Friday, October 13, 2017

Is the inflationary universe a scientific theory? Not anymore.

Living in a Bubble?
[Image: YouTube]
We are made from stretched quantum fluctuations. At least that’s cosmologists’ currently most popular explanation. According to their theory, the history of our existence began some billion years ago with a – now absent – field that propelled the universe into a phase of rapid expansion called “inflation.” When inflation ended, the field decayed and its energy was converted into radiation and particles which are still around today.

Inflation was proposed more than 35 years ago, among others, by Paul Steinhardt. But Steinhardt has become one of the theory’s most fervent critics. In a recent article in Scientific American, Steinhardt together with Anna Ijjas and Avi Loeb, don’t hold back. Most cosmologists, they claim, are uncritical believers:
“[T]he cosmology community has not taken a cold, honest look at the big bang inflationary theory or paid significant attention to critics who question whether inflation happened. Rather cosmologists appear to accept at face value the proponents’ assertion that we must believe the inflationary theory because it offers the only simple explanation of the observed features of the universe.”
And it's even worse, they argue, inflation is not even a scientific theory:
“[I]nflationary cosmology, as we currently understand it, cannot be evaluated using the scientific method.”
As alternative to inflation, Steinhardt et al promote a “big bounce” in which the universe’s expansion was preceded by a phase of contraction, yielding similar benefits to inflation.

The group’s fight against inflation isn’t news. They laid out their arguments in a series of papers during the last years (on which I previously commented here). But the recent SciAm piece called The Defenders Of Inflation onto stage. Lead by David Kaiser, they signed a letter to Scientific American in which they complained that the magazine gave space to the inflationary criticism.

The letter’s list of undersigned is an odd selection of researchers who themselves work on inflation and of physics luminaries who have little if anything to do with inflation. Interestingly, Slava Mukhanov – one of the first to derive predictions from inflation – did not sign. And it’s not because he wasn’t asked. In an energetic talk delivered at Stephen Hawking’s birthday conference two months ago, Mukhanov made it pretty clear that he thinks most of the inflationary model building is but a waste of time.

I agree with Muhkanov’s assessment. The Steinhardt et al article isn’t exactly a masterwork of science writing. It’s also unfortunate they’re using SciAm to promote some other theory of how the universe began rather than sticking to their criticism of inflation. But some criticism is overdue.

The problem with inflation isn’t the idea per se, but the overproduction of useless inflationary models. There are literally hundreds of these models, and they are – as the philosophers say – severely underdetermined. This means if one extrapolates any models that fits current data to a regime which is still untested, the result is ambiguous. Different models lead to very different predictions for not-yet made observations. Presently, is therefore utterly pointless to twiddle with the details of inflation because there are literally infinitely many models one can think up.

Rather than taking on this overproduction problem, however, Steinhardt et al in their SciAm piece focus on inflation’s failure to solve the problems it was meant to solve. But that’s an idiotic criticism because the problems that inflation was meant to solve aren’t problems to begin with. I’m serious. Let’s look at those one by one:

1. The Monopole Problem

Guth invented inflation to solve the “monopole problem.” If the early universe underwent a phase-transition, for example because the symmetry of grand unification was broken – then topological defects, like monopoles, should have been produced abundantly. We do not, however, see any of them. Inflation dilutes the density of monopoles (and other worries) so that it’s unlikely we’ll ever encounter one.

But a plausible explanation for why we don’t see any monopoles is that there aren’t any. We don’t know there is any grand symmetry that was broken in the early universe, or if there is, we don’t know when it was broken, or if the breaking produced any defects. Indeed, all searchers for evidence of grand symmetry – mostly via proton decay – turned out negative. This motivation is interesting today merely for historical reasons.

2. The Flatness Problem

The flatness problem is a finetuning problem. The universe currently seems to be almost flat, or if it has curvature, then that curvature must be very small. The contribution of curvature to the dynamics of the universe however increases in relevance relative to that of matter. This means if the curvature density parameter is small today, it must have been even smaller in the past. Inflation serves to make any initial curvature contribution smaller by something like 100 orders of magnitude or so.

