Cosmic controversy

An article in Scientific American that dealt with the theory of cosmic inflation provoked a sharp reaction from 33 physicists, and the authors' response.

The article is published with the approval of Scientific American Israel and the Ort Israel network 08.08.2017

The origins of space and time are among the most mysterious and controversial topics in science. Our article from the February 2017 issue, “The theory of cosmic inflation is in trouble” raises arguments against the conventional view, that the early cosmos underwent an extremely rapid expansion known as inflation. The authors of the article advocate a different scenario: that our universe did not start with a bang but with a spin, a jump from a previous cosmos that was shrinking. In the letter before you, a group of 33 physicists studying inflationary cosmology respond to this article. It is followed by an answer from the authors (an extended version of the answer can be found HERE).

 

The response letter to the article

In the article "The theory of cosmic inflation is in trouble” by Anna Iges, Paul J. Steinhardt, and Abraham Leib, the authors (hereafter: SHL) present arguments in support of a detection cosmology, proposed by Steinhardt et al. in 2001. They end the article with the amazing claim that inflationary cosmology "is a theory that cannot be evaluated using the scientific method" and further state that some of the scientists who accept the theory of inflation propose that "[science] will get rid of one of its founding features: the ability to be tested experimentally," and thus they work to "promote the idea of ​​some kind of non-empirical science.” We have no idea which scientists they mean. We disagree with some of their assertions in the article, however in this letter we will focus on our principle disagreement with these assertions regarding the ability to test inflation theory.

No one disputes the fact that inflation has become the dominant paradigm in cosmology. Many scientists around the world have been working for years to study models of cosmic inflation and compare these predictions with empirical observations. According to the High Energy Physics Database, INSPIRED, there are today more than 14,000 articles in the scientific literature, written by more than 9,000 great scientists, that use the words "inflation" or "inflationary" in their title or in the abstract of the article. In their claim, that inflationary cosmology lies outside the boundaries of the scientific method, Eshal invalidates not only the research of all the authors of this letter but also the research of an important group from among the scientific community. Furthermore, as is clearly evident from the work of several large international collaborations, not only is inflation testable, but it has also stood up to several tests and has so far passed each and every one of them.

Inflation is not a single theory, but actually a class of models based on similar principles. Of course, no one believes that all of these models are correct, and the question that must be asked is if there is at least one inflation model that seems justified, in the sense of the particle physics assumptions underlying it, and correctly describes the measurable properties of the universe. This situation is very similar to the first steps in the development of the standard model of particle physics, when a variety of quantum field theory models were investigated in search of one model that would fit the experiments.

Although in principle there is a wide spectrum of inflation models that stand up to our consideration, there is a very simple class of inflation models (called in the technical language models of "Single field and slow roll”) that all provide similar predictions for most of the observed magnitudes, predictions that were very clearly announced decades ago. These "standard" inflation models form a well-defined class that has been studied in depth. (Eshel expressed strong opinions as to what they consider to be the simplest models within this class, but simplicity is subjective, and we see no reason to limit attention to such a narrow subclass.) Some of the standard inflation models have already been dropped from the chapter following accurate empirical data , and this is part of the desired process of using observations to thin out the collection of viable models. However, many models within this class continue to show great success empirically.

The standard inflation models predict that the universe should have a critical mass density (that is, it should be geometrically flat), and they also predict the statistical properties of the light ripples we detect in the cosmic microwave background radiation (CMB). First, these ripples should be almost “independent of scale", meaning that they should have almost the same strength in all angular scales. Second, the ripples should be "adiabatic," meaning that the perturbations should be the same in all components: normal matter, radiation, and dark matter, all should oscillate together. Third, they should be "Gaussian," which is a statement about the statistical patterns of relatively light and dark areas. And finally, the models also make predictions about the polarization patterns in the cosmic background radiation, which can be divided into two groups, called E-modes and B-modes. The predictions for E-modes are very similar to each other in all standard inflation models, while the levels of B-modes, which are a measure of the gravitational radiation in the early universe, are very different from each other in the class of standard models.

