Symposium Brief: Untangling the Cosmos
On May 17, 2017, CIFAR, in partnership with the Ontario Science Centre, held a day-long symposium called “Untangling the Cosmos: How Research is Changing our Understanding of the Universe.” The event featured leading Canadian and international researchers in astronomy and cosmology — both theoreticians and experimentalists — discussing the deepest questions in their fields as well as their latest discoveries.
Moderated by Canadian science writer and broadcaster Jay Ingram, the event included presentations from: Dick Bond, CIFAR Fellow, Cosmology and Gravity Program and Professor at the University of Toronto and the Canadian Institute for Theoretical Astrophysics; Gil Holder, CIFAR Fellow, Cosmology and Gravity Program and Professor at the University of Illinois, Urbana-Champaign; Matt Dobbs, CIFAR Fellow, Cosmology and Gravity Program and Professor at McGill University; Daryl Haggard, Professor at McGill University; Lars Bildsten, CIFAR Fellow, Cosmology and Gravity Program and Professor at the Kavli Institute for Theoretical Physics and the Department of Physics, University of California, Santa Barbara; Vicky Kaspi, CIFAR Fellow, Cosmology and Gravity Program and Professor at McGill University; Luis Lehner, CIFAR Fellow, Cosmology and Gravity Program and Professor at the Perimeter Institute for Theoretical Astrophysics; Harald Pfeiffer, CIFAR Fellow, Cosmology and Gravity Program and Professor at the University of Toronto and the Canadian Institute for Theoretical Astrophysics; Barth Netterfield, CIFAR Fellow, Cosmology and Gravity Program and Professor at the University of Toronto; Scott Ransom, CIFAR Fellow, Cosmology and Gravity Program and astronomer at the National Radio Astronomy Observatory; Mark Chen, CIFAR Fellow, Cosmology and Gravity Program and Professor at Queen’s University; Renée Hložek, Professor at the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics.
This report provides a summary of the key insights presented by these researchers, and of the group discussions led by Jay Ingram that followed.
() PANEL A: The Cosmic Background
A long time ago, the universe was a very simple place. But from its origins in the big bang a bit less than 14 billions years ago, the complexity that we see around us today came into being. The driving force for this evolution — and this increase in complexity — has been the force of gravity.
Only a small fraction of the universe is made up of ordinary, visible matter. We now know that an unseen form of matter, known as dark matter, makes up most of a galaxy’s mass (the dark matter outweighs ordinary matter by a ratio of more than five to one). We’ve also discovered that another entity, dark energy, is counteracting the force of gravity, pushing all of the galaxies away from one another. The best guess is that dark matter is made up of fundamental particles that are constantly passing through us at some 200 km per second. Here on Earth, dark energy has a negligible influence – but in space, it has a dominating effect, likely determining the fate of the universe. Billions of years from now, this extra “push” could lead to the “heat death” of the universe.
Today, the universe is richly structured. Galaxies group together into clusters, and clusters clump together into superclusters. These in turn appear to form long stringy filaments, many tens of millions of light years in length, separated by enormous voids. With our best telescopes, however, we can see distant regions that appear to be devoid of galaxies. Because of the finite speed of light, our telescopes are effectively peering back in time – so far back that we can glimpse an era from before the formation of the first galaxies.
We can learn about the very early universe by studying the cosmic microwave background (CMB) — the faint microwave “echo” of the big bang. Photons from CMB have been traveling toward us since about 400,000 years after the big bang. The Planck spacecraft, launched in 2009, has produced extremely high-resolution maps of the CMB. These maps have been used to calculate the exact proportions of ordinary matter, dark matter, and dark energy in the universe. The maps also show very small inhomogeneities in the CMB — and computer simulations can be used to show how these tiny fluctuations eventually lead to the rich structure that we observe today. Despite tremendous advances in cosmology, there’s still a great deal we’re unsure about — especially regarding the first moments of the big bang.
Dark matter seems to be everywhere — but we can’t detect it directly. Because it doesn’t interact with electromagnetic radiation, dark matter is invisible; similarly, it doesn’t interact via any of the other known forces, with the exception of gravity.
Even though we can’t see dark energy, we can use it to study the universe. Because of its gravitational tug, dark matter bends light from more distant objects, as the light from those objects travels toward us. This effect, known as “gravitational lensing,” allows us to calculate the distribution of dark matter in the universe. Even the light coming to us from the CMB has been subject to this bending — which means that detailed studies of the CMB can also tell us how much dark matter is out there, and where it’s located. The South Pole Telescope will soon be producing more data that will aid in this investigation.