This is supposed to be an explanation, but it doesn’t explain anything, for now you can ask, well, why wasn’t the original curvature larger than some other number? The reason that some physicists believe something is being explained here is that numbers close to 1 are pretty according to current beauty-standards, while numbers much smaller than 1 numbers aren’t. The flatness problem, therefore, is an aesthetic problem, and I don’t think it’s an argument any scientist should take seriously.

3. The Horizon Problem

The Cosmic Microwave Background (CMB) has almost at the same temperature in all directions. Problem is, if you trace back the origin the background radiation without inflation, then you find that the radiation that reached us from different directions was never in causal contact with each other. Why then does it have the same temperature in all directions?

To see why this problem isn’t a problem, you have to know how the theories that we currently use in physics work. We have an equation – a “differential equation” – that tells us how a system (eg, the universe) changes from one place to another and one moment to another. To make any use of this equation, however, we also need starting values or “initial conditions.”*

The horizon problem asks “why this initial condition” for the universe. This question is justified if an initial condition is complicated in the sense of requiring a lot of information. But a homogeneous temperature isn’t complicated. It’s dramatically easy. And not only isn’t there much to explain, inflation moreover doesn’t even answer the question “why this initial condition” because it still needs an initial condition. It’s just a different initial condition. It’s not any simpler and it doesn’t explain anything.

Another way to see that this is a non-problem: If you’d go back in time far enough without inflation, you’d eventually get to a period when matter was so dense and curvature so high that quantum gravity was important. And what do we know about the likelihood of initial conditions in a theory of quantum gravity? Nothing. Absolutely nothing.

That we’d need quantum gravity to explain the initial condition for the universe, however, is an exceedingly unpopular point of view because nothing can be calculated and no predictions can be made.

Inflation, on the other hand, is a wonderfully productive model that allows cosmologists to churn out papers.

You will find the above three problems religiously repeated as a motivation for inflation, in lectures and textbooks and popular science pages all over the place. But these problems aren’t problems, never were problems, and never required a solution.

Even though inflation was ill-motivated when conceived, however, it later turned out to actually solve some real problems. Yes, sometimes physicists work on the wrong things for the right reasons, and sometimes they work on the right things for the wrong reasons. Inflation is an example for the latter.

The reasons why many physicists today think something like inflation must have happened are not that it supposedly solve the three above problems. It’s that some features of the CMB have correlations (the “TE power spectrum”) which depend on the size of the fluctuations, and implies a dependence on the size of the universe. This correlation, therefore, cannot be easily explained by just choosing an initial condition, since it is data that goes back to different times. It really tells us something about how the universe changed with time, not just where it started from.**

Two more convincing features of inflation are that, under fairly general circumstances, the model also explains the absence of certain correlations in the CMB (the “non-Gaussianities”) and how many CMB fluctuations there are of any size, quantified by what is known as the “scale factor.”

But here is the rub. To make predictions with inflation one cannot just say “there once was exponential expansion and it ended somehow.” No, to be able to calculate something, one needs a mathematical model. The current models for inflation work by introducing a new field – the “inflaton” – and give this field a potential energy. The potential energy depends on various parameters. And these parameters can then be related to observations.

The scientific approach to the situation would be to choose a model, determine the parameters that best fit observations, and then revise the model as necessary – ie, as new data comes in. But that’s not what cosmologists presently do. Instead, they have produced so many variants of models that they can now “predict” pretty much anything that might be measured in the foreseeable future.

It is this abundance of useless models that gives rise to the criticism that inflation is not a scientific theory. And on that account, the criticism is justified. It’s not good scientific practice. It is a practice that, to say it bluntly, has become commonplace because it results in papers, not because it advances science.

I was therefore dismayed to see that the criticism by Steinhardt, Ijas, and Loeb was dismissed so quickly by a community which has become too comfortable with itself. Inflation is useful because it relates existing observations to an underlying mathematical model, yes. But we don’t yet have enough data to make reliable predictions from it. We don’t even have enough data to convincingly rule out alternatives.

