Hello Space Fans and welcome to another edition of Space Fan News.
This week astronomers study the fifth gravitational wave event and its associated optical counterparts
and it looks like we can finally get rid of some theories about the expanding universe
and scientists at CERN are struggling to understand why they haven’t observed a asymmetry between
matter and anti-matter.
On August 17th 2017, if you were trying to find an astronomer, you were probably having
a hard time of it because on that day, a full one-quarter of the world’s astronomers and
observatories were busy.
Most of you have probably heard by now of the announcement of the detection of a fifth
gravitational wave event from the LIGO/Virgo collaboration that occurred a week ago last
Monday in a widely-anticipated news conference.
At first I couldn’t understand why everyone was so much more excited about this particular
event over the previous four but I soon found out.
The first four gravitational wave events detected by LIGO were of two intermediate-sized black
holes that merged to form a somewhat larger black hole and also emitted some gravitational
waves in the process.
The detection announced earlier this month was a much different and much more rare event.
The fifth detection was actually the result of two neutron stars located in a distant
galaxy known as NGC 4993 some 130 million light years away.
As their name suggests, neutron stars are made entirely of neutrons and they are packed
so tightly together that a star possessing the mass of our Sun would be squished to a
size only 10 to 15 km in diameter, and a teaspoon of this stuff would weigh about one-billion
tons on Earth.
I always find it funny that we use a unit of ‘teaspoon’ to visualize how heavy a
dense star is.
As if we could actually grasp how much that is.
Huh, yeah one teaspoon of sugar weighs about this much.
So a neutron star?
A billion tons.
Oh OK.
I can see that now.
Anyway, neutron stars are the smallest and densest stars we know about and are the collapsed
core of a star between 10 and 29 solar masses that has died.
Neutron stars that can be observed are very hot and typically have a surface temperature
around 600,000 K.
So neutron stars are quite interesting in and of themselves but when two or more of
them merge, it gets even better.
The thing is, colliding neutron stars is a really rare occurence, there aren’t that
many clumped together enough to do all that much merging so when they happen, it’s a
really big deal.
Which makes the observations made last August that much more impressive.
Less than two years after the first gravitational wave signal ever seen EVER was made, we see
this really rare event.
So, astronomers were understandably excited.
But there is so much more that happened that day.
Less than two seconds after LIGO and Virgo saw the gravitational wave, NASA’s Fermi
Spacecraft detected a gamma ray burst.
Astronomers have long suspected that short gamma ray bursts were the result of neutron
star collisions but now they had observations to back them up.
They caught one!
Seeing the gravitational waves first is important, you want the stars to smash together, then
emit radiation.
And that’s what happened here, the gamma rays came 1.7 seconds after the gw event.
After that, pandemonium in the astronomical community.
Astronomers around the world received text alerts that this was happening and many sprung
into action.
As I said, 25% of the world’s astronomers grabbed whatever telescope they had time on
and made observations.
Every conceivable wavelength from gamma rays, to ultraviolet to optical to infrared down
to radio was used.
At first, they didn’t have a good idea where this thing came from because LIGO and Fermi
aren’t that good at pinpointing things in the sky, but after some clever work using
optical observations from the general area of the sky indicated by these initial observations,
astronomers were able to use template images of the sky taken about a few months earlier
to compare what they were seeing on that August night.
And that’s when they discovered where it was coming from: an elliptical galaxy 130
million light years away known as NCG 4993.
Images taken in April with Hubble compared to those taken on that August night revealed
a definite optical brightening showing the optical wavelengths of the short gamma ray
burst.
So let’s summarize: gravitational waves are very hard to detect, even from black holes,
neutron stars are even harder.
And collisions of neutron stars are exceedingly rare.
And they saw a gamma ray burst 1.7 seconds later and astronomers around the world localized
the event to a single galaxy and imaged the crap out of it.
So, what did they learn?
The first thing they saw was that the gravitational wave signal was way longer than the ones generated
from merging black holes, about 30 times longer.
The gravitational wave signal lasted around 100 seconds.
Here is a rough timeline leading up to the collision.
Astronomers think initially there was a pair of stars: a normal star and a neutron star
happily going about their day until the normal star explodes, totally ruining their day and
leaving a pair of neutron stars whirling around each other in an elliptical orbit.
Then, finally, the neutron stars merged and created the gravitational waves detected by
LIGO and Virgo.
1.7 seconds later, the short gamma ray burst happens and was seen by Fermi.
Alerts went out and observatories all over the world saw the optical part of that SGRB
and omething called a kilonova that produced ultraviolet (violet), optical and infrared
(blue-white to red) emission.
What’s a kilonova you may ask?
I know I did.
A kilonova is more powerful than a nova and less powerful than a supernova.