The noteworthy fact is that, starting with the results of the Kobe satellite (COBE) who studied the cosmic background radiation in 1992, several experiments confirmed the hypothesis that these predictions (along with several other predictions that are too technical for us to discuss here) do accurately describe our universe. The average mass density of the universe has now been measured to an accuracy of about half a percent, and is completely consistent with the prediction of inflation. (When the idea of ​​inflation first came up, the uncertainty about the average mass density was at least three times that, so this is an impressive success.) The ripples of the cosmic background radiation were carefully measured by two more satellite experiments, WMAP וPlank, as well as many other ground or balloon-based experiments - all of them confirmed that indeed the primordial fluctuations are almost independent of the scale, and that they are adiabatic and Gaussian to a large degree of accuracy, just as the standard inflation models predicted (a long time before). The B mode of polarization has not yet been observed, a fact that is consistent with many of the standard models, even if not all, and the E mode is found to match the predictions. In 2016, the Planck satellite team (a collaboration of about 260 authors) summarized its conclusions and said that "the Planck results provide strong evidence in favor of the simple inflation models." And the question arises - if inflation is untestable, as Israel is trying to convince us, why has it passed so many tests and with such impressive success?

However, even though the success of the inflation models is not in doubt, Israel still claims that inflation cannot be examined. (We are shocked by ESL's assertion that the dramatic observational successes of inflation must be abolished, while at the same time they accuse inflation supporters of abandoning experimental science!) They claim, for example, that inflation is untestable because its predictions can be changed by choosing a different form to the inflationary energy density curve or choosing other initial conditions. But being a testable theory by no means requires that all its predictions be independent of the choice of variables. If we were to demand such independence in the variables, then we would also have to question the status of the standard model, its particle content, which was determined in experiments, and that it has 19 or more variables determined in the experiment.

One of the important points is that the standard inflation models could have failed any of the empirical tests described here, but they did not. Eshel writes that "a failed theory becomes more and more immune to experiments because of the attempts to patch it up," and imply that this claim has something to do with inflation. However, despite Eshal's quibbles, adapting theories to new data appearing in experiments is a routine procedure in experimental science, as for example the adaptation of the standard model designed to explain newly discovered quarks and photons. For now, in the case of inflationary cosmology, there was still no need to go beyond the standard inflation model class.

ESL state that inflation cannot be examined also because it leads to eternal inflation and a multiverse. But even though the possibility of a multiverse is an active field of research, this possibility does not in any way harm the ability of inflation to be tested experimentally. If the multiverse picture is valid, the proper way to understand the Standard Model would be to see it as a description of the physics in our visible universe. And similarly, the models of inflation refined by the latest observations will describe the ways in which inflation can occur in the particular part of our universe. Both theories could definitely remain within the framework of empirical science. Scientists will still be able to take new data coming in - from astrophysical observations and from experiments in particle physics - and compare it to precise quantitative predictions of certain models of inflation and particle physics. Note that this issue is distinct from the larger goal of developing a theoretical framework that can predict, without the aid of observational data, the particular models of particle physics and inflation that are supposed to describe our visible universe.

Like any other scientific theory, inflation does not need to answer all the questions imaginable. Inflation models, like all scientific theories, stand on a set of assumptions, and to understand these assumptions we must turn to a deeper theory. But this fact does not undermine the success of the inflation models. This situation is similar to standard hot big bang cosmology: the fact that this theory leaves some questions open, such as the near-critical mass density and the origin of the structure of the universe (questions for which inflation provides an elegant solution), do not undermine its many successful predictions, including the prediction of relative rates of light chemical elements. The fact that our knowledge of the universe is still incomplete is by no means a reason to ignore the impressive empirical success of the standard inflation models.