There hasn’t been a Nobelprize for inflation, and I think the Nobel committee did well in that decision.

There’s no warning sign you when you cross the border between science and blabla-land. But inflationary model building left behind reasonable scientific speculation long ago. I, for one, am glad that at least some people are speaking out about it. And that’s why I approve of the Steinhardt et al criticism.

* Contrary to what the name suggest, the initial conditions could be at any moment, not necessarily the initial one. We would still call them initial conditions.

** This argument is somewhat circular because extracting the time-dependence for the modes already presumes something like inflation. But at least it’s a strong indicator.

This article was previously published on Starts With A Bang. 


Philippe Mermod said...

What about Higgs inflation? Is it not worth investigating -- making no assumptions beyond the Standard Model and see if one can still fit the observations?

A. Mikovic said...

The Higgs particle is to heavy to be the inflaton. A recent search of LHC scattering results concluded that there is no inflaton, see

A way to obtain inflation without a scalar field is by adding R^2 term to the Einstein-Hilbert action (Starobinsky inflation).

Michaud Venant said...

"Inflation, on the other hand, is a wonderfully productive model that allows cosmologists to churn out papers."

And it gives you a the chance to write a click bait title for a blog... congratulations

Sabine Hossenfelder said...


If I was after "click bait," theoretical physics would be the last topic I'd consider writing about.

Sabine Hossenfelder said...


If you think Higgs inflation makes no assumptions beyond the Standard Model you should look at it again.

Philippe Mermod said...

This paper claims to make minimal assumptions:

Sabine Hossenfelder said...


Such type of couplings are highly implausible and believed to be inconsistent, see eg this paper. You can try to work around it, but the more you work the more constructed the model becomes.

I'm not against using the Higgs as inflaton. It's a nice idea but in that case too, models abound and I can't really see the use of any of that.

Theophanes Raptis said...

So is that all about The Three Grand Choices? Because so he liked? (the original World Programmer!)

CapitalistImperialistPig said...

OT, but are you going to say anything about your new book on Gravitational Waves?

Uncle Al said...

"We don’t know there is any grand symmetry that was broken in the early universe" Baryogenesis, Sakharov conditions; ~10^(-9) relative hadron-selective mirror-symmetry-breaking. Primordial scalar field (Mexican hat) decay has a detectable remnant pseudoscalar component.

Physics postulates exact vacuum mirror symmetry toward hadrons. Unending empirical chiral divergences are forever parameterized. Look elsewhere.

10^(-12) sensitive vacuum chiral divergence test. Single crystal quartz test masses, space group P3(1)21 (right-handed) versus P3(2)21 (left-handed), as the torsion balance rotor’s opposed vertical squares’ corners, right-handed only versus left-handed only. Everything measurable exactly cancels. Geometry maximally diverges in theory (DOI:10.1107/S0108767303004161 Section 3ff) and calculated (DOI:10.1063/1.1484559).

"Monopoles" Dark matter. Theory alone is not sufficient.

Sabine Hossenfelder said...


No, except for one sentence in which I mention the recent direct measurement.

David Brown said...

"But these problems aren't problems, never were problems, and never required a solution." I think that the Flatness Problem and the Horizon Problem definitely are problems and some version of ihflation resolves those 2 problems. My guess is that inflation definitely occurred but the inflation is Milgromian instead of Newtonian-Einsteinian. I say that Milgrom is the Kepler of contemporary cosmology — beyond a reasonable doubt.
“The failures of the standard model of cosmology require a new paradigm” by Kroupa, Pawlowski & Milgrom, 2013
Wikiquote, Pavel Kroupa
Wikiquote, Stacy McGaugh
Google "witten milgrom”.

John Baez said...

If the inflationary universe is not a scientific theory "anymore", and its original motivations are suspect, roughly when was it a scientific theory and why did it stop being one? (I'm really asking this; these are not rhetorical questions.)

