Gravitational waves may oscillate, just like neutrinos
phys.org
phys.org
Secondly, this comment grew a lot while I was composing it, and it turns into two comments: a crash-speed overview of some aspects of linearized gravity and a few paragraphs describing what's in this paper, forming a quick personal reaction from a pseudonymous nobody.
Thirdly, massive gravity theory is not really new, and the 2010 date in the phys.org writeup refers to a solution for massive gravity in 4-dimensional spacetime by de Rham, Gabadadze and Tolley ( https://arxiv.org/abs/1011.1232 ). Previously, stable solutions were only known in lower dimensioned spacetimes. However, there have been fits and starts of work on this broad family of gravity theories dating to Pauli & Fierz, who proposed a massive spin-two field on a flat spacetime in the 1960s (Blasi & Maggiore give some details at https://arxiv.org/abs/1706.08140 ).
In order to talk about this (dense, technical!) paper, I think one needs to know a little about about perturbatively quantized gravity, which I'll describe over a few paragraphs. You can skip ahead a bit by just accepting that gravitons exchanged by everything (including each other) and that in standard General Relativity gravitons are massless.
When you take a perturbation theory approach to the metric of General Relativity, you fix a background spacetime (which is almost always flat, i.e. described with the Minkowski metric), which gets the label \eta (the solvable), with deviations from \eta held in h (the perturbation). The metric becomes g = \eta + h. Some configurations of moving mass-energy can produce waves in h that propagate according to the (classical) massless wave equation on flat spacetime, analogously to (classical) electromagnetic waves on flat spacetime.
One can proceed to quantize h. Well, really first one expands g = \eta + h + h^2 + h^3 + ... where h^2 etc are quadratic, cubic, and higher-order terms in the perturbation. Next one omits as many of the higher-order terms as one can get away with. In linearized gravity, that means h^2 and up are ignored.
Essentially we turn classical waves in h into large numbers of particles we call gravitons, much as one turns classical electromagnetic waves into large numbers of photons. I omitted indices and a couple of other details (most notably gauge fixing) nevertheless g, \eta and h are all symmetric rank-two tensors. The result of the quantization of (symmetric rank-2 tensor) h is therefore a massless spin-2 gauge boson, which compares with the quantization of electromagnetic waves (as antisymmetric rank-2 tensors [4]) resulting in a massless spin-1 gauge boson.
This is perturbative quantum gravity, a real working quantum theory of gravity, but not a candidate for a more fundamental theory than General Relativity because higher order terms in h are important in a universe with compact massive objects, and we don't know how to include such higher-order terms into this type of quantization.
There is a key pattern between the spin-1 and spin-2 particles: each mediates an interaction between particles that carry a + or - charge.
spin like-charges opposite-charges
---- ------------ ----------------
1 repel attract
2 attract repel
In our universe, the electromagnetic interaction is much stronger than the gravitational interaction, but not everything has an electromagnetic charge. Most importantly, photons themselves do not have an electromagnetic charge. In perturbatively quantized gravity though, everything has a gravitational charge, including gravitons.
If we take perturbatively quantized gravity seriously, in our patch of the universe there is effectively nothing with the opposite gravitational charge of stuff here on earth, probably for reasons similar to why there is approximately no antimatter around us: segregation in the early universe. Essentially why we see no anti-gravity is that even our positrons and antiquarks have the same gravitational charge as everything else around us, so they interact with only one of the possible graviton charges.
That gravitons of the same charge attract one another leads to the higher order terms in h: their self-interaction is the source of non-linearity.
Finally, perturbatively quantized gravity allows us to define an energy scale for the gravitational interaction: it is "low energy" when the quadratic and higher-order terms in h simply do not matter; it is "high energy" or "strong gravity" when they do. The non-linearities higher-order terms in expanded h can be represented by one or more loops of gravitons in Feynman diagrams.
