Gravitational waves, or ripples in space-time, slip through Earth all the time, carrying secrets about the universe. But until a few years ago, we couldn't detect these waves at all, and even now, we have only the most basic ability to detect the stretching and squeezing of the cosmos.
However, a proposed new gravitational wave hunter, which would measure how particles of light and gravity interact, could change that. In the process, it could answer big questions about dark energy and the universe's expansion.
The three detectors on Earth today, all together called Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo, operate according to the same principle: As a gravitational wave moves through the Earth, it faintly stretches and squeezes space-time. By measuring how long a laser light takes to travel over long distances, the detectors notice when the size of that space-time changes. But the changes are minute, requiring extraordinarily sensitive equipment and statistical methods to detect.
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In this new paper, three researchers proposed a radical new method: hunting gravitational waves by looking for effects of direct interactions between gravitons — theoretical particles that carry gravitational force — and photons, the particles that make up light. By studying those photons after they've interacted with gravitons, you should be able to reconstruct properties of a gravitational wave, according Subhashish Banerjee, a co-author of the new paper and physicist at the Indian Institute of Technology in Jodhpur, India. Such a detector would be much cheaper and easier to build than existing detectors, Banerjee said.
"Measuring photons is something which people know very well," Banerjee told Live Science. "It's extremely well-studied, and definitely it is less challenging than a LIGO kind of setup."
No one knows exactly how gravitons and photons would interact, largely because gravitons are still entirely theoretical. No one's ever isolated one. But the researchers behind this new paper made a series of theoretical predictions: When a stream of gravitons hits a stream of photons, those photons should scatter. And that scattering would produce a faint, predictable pattern — a pattern physicists could amplify and study using techniques developed by quantum physicists who study light.
Linking the physics of the tiny quantum world with the large-scale physics of gravity and relativity has been a goal of scientists since Albert Einstein's time. But even though the newly suggested approach to studying gravitational waves would use quantum methods, it wouldn't fully bridge that tiny-to-large-scale gap on its own, Banerjee said.
"It would be a step in that direction, however," he added.
Probing the direct interactions of gravitons might solve some other deep mysteries about the universe, though, he said.
In their paper, the authors showed that the way the light scatters would depend on the specific physical properties of gravitons. According to Einstein's theory of general relativity, gravitons are massless and travel at the speed of light. But according to a collection of theories, together known as "massive gravity," gravitons have mass and move slower than the speed of light. These ideas, some researchers think, could resolve problems such as dark energy and the expansion of the universe. Detecting gravitational waves using photon scattering, Banerjee said, could have the side effect of telling physicists whether massive gravity is correct.
No one knows how sensitive a photon-graviton detector of this kind would end up being, Banerjee said. That would depend a lot on the final design properties of the detector, and right now, none are under construction. However, he said, he and his two co-authors hope that experimentalists will start putting one together soon.
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Originally published on Live Science.
Rubin’s most important scientific contribution was establishing that the orbiting speeds of gas clouds in the outer rims of the galaxies she examined remain constant (i.e., “flat”) to distances well beyond the visible starlight, rather than declining as in the outer parts of our Solar System. High orbital speeds in the outer parts of galaxies imply the existence of extra matter at large radial distances to insure these velocities.
As a result of Dr. Rubin’s work and later studies, we now know that galaxies are surrounded by enormous invisible halos of matter containing 5/6 of their mass which extend ten times farther out than the visible regions. Numerous arguments and thought experiments show that this so-called “dark matter” must be totally different from the ordinary, “baryonic”, matter of the periodic table. Although its nature is still unknown, it is being pursued in numerous experiments in particle accelerators and particle detectors around the world. The eventual realization that baryonic matter is only a partial component of the Universe, following the acceptance of numerous papers by Dr. Rubin and her collaborator, Kent Ford, showed that our understanding of the cosmos was shockingly incomplete and was one of the milestones that ushered in modern cosmology.
Dark matter had a somewhat checkered history before Rubin’s first paper on the subject was published in 1978 (Rubin, Ford, and Thonnard, Astrophysical Journal Letters, 225, 107, 1978). Astronomer Fritz Zwicky opened the subject in 1933 with the claim that galactic clusters would fly apart if extra matter were not present to provide more gravitational pull. A sprinkling of papers followed over the next three decades, culminating in the Santa Barbara Conference on “missing mass” in 1964, but the available data, mostly still confined to clusters and binary galaxies, were hard to analyze. The subject advanced in the early 1970’s with the early radio studies of the 21-cm line of neutral hydrogen to measure rotation speeds in the disks of gas in the outskirts of nearby galaxies. The disks in circular rotation were much simpler to analyze, and these early data hinted at the rotation curve discrepancy, but the number of sampled galaxies was small. A leader in these early radio papers was Morton Roberts at the National Radio Astronomy Observatory, who actively stimulated Rubin’s interest in the subject. The PhD thesis of Albert Bosma, which appeared in 1978 just before Rubin’s first paper, extended radio data to 24 galaxies using the Westerbork interferometer, in the Netherlands, and again saw flat outer rotation curves.
Subsequently, Babcock's optical rotation curve, and that of Rubin and Ford (1970), was extended to even larger radii by Roberts and Whitehurst (1975) using 21 cm line observations that reached a radial distance of ~30 kilo parsecs. These observations clearly showed that the rotation curve of the Andromeda Galaxy, or M31, did not exhibit a Keplerian drop‐off in velocity. In fact, its rotational velocity remained constant over radial distances of 16–30 kpc. These observations indicated that the mass in the outer regions of the Andromeda galaxy increased with the distance from the galactic center, even though the stellar optical luminosity of M31 did not.
Amidst this growing body of data indicating dark matter, Rubin’s work was particularly influential because of three factors. First was the clarity and directness of the papers, including beautiful illustrations of the raw spectra that she was measuring—the flatness of the rotation curves could not be denied. Second was the fact that Rubin and her colleagues followed up with several more papers over the next few years, each one enlarging the sample size and demonstrating the seeming ubiquity of flat curves of rotations. Third were Rubin’s presentations at numerous astronomical conferences, which, like her published papers, were clear, direct, pared down to essentials, and ultimately compelling, driving her dark matter thesis home.
Vera Rubin truly lit the way in dark matter discovery and she began her work with our galactic neighbor, M-31, Andromeda, that massive and beautiful star rich cousin.
Of course, there is a place for speculation, but without some means of testing, speculation can seem to be endless. Speculation leads to thoughtfulness, testing to hope and observation to proof, satisfaction, and a new round of speculation.
The article says "According to Einstein's theory of general relativity, gravitons are massless and travel at the speed of light. But according to a collection of theories, together known as "massive gravity," gravitons have mass and move slower than the speed of light. These ideas, some researchers think, could resolve problems such as dark energy and the expansion of the universe. Detecting gravitational waves using photon scattering, Subhashish Banerjee said, could have the side effect of telling physicists whether massive gravity is correct."
Banerjee suggests that a photon stream can be affected by gravitons and the scattering of the photons, if they can be measured in an apparatus yet to be built, will give us an idea of the mass of the graviton. Apparently, a heavy and slower graviton will open hitherto closed doors in the hunt for dark energy.
It's a bit esoteric but somewhat understandable considering that subatomic particles are deflected all the time at CERN and at Fermi Labs. Perhaps individual light quanta can be deflected by gravitons, who can say?