The universe is expanding, and 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 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.