In fact, they are 1000 times more powerful than a nova, hence the name.
But they are way more interesting than that.
The first kilonova was discovered in 2013 by the Hubble Space Telescope in association
with a short gamma ray burst that occurred that year and because they are associated
with sGRBs, they have jets of material that shoot out full of all kinds of interesting
things.
As you can imagine, these jets are rich in neutrons, and they also have a lot of heavy
elements.
Astronomers took spectra of the kilonova jets and saw what they had predicted for a long
time.
A lot of markers of lanthenides in their spectra.
What’s a lanthenide?
It’s the good stuff: gold, platinum, uranium, heavy metals, you know stuff no one really
cares about.
According to the observations taken last August, astronomers estimate that kilonova produced
200 Earth masses of gold, and nearly 500 Earth masses of platinum.
Astronomers were actually quite surprised at how well the observations of the lanthenide
spectra compared with theories.
But in addition to all this other good stuff that came out of this event: we have yet another
data point with regard to dark matter and dark energy that we can use.
In order to explain dark matter and dark energy, some astronomers have said that maybe there
is something about gravity we don’t understand at large scales that needs rethinking.
They called it Modified Newtonian Dynamics or MOND.
MOND was an idea that tried to tweak the well-understood notion of gravity to fit an accelerating universe
and to explain dark matter.
That would mean changing relativity, something that must be done carefully because relativity
has proven itself time and again, it is a framework that has stood up to scrutiny and
has proven very robust in describing how the universe works.
With every discovery, we seem to be announcing that Einstein was right!
Relativity works!
Well, one of the things that was confirmed with this LIGO neutron star merger was the
speed of gravity.
Relativity predicts that gravity should travel at the speed of light.
The fact that the short gamma ray burst was detected only 1.7 seconds after the gravitational
waves after travelling 130 million light years suggests very strongly that gravity is travelling
at the speed of light.
Otherwise, we’d have a big mismatch of arrival times.
So many astronomers are saying this puts the final nail in the coffin of modified gravity.
With this observations, we have a pretty good grasp of how it works on larger scales and
we need to start looking for more data that helps us understand dark matter and dark energy.
All in all, this event has really been amazing.
First we managed to capture an event that is really rare in the universe: neutron star
collisions, then we saw the associated gamma ray burst and kilonova which happened to be
pointing our way.
If those jets had been oriented elsewhere, we couldn’t have gotten the spectra.
As it was, astronomers got lucky and were able to confirm that heavy elements like gold
and platinum throughout the universe come to us from kilonova.
And if that wasn’t enough, for good measure we learned that gravity does indeed travel
at the speed of light.
All in all, August 17th 2017 was a good day for astronomers, and indeed for all of us.
One quarter of all astronomers around the world were busy that day and the weeks that
followed, and for very good reason.
There was one little niggling question though: 1.7 seconds seemed a bit long.
Why wasn’t sGRB sooner?
Next, scientists at CERN have been struggling to find asymmetry between matter and antimatter,
both of which were created in the Big Bang, but for some reason, normal matter won out.
For those that don’t know, when matter and antimatter collide, they annihilate each other
completely so the fact that we live in a universe of normal matter must mean that there was
a little more of it left over from the Big Bang than antimatter.
At least that was the assumption, and scientists have been looking hard to find some way in
which there could be more normal matter than antimatter in our universe.
They call it the “matter antimatter asymmetry problem” and they have yet to solve it.
What was different between the two?
Now to try and figure this out, the guys at CERN have some really neat toys.
They have this thing called a Penning trap, which holds - wait for it - antiprotons.
That’s right, it’s like a little box they can keep antimatter in and they had some sitting
around in these Penning traps since 2015 and they figured they better use em up before
they get out.
Their experiment involved a high-precision technique for measuring the antiproton’s
magnetic field and they compared it with regular protons.
Before the closest they could get to measuring that magnetic field was 2.792 847 350 nuclear
magnetons.
Which was different than the one for normal protons so there was hope that there was some
asymmetry.
Then they tried their new technique and got that the magnetic moment of the antiproton
was 2.792 847 344 1 nuclear magnetons.
A measurement over 350 times more precise than before.
The problem?
That’s exactly the same moment as the normal proton, down to the last decimal place they
could measure.
No asymmetry here.
Sigh.
So scientist say the next step is to keep going for more precision.
After all, they say, there has to be some reason why we’re here living and breathing.
That’s it for this week space fans, I want thank the Great Courses Plus for sponsoring
Deep Astronomy this month.
Thanks to all Patreon Patrons who make sure Deep Astronomy Content continues to be produced,
I couldn’t do it without you.
Thanks to all of you for watching and as always, Keep Looking Up!