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 true? That 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 approaches—it 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 going… and 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 expansion, and 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 is—or 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 casual—this 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 gravity—although indirectly—via 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

open, helping 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.