If I create an elegant, predictive scientific theory and then a crowd of fools create a vast number of less elegant variants of my theory that make all possible variants, does my theory cease to be scientific?

Could there be a scientific theory of the inflationary universe that's lost in a sea of foolish variants?

naivetheorist said...

now that you've finished hammering the final nail into the inflationary universe coffin 'theory', could you please use your hammer to smash to bits, the equally unscientific many-worlds, multiverse interpretation'.

btw- nice video on quantum gravity.

JimV said...

As I stated in the previous post that mentioned the "Flatness Problem", I agree that it is not a problem, but do see it as an indication (that Inflation has explanatory value). To repeat my argument:

There (probably) is a finite range of curvature (negative to positive) in which human life could have developed within the universe's current age. Assume a random, uniform probability on this interval. Under Inflation, any sub-range around and including zero curvature becomes more likely than it would be without Inflation. Thus our near-zero curvature is an indication that Inflation has explanatory value. (Whereas a curvature near the maximum of the range would be more difficult to explain under Inflation.)

To which I must add, Inflation or any other hypothesis which explains how curvature could have evolved to a much smaller value. If the Bounce hypothesis also does, then the two hypotheses have equal explanatory value, in this regard.

That said, certainly scientists and SciAm have the scientific right to write and publish criticisms of Inflation. The Inflation defenders should have submitted a rebuttal article to SciAm rather than decrying the article's publication. I must be missing something in their argument.

murli manohar verma said...

Inflation says nothing about the value if k,per se. It throws the entire term k/a^2 out of the Friedman equations.

MarkusM said...

It's very instructive to have look at the predictions of the mass of the Higgs
Nearly any mass was predicted, and so roughly 99% of the predictions were wrong. Had we found no Higgs particle, 100% of the predictions would have been wrong, not a big quantitative difference. Anyway, the predictive power of theory was close to zero.
Same with inflation, nearly anything has been predicted. Hence it's worth considering giving a damn on this kind of theoretical physics, which is what I do (life is too short :-)).

CGT said...

While the topic is 'inflation', another argument for the notion appeared today, unrelated to the usual justifications. It's at the arXiv here: with title: "Was inflation necessary for the existence of time?" Just fyi...

Ambi Valent said...


your intention is laudable, but you won't change any true believers' minds. There are a lot of them, and they will back each other up, probably sadly shaking their heads about you.

Though I personally think that worse than supporting an unproven hypothesis is rejecting one that is well-supported by all observation we have just because it disagrees with a product of logical fallacy that mixes coordinates from different realms that aren't supposed to be mixed.

Compare to the following example: Imagine a spaceship going from Earth to Alpha Centauri in 1 year of proper spaceship time, and choosing distance as measured from Earth and time as measured by a clock on the spaceship. Then although the example is theoretically possible and the coordinates are all true, all they will tell you what time the spaceship clock will measure at a specific distance from Earth, but will not allow you to conclude there are superluminal speeds and that a model that has a speed limit at c is therefore incomplete and should be rejected. Before you make any judgements about speed, you should choose a time coordinate that corresponds to your chosen space coordinate, and then you'll find the spaceship was still going slower than light. Similar transformations into corresponding coordinates should be used above.

Mobley Meadows said...

Once you eliminate the impossible whatever is left over, howerver improbable, must be true. So, it's time to look for the impossible starting right here. If any of it may be possible, set it aside until every thing known about the problem is compared to it. When you have a pile of possibles, measure them against themselves.

Unknown said...

When I think how simple the universe was in my youth, so long ago: spacetime, and galaxies, nothing more. No, not the good old days! How, how, I want to know what the dark matter is. Even just, is it new particles, or is it some twist on GR? Tell me, tell me!

Sabine Hossenfelder said...


I'm not much of a historian or sociologist. Also, I haven't been around for all that long. So I can really just guess. I roughly have the impression things started changing in the late 1990s. Maybe somewhat ironically, it was a misguided push to Popperian falsification that I recall sweeping through. A good theory, so the argument, must be testable. The problem being that too many people concluded if a theory is testable it's a good theory.

