A Crisis in Cosmology
(ASIAA Science Highlight released on 24th. October, 2019)
A group of astronomers led by University of California, Davis has obtained new data that suggest the universe is expanding more rapidly than predicted. The study comes on the heels of a hot debate over just how fast the universe is ballooning; measurements thus far are in disagreement.
Monthly Notices of the Royal Astronomical Society journal just publish their results. The first author, Geoff Chih-Fan Chen (陳之藩) , was a master student in ASIAA/NTU and is now planning to graduate from the Ph.D. program at UC Davis next year.
Chen mentioned that when he started the master program, his advisors, Sherry Suyu (蘇游瑄, now is Max Planck Research Group Leader at the MPA, an Assistant Professor at the TUM, and also a Visiting Scholar at the ASIAA) and Prof. Tzihong Chiueh（闕志鴻）suggested an interesting but challenging research topic: using Keck AO imaging of strong lensing time-delay lenses, obtained with Keck telescope, to measure the Hubble constant. "I never expected that I would be so into this topic even until now" Chen says, "Our latest results show that the Hubble constant measured by strong lensing time-delay with AO data is faster than the prediction from Plank results assuming LCDM model and our value is also consistent with the results from Adam Reiss, who use Type Ia supernova with the Cepheid distance ladder to measure the Hubble constant". Chen also says, "When two independent methodologies obtain a consistent value, more and more top-notch physicists and cosmologists start to believe that instead of having systematic issues in the measurements, there is really a crisis in cosmology."
The team’s new measurement of the Hubble Constant, or the expansion rate of the universe, involved a different method. They used NASA’s Hubble Space Telescope (HST) in combination with W. M. Keck Observatory’s Adaptive Optics (AO) system to observe three gravitationally-lensed systems. This is the first time ground-based AO technology has been used to obtain the Hubble Constant.
“When I first started working on this problem more than 20 years ago, the available instrumentation limited the amount of useful data that you could get out of the observations,” says co-author Chris Fassnacht, Professor of Physics at UC Davis. “In this project, we are using Keck Observatory’s AO for the first time in the full analysis. I have felt for many years that AO observations could contribute a lot to this effort.”
To rule out any bias, the team conducted a blind analysis; during the processing, they kept the final answer hidden from even themselves until they were convinced that they had addressed as many possible sources of error as they could think of. This prevented them from making any adjustments to get to the "correct" value, avoiding confirmation bias.
“When we thought that we had taken care of all possible problems with the analysis, we unblind the answer with the rule that we have to publish whatever value that we find, even if it's crazy. It's always a tense and exciting moment,” says lead author Geoff Chen, the graduate student at the UC Davis Physics Department.
The unblinding revealed a value that is consistent with Hubble Constant measurements taken from observations of “local” objects close to Earth, such as nearby Type Ia supernovae or gravitationally-lensed systems; Chen’s team used the latter in their blind analysis.
The team’s results add to growing evidence that there is a problem with the standard model of cosmology, which shows the universe was expanding very fast early in its history, then the expansion slowed down due to gravitational pull of dark matter, and now the expansion is speeding up again due to dark energy, a mysterious force.
This model of the expansion history of the universe is assembled using traditional Hubble Constant measurements, which are taken from “distant” observations of the cosmic microwave background (CMB) - leftover radiation from the Big Bang when the universe began 13.8 billion years ago.
An artist's depiction of the standard cosmological model. IMAGE CREDIT: BICEP2 COLLABORATION/CERN/NASA
Recently, many groups began using varying techniques and studying different parts of the universe to obtain the Hubble Constant and found that the value obtained from “local” versus "distant" observations disagree.
“Therein lies the crisis in cosmology,” says Suyu. “a difference in the Hubble constant between early and late-time universe means that there is something missing in our current standard model. For example, it could be exotic dark energy, or a new relativistic particle, or some other new physics yet to be discovered.”
Using Keck Observatory’s AO system with the Near-Infrared Camera, second generation (NIRC2) instrument on the Keck II telescope, Chen and his team obtained local measurements of three well-known lensed quasar systems: PG1115+ 080, HE0435-1223, and RXJ1131-1231.
Quasars are the brightest, most distant objects in the universe with flickering light-speed jets that are powered by a supermassive black hole ravenously eating material surrounding it.
Though quasars are extremely far way, astronomers are able to detect them through gravitational lensing, a phenomenon that acts as nature’s magnifying glass. When a sufficiently massive galaxy closer to Earth gets in the way of light from a very distant quasar, the galaxy can act as a lens; its gravitational field warps space itself, bending the background quasar’s light into multiple images and making it look extra bright.
At times, the brightness of the quasar flickers, and since each image corresponds to a slightly different path length from quasar to telescope, the flickers appear at slightly different times for each image – they don’t all arrive on Earth at the same time.
With HE0435-1223, PG1115+ 080, and RXJ1131-1231, the team carefully measured those time delays, which are inversely proportional to the value of the Hubble Constant. This allows astronomers to decode the light from these distant quasars and gather information about how much the universe has expanded during the time the light has been on its way.
Multiple lensed quasar images of HE0435-1223 (left), PG1115+ 080 (center), and RXJ1131-1231 (right). IMAGE CREDIT: G. CHEN, C. FASSNACHT, UC DAVIS
“One of the most important ingredients in using gravitational lensing to measure the Hubble Constant is sensitive and high-resolution imaging,” said Chen. “Up until now, the best lens-based Hubble Constant measurements all involved using data from HST. When we unblinded we found two things. First thing is that we had consistent values with previous measurements that were based on HST data, proving that AO data can provide a powerful alternative to HST data in the future. Second thing is that combining the AO and HST data gave a more precise result.”
“If more measurements continue to show the same results, we will change our understanding of the universe and have a deeper understanding of it.”
“A SHARP view of H0LiCOW: H0 from three time-delay gravitational lens systems with adaptive optics imaging,” Geoff C-F Chen et al., 2019 Sep. 12, Monthly Notices of the Royal Astronomical Society [https://doi.org/10.1093/mnras/stz2547, preprint: https://arxiv.org/abs/1907.02533].
Geoff C-F Chen, Christopher D Fassnacht, Sherry H Suyu, Cristian E Rusu, James H H Chan, Kenneth C Wong, Matthew W Auger, Stefan Hilbert, Vivien Bonvin, Simon Birrer, Martin Millon, Léon V E Koopmans, David J Lagattuta, John P McKean, Simona Vegetti, Frederic Courbin, Xuheng Ding, Aleksi Halkola, Inh Jee, Anowar J Shajib, Dominique Sluse, Alessandro Sonnenfeld, Tommaso Treu
An HST and ESA Joint Press Release in 2017: Cosmic lenses support finding on faster than expected expansion of the Universe
H0LiCOW project website: https://shsuyu.github.io/H0LiCOW/site/
Geoff Chih-Fan Chen, email: firstname.lastname@example.org