New LHC Data Coming This December; Inflation Theory Inventor Now Doubts Theory

Peter Woit's blog reports a couple of notable recent developments:

* First, we can expect new results from the LHC in mid-December reporting on the 2016 experimental date from the LHC which has now recorded lots of data from collisions at 13 TeV energies:

The 2016 LHC proton-proton run is now over, with delivered (41.07 CMS/38.4 ATLAS) and recorded (37.82 CMS/35.5 ATLAS) luminosities (in inverse fb) far above the goal for this year of 25. Together with last year’s data, the experiments now have 41.63 (CMS) and 39.4 (ATLAS) inverse fb recorded at 13 TeV, close to the LHC design energy of 14 TeV. It is likely that preliminary results will be reported at an “end-of-year jamboree” in mid-December, with more to come at the winter conferences.

This experimental data will help physicists determine if the world continues to fit the Standard Model or if there is significant evidence of beyond the Standard Model physics.

Since the new data are from close to peak LHC energies and are substantial enough to draw preliminary conclusions, this is the last moment when there is any real possibility that something dramatically new is likely to appear in the data.

After that, the LHC's reports are likely to be more about reducing statistical error that can confirm or disprove previous findings and make them more precise, and less about finding any radically new phenomena that are observable at the LHC because they have a cutoff energy scale that is below the peak energies that the LHC is capable of probing.

* Second, one of the inventors of the notion of cosmological inflation now things that this theory in its current form is a failure.

Paul Steinhardt gave a colloquium at Fermilab last month with the title Simply Wrong vs. Simple. In it he explained “why the big bang inflationary picture fails as a scientific theory” (it doesn’t work as promised, is not self-consistent and not falsifiable). This is a complicated topic, but Steinhardt is an expert and one of the originators of the theory, so if you want to understand the problems of some common arguments for inflation, watching this talk is highly recommended. Steinhardt’s talk was part of a Fermilab workshop, Simplicity II.

Of course, finding problems with cosmological inflation theory does not imply that a good alternative to explain that phenomena that the theory sought to explain is available. But, as a paper he co-authored late last year explains, he has identified a couple of viable alternatives to the "classic" inflationary paradigm that he helped to establish, which are called "bouncing theories."

The results from Planck2015, when combined with earlier observations from WMAP, ACT, SPT and other experiments, were the first observations to disfavor the "classic" inflationary paradigm. To satisfy the observational constraints, inflationary theorists have been forced to consider plateau-like inflaton potentials that introduce more parameters and more fine-tuning, problematic initial conditions, multiverse-unpredictability issues, and a new 'unlikeliness problem.' Some propose turning instead to a "postmodern" inflationary paradigm in which the cosmological properties in our observable universe are only locally valid and set randomly, with completely different properties (and perhaps even different physical laws) existing in most regions outside our horizon. By contrast, the new results are consistent with the simplest versions of ekpyrotic cyclic models in which the universe is smoothed and flattened during a period of slow contraction followed by a bounce, and another promising bouncing theory, anamorphic cosmology, has been proposed that can produce distinctive predictions.

Anna Ijjas, Paul J. Steinhardt "Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies" (30 Dec 2015).

As the introduction to that article explains (emphasis added):

The Planck2015 [1] and Planck2013 [2] observations, combined with the results by the Wilkinson Microwave Anisotropy Probe (WMAP), Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT) teams have shown that the measured spatial curvature is small; the spectrum of primordial density fluctuations is nearly scale-invariant; there is a small spectral tilt, consistent with a simple dynamical mechanism that caused the smoothing and flattening; and the fluctuations are nearly gaussian. These features are all consistent with the simplest textbook inflationary models. At the same time, Planck2015 also confirmed that r, the ratio of the tensor perturbation amplitude to the scalar perturbation amplitude, is less than 0.1, a result that virtually eliminates all the simplest textbook inflationary models. The development is notable because, as emphasized by Ijjas et al. [3], it is the first time that the classic inflationary picture has been in conflict with observations. 

The results have led theorists to consider alternative plateau-like inflationary models whose parameters can be adjusted to reduce the expected value of r while retaining all the rest of the classic predictions. However, as we explain in this brief review, the remaining models raise new issues. They require more parameters, more tuning of parameters, more tuning of initial conditions, a worsened multiverse-unpredictability problem, and a new challenge that we call the inflationary ‘unlikeliness problem.’ 

One response to these problems has been that they should be ignored. The classic inflationary picture should be abandoned in favor of an alternative ‘postmodern’ inflationary picture that allows enough flexibility to fit any combination of observations. The classic and postmodern inflationary pictures are so different that they ought to be viewed as distinct paradigms to be judged separately. 

A more promising response to Planck2015 has been to develop “bouncing” cosmologies in which the largescale properties of the universe are set during a period of slow contraction and the big bang is replaced by a big bounce. For example, a new, especially simple version of ekpyrotic (cyclic) cosmology has been identified that fits all current observations, including nearly Gaussian fluctuations and small r [4–7]. Also, anamorphic bouncing cosmologies have been introduced that use yet different ways to smooth and flatten the universe during a contracting phase and generate a nearly scale-invariant spectrum of perturbations [8]. 

We will first review the problems that Planck2015 imposes on classic inflation, the version that most observers consider. We will briefly discuss the conceptual problems of initial conditions and multiverse that have been known and unresolved for decades. Then we will turn to the tightening constraints resulting from Planck2015 and other recent experiments. We will review and critique postmodern inflation that some theorists now advocate. Finally, we will turn to the promising new developments in bouncing cosmologies, both ekpyrotic and anamorphic approaches.

The review article follows with a nice brief introduction to "classic" inflation theory, before exploring other topics.