The Canadian Hydrogen Intensity Mapping Experiment (CHIME) will soon be mapping the entire sky at radio frequencies. The telescope, now nearing completion at the Dominion Radio Astrophysical Observatory in British Columbia, has no steerable dish — instead, it consists of five aluminum half-cylinders, running in parallel, all of them aimed straight up. As the Earth rotates, CHIME’s field of view will sweep across the entire sky, while sophisticated computer analysis will reveal the direction in the sky from which a particular signal originated.
One of the main goals for CHIME is to determine how the strength of dark energy may have changed over time — which in turn may allow physicist to figure out what dark matter actually is.
Insights From First Panel Discussion
Gravitational waves may allow us to peer back almost to the big bang itself. These elusive waves, discovered for the first time in 2015, have opened a new window on the universe. Although detecting so-called primordial gravitational waves from the big bang may prove difficult, there’s a good chance of detecting the influence of gravitational waves on the CMB, perhaps via the South Pole Telescope, Professor Dobbs said.
The expansion of the universe means that our view of the cosmos will change over time. Because of the finite speed of light, the proportion of the universe that astronomers will be able to study with their telescopes will get smaller, Professor Bond explained.
Other universes might exist — but there is no evidence for this, so far. Instead, it makes sense to think of the cosmos as a single, connected universe, even if some regions remain cut off from any possible observation, because of the finite speed of light, Professor Bond said in response to a question from the audience.
Cosmology is unlikely to directly affect anyone’s daily life — but it’s still worth doing. It’s a privilege to live in a society that values pure research enough to fund it, Professor Dobbs said, in response to a question from the audience. As well, scientific knowledge, which helps us understand out place in the universe, is valuable in its own right, just as art and literature are, added moderator Jay Ingram.
() PANEL B: Strange Objects
Black holes don’t suck. Although this is the popular conception of these esoteric objects, they do not in fact act like cosmic vacuum cleaners. Rather, matter near a black holes orbits around it and can form an “accretion disk.” Under the force of gravity, matter can fall from this disk into the black hole itself. When this happens, bursts of radiation are released, which is how black holes were originally detected. As well, the existence of a black hole can be inferred by observing the motion of nearby stars.
Black holes are, in a sense, simple. Physicists believe they only have a few properties, such as mass, spin (or angular momentum), and charge (for theoretical black holes only). A black hole is made of ordinary matter that just happens to be highly compressed; if our sun was shrunk into a ball about three kilometres across, it would become a black hole (that is, its escape velocity — the speed needed to escape its gravitational pull — would be greater than the speed of light).
So-called “supermassive” black holes can weigh as much as several million suns. One of them, known as Sagittarius A*, is located at the heart of our Milky Way galaxy. The Event Horizon Telescope, a globe-spanning array of radio telescopes that has recently begun collecting data, will give us our closest look yet at Sagittarius A*.
A supernova – an exploding star – occurs when a massive star exhausts its nuclear fuel supply, and explodes. A supernova occurs approximately once every second somewhere in our universe. In our own galaxy the rate is approximately once every hundred years. When a supernova happens, it can be as bright as an entire galaxy.
Survey telescopes, which scan the entire sky, can be used to detect supernovae. An important technique, known as “difference imaging,” involves comparing current images with archival images of galaxies. An object that is seen to be much brighter now compared to the archival image may be a supernova explosion.
The Zwicky Transient Facility (ZTF) will soon be aiding in the search for supernovae. The ZTF, located on Mount Palomar in California, can scan the entire sky every night in search of objects that rapidly change in brightness, such as supernovae and other transient phenomena such as variable stars and asteroids.
Fast Radio Bursts (FRBs) are one of the most mysterious, and exciting, recent discoveries in astronomy. First detected in 2007, FRBs are brief “blips” of energy, of unknown origin, lasting only a few milliseconds. Astronomers have been able to show that FRBs must be extremely distant, based on the “dispersion” of the radio signals they produce (that is, because of the delay between radio waves of various frequencies within each FRB signal). The first FRBs were detected using the Parkes Radio Telescope in Australia; more recently, they have been observed using other radio telescopes, such as the Arecibo dish in Puerto Rico and the Very Large Array (VLA) in New Mexico.
