Astronomers regularly observe gravitational waves (GW)—ripples in space and time—that are caused by pairs of black holes merging into one. Einstein’s theory of gravity predicts that GW, which squeeze and stretch space as they pass, will permanently distort space, leaving a ‘memory’ of the wave behind. However, this memory effect has not yet been detected as it’s extremely small, leaving the faintest traces.
Researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University have finally developed a method to search and detect GW memory. Led by OzGrav PhD student Moritz Huebner, the recently published paper explains the tricky conquest of searching for memory by analysing data from numerous different observations. Huebner will be presenting these results at the Australian National Institute for Theoretical Astrophysics (ANITA) in Canberra this Thursday 6 February 2020.
The scientific models expect memory to leave an extremely faint trace on the detectors which is far smaller than the waves from the black hole collision itself. Therefore, data needs to be combined from many different gravitational wave events. To do this, the team used some of the most precise GW and memory models developed from the study of black hole mergers.
‘Our algorithms carefully comb through the data and measure the exact evidence for the existence of GW memory,’ said Huebner.
For each individual observation, this painstaking method can take hundreds of hours on a normal computer chip to explore all the possibilities of how a GW signal came about—this prompted the researchers to focus on fine-tuning the setting to reduce the amount of computing hours without compromising the search. So far, the results of the search applied to the first ten black-hole collisions—detected by LIGO and Virgo between 2015 and 2017—have proven inconclusive. LIGO and Virgo are not yet sensitive enough to make any statements about GW memory.
So, will we ever be able to detect memory?
‘Thankfully, we can now use data from the first ten black-hole collisions and have a decent idea of how many observable GW events there will be in the future. We can also calculate how much evidence of memory can be detected in each event,’ said Huebner.
Throughout the study, the researchers also discovered that their new search method must take data from approximately 2000 black hole mergers to detect memory. While this might sound implausible, the team expects to hit this number by the mid-2020s.
Plus, LIGO and Virgo are continuously being upgraded and have seen more than 40 mergers since April 2019, when the third observation run started. With further technological advances and the Japanese KAGRA observatory soon coming online, the team is confident that they’ll detect multiple binaries every day which will finally lead to revealing GW memory.
Link to journal: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.101.023011
Scientists from the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) reveal the eccentricity of binary black holes: the shape of the orbit formed when two black holes fall into a dance as they spiral towards each other and eventually collide. While the most common orbit is thought to be circular, about one in 20 are in egg-shaped eccentric orbits, which can indicate completely different binary life histories.
Since the first detection of gravitational waves (GW) in September 2015, LIGO and its European counterpart Virgo have published the discovery of ten merging black-hole binaries. The latest run has already uncovered more than 30 new detections, with more forecast by April 2020.
OzGrav PhD student (and first author) Isobel Romero-Shaw recently published a study on the origins of GW 190425 – an event which was only announced this month (January 2020) by the LIGO/Virgo collaboration.
The GW signals provide a wealth of information about the pre-merger binaries; however, no one has yet deciphered how these black holes pair up in the first place.
New research, published in the journal Monthly Notices of the Royal Astronomical Society, reveals an important clue to how these black hole binaries are formed, how long they’ve been ‘together’ and what happens when they finally collide.
The study, led by Romero-Shaw, OzGrav Chief Investigator Eric Thrane and Associate Investigator Paul Lasky—all from Monash University—looked at data from the first and second rounds of observation of LIGO and Virgo, in particular, the ten black hole collisions that these two observation runs confirmed. They found that the orbits of all ten of these systems were remarkably circular, which is consistent with the expectation that about one in 20 orbits are not.
Artist’s depiction of two black holes falling into a dance as they spiral towards each other and eventually collide. Credit: Isobel Romero-Shaw
The current LIGO/Virgo run has already detected more than 30 additional collision signals. According to Romero-Shaw, the large amount of data coming from the third observing run ‘will mean we are much more likely to see eccentric collisions of black holes, which will give us real insight into how these systems form’.
According to Thrane, the more common circular orbits come from black holes who have been together from when they were garden-variety stars before they exploded and became black holes. Thrane explains: ‘These binaries are like siblings if you like. They grew up together and their orbit is circular’.
Eccentric orbits occur when black holes fall under each other’s gravitational influence by chance as they are zipping around galaxies. ‘These are more like adults who meet later in life and pair up. Their orbital relationship is more interesting -- much like in life,’ he added.
Importantly, when these two objects collide, the shape of their orbit means their gravitational-wave signal looks different. These detected explosions can now be used to retrospectively study the objects that collided.
Lasky said that the current LIGO and Virgo observing run is detecting ‘large numbers of these binaries and by April 2020—when the run finishes—we will have a far greater insight into what these events mean’.
VIDEO EXPLAINER: https://www.youtube.com/watch?v=LmW4Hd4sJvg
Credit: Johan Samsing