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Thanks to state-of-the-art telescopes like the James Webb,
we can look so deep into time and space that we can see
some of the very first galaxies, and hunt for the very first stars.
And other telescopes can take us even farther back in time,
to a point 380,000 years after the universe was born,
when light began to shine through space.
For a universe that’s roughly 13.8 billion years old,
that’s a pretty solid baby picture.
But for scientists, it’s not good enough.
They want to study just slivers of a second after the universe was born,
when everything we know was packed into an area the size of a marble,
space was opaque, and subatomic particles like protons and neutrons
were a thing of the future.
The problem is, because the universe was opaque back then,
we will never see this moment in time.
But we might be able to observe it anyway,
using a completely different kind of signal.
Gravitational waves.
Astronomers think that this moment in time
created wrinkles in the fabric of space.
And if it did, those wrinkles are still out there.
If we manage to find them, it would clarify how the universe came to be,
and exactly what happened in the fractions of a second
that defined the entire course of the universe.
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For nearly a century, gravitational waves were purely hypothetical.
Einstein published his theory of general relativity in 1915,
and a completely mind-bending idea soon popped out of the math.
The math showed that any object with mass
that was accelerating through spacetime would warp spacetime.
And that warping would ripple out into
the greater universe at the speed of light.
As spacetime squeezed and stretched,
the distances between objects would change.
Of course, we don’t see the world around us
warp every time a truck passes by or a plane takes off.
Gravitational waves are weak.
But according to general relativity, the more massive an object,
and the faster it is moving, the stronger its gravitational waves will be.
So scientists eventually started to think that gravitational waves
from the most extreme events in the universe might just be detectable,
if you could build a machine capable of measuring
changes smaller than a proton.
Enter, LIGO. the Laser Interferometer Gravitational Wave Observatory.
Actually, it is a pair of observatories that work together.
One’s in Washington state and the other’s in Louisiana.
Each has two arms that are four kilometers long
and meet to form an L-shape.
And they’re shaped that way because when
a gravitational wave passes through space,
that space will stretch in one direction and
squeeze in the perpendicular direction.
So if a gravitational wave were to ripple through the observatory,
one arm would get longer while the other got shorter.
In 2015, almost exactly 100 years after
Einstein’s theory of general relativity dropped,
LIGO picked up a brief little chirp, made by the gravitational waves
of two merging black holes, each roughly 30 times the mass of our Sun.
The discovery was not only a major confirmation of general relativity.
It opened up a whole new way of exploring the universe,
using gravity as a probe instead of light.
And once it was clear that any kind of gravitational wave
was definitely real and definitely detectable,
the idea of finding primordial gravitational waves,
the ones created just after the dawn of reality as we know it
didn’t sound that far-fetched.
But if any of these primordial waves were out there,
we couldn’t rely on LIGO to pick them up.
Just like human ears can only pick up sound waves within
a certain frequency range, LIGO can’t detect all gravitational waves.
It’s limited to frequencies between roughly ten
and 10,000 peaks per second.
And the objects that churn out those frequencies are objects
like your garden-variety black hole or neutron star,
spiraling ever closer together until they smush.
Primordial gravitational waves, on the other hand,
are an entirely different beast.
These would have been made during, or right after,
a hypothetical period known as inflation that happened
just after the birth of the universe.
During inflation, the observable universe swelled exponentially,
going from a subatomic speck to about the size of a marble.
It was all over in a fraction of a second, like this many seconds,
but during this flash of expansion, our universe was dominated
by constant, subatomically-small fluctuations in energy
that warped spacetime just like an object with mass does, today.
And it’s impossible for our Earthling brains to truly
envision this baby universe, so for now, let’s just imagine a lake.
Plus a bunch of raindrops falling down onto that lake,
creating ripples in the smooth surface.
According to our current knowledge of how physics works,
as spacetime writhed in this early universe,
gravitational waves rippled outward.
And as the universe doubled in size many times over during
this split second, those hypothetical ripples turned into longer waves.
So, in our metaphor we’d also need our lake to be growing,
but that’s okay. It’s all inside of our minds,
we can totally make that happen. Imagine the lake growing!
Currently, cosmologists don’t know exactly how inflation played out.
They’re not even 100% sure it happened.
And that’s not too surprising since we’ve never been
able to observe that far back in time.
There could be multiple sources of these
inflation-era gravitational waves.
Some may have even been generated by inflation ending.
But no matter how this split second of cosmic history went down,
it was likely a time when space was getting stretched and contorted
in extreme ways and this was bound to produce gravitational waves.
Then, as our tiny baby universe swelled
to its current size, those waves stretched out.