Astronomers don’t know what FRBs actually are. They might be exploding stars, or colliding neutron stars, or some other exotic astrophysical phenomenon. Recent observations with the Gemini telescope in Hawaii suggest that FRBs may be associated with faint “dwarf galaxies” — an unexpected finding.
They’re not necessarily one-off events: at least one FRBs repeats. The FRB detected in 2015 using the Arecibo telescope has a repeating signature — that is, it emits multiple bursts of energy (though not at regular intervals). These repetitions allowed for the FRB’s location in the sky to be pinpointed with great precision. But it is unclear if all FRBs repeat, or if there are two categories of FRB, only one of which repeats.
The CHIME telescope may soon shed some light on FRBs. Although designed for a different purpose, CHIME is almost ideally suited to detecting fleeting phenomena like FRBs. When it goes on-line, CHIME may detect a couple of dozen FRBs per day.
Insights From Second Panel Discussion
An FRB could, in principle, happen in our own galaxy. However, such a “local” FRB might be hard to detect against the backdrop of terrestrial “noise,” such as radio stations and microwave ovens, Professor Kaspi explained in response to a question from Jay Ingram.
If a supernova exploded close enough to earth, it could impact life on our planet. The possible effects of such a nearby supernova — say, within a few dozen light years – have been studied in detail, Professor Kaspi said in response to a question from an audience member. However, we know of no nearby stars that are in danger of exploding.
The Event Horizon Telescope will give astronomers the sharpest views yet of a black hole. Specifically, the EHT should allow us to see a black hole silhouetted against the accretion disk, Professor Haggard explained.
() PANEL C: Gravitational Waves
The idea of gravitational waves goes back to Einstein. His theory of gravity, known as the general theory of relativity, predicts that when a massive object is accelerated, it will produce ripples in the fabric of space-time which we call gravitational waves.
Gravitational waves are hard to detect, because gravity is such a weak force. Even the most violent astrophysical events, such as colliding black holes, cause only very slight ripples in space-time. As a gravitational wave passes by, space literally gets compressed or stretched – but only by a very small amount.
Gravitational wave astronomy is still in its infancy. The LIGO observatory finally detected gravitational waves (from a pair of colliding black holes) in 2015. But new observations may allow us to glimpse the formation of black holes, or to peer into the big bang itself.
The Laser Interferometer Gravitational-wave Observatory (LIGO) was specially designed to detect these space-time ripples. Each LIGO detector is shaped like an “L,” with arms four kilometres long. Within each arm, laser beams are reflected back and forth between mirrors attached to weights. A passing gravitational wave causes a slight change in the distance the laser beam travels. The displacement is very slight – no more than 1/1000th of the size of an atomic nucleus. However, this was enough to create an interference pattern in the recorded laser light.
Having two identical detectors helps rule out a false signal. LIGO employs two detectors, one in Hanford, Washington, the other more than 3,000 km away in Livingston, Louisiana. When gravitational waves from a pair of colliding black holes passed by Earth in the fall of 2015, they triggered a tiny “blip” at both detectors.
Black holes are heavy, but small. The merger event observed in 2015 involved a black hole with 10 million times the earth’s mass, and another with 12 million times the earth’s mass. However, they’re small in size, having diameters of just a few hundred km. They were orbiting each other about ten times per second, before finally colliding and merging, releasing a burst of gravitational wave energy.
LIGO is just the beginning. Already, new gravitational wave detectors are being built in Europe, India, and Japan
The surface of the earth is a good place to do astronomy with visible light and with radio waves – but not for studying other wavelengths. Unfortunately, the earth’s atmosphere absorbs most infrared and ultraviolet light, as well as millimetre and sub-millimetre wavelengths and also high-energy radiation such as X-rays and gamma rays.
A balloon that can rise above the earth’s atmosphere can study these other wavelengths. At a cost far below that of an orbiting satellite, a balloon-borne probe can achieve many of the same results. Specially designed balloons can lift a 3500 kg payload to an altitude of 40 km, and can remain aloft for 50 days.
The latest such balloon mission is called “SPIDER.” SPIDER, which flew for the first time in 2015 over Antarctica, is searching for gravitational waves from the very early universe. The probe looks for distortions in the CMB created by gravitational waves. Data from SPIDER is currently being analyzed, with results expected soon; the probe is due to fly again in late 2018.
Insights From Third Panel Discussion
Theoretical models can be used to predict what SPIDER ought to see. In particular, we have models that predict the “angular power spectrum” of the distortions that ought to be observed in the CMB due to the effect of gravitational waves (i.e., the proportion of effects of various angular sizes), Professor Netterfield explained.