Unfortunately, that isn't so. I can cook up a lot of "theories" that are totally testable and yet not any good. Like, I could have a theory that Donald Trump will die from heart failure tomorrow. Totally predictive, totally falsifiable. And totally non-scientific. No one with a brain would believe it. Why? Because I haven't given you any reason to actually trust in this prediction.

And it's somewhere there where things went wrong with inflationary models. Somewhere around model 20 or 30 I am pretty sure it occurred to people that really they could cook up pretty much everything for future observations and make it compatible with past observations, so there was no reason to trust any of these predictions. People cooked up models (still do!) saying "But you can go and test it." At that point someone should have written a paper quantifying the ambiguity (or maybe someone did and I never heard of it?) and they should have settled on some minimal model but that never happened.

I'm not sure that misunderstanding Popper is the whole story. It adds to this that physicists trust in math, a lot. If you can calculate something, they believe that makes it scientific. That's why my book is called "Lost in Math."

In any case, I scraped the part about the motivations for inflation from the book because I thought it was somewhat confusing to lump it together with the finetuning arguments in particle physics. In particle physics people aren't discussing about initial conditions, so it's a different story.



Sabine Hossenfelder said...

PS: As to your question whether there could be a scientific theory lost in the ocean of models. Well, basically, yes. This isn't my area of research (I have never worked on anything to do with inflation) but what you'd really want to do is figure out what are the minimal assumptions necessary to reproduce observation, and if - based on only these - there are any conclusions you can draw for future observations. Maybe you can, maybe you can't.

Eg, there is some interesting work by Kinney et al that identifies some general conditions necessary for (nearly) scale-invariant perturbations. This is pretty good, but of course the spectral index isn't the only observable. So I think one should do something similar taking into account all observations.

Sabine Hossenfelder said...


What you say is totally wrong! The mass RANGE of the Higgs was predicted very accurately. This is why the LHC was built to begin with. And that prediction was totally correct. What you mean to say is that they couldn't predict the exact mass better than to within 30 or so GeV.

Sabine Hossenfelder said...


Not sure what you mean. I have no problem with the many worlds interpretation. I don't find it particularly useful, but I don't see the point arguing about interpretations.

MarkusM said...

no, what I say is totally right !
See p. 53 of the following document (with headline: "Inflation has many variants..."):

Sabine Hossenfelder said...


Sorry, I don't understand your comment. I was replying to your statement about the Higgs mass that "Nearly any mass was predicted". That just isn't so. Nearly any mass was predicted within a 30 GeV window, but the window in and by itself was an excellent prediction!

Koenraad Van Spaendonck said...

@Ambi Valent

'Though I personally think that worse than supporting an unproven hypothesis is rejecting one that is well-supported by all observation we have...'

Here's Magueijo and Randall disagreeing :

Look from 03m30s. " Dark matter is really painted on.."

And that's where things go wrong, that's where they lowered the bar, it's curve-fitting and then claiming it's a model which matches observations.
That's no what a decent scientific model should be about.

Best, Koenraad

Uncle Al said...

@Koenraad Van Spaendonck Theory shunning falsification is a sales pitch earning commission on advertising. Lisa Randall’s lecture tour selling her book included a slide with an impossible lithium isotope. I told her, she shined me off. Who was wrong?

Euclid cannot plot ocean navigation or ballistic trajectory (non-Euclidean great circles). 2000 years of parameterization suppressed progress. Otto Stern was a Nobel Laureate/Physics for showing the Dirac equation failed for protons – quarks!

Observation defines science. Carefully wrongly observe when empirically defective theory claims precedence. Theory is necessary but not sufficient.

Ambi Valent said...

@Koenraad Van Spaendonck

When saying well-supported by observation I didn't mean dark matter, I meant Schwarzschild coordinates. General Relativity has been tested a lot with predictions of object movement based on Schwarzschild coordinates (relativistic Kepler orbits, gravitational lensing, Shapiro delay), and the predictions were supported ny the observation.