From this we have learned that "strong gravity" appears when the dilating effects of curvature are apparent even at Planck lengths or Planck time, or equivalently "high energy" appears around a tiny object with at least Planck energy. In practice this only appears very close to whatever quantum state there is in the place of classical gravitational singularities. Crucially this is well inside the event horizons of black holes, or in the very early universe. Consequently, perturbatively quantized gravity, and General Relativity, are both reliable "low energy" effective field theories practically everywhere we presently have the technology to study.
Next, on to bigravity.
It can be fun and interesting to investigate theories of gravity other than General Relativity, and one popular strategy is to introduce extra degrees of freedom. This paper deals with a family of such theories called bimetric gravity (or "bigravity" as in the paper) where the extra degree of freedom is a (classical) second rank-two tensor field.
Typically one runs into a bimetric theory where the second metric is energy-dependent, and is simply not noticeable in weak gravity. Where one finds strong gravity -- notably, in the very early universe -- the second metric becomes important. A popular approach is that the second metric "steers" the first metric. In our classical expansion g = \eta + h, the perturbations that obey the equations for massive wave propagation instead obey the massless wave equations. This drag on the waves means that (classical) electromagnetic waves outrace (classical) gravitational waves when the second metric is operative. This is often used in models to solve the horizon problem: why are readings of the cosmic microwave background at opposite sides of our view of the universe at the same temperature? Because light was fast enough to carry energy between distantly separated matter, thermalizing it before gravitational collapse could begin in earnest. [1][2]
When we quantize a bimetric theory, we get two gravitons: one that is the standard massless spin-2, and the other which is a massive spin-2. The massive graviton only interacts with the massless graviton, and in weak gravity there simply aren't many of either type of graviton, so you don't see the interaction. In strong gravity the densities of both types of graviton inevitably lead to interactions. As in standard perturbatively quantized gravity, the massless graviton mediates the gravitational interaction of everything in the universe (including both types of graviton).
We can now look at this particular bigravity paper's non-novelty in its Abstract: "... a second ... tensor ... [couples with the standard metric tensor] ... such that one massless and one massive linear combination arise. Only one ... tensor ... [couples] to matter" and have an idea what that means. The novelty here is that the second -- massive -- tensor is not energy-dependent, it's always there but gets "washed out" by the behaviour of the tensor fields.
This novel behaviour, "oscillations", make sense in perturbatively quantized gravity: we get two different gravitons, g_standard and g_massive, where g_massive only interacts with g_standard, and g_standard interacts with everything; when g_standard and g_massive are in the same point in the background spacetime, there is a linear effect [3] . g_standard continues to propagate at the maximum speed (c), while wave packets of g_massive have a lower group velocity. This is a tight analogy with neutrino oscillations where heavier neutrinos move more slowly than the lightest neutrino, and where a heavy sterile (no weak charge) neutrino exists.
This is a pretty wild claim. The authors believe their theory is self-consistent, and propose some observables that are in detectable through gravitational wave astronomy that we are close to being able to do. Roughly, a spinning barbell arrangement of matter (two black holes in orbit can be seen as the weights on the end of a vanishingly thin bar) sheds gravitational waves -- and under the Max-Platscher-Smirnov model there is a drag on their propagation compared under standard General Relativity. As a practical example, this leads to a dependence on several parameters, including travel time for the massive component, of gravitational waves detectable by LIGO: the final "bloop" [3] will sound different for similar-mass mergers at different redshifts.
There are a lot of other likely observables including ones that should appear in the cosmic microwave background. Bimetric theories are notorious for generating modes that seem unreasonable (and thus are unlikely to be there when you look for them). However, the idea is a neat and provocative one that is put across well, and so was worth publishing on that basis.
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[1] https://www.wikiwand.com/en/Horizon_problem#/Variable_speed_...
[2] https://www.wikiwand.com/en/Variable_speed_of_light
[3] https://www.youtube.com/watch?v=TWqhUANNFXw
[4] https://www.wikiwand.com/en/Electromagnetic_tensor ("... an antisymmetric rank-2 tensor field—on Minkowski space.")
[5] https://www.wikiwand.com/en/Electrovacuum_solution ("... in which the only nongravitational mass-energy present is the field energy of an electromagnetic field")