m not sure how much of a “schism” (to use Ijjas et al.’s word) there actually is, but it seems like an appropriate subject for a totally unscientific Friday lunchtime opinion poll:

## Cosm(et)ology

Posted in The Universe and Stuff with tags , , , , on January 5, 2012 by telescoper

I’ve mentioned before that in some parts of the world, cosmology is likely to be confused with cosmetology. I now think cosmologists are missing a trick, and should in fact be borrowing terms from the world of cosmetics.

I suggested on Twitter that the Dark Energy responsible for making the Universe expand more rapidly should henceforth be known as Volumiser, a term which could also apply to the scalar field that drives cosmic inflation. I do think, however, that it might be better to think of inflation as an anti-wrinkle agent, as it smooths away any primordial irregularities.

Anyway, esteemed Nobel laureate and even more esteemed wine producer Brian Schmidt offered this contribution to show that someone has already had the idea…

I’m sure Quintessence isn’t the only example of physics that’s entirely made up, and no doubt you’ll be offering your own suggestions through the comments box…

## The Meaning of Inflation

Posted in Biographical, The Universe and Stuff with tags , , on June 4, 2010 by telescoper

Our little meeting here in Copenhagen is more-or-less over and I’ve now got a free day to enjoy my birthday. It’s a lovely sunny morning and I’m looking forward to being a tourist. Yesterday we had a busy day of talks and discussions followed by a pleasant dinner in a nearby restaurant. One of the good things about small informal meetings like this is that you really get the chance to ask proper questions and have a meaningful dialogue, although sometimes things get a bit heated – especially when people like Leonid Grishchuk are present!

Leonid’s talk yesterday contained various polemical statements about cosmic inflation involving words like “bullshit” and “nonsense”. In the subsequent discussion the question arose as to what, precisely, the word inflation means.

In a nutshell, cosmic inflation is the name given to a short period of rapidly accelerating expansion in the very early Universe that caused it to expand by an enormous factor and also laid down a spectrum of fluctuations through quantum-mechanical processes.  Inflation is a part of the standard “Big Bang” cosmological model, and there is a great deal of circumstantial evidence for it having happened and it’s a very elegant theory. I think it’s safe to say that there isn’t definitive proof but it’s certainly a thriving industry associated with its many versions.

However, the point is that there are many variants of the basic inflationary universe scenario – involving different fields, energy scales and so on – and, although they share some common features, they also differ dramatically from one to the other. What, it was asked, are the essential elements of inflation and what bits are just the trimmings?

In order to contribute meaningfully to the discussion I called upon the assistance of the Oxford English Dictionary to see how it defines inflation. The result was unexpectedly hilarious. Here are the first four definitions as they appear in the OED’s online edition:

1. The action of inflating or distending with air or gas
2. The condition of being inflated with air or gas, or being distended or swollen as if with air
3. The condition of being puffed up with vanity, pride or baseless notions
4. The quality of language or style when it is swollen with big or pompous words; turgidity, bombast

I was quite surprised that definitions to do with economics only appear further down the list, but cosmology’s position even lower down wasn’t unexpected.   However, the leading entries are brilliant, especially definition number 3, which I think is hilarious. I’ll never be able to mention inflation again without thinking of that!

I fear I may have given Leonid quite a bit of ammunition for future anti-inflation rants although if he uses the phrase “baseless notions” in future talks he should perhaps also be careful  to steer clear of “bombast”…