Ten years before the detection of gravitational waves, two postdocs from KITP at UC Santa Barbara had a novel idea



Ten years before the detection of gravitational waves, two postdocs from KITP at UC Santa Barbara had a novel idea

Two neutron stars collide, emitting gravitational waves and electromagnetic radiation detected on Earth in 2017. Credit: FERMILAB

The history of science is full of stories of enthusiastic researchers who slowly win over skeptical colleagues to their point of view. Astrophysicist Scott Hughes can relate to these stories.

"During the first 15 or 16 years of my career, I was talking to astronomers, and I always had the impression that they were politely interested in what I had to say, but I considered myself a bit of an outlandish eye enthusiast. a pack of unicorns that my friends and I were raising, "Hughes said.

"Now," he continued, "there are people who say:" Ooh, all those unicorns you found, can I use them to solve my problem? Do your unicorns have wings? Are they bright? "

These unicorns are gravitational waves, an area of ​​physics in which Hughes specializes. While working as postdoctoral researchers at the Kavli Institute of Theoretical Physics (KITP) at UC Santa Barbara, Hughes and his colleague, Daniel Holz, were the first to propose the use of the phenomenon, in combination with observations based on telescopes, to measure the constant of Hubble. A fundamental quantity involved in describing the expansion of the universe.

As the universe expands, the celestial objects move away from us. This stretches the wavelength of the light that we detect from these objects, causing the frequency to decrease like a siren in a pbading ambulance. The faster the object recoils, the more its light will move towards the red end of the spectrum. The Hubble constant relates the distance of an object from Earth to this displacement to red and, therefore, the speed of the object as it moves away.

One of the best tools of an astronomer to calculate this is a standard candle, any clbad of objects that always have the same standard brightness. If scientists know the brightness of an object, they can determine its distance by measuring how weak it looks to us on Earth.

For decades, scientists have tried to obtain accurate measurements of the Hubble constant to investigate why the universe is expanding and, in fact, accelerating. This is ultimately resolved by measuring the redshifts of the objects and comparing them with independent measurements of the distances of the objects to us. However, these two more accurate measurements that scientists currently have for the Hubble constant do not agree, an endless source of frustration for cosmologists.

A proposal

This was the cosmological picture in the early 2000s when Holz and Hughes held positions as postdoctoral researchers at KITP. "Scott had been thinking about gravitational waves for a while," Holz said. "He was the expert and I was much more focused on cosmological issues." But Hughes' enthusiasm soon aroused Holz's curiosity, and the two began to talk about the cosmology of gravitational waves in the office and on walks along the Santa Barbara cliffs.

Holz and Hughes attribute their close collaboration to the construction of Kohn Hall's new wing in 2001. Initially, all postdocs at KITP had their own offices, Hughes explained, but construction forced them to double. "Suddenly, we spent much more time together."

A 2002 KITP program on cosmological data stoked the flames of its interest in the subject. When Hughes retired to join the faculty at MIT, they had completed the first draft of his article detailing how to calculate the Hubble constant with gravitational waves. After two years of gestation they finally published the study in The Astrophysics Magazine.

Ten years before the detection of gravitational waves, two postdocs from KITP at UC Santa Barbara had a novel idea

Three gravitational wave detectors (yellow) and approximately 70 terrestrial and space light observatories (blue) focused on the neutron stars that fuse. Credit: LIGO

"I had a great time writing that paper with Scott," said Holz. "I learned an incredible amount, so much that I was convinced that gravitational waves were the future and that I should get involved."

The idea of ​​using gravitational wave sources to measure the Hubble constant was not new. The concept was first proposed in a visionary article in 1986 by Bernard Schutz. And a number of other notions regarding gravitational waves also hovered around literature in the early 2000s. But what Holz and Hughes did was to synthesize all these ideas and emphasize the possibility of combining gravitational wave data with observations. of tracking using light.

The study was also the first to use the term "standard siren". Hughes recalled discussing the article with Caltech astrophysicist Sterl Phinney, who commented, "Hmm, it's like a standard candle, but you hear it, you should call it a standard siren." Holz, independently, had an almost identical conversation with physicist Sean Carroll, a former postdoctoral KITP. Holz and Hughes included the term in their paper, and got stuck. The phrase has become ubiquitous in cosmology.

"The term & # 39; siren standard & # 39; could be our most lasting contribution, Scott," said Holz. "I'll take it," Hughes laughed.

Using gravitational waves to measure the Hubble constant has many advantages over other methods. Certain supernovas provide decent standard sails, "but, like a standard sail, supernovas do not understand each other very well," Holz said. "The main thing that makes standard sirens interesting is that they are understood from the first principles, directly from the theory of general relativity."

When standard candles are used, scientists have to calibrate the distances of certain kinds of objects using the information of others, and in this way they make their way to an adequate distance measure. Astronomers call this method a "distance scale," and errors and uncertainty can appear at many points in the calculations.

