Beautiful but strange: The dark side of cosmology (Science)

By Catherine Zandonella, Office of the Dean for Research

It’s a beautiful theory: the standard model of cosmology describes the universe using just six parameters. But it is also strange. The model predicts that dark matter and dark energy – two mysterious entities that have never been detected — make up 95% of the universe, leaving only 5% composed of the ordinary matter so essential to our existence.

In an article in this week’s Science, Princeton astrophysicist David Spergel reviews how cosmologists came to be certain that we are surrounded by matter and energy that we cannot see. Observations of galaxies, supernovae, and the universe’s temperature, among other things, have led researchers to conclude that the universe is mostly uniform and flat, but is expanding due to a puzzling phenomenon called dark energy. The rate of expansion is increasing over time, counteracting the attractive force of gravity. This last observation, says Spergel, implies that if you throw a ball upward you will see it start to accelerate away from you.

The components of our universe
The components of our universe. Dark energy comprises 69% of the mass energy density of the universe, dark matter comprises 25%, and “ordinary” atomic matter makes up 5%. Three types of neutrinos make up at least 0.1%, the cosmic background radiation makes up 0.01%, and black holes comprise at least 0.005%. (Source: Science/AAAS)

A number of experiments to detect dark matter and dark energy are underway, and some researchers have already claimed to have found particles of dark matter, although the results are controversial. New findings expected in the coming years from the Large Hadron Collider, the world’s most powerful particle accelerator, could provide evidence for a proposed theory, supersymmetry, that could explain the dark particles.

But explaining dark energy, and why the universe is accelerating, is a tougher problem. Over the next decade, powerful telescopes will come online to map the structure of the universe and trace the distribution of matter over the past 10 billion years, providing new insights into the source of cosmic acceleration.

Yet observations alone are probably not enough, according to Spergel. A full understanding will require new ideas in physics, perhaps even a new theory of gravity, possibly including extra dimensions, Spergel writes. “We will likely need a new idea as profound as general relativity to explain these mysteries.”

When that happens, our understanding of the dark side of cosmology will no longer accelerate away from us.

Read the article

Citation: Spergel, David. The dark side of cosmology: Dark matter and dark energy. Science, 6 March 2015: Vol. 347 no. 6226 pp. 1100-1102 DOI: 10.1126/science.aaa0980.

–David Spergel is the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation, a professor of astrophysical sciences, and chair of Princeton’s Department of Astrophysical Sciences. His research is supported by the National Science Foundation and NASA.

Pennies reveal new insights on the nature of randomness (PNAS)

By Tien Nguyen, Department of Chemistry

The concept of randomness appears across scientific disciplines, from materials science to molecular biology. Now, theoretical chemists at Princeton have challenged traditional interpretations of randomness by computationally generating random and mechanically rigid arrangements of two-dimensional hard disks, such as pennies, for the first time.

‘It’s amazing that something so simple as the packing of pennies can reveal to us deep ideas about the meaning of randomness or disorder,” said Salvatore Torquato, professor of chemistry at Princeton and principal investigator of the report published on December 30 in the journal Proceedings of the National Academy of Sciences.

In two dimensions, conventional wisdom held that the most random arrangements of pennies were those most likely to form upon repeated packing, or in other words, most “entropically” favored. But when a group of pennies are rapidly compressed, the most probable states are actually highly ordered with small imperfections—called a polycrystalline state.

“We’re saying that school of thought is wrong because you can find much lower density states that have a high degree of disorder, even if they are not seen in typical experiments,” Torquato said.

Torquato and coworkers proposed that randomness should be judged from the disorder of a single state as opposed to many states. “It’s a new way of searching for randomness,” said Morrel Cohen, a senior scholar at Princeton and the editor assigned to the article.

Using a computer algorithm, the researchers produced so-called maximally random, jammed (rigid) states as defined by a set of “order metrics.” These measurements reflect features of a single configuration, such as the fluctuations of density within a system and the extent to which one penny’s position can be used to predict another’s.

The algorithm generated random states that have never been seen before in systems with up to approximately 200 disks. Theoretically, these maximally random states should exist for even larger systems, but are beyond the computational limits of the program.

These findings hold promise especially for the physics and chemistry of surfaces. Randomly dispersed patterns can be relayed to a 3D printer to create materials with unique properties. This may be desirable in photonics—analogous to electronics, but with photons instead of electrons—where the orientation of particles affects light’s ability to travel through a material.

This work also provides a tool for measuring degrees of order that may be applied to broadly to other fields. For example, the degree of disorder in the spatial distribution of cancer cells versus healthy cells could be measured and compared for possible biological links. The next challenge in this line of research will be for experimentalists to replicate these findings in the laboratory.

Read the article.

Atkinson, S.; Stillinger, F. H.; Torquato, S. “Existence of isostatic, maximally random jammed monodisperse hard-disk packings,” Proc. Natl. Acad. Sci., 2014, 111, 18436.

This work was supported in part by the National Science Foundation under Grants DMR- 0820341 and DMS-1211087. This work was partially supported by Simons Foundation Grant in Theoretical Physics 231015.