Thread by @hankgreen

Wanna understand some of the biggest science news of the year (maybe the decade?) better than 99% of people? That new should be coming out tomorrow evening, but we know a bit about it...so here's a thread:

https://twitter.com/EliseValdes/status/1673736935053967367

For almost all of the history of astronomy we have learned everything we know about the universe by looking at photons. Whether Galileo was looking at the visible light of Jupiter or the JWST was looking at the infra-red of the Orion Nebula (which it did recently...check it out.)

But recently, we got a new way...we can detect actual ripples in spacetime, and we do it using a tool called LIGO that is basically two, perpendicular, three mile long arms that are /exactly/ the same length.

(there's actually two of them to ensure good data.)

If one of these spacetime ripples passes over LIGO, one of the arms will get a little longer and shorter than the other one. We're talking less than the width of a proton...but enough to detect!!

With this, we can detect when two black holes orbit and then fall into each other. This is the easiest thing to detect with gravitational waves, but it's still very hard to detect. But the biggest deal here is...we proved there is a whole new way to look at the universe.

But just like visible light is useful, but a very narrow look at things...the reality is, there's way more going on with gravitational waves than we can see with instruments as small as LIGO. To detect lower frequency waves, though, we'd need detectors millions of times bigger.

Luckily, the universe gave us one. And, even better, a bunch of scientists were, not just clever enough, but PERSISTENT enough to figure out how to use it.

Because, in the night sky, there are these things called pulsars...and they pulse.

They spin super fast and (importantly) super regularly. And they're distributed all over the galaxy. And when spacetime shrinks or expands as a gravitational wave passes between them and us, theoretically, they should appear (to us) to speed up or slow down JUST A TINY BIT.

Some folks figured this out years ago, and so...they started timing pulsars....and they didn't stop. They looked up once a month for the last fifteen years to check, not ever sure if there would be any way to correct the data enough, or if there would even be a signal to detect!

And they FOUGHT LIKE HELL to get funding for something super /out there/ and experimental when there were lots of proven types of astronomy that were easier to land dollars for. But, in the end, more people joined...

Now there are Pulsar Timing Array (PTA) teams all over the world. @NANOGrav in the US, @APT_GW in Africa, @EPTAGW in Europe, @InPTA_GW in India, @ARC_OzGRav in Australia...sorry if there are others I don't know about!

Pulsar signals are radio waves, so crucially important to this project are radio telescopes all around the world, including, before its collapse, the Arecibo telescope in Puerto Rico.

And, what is going to be announced tomorrow evening...I don't know. But now you know a little about Pulsar Timing Arrays, which should better prepare you for the news...and I am very excited to talk about what they did or didn't find tomorrow night, when the embargoes lift!!

OK! THE EMBARGO HAS LIFTED AND WE'RE MOVING!!
I'm just gonna add to this thread since it has all of the "how" and now we can talk about the "what."

The "what" is that, after 15 years of collecting pulsar timing data, and people realizing that, yes indeed this is potentially an extremely powerful tool for learning about our universe, they have detected and begun learning things from the "Gravitational Wave Background."

That background seems to be built /mostly/ of waves coming from super-massive black holes orbiting and slowly moving toward each other. All of these are in the centers of /other/ galaxies.

(I don't think a pulsar timing array actually can't detect things in our galaxy because the array is...made of pulsars distributed throughout our galaxy.)

The data are super consistent with models of what the GWB should look like, meaning...excellent confirmation of some current understandings. Though it also seems like there are significantly more supermassive black holes than we thought there would be.

But as we get more data and process it more carefully, we should be able to start to tease individual signals out from each other...right now the signal is like a giant chord being played, and picking out individual notes is going to tell us SO MUCH.

But there is also a paper called "The NANOGrav 15-year Data Set: Search for Signals from New Physics" which is possibly the thing that folks have been most excited about. It's a monster of a paper...it has a LOT of authors!

It is the first attempt to start to tease out the black hole signals from deeper signals that could originate from the very first moments of the existence of the universe. They're interested in this because our current main way of understanding the universe is...missing stuff.

That system for understanding stuff is called the Standard Model, so there are a bunch of "Beyond Standard Model" models that are guesses at what the universe really is and how it really works, but that's VERY hard to test for.

And only having one thing to look at in outer space (various kinds of light) holds us back from the kinds of observations that might test for these Beyond Standard Model ideas.

The thinking is that these gravitational signals from the first moments of the universe could tell us which of the ideas are correct. They haven't been able to do that yet (though they were already able to, it seems, rule one of them out.)

So that's some of the details.

But most importantly for me, this is just scratching the surface of what we are going to discover using this technique. What we're watching right now is not unlike being the first human eye that ever pushed against a telescope.

It's the creation of an entirely new and EXTREMELY POWERFUL tool for observing the universe. It took 15 years of data collection to get data good enough to start to say stuff about, but more data is going to be coming much faster.

This is a graph of the pulsars that have been added to @NANOGrav's PTA and every year we add more data from more pulsars, we get better resolution and more data to process.

The fact that they're already using this to interrogate new physics is spine tingling. And that soon we'll be able to tease out signals from specific galaxies and then observe those galaxies with traditional telescopes...that's gonna be good!!!

Congratulations to the thousands of people who have worked on this project. And a special recognition that, with any project this size, there will be people who worked on it who weren't around to see this day.

But I'm confident that none of us will be around to see every discovery made with this new technique, because humans will be using it (and extensions of it) to examine and understand their universe for, I think, centuries.