Thread by @realscientists

Hi everyone, so this morning I want to talk about gravitational waves. Ooooooh! These are waves travelling through spacetime that warp space and are created by all objects moving but to get the really strong ones to detect you need to have really massive objects like...

...black holes or neutron stars to be orbiting around each other to detect them. In theory the Earth and Moon and Sun all create gravitational waves (GWs) but they'er just way too weak. So Einstein's theory of relativity predicted them but it's taken over 100 years to make a...

...device sensitive enough to detect them. The breakthrough was in 2015 when @LIGO detected a signal during an engineering run. The signal is shown here a long with it turned into sounds, you can hear the chirp going "wwwwhhhhhooooOOOPPP!". :o)

If we were close enough to see it his is what we would see:

So this is all quite cool and we now have 5 of these mergers detected and some of them even have their own twitter accounts: @iamgw150914

Anyway so the detection is amazing science in...

...terms of measuring distances down to less than a thousandth of the size of a nucleus, like a thousand, billion, billionth of a metre. It's an amazing achievement in quantum optics and experimental design and there are still improvements and work to be done. However for me...

...I find it exciting that we are measuring the masses of black holes and neutron stars highly accurately and getting a window on these remnants of stars that we never had before. Here is @LIGO's current list of measurements as well as masses from binary stars. You can see...

...that the ones form the GW events are much higher in mass than those in binaries which worried/surprized a lot of people when they were first detected.

Now to go back a few years rumours had been around for months at the end of 2015 and early 2016 that something had been found

...by LIGO and this was a big deal, gravitational waves are awesome for so many reasons and the first detection is really important. The thing is the news was so big that it's always going to be difficult to keep it quiet, I really think conspiracy theorists give people too...

...much credit for being able to keep secrets. So this ended up to an email from someone being posted on facebook saying that the masses detected of the black holes were 36+29 solar masses. This began me searching through my models of my...

...stellar models in my Binary Population and Spectral Synthesis (BPASS) code to see if we did have any of these binaries that would merge within the age of the Universe. I was a bit shocked that we found some... and then we had to decide what to do and we wrote it up.

Why? Well because the sense I got from people was that those masses were completely too high and unexpected but then they were in my models of binary stars that were computed for a completely different reason. So it matched with my idea of using the same models to compare to...

...many different observations.

[This thread will be continued after a short intermission...]

And we're back! Had to talk to my student, who is working on a GW project. :o)

Unfortunately I think because we had the paper out so quickly I think we annoyed a few people which I'm sorry about but it was just SOOOOO EXCITING!

Why? Well I've tested my model against stars when they're shining light, I've tested them against stars exploding, I've tested them against observations of the stars at the very edge of the observable Universe and now I can do this test and compare the black hole mergers I...

...predict from the models and constrain the type of stars they came from. It's really difficult to do this, to model this very last stage of the life story of these stars you need to model their interactions through all their lifetimes, two supernovae to get to the end.

There are so many things we're unsure and uncertain of in how stars evolve so we have to take our best guess and the fact we can get to something that even closely matches what we detect is astounding and tells us we're at least on the right track.

So here on this figure I've plotted up the masses of the 5 detected black hole mergers over predictions from my code for the number of events we should see at different metallicities of stars which really translates into different generations of stars. Lower Z=earlier generation.

This figure was taken from arxiv.org/abs/1710.02154

What we can see is that you need to have earlier generations of star to have the more massive black holes. Also here I've separated out the stars with the "primary and secondary" which are the initially more or less massive...

...stars, there is no way we can tell which is which from observations but I did this plot here so you can see if one is more likely than the other. And at the lowest metallicities it becomes more likely that the more massive black hole arises from the initially less massive...

...star! It's this kind of modelling that makes it interesting. Anyway we're currently now trying to predict the rate of these events and more from these models. And have done so in a recent paper here: arxiv.org/abs/1807.07659

So on the left are the neutron-star-neutron star...

...merger rates and on the right the black-hole-black-hole merger rates. The observations are the points with error bars and the lines are our BPASS model predictions for today (solid) and at a redshift of z=0.5 (dashed). So for the black holes we're a little high but okay.

But for the neutron stars you'll note there is a HUGE scatter. That's because there are many type of events that we expect merging neutron stars to cause and many different way to constrain their rates so these predictions are highly uncertain and span 3 orders of magnitude!

But the LIGO estimate in that box on the right is at the upper end of these measurements and we only just agree with the rate. The problem is we don't know if we were "lucky" with detecting a neutron star merger or if they are very high.

The problem with predicting the rate...

...from stellar models is the binary has to survive two supernovae, two stellar explosions that might unbind the binary. So surviving double neutron star binaries are rare. However in work with John Bray one of my previous students we have found that we can get a rate of...

...3000 mergers/yr/Gpc^3 rather than the 300 we predict here, just by using a different model for the "kicks" the neutron stars get in their formation. We're now waiting for the next observing run, if the rate is as high as we saw from GW170817 we learn something about...

..how neutron stars are formed and that's going to be very interesting. :oD

The caveat being that our understanding of star formation in the Universe is also very uncertain. If we have more stars we have a higher rate, if we have few stars forming early on we have a lower rate.

All these things are connected but hopefully by trying to separate out and fit everything at the same time some of the degeneracies in uncertainty will cancel out and we can really understand something about the Universe at the same time as the stars in it. :o)

The next observing run stars early next year and I think we're all pretty excited!

Okay this thread is already pretty long but one last thing. With the neutron-star mergers as there is "stuff" and not just a black hole we can see the merger too. With GW170817 we were able to see a "kilonova" from the merging event too which means we know something about the...

...host galaxy too and this is what is looked like. Taken from here. sci-news.com/astronomy/ligh…

It's an elliptical galaxy with no recent star formation. The stars must have been very old as they would take billions of years to merge. But understanding the stellar content of the...

...galaxy we can work out when most of the stars were formed and thus the most likely age of the stars that merged. While for 1 systems this isn't so useful with many of the mergers detected we'd be able to build up a useful understanding of the population.

But to do this...

...you need to have a model that predicts what stars look like when you take a spectrum with a telescope. So you need a spectral synthesis code. The problem is all the publicly available spectra synthesis codes all assume that stars are single stars....

But the progenitor of GW170817 was a binary star, thus we actually need a binary population and spectral synthesis code to predict the rate of these different types of GW mergers and what the population of binary stars will look like in the host galaxy when we look at it.

Um... as far as I know I've got the only one where I'm trying to make as much of our data publicly available so people can go do this. BPASS is perfectly suited to this and I just wish I had time to do this science but there are too many other exciting things to do!

There is a small chance that I might win funding this year to do it but it's roughly 50/50 if I do or not. If not I'll just have to make the GW event data from BPASS public so someone else can do it. But it's exciting to think that a project I started out over 10 years ago to...

...predict the right number of different supernova types and match the observed spectra of high redshift galaxies is today the one code that is naturally suited to study GW events and their host galaxies at the same time.

Phew, thank you for listening. I think I need a second coffee and/or a lie down. :oD