And by rejecting I mean that Schwarzschild coordinates are being discarded when people say they're disagreeing with the comoving coordinate system which - like the Alpha Centauri example above - consists of externally measured distance and locally measured proper time.

Now the comoving coordinate system isn't completely wrong as it contains all the correct information. But while the coordinate systems are representations of spacetime reality they are not equally accurate when taken literally, just like the globe and world maps are all representations of Earth but not equally accurate. If your predictions based on the map differ from your predictions based on the globe, it would be crazy to decide the globe must be the wrong one as its spherical shape is well-supported by observation.

Similarly one can't discard the observationally well-supported Schwarzschild global coordinates just because one wants the comoving coordinates being absolute - one must read them differently, such as "when this point in space is reached, this local proper time is measured". For decades it has been known that if you continue with the interpretation that comoving coordinates are superior, you get several problems and paradoxes. But most people just shrug and claim that's just the way it is instead of returning to the pure Schwarzschild coordinates where these problems and paradoxes don't appear.

Koenraad Van Spaendonck said...

Ok Ambi Valent,point taken.

Psylife said...

If the Big-Bang theory cracks, the Big-Crack theory bangs?

Kayanote said...

In a quark matter meeting in Seattle in 1983 someone distributed a slip of paper with the title "Junior Nobel Prize Hunting Kit" containing a large number of alternatives one could choose to define a model hopefully leading to a Nobel prize. I should still have this slip in my possession. So already 34 years ago one could make fun of the multitude of inflationary models. However, the scientific process has been effective and nowadays most of those alternatives look totally outdated. So I would say that at least in 1983 inflation was still science (this is to John Baez).

Sabine Hossenfelder said...


The Big-Bang (if there was one) happend before inflation. Different story really.

Psylife said...

If the expanding universe is false, and the static universe does not fit with astronomical observations, what do you suggest Ms.Sabine Hossenfelder a big gravitational crunch that we are living it now, or do you have a better idea?

Sabine Hossenfelder said...


It's Dr. Sabine Hossenfelder, not Ms. And I don't know, I'm not in the business of making billion-year extrapolations.

Ambi Valent said...


would the alternative to inflation mean that the universe is older than we thought, or that it started off extremely large (maybe infinite)? Or is it just that there are no indications either way that could lead to a prediction that can be tested?

Sabine Hossenfelder said...


Depends on what you mean by "universe." I'm not trying to be annoying but in alternatives that are cyclic, the universe gets reborn periodically. So does this mean there is only one universe which lives eternally or are these all different universes? Also, in inflation, the age of *our* universe is something like 13.8 billion years, but the whole multiverse is much older (and no one knows how old). So, really depends on what you ask. In some cases there's just an infinite amount of time before the "bang," in which case I guess the total time is infinite.

Either way, note that all of this only changes what happens before the creation of matter. It's super speculative territory.

Ambi Valent said...


so currently, all testable hypotheses only allow us to extrapolate backwards to the creation of matter, and beyond that, we have nothing testable?

Sabine Hossenfelder said...


Currently testable is only back to the EW phase transition, which is like 14 or so orders of magnitude below inflation.

Of course you can extrapolate beyond that whatever you like. Whether you should trust in it is another question entirely. Note that inflation is *not* just an extrapolation back of what we know, but it introduces additional fields and a potential for these fields and some supposed explanation for why we don't see these fields.

Phillip Helbig said...

"The flatness problem, therefore, is an aesthetic problem, and I don’t think it’s an argument any scientist should take seriously."

Indeed; I actually wrote a whole paper about this. If you agree with me, cite me! :-) Don't be afraid!

I also agree that the monopole problem is a non-starter unless there is a firm prediction that they should be there; the GUTs which were around when Guth (must be a pun there somewhere!) solved the monopole problem have been ruled out for a while now.

