Thank you to Opera for supporting PBS. A few weeks ago the world's gravitational
wave astronomers announced something pretty wild. The moderately confident detection of
pervasive ripples in the fabric of space time that presumably fills the cosmos, detected by
watching for subtle connections between the signals from rapidly spinning cores of dead stars
in our galactic neighbourhood. In other words, the gravitational wave background has probably
been detected using a pulsar timing array. The likely detection of the gravitational
wave background is huge. Several channels have gotten to this news item
before us, but, in our defence, we did an episode on the slightly more
tentative detection two years ago. Now the detection is on firm footing and the basics have
been thoroughly covered by ourselves and others, we can dig a little deeper. Today I want to
talk a bit about what it took to spot the gravitational wave background, and then more
about what it tells us about our universe. First up, let’s take a moment to appreciate
the awesomeness of this achievement. We started with a deceptively simple idea. And
by we I mean humanity, but specifically the incidence of humanity named Albert Einstein.
The following thought popped into his head: “For a man falling from the roof of a
house, there is no gravitational field.” This became the equivalence principle,
which basically states that the feeling of weightlessness you have while falling is
the same as weightlessness in the absence of gravity. And the feeling of heaviness
when accelerating is the same as that when stationary in a gravitational field. At least
as far as the laws of physics are concerned. From the equivalence principle and one more
axiom that the speed of light is constant for all observers, an inevitable chain of reasoning
led Einstein to the general theory of relativity, which explains gravity as being due
to the warping of space and time. The equations of GR give us so much
more than gravity—they predict that gravitational fields slow clocks and deflect
light, reveal the inevitability of black holes, and also predict that the fabric
of spacetime should carry waves. Gravitational waves were the last great prediction
of general relativity to be experimentally verified, and that happened only in 2016
when LIGO spotted the spacetime ripples caused when a pair of black holes spiraled together
and merged over a billion light years away. LIGO did this by measuring the literal stretching
and squishing of space, using what amount to a pair of ultra-precise rulers a few kilometers
in length, set at right angles to each other. In the 8 years we’ve been doing this, we’ve
observed the gravitational waves resulting from the final inspiral of pairs of black holes
and/or neutron stars. These have mass from a few to a few tens of times the mass of the Sun.
Our Earth-based facilities were built to be sensitive to these, because we knew there should
be lots of these sorts of gravitational waves. Now as waves, gravitational waves
have wavelengths. An observatory will be sensitive to wavelengths that have a
similar size to the detector arm—to its rulers. And inspiralling stellar corpses generate
wavelengths roughly equal to their orbital period times the speed of light—which is a few
kilometers in the last seconds of that inspiral. The larger the orbit the longer the
wavelength. The gravitational waves produced by binary stellar mass black
holes when they’re further apart should be visible to the Laser Interferometer
Space Antenna—LISA—with its 2.5 million km long arms of its laser-connected
spacecraft. At least, after it launches. But there are also gravitational waves that
stretch for light years—waves that no human-built device could hope to detect because we can’t build
galactic-scale rulers. However by happy chance the galaxy has obliged and provided us with a
network of natural rulers—the pulsars. Well, more clocks than rulers. Pulsars are rapidly
rotating and precessing neutron stars whose jets sweep past the Earth, resulting in blips of
electromagnetic radiation that repeat with extreme regularity, sometimes several hundred times a
second. Those ridiculously fast ones are called millisecond pulsars, and they are the most precise
clocks in the universe, natural or unnatural. But we wanted a ruler, not a clock. But with
the conveniently constant speed of light, a clock becomes a ruler if we just measure the
travel time of light. If a gravitational wave passes by the stream of incoming signals
from a pulsar, it will stretch and compact the space between those pulses. Measuring
the change in pulse arrival time measures the gravitational wave. This can, in principle,
be used to spot individual gravitational waves. But that’s not what this new result is. Several
international collaborations have now been watching dozens of pulsars for over 15 years using
many of the largest radio telescopes on Earth. These pulsar timing arrays don’t yet have a
