# Surprises from the quantum to the cosmos

Day two of the Convergence conference started with a dive into the details of quantum condensed matter.

By David Harris

## From Quantum...

Over the past 20 years, the coldest material in the universe, Bose Einstein Condensates, have been used in the lab in increasingly sophisticated ways beyond just making them in the first place. Immanuel Bloch described how lattices of these globs of cold atoms can be used for a wide variety of applications, including the most precise clocks ever made, which are accurate to one second in the 13.7 billion year age of the universe.

Among the intriguing possibilities for these cold atom lattices is using them as a test bed for a range of theoretical ideas. For example, the phase transition that occurs as atoms are cooled down to a quantum mechanical state can reveal details of the interface between the classical and quantum worlds. Bloch says that there are many surprises to come in these experiments, especially in non-equilibrium dynamics, because there isn’t yet enough theory to predict what will happen in many experimental configurations.

Subir Sachdev continued the theme of quantum matter, introducing three types: ordinary quantum matter, which merely obeys the strange laws of quantum physics; topological quantum matter, in which matter is quantum entangled with other matter at a distance in a way that is dependent on the topology of the material, leading to surprising electronic effects; and strange metals, which exist as materials that would superconduct at low temperatures but which are kept above the transition temperature to superconducting.

These strange metals can be used as analog systems to explore various systems in cosmology and high-energy physics. For example, the materials can be made to behave mathematically identically to black holes, and therefore potentially be used as a test case for theories about black hole behaviour.

Neal Weiner gave a whirlwind tour of the world of dark matter. Perhaps surprising to some people, he showed how the study of dark matter is still wide open, with no clear indications of what it is. Dark matter is being sought by the Fermi Gamma-ray Space Telescope, the Large Hadron Collider, underground “direct detection” experiments, and astronomical surveys of large-scale structure that look for deviations from the standard cosmological model of the universe. All of these have the potential to find dark matter or they could all come up empty.

Axions received a bit more attention at the Convergence conference as a well-motivated candidate for dark matter and Weiner showed how the ADMX experiments will cover “a good chunk” of the space of possible characteristics of dark matter. Weiner emphasized that it is important to think in general terms about dark matter because conventional wisdom has been wrong before. In particular, the assumption that dark matter is relatively simple is appealing but not very well motivated. After all, and as Weiner entertainingly demonstrated, the existing model/understanding of visible matter is much more complex than most models of dark matter, despite the fact that dark matter is much more prevalent in the universe than visible matter.

## ...To Cosmos

[Bad Link: Plugin Not Found]Avery Broderick surprised some by showing how the Event Horizon Telescope “is not a proposal, not a future experiment. It is happening now.” The telescope is actually based on collecting data from the archives and current and future observations of a set of telescopes spread around the world. It will provide the best test of general relativity so far and actually provide images of the event horizon around a black hole. Due to the radiation emitted around black holes, they are not anywhere near as black as their name suggests, and the data are already providing precise information about the structure and nature of the space around black holes. Perhaps most excitingly, the Event Horizon Telescope can do spacetime tomography. In other words, it can measure the shape of spacetime in regions of strong gravity. Broderick convincingly announced the beginning of “the era of empirical strong gravity research.”

Kendrick Smith provoked some of the strongest discussion of the conference so far with his statements and predictions about what will be discovered in the future about cosmology. He said that the constraints on gravitational waves are about to get “super interesting” and could help winnow down the number of potential cosmological models.

Smith’s dream is to reduce the vast number of inflationary models of the early universe and the competing non-inflationary models down to one, or at least a very small number, of models. It was the topic of inflation that worked up the audience, with some differences of opinion about whether inflationary models are in fact scientific models at all, seeing as they can be tuned to match almost any observation and can’t easily be falsified. Smith predicted that in 30 years, observations will still be consistent with several possibilities for the early universe, including inflation, OR inflation will have been observed and its alternatives ruled out. However he believes that inflation will not be ruled out with one of its alternatives observed, to the consternation of the proponents - present in the room - of some of those alternatives.

Smith furthermore predicted that we are at the dawn of the “21-cm era of cosmology” referring to the wavelength of light coming from hydrogen atoms, the most common atom in the universe. He said he believes “there is no fundamental limit to how well we might measure most cosmological parameters” in this era. In fact, physicists “can keep adding zeros to measurements as long as we’re willing to keep developing detector technology.”

Sara Seager wrapped up the day’s technical talks with a discussion of extra-solar planets, enthusing about the wealth of exciting questions to ask. In fact, some exoplanet problems are going neglected because there is too much other exciting work going on in the field. In her summary of the state of observations, she presented the takeaway message that small planets (about twice the size of Earth) are nearly 10 times as common as Jupiter-sized planets, which is a big challenge for planet formation theories.

One of the most exciting areas of study, according to Seager, is the search for possible life on exoplanets. That study is being conducted by searching for the spectral signs of hundreds of biosignature gases. So far, about 20 planets are known to be in their stars’ habitable zones, neither too hot nor too cold to sustain life.

Finishing up her talk, Seager outlined a bold plan to launch a “starshade,” a flower-petal-shaped sun-blocking shield into space. It would be positioned thousands of kilometres beyond a telescope, eclipsing a distant star that had surrounding exoplanets. That way the bright star would be occluded while the exoplanet would be visible.

As the conference has progressed, scientists seem to have been more willing to speculate and make predictions about the future of physics, although all in the same spirit of trying to work beyond the successful simplicity of theoretical models currently in use. Tomorrow wraps up with a talk about the 100th anniversary of the work of Emmy Noether, and a panel discussion that is sure to be lively.

David Harris is a theoretical physicist turned science journalist and communicator. He is the on-site rapporteur for the Convergence conference at Perimeter Institute. Follow him on Twitter.

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"As the conference has progressed, scientists seem to have been more willing to speculate more, and make predictions about the future of physics."