Could Gravitons Solve The Mystery Of Dark Matter?

by akoloy

One of probably the most puzzling observations concerning the Universe is that there isn’t sufficient matter — at the least, matter that we all know of — to elucidate how we see issues are gravitating. On Solar System scales, General Relativity and the lots we observe do the job simply positive. But on bigger scales, the interior motions of particular person galaxies point out the presence of extra mass than we observe. Galaxies in clusters transfer round too shortly, whereas X-rays reveal an inadequate quantity of regular matter. Even on cosmic scales, further mass needs to be current to elucidate gravitational lensing, the cosmic internet, and the imperfections within the Big Bang’s leftover glow. While we usually invoke a brand new particle of some sort, one intriguing thought is solely gravitational: might darkish matter be made from gravitons alone? That’s what Neil Graham desires to know, as he writes in to ask:

“Why couldn’t dark matter be gravitons? Gravitons are undefined as is dark matter. We know dark matter has gravity. Why couldn’t it made of the mythical graviton particles?”

Why couldn’t darkish matter be gravitons? Or, higher but, might gravitons make up some or all the darkish matter? Let’s take a look at what we all know, and see what prospects stay.

The very first thing we’ve to contemplate is, astrophysically, what we already know concerning the Universe, as a result of the Universe itself is the place we get all the data we find out about darkish matter. Dark matter needs to be:

  • clumpy, which tells us that it must have a non-zero relaxation mass,
  • collisionless, within the sense that it can not collide (very a lot, if in any respect) with both regular matter or photons,
  • minimally self-interacting, which is to say there are slightly tight restrictions on how considerably darkish matter can collide and work together with different darkish matter particles,
  • and chilly, that means that — even at early instances within the Universe — this materials must be transferring slowly in comparison with the pace of sunshine.

Furthermore, after we take a look at the Standard Model of elementary particles, we discover, fairly definitively, that there aren’t any particles that exist already that may make good darkish matter candidate.

Any particle with an electrical cost is eradicated, as are the unstable ones that may decay. Neutrinos are too mild; they had been born sizzling and would characterize a really totally different sort of darkish matter than we’ve, plus, primarily based on our cosmic measurements, they will solely make up about ~1% of the darkish matter, at most. Composite particles, just like the neutron, would clump and cluster collectively, shedding momentum and angular momentum too considerably; they’re too “self-interacting.” And the opposite impartial particles, like gluons, would additionally couple too strongly to the opposite regular stuff on the market; they’re too “collisional.”

Whatever it’s that darkish matter is made from, it isn’t any of the particles that we all know of. Without these constraints — for the reason that null speculation is fairly definitively dominated out — we’re free to take a position about what darkish matter could be. And whereas it’s definitely not the preferred possibility, there are many the explanation why one would possibly need to contemplate the graviton.

Reason #1: gravity exists, and may be very possible quantum in nature. Unlike lots of the darkish matter candidates which might be extra generally talked about, there’s far much less hypothesis related to the graviton than virtually another thought in beyond-the-Standard-Model physics. In reality, if gravity, like the opposite recognized forces, seems to be inherently quantum in nature, then the existence of a graviton is required. This stands in distinction to many different choices, together with:

  • the lightest supersymmetric particle, which might require supersymmetry to exist regardless of the mountain of proof that it doesn’t,
  • the lightest Kaluza-Klein particle, which might require further dimensions to exist, regardless of an entire lack of proof for them,
  • a sterile neutrino, which might require further physics within the neutrino sector and is extremely constrained by cosmological observations,
  • or an axion, which might require the existence of at the least one new sort of elementary discipline,

amongst many different candidates. The solely assumption we want, in an effort to have gravitons within the Universe, is that gravity is inherently quantum, slightly than being described by Einstein’s classical principle of General Relativity on all scales.

Reason #2: gravitons aren’t essentially massless. In our Universe, you’ll be able to solely clump collectively and kind a certain construction, gravitationally, in case you have a non-zero relaxation mass. In principle, a graviton could be a massless, spin-2 particle that mediates the gravitational pressure. Observationally, from the arrival of gravitational waves (which themselves, if gravity is quantum, ought to be made from energetic gravitons), we’ve very strong constraints on how large a graviton is allowed to be: if it has a relaxation mass, it needs to be decrease than about ~10-55 grams.

But as tiny as that quantity is, it’s solely in step with the massless answer; it doesn’t mandate that the graviton is massless. In reality, if there are quantum couplings to sure different particles, it could prove that the graviton itself has a relaxation mass, and if that’s the case, they will clump and cluster collectively. In giant sufficient numbers, they may even make up half or all the darkish matter within the Universe. Remember: large, collisionless, minimally self-interacting, and chilly are the astrophysical standards we’ve on darkish matter, so if gravitons are massless — and whereas we don’t count on them to be, they might be — they might be a novel darkish matter candidate.

Reason #3: gravitons are already extraordinarily collisionless. In physics, any time you might have two quanta that occupy the identical area on the identical time, there’s an opportunity that they’ll work together. If there’s an interplay, the 2 objects can trade momentum and/or power; they may fly off once more, stick collectively, annihilate, or spontaneously create new particle-antiparticle pairs if sufficient power is current. Regardless of which sort of interplay happens, the cumulative chance of all the things that may happen is described by one vital bodily property: a scattering cross-section.

If your cross-section is 0, you’re thought-about non-interacting, or utterly collisionless. If gravitons obey the physics we expect them to obey, we will really compute the cross-section: it’s non-zero, however detecting even one graviton is exceedingly unlikely. As a 2006 study demonstrated, a Jupiter-mass planet in tight orbit round a neutron star would work together with roughly one graviton per decade, which is collisionless sufficient to suit the invoice to explain darkish matter. (Its cross-section with photons is comparably laughable in how minuscule it’s.) So, on this entrance, gravitons haven’t any drawback as a darkish matter candidate.