During the more than 35 years of the inflationary theory's existence, it has gradually become the main cosmological paradigm for describing the early stages of the evolution of the universe and its large-scale structure. No one claims that inflation is an indisputable fact; Scientific theories are not proven in the way that mathematical theorems are proven, but over time, successful theories become more and more substantiated by improved experimental tests and by theoretical advances. This is what happened with inflation. Progress continues, supported by the vigorous efforts of many scientists who have chosen to take part in this colorful branch of cosmology.

Empirical science is alive and kicking!!

 

signed the letter:

George Epstein - Professor of Physics, Kawli Institute for Cosmology, University of Cambridge; Member of Planck's scientific team

C. Richard Bond - Professor at the University of Toronto and the Canadian Institute for Descriptive Astrophysics, and Director of the Cosmology and Gravitation Program at the Canadian Institute for Advanced Research; Member of the Plank collaboration

Francois Bouche - Research Director at the Paris Institute of Astrophysics, the French National Institute for Scientific Research (CNRS) and the Pierre and Marie Curie University of the Sorbonne (UPMC); Deputy Principal Investigator of the High Frequency Instrumentation (HFI) Consortium of the Planck satellite and member of the Planck Science Team

Charles L. Bennett - Professor of Physics and Astronomy at Johns Hopkins University, holder of the Bloomberg Distinguished Chair (BDP) and the chair of the university's Alumni Centennial Professor; Principal Investigator of the Wilkinson Anisotropic Background Radiation Probe Mission (WMAP); Deputy Principal Investigator and member of the scientific work team in the Cobi mission (COBE), surveyor of the cosmic background radiation

Alan H. Goth - Head of the Victor P. Weiskopf Department of Physics, Massachusetts Institute of Technology (MIT)

Stephen Hawking - Professor Emeritus, Head of the Lukes Chair of Mathematics and Research Director in the Dennis Stanton Avery and Sally Tsui Wong-Avery Department of Applied Mathematics and Theoretical Physics, University of Cambridge

Edward Witten - Head of the Charles Simoni Department of Physics, Institute for Advanced Research, and recipient of the Fields Medal (1990)

Steven Weinberg - Senior Professor and Chair of the Jack S. Josie–Welch Foundation Chair and Director of the Theoretical Research Team, Department of Physics, University of Texas at Austin, and Nobel Laureate in Physics (1979)

Rainer Weiss - Professor of Physics (Emeritus), Massachusetts Institute of Technology (MIT); Chairman of the scientific work team of the Kobe mission (COBE), surveyor of the cosmic background radiation; Founding partner of the Laser Interferometry Gravitational Wave Observatory (LIGO)

Alexander Vilenkin - Head of the L. and J. Bernstein Department of Evolutionary Sciences and Director of the Institute of Cosmology, Tufts University

Frank Wilchek - Head of the Herman Peschbach Department of Physics, Massachusetts Institute of Technology (MIT), and winner of the Nobel Prize in Physics (2004)

Mathias Zeldriaga - Professor of Physics, Institute for Advanced Research

Michael S. Turner - Holds the Bruce W. Rauner Chair for Distinguished Service in the Department of Astronomy and Astrophysics and the Department of Physics, University of Chicago

Andrey D. Linda - Head of the Harold Trapp Ferris Department of Physics, Stanford University

David H. Leith - Professor of Physics (Emeritus), Lancaster University

John C. Mather - Senior astrophysicist and Goddard Fellow, NASA's Goddard Space Flight Center, and Nobel Prize laureate in Physics (2006); COBE Project Scientist, Cosmic Background Radiation Surveyor and Senior Project Scientist at the James Space Telescope

Juan Maldesana - Head of the Carl P. Feinberg Chair in Physics, Institute for Advanced Research

Yasunori Nomura - Professor of Physics and Director, Berkeley Center for Theoretical Physics, University of California, Berkeley

Alexei Strobinsky - Principal Investigator, Landau Institute for Theoretical Physics, Moscow

