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  • The universe is expandingand that expansion is speeding up...

  • which is the opposite of what we expected

  • We haven't been able to figure out exactly why, but we have thought

  • it might have something to do with dark energy.

  • And a certain paradigm-shifting hypothesis may

  • just resolve all of these uncertainties,

  • and if it's trueThat means gravity has mass.

  • Okay, but let's back up. There are many weird and wonderful mysteries out there in

  • space, but one of the most perplexing is the fact that the universe is expanding.

  • We know this because we can see the Doppler effect in action when we observe distant galaxies

  • You may know of the Doppler effect as that distortion of sound that happens when say,

  • an ambulance approachesit sounds different when it's far away

  • versus when it's right next to you in the street.

  • That same thing happens with light waves, so we can tell where galaxies

  • outside of our own are goingand how fast

  • These observations fit within Einstein's theory of general relativity. In fact, the

  • universe's expansion is one predicted outcome of his theory. But here's the catch: accelerating

  • expansion DOESN'T fit with our current theory of gravity. According to Einstein, the universe's

  • expansion should actually be slowing down.

  • There are lots of different ideas about what could be causing the inexplicable acceleration

  • of this expansionand one commonly suspected culprit is something called dark energy

  • ...and we're not entirely sure what that is, either

  • But now, we might finally have an explanation.

  • A theory proposed by professor Claudia de Rham at Imperial College London

  • suggests something pretty radical: that gravity has mass

  • Within the Standard Model, there are four fundamental forces that pretty much govern

  • the way the universe works.

  • Three of those four forces have "carrier particles" that we can observe:

  • the weak force has bosons, the strong force has gluons , and the electromagnetic

  • force has photons. That fourth force is gravity, and hypothetically its carrier particle would

  • be called a graviton...if we could find it

  • Which we haven't yet.

  • In Einstein's theory of general relativity, gravitons should be massless, but in de Rham's

  • proposal, the graviton has mass

  • This would resolve our confusion about the universe's expansion

  • If gravitons have mass, then gravity as a force isn't as easily able to propagate

  • over long distances

  • The math tells us that gravity is actually weaker on cosmological scales if gravity has mass.

  • So gravity wouldn't be able to slow down the expansion of our universe

  • And this also means that, maybe, we now know what dark energy isor rather, we've known

  • all along because… it's gravity

  • If gravity itself is a particle with mass, it takes up space, it is STUFF,

  • then it could be fulfilling the role we've assigned to this mysterious 'dark energy.' And

  • scientists are calling this the theory of "massive gravity."

  • So, just very casualthis work may have upended some of the most fundamental things

  • we believed about the universe and solved some of its biggest mysteries...

  • no big deal.

  • But it's not just interesting for that reason

  • The idea was originally proposed in the 1930s, but de Rham and her co-authors reignited the

  • debate around it back in 2011 with a pretty controversial paper. There was a big backlash from others

  • in the cosmology community who didn't believe this could possibly be true

  • But in the almost ten years since, the work has withstood the tests it's been put through.

  • It's been steadily gaining credibility, and sparking conversation about the nature

  • of our universe more than ever before.  

  • The math checks out, but now we need to look for evidence we could observe and record that

  • might empirically support this theory

  • Luckily, we live in an era where we can measure gravityalthough indirectlyvia gravitational

  • waves, with tools like LIGO

  • The future of gravitational wave astronomy may allow us to measure and observe gravity

  • and its behavior and its effects on the universe in ways that may blow this whole case wide

  • openhelping us settle this question of whether gravity

  • is as massive as it seems it could be

  • For even more on gravitational wave science and scientific controversy, check out this

  • video here, and let us know what other mysteries of the universe you want to see us cover next

  • down in the comments below.

  • Subscribe to Seeker for all your paradigm-shifting news and as always

  • thanks for watching.

  • I'll see you next time.

The universe is expandingand that expansion is speeding up...

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