LIGO is particularly sensitive to black hole collisions. Because of the enormous energy associated with these collisions, such mergers happen relatively quickly, producing signals that can be detecting with LIGO, compared to events that have lower energy and progress more slowly, Professor Lehner said, in response to an audience question.
We might never “control” gravity. In science fiction films, we often see a spaceship hovering effortlessly above a planet. In practice, this seems impossible, because gravity – as far as we know – is always an attractive force, and never a repulsive one, Professor Pfeiffer said, in response to a question from the audience.
() PANEL D: The Extreme Universe
Pulsars are among the most exotic astrophysical objects in the universe. Discovered by Jocelyn Bell in the 1960s, pulsars are rapidly-spinning, ultra-dense stellar cores that have shed their outer layers in a supernovae explosion. They can contain as much mass as a solar system, compressed into a sphere the size of a city. Their surface temperature can be millions of Kelvins, and their surface gravity can be 100 billion times stronger than Earth’s.
Pulsars function like “cosmic lighthouses.” As a pulsar spins, it can periodically aim a beam of radio waves toward Earth. Pulsars have been measured to spin as rapidly as 700 rotations per second.
Their spin is highly regular, allowing them to be used as clocks. Measurements of pulsar spin rates are among the most precise measurements in all of physics. A spinning pulsar can be used to keep time just as well as, or even better than, an atomic clock. As a result, pulsars may play a vital role in cosmological observations in which precise timing is needed, such as tests of general relativity.
It might be possible to use pulsars to detect passing gravitational waves. If this line of research bears fruit, it could open a second window into the gravitational-wave universe
Astronomers usually look upward — but particle physicists can also study the cosmos from deep beneath the earth. At the SNOLAB facility near Sudbury, Ontario — original developed to study neutrinos — physicists are building the world’s largest dark matter detector. Known as DEAP-3600 (Dark Matter Experiment using Argon Pulse-shape), the detector will use 3.6 tons of liquid argon in an effort to snare dark matter particles. By locating the facility deep beneath the earth, cosmic rays, which would otherwise flood the detectors, can be filtered out.
At the moment, the nature of dark matter is unknown. Our best guess is that dark matter is made up of weakly-interacting massive particles; more speculatively, it might be made of exotic particles known as axions, or it could be linked to the existence of extra space-time dimensions. DEAP-3600 may provide an answer.
New kinds of telescopes and detectors are opening up new windows on the universe. These include the Large Synoptic Survey Telescope (LSST), a wide-field 8.4-metre telescope, located in Chile, which will scan the entire southern sky every few nights; the Atacama Cosmology Telescope, the South Pole Telescope, orbiting spacecraft such as Planck, and balloon-borne experiments such as SPIDER — all of which are studying the CMB; along with gravitational wave detectors such as LIGO and dark matter detectors like DEAP-3600.
The Universe is like a puzzle. It has many seemingly exotic components, such as dark matter and dark energy. However, thanks to innovative telescopes and detectors, and continuing theoretical investigation, the pieces of this puzzle are beginning to fall into place.
Insights From Fourth Panel Discussion
It’s a minority view — but we could be wrong about dark matter. Several theorists have suggested that dark matter might not exist; rather, some adjustment to Einstein’s theory of gravity may be needed. However, while such attempts to develop modified gravity theories are interesting, so far they’ve failed to explain all of the different kinds of observational evidence that point to the existence of dark matter, Professor Hložek said, in response to an audience question. As Professor Ransom pointed out, however, it’s clear that general relativity is in some sense incomplete, because of its failure to mesh with quantum mechanics. Thus, the quest for a more complete, unified theory continues.
All of today’s advanced telescopes and detectors rely on enormous computational power. The ability to handle “big data” has been an essential ingredient in all of today’s leading astronomy and cosmology experiments, Professor Ransom said, in response to an audience question.
International collaboration is propelling discovery. It’s also allowing scientists to cross-check their findings with colleagues from around the world. A recent example of this is when scientists from the BICEP gravitational wave experiment collaborated with those involved with the Planck spacecraft. As a result, the BICEP team realized that their initial claim — that they had detected the imprint of gravitational waves on the CMB — was wrong, Professor Hložek said, in response to an audience question. At the moment, the largest international astronomy project is the Square Kilometre Array (SKA), a multi- radio telescope project planned for Australia and South Africa; the array would be 50 times more sensitive than any other radio detector.