Eva Silverstein - Professor of Physics, Stanford University

George P. Smoot III - Professor of Physics (Emeritus), founding director of the Berkeley Center for Cosmological Physics, and winner of the Nobel Prize in Physics (2006); Principal investigator of the COBE mission, surveying the cosmic background radiation

Leonardo Santora - Associate Professor of Physics, Stanford University

Misao Sasuke - Professor at the Yukawa Institute of Theoretical Physics, Kyoto University

Leonard Susskind - Head of the Felix Bloch Department of Physics and director on behalf of the Wells family of the Stanford Institute for Theoretical Physics at Stanford University

Hirania Peiris - Professor of Astrophysics, University College London and Director of the Oskar Klein Center for Cosmoparticle Physics, Stockholm; Member of the Wilkinson Anisotropic Background Radiation Probe (WMAP) collaboration and the Planck collaboration

Malcolm Perry - Professor of Theoretical Physics, University of Cambridge

Renata Kalosh - Professor of Physics, Stanford University

Eiichiro Komatsu - Director of the Department of Physical Cosmology, Max Planck Institute for Astrophysics in Gerching; Member of the Wilkinson Anisotropic Background Radiation Probe (WMAP) collaboration

David A. Kaiser - Head of the Germshausen Department of History of Science and Professor of Physics, Massachusetts Institute of Technology (MIT)

Lawrence Krauss - Founding Professor in the School of Earth and Space Sciences and Department of Physics, and Director of the Origins Project at Arizona State University

Sean Carroll - Research Professor of Physics, California Institute of Technology (Caltech)

Martin Rees - Astronomer Royal of Great Britain, former President of the Royal Society of London and Professor (Emeritus) of Cosmology and Astrophysics at the University of Cambridge

Lisa Randall - Head of the Frank B. Baird Jr. Chair of Sciences, Department of Physics, Harvard University

 

Authors' answer:

We have great respect for the scientists who signed the letter responding to our article, but we were disappointed by their response, which misses our main point: the differences between the inflationary theory as it was believed to be possible in the past and the theory as it is understood today. The claim that inflation has been proven refers to the outdated theory, before we understood its fundamental problems. We believe with all our hearts that in a healthy scientific community it is possible to have a respectful debate, and therefore we reject the claim that by pointing out problems we are undoing the work of all those who developed the theory of inflation and made accurate measurements of the universe possible.

Historically, thinking about inflation was based on a series of misconceptions. It was not understood that the result of inflation is very sensitive to initial conditions. And it was not understood that inflation typically leads to eternal inflation and consequently to a multiverse, i.e. to an infinite variety of outcomes. Articles that claim that inflation predicts one way or another ignore these problems.

Our point is that we should talk about the current version of inflation, about the problems with it, and not about outdated theories that have lost their cool. Logically, if the outcome of inflation is very sensitive to initial conditions that are not yet understood, as the respondents admit, then its outcomes cannot be determined. And if inflation creates a multiverse in which, if we quote a previous statement by one of the respondents (Goth), "everything that can happen will happen" - then there is no sense in talking about predictions. Unlike the standard model, even after correcting all the variables, each inflationary model creates an infinite variety of results without any one of them having priority over its friend. This makes inflation immune to any observational test. For more details, see our article from 2014, "Inflationary Schism", a pre-print version of which can be found on the website https://arxiv.org/abs/1402.6980 .

We are three independent thinkers, representing three different generations of scientists. Our article is not intended to stir up old debates, but rather to discuss the implications of the latest observations and to address unresolved issues that open up opportunities for a new generation of young cosmologists to make their mark. We hope that readers will come back and look at the concluding paragraphs of our article. We came out against the possibility of exercising authority and supported a sober recognition of the failures of the current concepts, a renewed effort to settle these problems and an impartial examination of different ideas that do not suffer from these problems. We continue to adhere to these principles.

 

The views expressed here are those of the author(s) and do not necessarily reflect the position of Scientific American.