UChicago researchers estimate, based on LIGO's rapid first discovery of a first neutron star collision, that they could have an…
UChicago researchers estimate, based on LIGO’s rapid first discovery of a first neutron star collision, that they could have an extremely precise measurement of the universe expansion rate within five to ten years. Credit: Robin Dienel / Carnegie Department of Science
Twenty years ago, researchers were shocked to realize that our universe not only expands, but it grows over time.
It was surprisingly difficult to determine the exact expansion rate, called the Hubble constant after the famous astronomer and UChicago alumni Edwin Hubble. Since then, researchers have used two methods to calculate the value, and they spell disturbing different results. But last year’s surprising capture of gravity waves emanating from a neutron star collision offered a third way to calculate the Hubble constant.
It was only a single data point from a collision, but in a new paper published October 1
7th in Nature three University of Chicago researchers estimate how fast researchers saw the first neutron star collision they could have a very accurate measurement of the Hubble constant within five to ten years.
“The Hubble Constant tells you the size and age of the universe, it has been a sacred grail since cosmology’s birth. Calculation of this with gravity waves can give us a whole new perspective on the universe,” says study author Daniel Holz, a UChicago professor in physics that co-authorized the first such calculation from the 2017 discovery. “The question is: When is the game change for cosmology?”
In 1929, Edwin Hubble announced that, due to his observations of galaxies beyond the Milky Way, they seemed to move away from us – and farther away from the Galaxy, the faster it was. This is a cornerstone of the Big Bang theory, and it started an almost century long search for the exact pace this occurs.
To calculate the pace of the universe, researchers need two numbers. One is the distance to one distant object, the other is how fast the object moves away from us because of the universe’s expansion. If you can see The one with a telescope, the second quantity is relatively easy to determine because the light you see when looking at a distant star is shifted in red as it decreases. Astronomers have used this trick to see how fast an object moves for more than a century. It’s like the Doppler effect, where a siren changes pitch as an ambulance pass.
“Key Issues in Calculations”  But getting an accurate measurement of the distance is much more difficult. Traditionally, astrophysics have used a technique called the cosmic distance steps, where the brightness of some variable stars and supernovae can be used to build a series of comparisons that reach the object in question. “The problem is that if you get under the surface there are many steps with many assumptions along the way,” says Holz.
Perhaps the supernuts used as markers are not as consistent as thoughts. Perhaps some types of supernova fail for others, or there is something unknown in our measurement of distances to nearby stars. “There is a lot of complicated astrophysics there that can throw readings in a number of ways,” he said.
The second major way to calculate the Hubble constant is to look at the cosmic microwave oven – the lightbulb created at the beginning of the universe, which is still weakly detectable. Although useful, this method also builds on assumptions about how the universe works.
Prof. Daniel Holz discusses the role of UChicago researchers in LIGO’s historical detection of gravity waves from two colliding neutron stars.
Surprisingly, even if researchers who make each calculation are convinced of the results they do not match. One says the universe expands almost 10 percent faster than the other. “This is a major issue in cosmology right now,” said the first author of the study, Hsin-Yu Chen, after a PhD student at UChicago and now a colleague with Harvard University’s Black Hole Initiative.
Then, the LIGO detectors discovered their first ripple in space-time fabric from the collision of two stars last year. This not only shook the observatory, but the area of the astronomer itself: Being able to both feel the gravity wave and see the light of the collision aftermath with a telescope gave the researchers a powerful new tool. “It was kind of embarrassment for wealth,” says Holz.
Gravitational waves offer a completely different way of calculating the Hubble constant. When two massive stars crash into each other, they send out ripples in the space of space that can be detected on Earth. By measuring that signal, researchers can get a signature of the mass and energy of the colliding stars. When comparing this reading with the power of gravity waves, they can deduce how far it is.
This measurement is cleaner and has fewer assumptions about the universe, which should make it more accurate, Holz said. Together with Scott Hughes at MIT, he suggested that this measurement be made with gravitational waves along with telescope readings in 2005. The only question is how often researchers can capture these events and how good data from them would be.
“It’s Just Getting More Interesting”
The paper predicts that when researchers have detected 25 readings from neutron star collisions, they will measure the universe’s expansion within an accuracy of 3 percent. With 200 readings, it narrows to 1 percent.
“It was a bit of a surprise to me when we got into the simulations,” Chen said. “It was clear that we could achieve precision, and we could reach it quickly.”
An exact new number for the Hubble constant would be fascinating regardless of the answer, the researchers said. For example, one possible cause of mismatch in the other two methods is that gravity’s nature may have changed over time. Reading can also throw light on dark energy, a mysterious force that is responsible for the universe’s expansion.
“With the collision we saw last year, we were lucky – it was close to us so it was relatively easy to find and analyze,” says Maya Fishbach, a UChicago PhD student and the other author of the newspaper. “Future detections will be far further away, but when we get the next generation telescope, we should also be able to find counterparts to these remote detections.”
The LIGO detectors are scheduled to launch a new observatory run in February 2019, associated with their Italian counterparts at VIRGO. Thanks to an upgrade, the detectors’ sensitivities will be much bigger and increase the number and distance of astronomical events they can retrieve.
“It will only be more interesting from here,” said Holz.
Gravitational waves provide a real pace of extra dimensions
Hsin-Yu Chen et al. A two percent Hubble constant measurement from standard sirens within five years, Nature (2018). DOI: 10,1038 / s41586-018-0606-0