David Gross, a particle physicist and former director of KITP was talking about the rate at which our universe is expanding. But Gross wasn’t worried about the expansion itself. We have already known for decades that the universe is collapsing exponentially because the celestial bodies surrounding our planet are constantly moving away from us and from each other. No, Gross was worried about math. To determine exactly how fast this cosmic drift is happening, scientists must calculate an important value called the Hubble constant — but even today, no one can agree on the answer. Thus, the astronomy community was permeated by a “crisis,” but it was a dilemma based on innovation. Since that tense conference, experts everywhere have clearly adjusted how they view the Hubble constant equations as an attempt to restore peace among the stars. And on Monday, one such team presented a highly unusual idea for solving the dispute, as described in a paper published Aug. 3 in the journal Physical Review Letters. Basically, astronomers from the University of Chicago believe that when black holes lurking in deep space collide with each other — as they sometimes do — the gravitational leviathans reverberate through the fabric of space and time that they can leave behind. traces of information vital to deciphering the Hubble constant. In the end, if scientists can figure out the true Hubble constant, they can also find answers to some really big questions about our universe, like: How did it evolve into the amazing realm we see today? What does it naturally consist of? What might it look like billions of years from now, long after humanity has ceased to exist and therefore cannot lay eyes on it?
Reading between the lines of spacetime
Every now and then, two supermassive black holes collide. This means that a pair of the most incomprehensibly massive objects in the universe are combining into an even more incomprehensibly massive object. When this happens, the merger sends ripples through the fabric of space and time — as coined by Albert Einstein’s general relativity — just as dropping a rock into a pond would send ripples through water. Animation of gravitational waves produced by a fast binary orbit. NASA Just four years before Gross and the other physicists hosted their stressful debate on the ongoing Hubble enigma, two powerful observatories managed to capture these black hole-induced ripples from down here on Earth. They are called the US Symbolometer Gravitational-Wave Observatory and the Italian Virgo Observatory. In recent years, both LIGO and Virgo have detected the ripples from nearly 100 pairs of black hole collisions, and these measurements may help us estimate the rate at which the universe is expanding, according to Daniel Holz, an astrophysicist at the University of Chicago and co-author of the new study. They may shed light on the Hubble constant. “If you took a black hole and put it earlier in the universe,” Holtz said in a press release, “the signal would change and look like a bigger black hole than it actually is.” This means that if a black hole collision were to occur in space (egress) and the signal travels for a long (long) time, the gravitational waves originating from the event would have been affected by the expansion of the universe since the event. If you think back to lake ripples, for example, dropping a rock into a lake usually creates tighter ripples right at the point of contact. But if you continue to watch these ripples extend outward, they become somewhat wider and blunter. So if we can somehow measure the changes in black hole collisional ripples, we might be able to understand the rate at which some of these changes occur. This would help us understand the rate at which the expansion of the universe could have affected them, and ultimately the rate at which the universe is legitimately expanding. “So we measure the masses of nearby black holes and understand their characteristics, and then we look further away and see how much these further ones seem to have shifted,” Jose María Ezquiaga, NASA Einstein Postdoctoral Fellow, Kavli Institute for Cosmological Physics. collaborator and co-author of the new study, said in the release. “That gives you a measure of the expansion of the universe.”
Is there a catch?
But there’s a small caveat — this technique, which the researchers call the “standardized siren” method, can’t be applied right now. In fact, LIGO and Virgo will really have to bend and work to even imagine a future where it becomes commonplace. “We need preferably thousands of these signals, which we should have in a few years, and even more in the next decade or two,” Holz said. “At that point, it would be an incredibly powerful method for learning about the universe.” Although one promising aspect of the standard siren method is that it is based on Einstein’s general theory of relativity — time-tested rules that are considered inviolable by many, and therefore incredibly reliable. From the left, an illustration of how relative amounts the moon can warp spacetime, then the Earth, the sun, and a black hole all the way to the right. Zooey Liao/CNET By contrast, most other scientists grappling with Hubble’s ongoing crisis rely on stars and galaxies, the researchers said, which involves very complex astrophysics and introduces a sincere possibility of error. But, notably, there have been some other experts who zero out gravitational waves as measurements of the Hubble constant. In 2019, for example, a separate crew of astronomers looked at ripples in space and time from a neutron star merger picked up by LIGO and Virgo in 2017. They were trying to figure out how bright the collision was when the countdown occurred from gravitational waves and finally arriving at a consistent Hubble estimate. And in the same year, another group suggested that we only need about 25 neutron star collision measurements to determine the constant to within 3%.