sure signal from a single gravitational wave, but essentially all these teams agree that their
data reveal something that’s arguably even cooler. All of spacetime across the pulsar network, and probably across the universe, is a bit
wibbly wobbly. They claim detection of the stochastic gravitational wave background—the
jumbled overlap of many many very weak but very long wavelength gravitational waves that
must originate from across the known universe. We don’t know what creates the background
yet—it could be echoes from the inflationary epoch which kickstarted the Big Bang, or
universe-wide phase transitions right after that. It could be cosmic string collisions in
which fissures in spacetime tangle and split, or the frolicking of galactic gigawhales in
galaxies far far away. Well probably not that, but we can hope. Most likely, however, this
gravitational wave background results from binary black holes. Although in this case,
it’s not from the individual 10s-of-solar-mass black holes seen by LIGO. We’re probably
seeing the reverberating tremors caused by binary pairs of behemoth supermassive
black holes in the hearts of galaxies. These SMBHs with masses of millions
to billions that of our sun, are close to my own heart as a researcher,
so I’m more than a little excited about what we can learn about them, and I’m going to
spend some time talking about them today. But first, let’s start with an analogy to
get a better picture of all this craziness. Take the surface of a still lake and very
rapidly stir it at one point with a pin point. The expanding ripples are like the
gravitational radiation detected by LIGO. Now, instead of one pin-point spiral, stir
the surface of the lake with many many, I dunno, tree trunks or something, but much
more slowly. The entire lake is now covered in a jumble of very low-frequency ripples
that aren’t distinguishable from each other. This is similar to what the stochastic
gravitational wave background should look like if it's caused by
binary supermassive black holes. LIGO is tiny compared to the resulting spacetime
ripples. Both of its arms are affected to the same degree by lightyears-long oscillation,
and so it doesn’t notice their passage. But the relative distances to pulsars
are affected by these enormous waves, and so they should cause observable shifts in
the timing of their pulses as we observe them. These ripples are messy—apparently random, or
“stochastic”. So how can we be sure we’re even seeing gravitational waves? After all, there
are various reasons why the rate of a pulsar’s signal may change. Pulsar rotations rates
can slow down or speed up, and the travel time of their signals to us can be affected
by more than gravitational waves. For example, passing through a region of ionized gas slows the
radio light. But all of these things should affect each pulsar individually, or at worst affect
groups of pulsars in one particular direction. However, gravitational waves cause the pulse
rates of pulsars across the galaxy to change in ways that are correlated with each other.
Imagine signals traveling to us from different pairs of pulsars. Those signals could be traveling
together if the pulsars are near each other on the sky. They could be traveling to us from opposite
directions on the sky. Or the signals could be traveling at right angles to each other. Or they
could be situated in between those extreme cases. Any given gravitational wave that makes up
part of the background will also be traveling through the galaxy in some direction
relative to both of the pulsar signals. In some cases, that relative direction will
cause both pulse rates to be affected in the same way—correlated, and in some cases they’ll
be affected in opposite ways or — anticorrelated. For example, you would get a correlated
pulsar timing shift if both pulsar signals are surfing the same gravitational wave, or if
a 180-degree-separated pulsar pair encounters a gravitational wave moving at right angles to
both signals. And you’d get an anti-correlated shift if the pulsar signals are traveling
at right angles to each other because of the way gravitational waves alternatively stretch
and squish space at 90 degrees as they pass. The correlation or anticorrelation due to a single
gravitational wave is extremely difficult to pick out from all the sources of noise. However, if you
look at enough pairs of pulsars for enough time, you expect to see a statistical correlation
in what we call the pulsar timing residual— that'sthe amount of deviation from the very
precise expected arrival time of these pulses. This is the Hellings-Downs curve. It’s
the theoretical correlation between pulsar timing residuals for pairs of pulsars as
a function of their separation on the sky. Pulsars with little separation should be highly
correlated. Pulsars with 180 degree separation should be somewhat correlated. Pulsars with
90 degree separation should be anticorrelated. OK, so how are the real pulsars behaving?
