by Lori Ann White
The universe is full of mysteries that merit an exclamation or two of wonder and delight. Black holes, supernova explosions, planets around other stars, the thought that most of the matter surrounding us is some kind of stuff that we can’t detect—these are just a few of the cosmic marvels that warrant a “Wow!” or a “Neat!” or a “Gee whiz!”
Or even a “Holy cow!”
One group of scientists has found a way to permanently remind themselves of this.
The H0LiCOW collaboration just released news that they’ve measured what KIPAC and H0LiCOW collaboration member Phil Marshall calls “a key property of the universe”: the Hubble constant, which tells how fast the universe is expanding. According to their measurements, our universe is currently expanding at 71.9 km/s/Mpc, within about 3.8% accuracy, which means that each second, our universe is adding very close to 71.9 kilometers of space per megaparsec (a megaparsec is one million parsecs, and a parsec is about 3.3 light years) in every direction. This expansion is increasing, a phenomenon attributed (for now, at least) to the influence of a new component of the universe, dark energy.
What’s even wilder? A mismatch between their measurement and another made using a different technique could indicate new physics.
A key property of the universe
The Hubble constant (i.e. the Hubble parameter as determined at the current time in our Universe), known as H0 (say, “H-not”), is the key to this expansion, says Marshall.
“Our program here is to try to understand what’s making the expansion of the universe accelerate—what this dark energy is. So we’re interested in making more and more accurate measurements of H0.”
To make their most recent measurement, the H0LiCOW collaboration studied images produced by strong gravitational lenses: galaxies which are so massive that light from even more distant objects bends around them. Because strong gravitational lenses are generally not perfect spheres, the light can take four paths of different lengths to reach a telescope, resulting in multiple images (the illustration below does show a perfectly spherical lens, which results in two images).
Strong gravitational lensing: Bending of light from a background object by a massive galaxy cluster results in multiple images of the more distant object that appearing to be displaced from its actual location in the sky. (Credit: NASA/CXC/M.Weiss.)
The H0LiCOW collaboration takes advantage of this fact by studying lensed quasars, which are thought to be very early, far-distant galaxies with central supermassive black holes that power huge outflows of energy. The energy output of a quasar can vary over astoundingly over short time periods—months, days, even hours. And because the light from a lensed quasar takes paths of different lengths to reach us, each separate image of the quasar flickers at a different time. The different path lengths can be determined using basic geometry and knowledge of the speed of light.
“Our first ‘Holy cow!’ moment came when we realized this technique works!” Marshall says.
Thus far the collaboration has studied five such lenses. The H0 measurement is based on three lenses, while the other two are still being analyzed.
HE0435-1223, one of the lensed quasars studied by the H0LiCOW collaboration, is among the five best lensed quasars discovered to date. The foreground galaxy creates four almost evenly distributed images of the distant quasar around it. (Credit: ESA/Hubble, NASA, Suyu et al.)
A significant measurement
The collaboration’s accomplishment in measuring the constant is significant for several reasons, says Marshall.
“It’s the most accurate measurement ever made using this technique,” he says, “and it’s complementary to measurements using Cepheid variables [a type of star known as a kind of cosmic yardstick] and supernovae in nearby galaxies.” Having more than one method of measuring H0 is vital. “Cross-checks are very important to make sure of accuracy.”
The H0LiCOW H0 measurement is also important because it differs to an intriguing degree from another H0 measurement based on the cosmic microwave background, the relic light from the very beginning of the universe. This discrepancy could point to some fundamental physics we have yet to understand or take into account.
“The CMB research looks at certain cosmic parameters while strong lenses look at others,” Marshall says, “and the combination helps us tell the difference between a few different things that have had an on the expansion of the universe. One is dark energy; another is the total mass of neutrinos in the universe.”
As for why all of these different ways to measure H0 are necessary, think of the different experiments in cosmology as different colors of paint on a palette. The ability to combine colors results in a painting that’s richer and more accurate than trying to capture a scene with only primary pigments.
Removing observer bias
Finally, Marshall considers the H0LiCOW measurement important because the collaboration has used the technique of blind analysis to arrive at their figure. This technique, commonly used in particle physics but fairly new to astrophysics and cosmology, helps remove any influence that observer biases might have on their results.
“The first lens we analyzed wasn’t blind,” Marshall says, “but the second one was, because we realized how important it was to measure H0 blind.” It was a good thing, too. “The first two measurements were different enough to make us look harder for systematic errors.”
H0 for the third lens, also arrived at using blind analysis, ended up right in the middle, Marshall says. “That’s good news for strong lensing cosmology, because we would seem to be on the right track,” Marshall says.
He considers blind analysis such an important technique for preventing systematic errors that he’s helping host an upcoming KIPAC workshop at SLAC: Blind Analysis in High Stakes Survey Science: When, Why, and How? which will be held in mid-March, and be led by KIPAC fellow Elisabeth Krause.
Meanwhile, the H0LiCOW collaboration will keep refining the value of H0. “We’ll carry on doing blind analyses of strong lenses,” Marshall says. He’s looking forward to studying the new lenses that current and future surveys such as the Dark Energy Survey and and the Large Synoptic Survey Telescope survey will find.
“We’re closing in on H0,” Marshall says. “All of these cosmological parameters are interrelated. It boils down to understanding the entirety of the universe.”
The H0LiCOW collaboration consists of: S. H. Suyu (Max Planck Institute for Astrophysics, Germany; Academia Sinica Institute of Astronomy and Astrophysics, Taiwan; Technical University of Munich, Germany), V. Bonvin (Laboratory of Astrophysics, EPFL, Switzerland), F. Courbin (Laboratory of Astrophysics, EPFL, Switzerland), C. D. Fassnacht (University of California, Davis, USA), C. E. Rusu (University of California, Davis, USA), D. Sluse (STAR Institute, Belgium), T. Treu (University of California, Los Angeles, USA), K. C. Wong (National Astronomical Observatory of Japan, Japan; Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), M. W. Auger (University of Cambridge, UK), X. Ding (University of California, Los Angeles, USA; Beijing Normal University, China), S. Hilbert (Exzellenzcluster Universe, Germany; Ludwig-Maximilians-Universität, Munich, Germany), P. J. Marshall (Stanford University, USA), N. Rumbaugh (University of California, Davis, USA), A. Sonnenfeld (Kavli IPMU, the University of Tokyo, Japan; University of California, Los Angeles, USA; University of California, Santa Barbara, USA), M. Tewes (Argelander-Institut für Astronomie, Germany), O. Tihhonova (Laboratory of Astrophysics, EPFL, Switzerland), A. Agnello (ESO, Garching, Germany), R. D. Blandford (Stanford University, USA), G. C.-F. Chen (University of California, Davis, USA; Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), T. Collett (University of Portsmouth, UK), L. V. E. Koopmans (University of Groningen, The Netherlands), K. Liao (University of California, Los Angeles, USA), G. Meylan (Laboratory of Astrophysics, EPFL, Switzerland), C. Spiniello (INAF – Osservatorio Astronomico di Capodimonte, Italy; Max Planck Institute for Astrophysics, Garching, Germany) and A. Yıldırım (Max Planck Institute for Astrophysics, Garching, Germany)