# School of Natural Sciences

## BICEP: Spacetime Ripples or Galaxy Dust?

### Doubts Arise Over Claims of Evidence for Cosmic Inflation

“Space Ripples Reveal Big Bang’s Smoking Gun,” read the *New York Times* headline last March 17. In a seemingly momentous news conference at the Harvard–Smithsonian Center for Astrophysics, researchers using a BICEP (Background Imaging of Cosmic Extragalactic Polarization) telescope at the South Pole announced that they had detected the first direct evidence for cosmic inflation, a theory about the very beginnings of the universe first proposed in 1979.

The BICEP announcement claimed that the first images of gravitational waves, or ripples in spacetime, had been detected, a tantalizing and long hoped-for connection between quantum mechanics and general relativity. The landmark claim ignited the field and led to talk of a new era of cosmology.

At the Institute for Advanced Study, Raphael Flauger, Member (2013–14) in the School of Natural Sciences, began looking closely at the data. The year prior, Flauger had analyzed the first round of cosmic microwave background data released by the Planck satellite, a mission of the European Space Agency, which the BICEP team had used in its findings.

## From B-Mode Cosmology to the Fate of Spacetime

The Institute’s thirteenth annual Prospects in Theoretical Physics (PiTP) summer program for graduate students and postdoctoral scholars, which focused on string theory, was truly extraordinary in that it overlapped with Strings 2014. This is one of the field’s most important gatherings, which the Institute hosted with Princeton University, convening international experts and researchers to discuss string theory and its most recent developments. Six hundred attendees gathered for Strings 2014, which made it one of the largest Strings conferences since their inception in 1995.

Strings 2014 talks, which covered topics from B-mode cosmology and the theory of inflation to quantum entanglement, the amplituhedron, and the fate of spacetime, may be viewed at https://physics.princeton.edu/strings2014/Talk_titles.shtml. The program for PiTP and videos of its string theory talks may be viewed at https://pitp2014.ias.edu/schedule.html.

As part of the PiTP program, the Institute showed a screening of *Particle Fever*, a new film that follows six scientists, including the Institute’s Nima Arkani-Hamed, during the launch of the Large Hadron Collider and fortutiously captures the discovery of the Higgs particle. Peter Higgs, who predicted the existence of the particle fifty years ago, gave one of his first seminars on the topic at the Institute in 1966.

## From Fluid Dynamics to Gravity and Back

### by Shiraz Minwalla

*How the Movement of Water Molecules Corresponds to Ripples in Spacetime*

There is an interesting connection between two of the best-studied nonlinear partial differential equations in physics: the equations of hydrodynamics and the field equations of gravity.

Let’s start with a brief review of hydrodynamics. At the microscopic level a tank of water is a collection of, say, 10^{25} molecules that constantly collide with one another. The methods of physics may be used to model this collection of water molecules as follows: we set up equations that track the position and momentum of each of the water molecules and predict their time evolution. These conceptually complete equations have of order 10^{25} variables and so are clearly too difficult to handle in practice.

Does it then follow that tanks of water cannot be usefully studied using the methods of physics? As every plumber knows, this conclusion is false: a useful description of water is obtained by keeping track of average properties of water molecules, rather than each individual molecule.

Think of a tank of water as a union of non-overlapping lumps of water. Each lump is big enough to contain a large number of molecules but small enough so that gross macroscopic properties of the water (energy density, number density, momentum density) are approximately uniform. The fundamental assumption of hydrodynamics is that under appropriate conditions, all the “average” properties of any lump are completely determined by its conserved charge densities (in the case of water, molecule number density, energy density, and momentum density). In particular, the conserved current for molecule number j^{µ} and the conserved current for energy and momentum T^{µν} are themselves dynamically determined functionals of local thermodynamical densities in a locally equilibrated system (fluctuations away from these dynamically determined values are suppressed by a factor proportional to the square root of the number of molecules in each lump). The equations that express conserved currents as functionals of conserved densities are difficult to compute theoretically but are easily measured experimentally and are known as constitutive relations.

When supplemented with constitutive relations, the conservation equations ∂_{µ} j^{µ} =0, and ∂_{µ} T^{µν}=0(2) turn into a well-posed initial value problem for the dynamic of conserved densities. They are the equations of hydrodynamics. Let me reemphasize that the effect of the ignored degrees on the evolution of conserved densities is inversely proportional to the square root of the number of molecules in a lump, and so is negligible in an appropriate thermodynamic limit, allowing the formulation of a closed dynamical system for conserved densities.

My research concerns how the equations of hydrodynamics pop up in an apparently completely unrelated setting: in the study of the long wavelength dynamics of black holes governed by Einstein’s equations with a negative cosmological constant.

Einstein’s gravitational equations describe the dynamics of the geometry of spacetime. The ripples of spacetime (gravitational waves) have interesting dynamics even in the absence of any matter. For most of this article, I will be referring to Einstein’s equations in the absence of matter.

## Using Protein Sequences to Predict Structure

### By Lucy Colwell

*How do proteins self-assemble into functional molecules?*

Proteins are typically cited as the molecules that enable life; the word protein stems from the Greek *proteois* meaning “primary,” “in the lead,” or “standing in front.” Living systems are made up of a vast array of different proteins. There are around 50,000 different proteins encoded in the human genome, and in a single cell there may be as many as 20,000,000 copies of a single protein.^{1}

Each protein provides a fascinating example of a self-organizing system. The molecule is assembled as a chain of amino acid building blocks, which are bonded together by peptide bonds to form a linear polymer. Once synthesized, this polymer spontaneously self-assembles into the correct and highly ordered three-dimensional structure required for function. This ability to self-assemble is remarkable—each linear polypeptide chain is highly disorganized, and has the potential to adopt an array of conformations so vast that we cannot enumerate them, yet within less than a second a typical protein spontaneously assumes the correct, highly ordered three-dimensional structure required for function. The identity and order of the amino acids that make up this polypeptide, that is the protein sequence, typically contain all the information necessary to specify the folded functional molecule.^{2}

## What's Next?

*New physics suggests a profound conceptual revolution that will change our view of the world.*

The following excerpts are drawn from Professor Nathan Seiberg’s public lecture “What’s Next?” available at https://video.ias.edu/seiberg-2013/.

I do not know what the future will bring. I guess nobody knows; and we do not know what will be discovered, either experimentally or theoretically, and that’s actually one of the reasons we perform experiments. If we knew for sure what the outcomes of the experiment would be, there would be no reason to perform the experiment. This is also the reason scientific research is exciting. It’s exciting because we’re constantly surprised either because an experiment has an unexpected outcome or theoretically someone comes up with a new insight. . . .

We are in an unusual and unprecedented situation in physics. We have two Standard Models. The Standard Model of particle physics describes the shortest distances and the Standard Model of cosmology describes the longest distances in the universe. These models work extremely well over the range of distances for which they were designed to work. However, there are excellent arguments that this story is not complete, and there must be new physics beyond these models. . . .