In contrast, gravitational waves can provide a direct measure of the distance of an object. "You just write the equations and solve them, and then you finish," Holz said. "We've been testing general relativity for a hundred years, it really works and it says," here's how far the source is. "There's no ladder away, there's nothing like that playing."

According to Holz, all the early work on measuring the Hubble constant using gravitational waves was somewhat speculative. They were proposals for the distant future. "We had not even detected gravitational waves, much less waves of two neutron stars, let alone an optical counterpart," he said. But the interest and enthusiasm for the technique was growing.

Hughes remembers that colleagues approached him after his talks and asked him about the possibility of observing a standard siren in the next decade. He did not know, but he did say that with a better understanding of the optical counterpart, they could probably locate an event within 10 to 20 square degrees. "And I think if you have that, every piece of big glbad on Earth is going to look at that place in the sky," Hughes had said. "And, in the end, that's exactly what happened."

And then it happened

On August 17, 2017, less than two years after detecting the first gravitational waves, the LIGO and Virgo observatories registered a signal of the fusion of neutron stars. Thanks to a warning system, which Holz helped to establish, there was a wave of activities as almost all the major terrestrial and space observatories focused on the event. The scientists collected data on fusion in each region of the electromagnetic spectrum.

Ten years before the detection of gravitational waves, two postdocs from KITP at UC Santa Barbara had a novel idea

Obtaining precise measurements of distance requires building a ladder away using several different techniques for several ranges. Credit: MATT PERKO

"It really is one of those things that, if it had happened before I retired, I would have been happy," Hughes said. "But it actually happened before I turned 50."

Suddenly, the cosmology of gravitational waves was a real field, and standard sirens were another part of the toolkit. "But that something becomes part of the toolkit so quickly? That's extraordinarily unusual," Holz said.

It turns out that cosmologists need another tool, because they currently have two different values ​​for the Hubble constant. The methods that use the cosmic microwave background (little light remaining from the big bang) yield a value of around 68. Meanwhile, calculations using Type Ia supernovas, a variety of standard candles, yield a little more than 73 .

Although they appear close, the two values ​​really differ by three standard deviations, and both have fairly tight error bars. The disagreement has to the cosmologists more and more worried since the bars of error in these two values ​​only become stricter. It could signal a fundamental problem in our understanding of the universe, and it is the subject of a KITP conference in July.

However, there are some intrinsic differences between the two techniques. The cosmic microwave background reflects the conditions of the early universe, while the supernovae paint an image of the current universe. "There is a possibility that something very strange and unexpected may have happened between the early and late days of the universe, and that is why these values ​​do not agree," Holz said. But cosmologists simply do not know for sure.

Obtaining another independent value for the Hubble constant will help clarify this puzzle. "Because it's so clean and so direct, that measure will be a very compelling number," Holz explained. "At a minimum, you will inform this discussion, if not just solve it completely."

Holz and his colleagues, Hsin-Yu Chen and Maya Fishbach, have just published an article in the journal Nature, and discover that 20 to 30 observations would allow scientists to calculate the Hubble constant with an accuracy of 2 percent, enough adjusted to start comparing it. The two values ​​of the cosmic microwave background and the supernovas.

This summer, Holz is co-organizing a KITP program in the new era of gravitational wave physics and astrophysics, and the new field of standard siren cosmology will be an important topic of discussion. In fact, Holz also helped organize the KITP rapid response program that brought together researchers shortly after LIGO's first gravitational wave detection.

Holz and Hughes attribute their success to their experiences at KITP. "While we were working together at KITP, we were both enthusiastic about measuring the Hubble constant by using gravitational waves," said Holz. "And that's exactly what KITP is about: bringing together different people with different origins, removing the boat and seeing what happens."

Over the past decade, Holz's career has focused on the standard siren cosmology. "And the amazing thing is that we really have done it," he said. "I helped write the document that made the first standard mermaid measurement, which was exactly what Scott and I had hypothesized a few years ago."

"If we had not both been in the KITP, there's no way I'm spending a good part of my life on LIGO teleconferences right now," Holz said. "But I would not have it otherwise."


How to use gravitational waves to measure the expansion of the universe.


More information:
Hsin-Yu Chen et al. A constant Hubble measurement of two percent of standard sirens within five years, Nature (2018). DOI: 10.1038 / s41586-018-0606-0

Bernard F. Schutz. Determination of the Hubble constant from gravitational wave observations, Nature (2004). DOI: 10.1038 / 323310a0

Daniel E. Holz et al. Using standard gravitational wave sirens, The astrophysics magazine (2005). DOI: 10.1086 / 431341

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Ten years before the detection of gravitational waves, two postdocs from KITP at UC Santa Barbara had a novel idea (2019, April 2)
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