What started as microscopic ripples would be light-years long, now.
And that’s why finding these ancient waves is not a job for LIGO.
Each one is billions of times longer, the frequencies are
billions of times lower than the waves LIGO can detect.
In fact, there is no way to build a LIGO-type detector on Earth
that is long enough to pick up primordial gravitational waves.
The arms would need to stretch beyond Earth itself.
Fortunately, astronomers have a way to do just that.
It’s called a pulsar timing array.
A pulsar is a spinning neutron star that shoots
electromagnetic radiation from its poles.
And from Earth’s vantage point, that spinning
makes the radiation appear to flash.
So basically, pulsars are cosmic lighthouses.
And some of these lighthouses are flashing so steadily–
the pulsars are spinning at such a precise speed–
that their signals run like clockwork.
Or, they do until something disturbs the space between them and us.
And sometimes, that something is a gravitational wave.
Just like when a gravitational wave rolls through the Earth
and ever-so-slightly stretches and compresses the arms of LIGO,
it can do the same thing to interstellar space.
It changes the distance that light travels from a pulsar to the Earth.
And that tweaks the arrival time.
A flash either arrives too soon, or too late.
Now, gravitational waves aren’t the only thing that can mess
with a pulsar’s timing, so astronomers have to account for a lot of
other possibilities before declaring that a gravitational wave
is behind any wonky signal.
But in principle, an array of extremely regular pulsars
can work a lot like LIGO, just with a bunch more arms,
and blown up to an interstellar scale.
And this is how astronomers finally discovered evidence of
low-frequency gravitational waves in 2023.
As part of a collaboration called NANOGrav, researchers around
the world compiled 15 years of data from 67 pulsars.
And what they found wasn’t one isolated wave rolling through,
like what LIGO spots.
Instead, the signal was sort of a background noise,
a pattern of irregularities coming from all directions that suggests
all of spacetime is subtly and slowly quivering.
But unfortunately, the NANOGrav team doesn’t know
what’s creating this gravitational wave background.
It’s like if you had a bunch of rowboats and swimmers and
gusts of wind creating waves in a lake
and you wanted to figure out which wave came from which source,
but you could only look at the water, and not the things on the water.
And when a bunch of waves interfere with each other,
it’s not easy to tease them apart.
The leading hypothesis is that these
low-frequency waves are coming from
supermassive black holes at the centers of merging galaxies.
Unlike the smaller black holes LIGO detects,
these behemoths spend millions of years slowly spiraling
around each other before they finally collide.
Based on computer simulations, spiraling supermassive black holes
could produce the signal that the NANOGrav team observed.
And if a supermassive black hole merger is what caused these waves,
it would be the first direct evidence of such an event ever happening.
But for now, there are still a few other possibilities on the table.
One of them is that these gravitational waves, or at least some of them,
are primordial—created in the first second of the universe’s existence.
Scientists on the NANOGrav team also simulated
what a low-frequency gravitational wave background
would look like if it was created by inflation.
And those simulations reproduced what the researchers observed.
As their pulsar timing array provides more data, the NANOGrav team
hopes they’ll be able to pin down the true origin of this signal.
Something that’ll really help is figuring out how to tease apart different
sources of gravitational waves based on the frequencies they generate.
Going back to the lake analogy, you can imagine that waves
from a passing boat might look different than waves stirred up
by the wind or by a person doing a bellyflop.
So if scientists can figure out the typical profile of a gravitational wave
produced by a supermassive black hole merger,
they'll be able to filter out those signals
and just look for primordial waves.
If they ever find them, we’d have some of the
strongest evidence yet that inflation really did happen.
And hopefully, the details of those primordial waves
would teach us what exactly went down back then.
By studying a signal that’s basically as old as the known universe,
we’d finally be able to confirm whether or not some fundamental laws
of physics work the way theoretical physicists think they do.
Which would be a lot better than looking at a picture
from when the universe was 380,000 thousand years old
and trying to extrapolate backward.
But first, we need more data. Astronomers need to find more pulsars
that are reliable enough to add to NANOGrav’s array,
and watch the array for even longer to see
if meaningful patterns emerge.
Humanity will have to wait a while before anyone announces
they’ve definitely detected a primordial gravitational wave.
But we might not have to wait as long as you might think,
because NANOGrav isn’t the only collaboration doing this work!
Teams in China and Australia are also monitoring
their own pulsar timing arrays.
And all these teams are working to combine their data
to try and get an even clearer picture.
And even though it’s using gravity instead of light to take that picture,
making it a little hard to print out,
I’m ready to add an even older baby picture
to the universe’s photo album.
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