CHIME | ACT | Other

Canadian Hydrogen Intensity Mapping Experiment (CHIME)

Photograph of the CHIME Pathfinder.
The CHIME Pathfinder. (Credit: Andre Recnik)

One of the biggest puzzles in contemporary cosmology is why the universe is accelerating in its expansion. For almost a century it has been evident that there is indeed cosmic expansion, but given the attractive force of gravity, it was long assumed that the rate of growth must be slowing down. However, since the late 1990's, it has become clear that expansion has been speeding up for the last few billion years. Something is counteracting gravity's attraction. Typically, this mysterious force is called ‘dark energy’, but this is little more than a name for our ignorance. While Einstein's theory of gravity allows for a ‘cosmological constant’ that could explain the accelerated expansion, observations to-date do not allow us to distinguish between this hypothesis and the possibility that new, previously undiscovered physics is responsible.

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a new radio observatory in the Okanagan Valley of British Columbia that is designed to shed light on the nature of dark energy. In its final form, it will consist of four, 100-metre-long and 20-metre-wide cylinders, populated with a total of 1024 dual-polarisation radio antennas. Using this huge collecting area—ten thousand square metres—CHIME should be able to measure the radio waves emitted by billions of years ago by the clouds of hydrogen dispersed throughout distant galaxies. This signal is an excellent proxy for the distribution of matter at a given epoch in cosmological history: in particular, it should be possible to detect a higher-than-average density of matter on physical scales of about 500 million light years. We know this length scale quite precisely, since it was fixed very early on in the universe's history and can be calculated from the well-understood physics of the sound waves in the dense plasma which emerged from the big bang. However, the observed scale on the sky depends on the history of cosmic expansion between us and the emitting hydrogen. Thus, it is in principle possible for CHIME to measure how the rate of expansion has been evolving over a significant fraction of the universe's history.

Such a measurement is, however, difficult. The size of the signal emitted by cosmic hydrogen is so small that the amount of energy it will impart to CHIME in one year is roughly comparable to the energy it takes to lift a paperclip one millimetre off the ground. Our own galaxy emits radio waves that can be confused with those we are interested in measuring, requiring careful modelling and removal of this source of ‘contamination’. And CHIME will push current computational power to the brink, since it will be digitising and processing two trillion samples per second. In order to figure out how to address these and other challenges, we have built a ‘pathfinder’, consisting of two cylinders, each 37 metres in length and populated by a total of 128 antennas. Not is the pathfinder helping us to develop hardware, software and know-how to operate full CHIME, but it itself should be capable of making cutting-edge maps of cosmic structure which can begin refining our knowledge of the expansion history of the universe.

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The Atacama Cosmology Telescope (ACT)

The Atacama Cosmology Telescope
The Atacama Cosmology Telescope.

Long before stars or galaxies had formed, the universe was a hot, dense gas that glowed at a few thousand degrees. Today, we can observe the light from this primordial glow, called the cosmic microwave background (CMB) radiation. Since it has come to us through a universe that has expanded in size by a factor of over a thousand, the wavelength of the light has been stretched from the visible part of the spectrum to the microwave—on the order of millimetres. Observations of the CMB over the past two decades have provided solid support for the Big Bang theory while also yielding a surprising amount of information about the early universe and about the make-up of the universe on its largest scales. On small scales, the CMB not only bears a record of the early universe, but also of the more recent universe, since its light has passed through clusters of galaxies and other large structures which leave their own distinctive imprints on the primordial signal.

The Atacama Cosmology Telescope (ACT) is a six-metre telescope on Cerro Toco, in the mountainous desert of northern Chile, dedicated to making high-resolution measurements of the CMB. Observations made with ACT from 2007 to 2010 have made significant contributions to our understanding of cosmology. We have made precise and detailed maps of the CMB which have contributed to tightening the precision on many of the fundamental parameters that characterise the universe, such as the running of the spectral index, the number of neutrinos and the amount of primordial helium, and have placed constraints on the state of the very early universe. We were the first group to detect gravitational lensing of the CMB by intervening cosmic structure, to provide evidence for dark energy using only the CMB and statistically to measure the motion of galaxy clusters relative to the expansion of the universe via the kinematic Sunyaev-Zeldovich effect. We have detected many clusters of galaxies (including a remarkably huge one) and done numerous follow-up studies of their properties. We have measured the microwave brightnesses of distant point sources and determined some of their important statistical properties.

The ACT collaboration hopes to add to these achievements with a major upgrade of the telescope's camera. This new receiver, ACTpol, was installed on the telescope in the austral winter of 2013. It is not only significantly more sensitive than the original receiver, but it is also capable of measuring polarisation of the CMB. These new features will make the telescope even better at measuring the properties of the early universe. ACTpol should be capable of measuring the number of neutrino species that were present in the early universe and continue the search for new galaxy clusters. It will be an excellent instrument for measuring the lensing of the CMB by cosmic structure, which can help us better understand the nature of dark energy.

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Other Projects

Ephemeris Library

A few years ago, I discovered that there were no readily-available C libraries for using solar-system ephemerides and reducing to observing coordinates such as right ascension and declination. (An ephemeris (pl., ephemerides) is a table of positions of heavenly bodies, such as planets, at a series of discrete times, from which one precisely can interpolate their position at any given time.) My solution was to take some ancient but accurate code and transform it into a full-fledged C library (with python bindings included). The library is called aephem and can be downloaded here.

I try and keep this library somewhat up-to-date, and am slowly adding new features. See the aephem website for more information or to download it.

Creation ex Nihilo

I have been collaborating with a group of theologians on a project exploring the doctrine of creation from nothing (CEN) in the light of modern research. My own role has been to articulate the state of contemporary physical cosmology to give context to the metaphysical claims of CEN. In addition to attending to the results of natural science, the project also seeks to integrate modern progress in Biblical studies, patristics, Jewish scholarship and systematic theology. Most recently there was a conference at Notre Dame University on the subject.

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