The horizon problem is more interesting. Yes, we don't have a theory of quantum gravity. Yes, one can solve any problem by an appeal to initial conditions, but this is rightly regarded as not good practice. Even without a theory of quantum gravity, I think that there is some meat to the claim that homogeneous and isotropic initial conditions would be strange. (Note that homogeneous but anisotropic initial conditions are not a problem, as shown by Barrow more than 20 years ago.) Suppose that all stars between 16th and 17th magnitude, as seen from Earth, form constellations which spell out the complete works of Shakespeare. Yes, this can be explained by initial conditions at a time when quantum gravity was important, and no, we don't have a theory of quantum gravity. So we shouldn't worry about it? I think not.

I was long a sceptic of inflation. I think that the robust prediction (not that there is not some wild model which predicts something else, but this was a firm, generic prediction, long before there was any observational evidence) that the spectral index is 0.96, spectacularly confirmed, tipped the scales in my mind. As you mention, there are other indications that something like inflation probably happened, apart from the three "original problems".

Inflation is not a theory, but a paradigm. There is evidence that the paradigm is correct, even if people develop unconstrained theories based on it. Perhaps John Baez will agree with me if I rephrase his comment as "don't throw the baby out with the bathwater".

Perhaps the most serious argument against inflation is that it requires more special initial conditions than the special initial conditions it should solve, as first pointed out by Penrose.

While it is good to have constructive criticism and present an alternative theory, Steinhardt and colleagues seem to be claiming "since inflation has problem, our new theory must be right". Albrecht was also one of the early inflationists and is now something of a critic, but much more balanced.

kashyap vasavada said...

Hi Bee,
In reply to Ambi. you mentioned "Currently testable is only back to the EW phase transition, which is like 14 or so orders of magnitude below inflation." Is the time of this event during the first second fairly reliable or it depends on various models including inflation? Quarks, leptons and bosons must have been created long before that. Right?

uair01 said...

What a disappointment that I'll have to wait until next year to read your book! I only saw it by coincidence, while looking through the comments. It will get a nice place between "Not even wrong" and "The trouble with physics" :-)

pete said...

"Indeed, all searchers for evidence of grand symmetry – mostly via proton decay – turned out negative. This motivation is interesting today merely for historical reasons."

As far as the above point, Sabine, is this actually the consensus? I remember a Quanta Magazine article on GUTs from last year and they mentioned that there is still a ton of space left to constrain things like proton decay and the like:

Here are a couple parts:

Super-K’s latest result, which sets the lower limit on the proton’s lifetime just above 1034 years, moves into the region of interest of many models — including that of flipped SU(5), which predicts that protons will take between 1034 and 1036 years to decay. “I’m very excited about this,” said Nanopoulos, one of the researchers who developed flipped SU(5) in the early 1980s.

Grand unification hasn’t died, exactly. The circumstantial evidence is as compelling as ever. But the idea could remain in perpetual limbo, rather like the proton.

Wanted to get your thoughts on this, especially when it comes to the massive impact representation theory and symmetries have had in fundamental physics. It seems very unlikely that we'll somehow start going in a direction that doesn't include groups, and if that's the case then wouldn't it by definition mean we are going to continue trying to ascertain whether larger group structures (that contain SU(3)xSU(2)xU(1)) can help us on the quest for a TOE/quantum gravity?

Any input it greatly appreciated!

Uncle Al said...

@pete SUSY arising from universal exact symmetries uncreates baryogenesis. Wrong.

The Hubble constant's exact value remains contentious, as does cosmic expansion rate over time. What empirically demands they are exactly homogeneous and isotropic?

LIGO binary neutron star merger! Telescopes saw an off-axis gamma ray burst and an expanding heavy elements cloud. The merger’s distal ends centripetally fragmented. Neutron-rich nuclear matter furiously beta-decayed down the Periodic Table. Look for Island of Stability superheavy elements.

Phillip Helbig said...

"The Hubble constant's exact value remains contentious, as does cosmic expansion rate over time. What empirically demands they are exactly homogeneous and isotropic?"

Nothing. And no serious person claims that they should be.