This is the result published by the NANOgrav collaboration. This is for
every combination of pairs for 67 pulsars observed over 15 years. It’s very
consistent with our Hellings-Downs curve. NANOgrav claims that this is from 3.5
to 4 sigma depending on the statistical analysis used. That means it’s not
quite a slam-dunk 5 sigma detection, in that this apparent correlation could still have
popped out of random noise by a 1 in thousands chance. But it’s looking increasingly likely
that the correlations are real. The same results have been observed by other pulsar timing array
experiments, with varying degrees of confidence. Now we can start looking into what we learn
about the universe from this observation. But before we do that, let’s pause for a moment to
appreciate how crazy this achievement really is. Remember that we started with a simple
thought experiment about the experience of someone falling off a
roof. That “what if” scenario led us all the way to an actual observation
of galactic-scale spacetime ripples. But spacetime ripples from what? I’ve been talking
about binary supermassive black holes, because that’s what the NANOgrav team thinks this is. The
type of timing correlation that was observed is what you’d expect from many, many sources of
gravitational waves that are a) powerful and low frequency, b) randomly distributed across
the cosmos, and c) randomly polarized - so no preferred direction for the stretching and
squishing of space from any given wave. Any such population of sources should
give you this characteristic curve. So why binary SMBHs? Well they do potentially
fit the requirements, but just as importantly, we know they should exist. We know that every
galaxy contains a huge black hole at its center. And we know that bigger galaxies are made from
smaller galaxies combining, and that bigger galaxies have bigger black holes. It only stands
to reason that there are a good number of binary supermassive black holes out there, even if we
haven’t seen them directly detected them yet. Any other source of gravitational wave background like this is much more speculative—and we
talk about those in our previous episode. OK, so what does this signal tell us about
giant black holes, assuming they’re the cause? The pulsar timing data contains more information
than the Hellings-Downs correlation. We also learn about the frequencies of the underlying waves.
For binary black holes, the frequency of the outgoing gravitational wave is basically the rate
at which the monsters orbit each other. If we can see what different frequencies make up the jumbled
mess of the gravitational wave background we can learn something about those black hole orbits.
In particular, we can learn about how they spiral together and eventually merge. For example,
if those binaries spend a lot of time orbiting each other at a great distance, then there
should be a very strong low frequency signal. This is the NANOgrav frequency spectrum.
The grey … funny shapes represent the strength of each frequency observed
in the gravitational wave background. The dashed line is what we expect from
a simple model of how supermassive black holes grew and formed binary pairs over cosmic
history. It’s not inconsistent with the data, but there’s a hint of difference compared to
the simple model prediction. Perhaps too much high frequency signal, or too little low frequency
signal. The NANOgrav collaboration speculate that the latter could be due to the binary supermassive
black holes interacting with the stars of their surrounding galaxies, causing them to spiral
together quicker than without that interaction. There’s also hint that the gravitational
wave background is a bit stronger than expected from the simple binary black hole model, which means the SMBH pairs may be more massive
than expected or there may be many more of them. But this is all very loose, and there isn’t
enough data yet to make any conclusive statements. But that data is coming. With this spectacular
result you can be sure our pulsar timing array projects will continue. Now, the longer we
watch, the larger these arrays get. That’s because gravitational waves will have time to
traverse larger distances and affect the timing of more distant pulsars. For example, in NANOgrav’s
12.5 year data release they included 47 pulsars, while at 15 years they could include 67.
As the pulsar timing array gets larger, and as we track the correlations
between pulsar pairs for longer, we hope this detection of the gravitational
wave background becomes rock solid. Then we can really start to pin down its origin, and use
our new galaxy-scale observatory to study those mysterious cosmic cataclysms that are sending
tremors through the fabric of all of spacetime. Thank you to Opera for supporting PBS. One
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