Reason #4: gravitons have terribly low self-interactions. One of the questions I generally get requested is whether or not it’s potential to surf gravitational waves, or whether or not, if two gravitational waves collided, they’d work together like water waves “splashing” collectively. The reply to the primary one is “no” and the second is “yes,” however barely: gravitational waves — and therefore, gravitons — do work together on this means, however the interplay is so small that it’s utterly imperceptible.

The means we quantify gravitational waves is thru their strain amplitude, or the quantity {that a} passing gravitational wave will trigger area itself to “ripple” when issues cross by it. When two gravitational waves work together, the principle portion of every wave simply will get superimposed atop the opposite one, whereas the portion that does something apart from cross by each other is proportional to the pressure amplitude of every one multiplied collectively. Given that pressure amplitudes are usually issues like ~10-20 or smaller, which itself requires an amazing effort to detect, going 20+ orders of magnitude extra delicate is nearly unimaginable with the restrictions of present expertise. Whatever else could be true about gravitons, their self-interactions could be disregarded.

But among the properties of gravitons pose a problem for them to be a viable darkish matter candidate. In reality, there are two main difficulties that gravitons face, and why they’re not often thought-about as compelling choices.

Difficulty #1: it’s very tough to generate “cold” gravitons. In our Universe, any particles that exist may have a specific amount of kinetic power, and that power determines how shortly they transfer by the Universe. As the Universe expands and these particles journey by area, certainly one of two issues will occur:

  • both the particle will lose power as its wavelength stretches with the growth of the Universe, which happens for massless particles,
  • or the particle will lose power as the space it could possibly journey in a given period of time decreases, as a result of ever-growing distances between two factors, if it’s an enormous particle.

At some level, no matter the way it was born, all large particles will ultimately transfer slowly in comparison with the pace of sunshine: turning into non-relativistic and chilly.

The solely technique to accomplish this, for a particle with such a low mass (like an enormous graviton would have), is to have or not it’s “born cold,” the place one thing happens to create them with a negligible quantity of kinetic power, regardless of having a mass that have to be decrease than 10-55 grams. The transition that created them, due to this fact, have to be restricted by the Heisenberg uncertainty principle: if it their creation time happens over an interval that’s smaller than about ~10 seconds, the related power uncertainty might be too giant for them, and so they’ll be relativistic in spite of everything.

Somehow — maybe with similarities to the theoretical technology of the axion — they have to be created with an especially small quantity of kinetic power, and that creation must happen over a comparatively lengthy period of time within the cosmos (in comparison with the tiny fraction-of-a-second timespan for many such occasions). It’s not essentially a dealbreaker, however it’s a tough impediment to beat, requiring a set of recent physics that isn’t straightforward to justify.

Difficulty #2: regardless of our theoretical hopes, gravitons (and photons, and gluons) are all most likely massless. Until one thing’s been experimentally or observationally established, it’s significantly tough to rule out options to the main thought of the way it should behave. With gravitons — as with photons and gluons, the one different really massless particles we all know of — we will solely place constraints on how large they’re allowed to be. We have higher limits of various tightness, however haven’t any technique to constrain all of it the best way to “zero.”

What we will word, nonetheless, is that if any of those theoretically massless particles do have a non-zero relaxation mass, we’d must reckon with plenty of uncomfortable details.

  • Gravity and electromagnetism, if the graviton or photon are large, will not be infinite-range forces.
  • If the force-carrying particle is very large, then gravitational waves and/or mild wouldn’t journey at c, the pace of sunshine in a vacuum, however slightly a slower pace that we’ve merely didn’t measure to date.
  • And you get a theory other than General Relativity within the restrict that you simply take the graviton’s mass to zero, a pathology that requires a number of arguably more uncomfortable assumptions to remove. (In specific, they do not allow the Universe to be flat, which we observe; solely open, and that itself accommodates instabilities which could be dealbreakers.)

While the thought of large gravity has gotten a variety of curiosity over the previous decade, together with from current progress spurred largely from the research of Claudia de Rham, it stays a extremely speculative concept that might not be workable inside the framework of what’s already been established about our Universe.

What’s outstanding is that we not ask questions like, “why couldn’t dark matter be gravitons?” Instead, we ask, “if we wanted the dark matter to be gravitons, what properties would it need to have?” The reply, like all darkish matter candidates, is that it needs to be chilly, collisionless, with extremely restricted self-interactions, and large. While gravitons definitely match the invoice of being collisionless and barely self-interacting in any respect, they’re typically assumed to be massless, not large, and even when they had been large, producing chilly variations of gravitons is one thing we nonetheless don’t know how you can do.

But that isn’t sufficient to rule these eventualities out. All we will do is measure the Universe on the stage we’re able to measuring it, and to attract accountable conclusions: conclusions that don’t exceed the attain of our experimental and observational limits. We can constrain the mass of the graviton and uncover the implications of what would happen if it did have a mass, however till we really uncover the true nature of darkish matter, we’ve to maintain our minds open to all prospects that haven’t definitively been excluded. Although I wouldn’t wager on it, we can not but remove the chance that gravitons that had been born chilly are themselves chargeable for the darkish matter, and make up the lacking 27% of the Universe we’ve lengthy been trying to find. Until we all know what darkish matter’s true nature is, we have to discover each risk, regardless of how implausible.

Send in your Ask Ethan inquiries to startswithabang at gmail dot com!

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