Gravitational Wave Background Discovered?

it was pretty impressive when we built this giant machine that spotted gravitational waves from colliding black holes well we've just taken it to the next level with a galaxy spanning gravitational wave detector that may have detected a foundational element of space itself the gravitational wave background when the laser interferometer gravitational wave observatory ligo turned on in 2015 it quickly spotted exactly what it was built to detect vast ripples in the fabric of space-time produced by colliding black holes a billion light years away since then ligo and its partner virgo have detected 50 similar signals from merging black holes and neutron stars across the universe these sweep past the earth every few days causing space itself to oscillate lengths and widths contract and expand in turn by the tiniest fraction one one thousandth of the width of a single proton over the four kilometer arms of ligo antennas and these are only the most powerful gravitational waves that we can detect produced by only relatively nearby mergers the reality of these strong space-time ripples reveals an awesome truth we know that black holes and neutron stars have been merging for all of cosmic history and supernovae have been exploding and stellar remnants have been spinning and probably at or before the instant of the big bang insanely energetic events took place and all of these produce gravitational waves that means the entire universe must be thrumbing with these vibrations all the time and everywhere we call this underlying buzz the gravitational wave background and it could reveal the lives of the most massive black holes in the universe and provide a glimpse into the time before the big bang and reveal the structure of space time itself oh and we may have just detected it ligo itself may be able to detect a small part of the gravitational wave background or the gwb but it's not there yet but it turns out that we already have another device that may have done the job it's a gravitational wave observatory that spans the entire galaxy and recruits some of the most bizarre stars in the universe as our measuring devices i'm talking about the pulsar timing array and about the first tentative detection of the gravitational wave background revealed in early january by the nanograv collaboration but it's going to take a minute to get to that first let's talk about pulsar timing arrays and why we need them ligo has a fundamental limit it can only see those gravitational waves with frequencies around 1 to ten thousand hertz corresponding to relatively compact waves from tens of kilometers to hundreds of thousands of kilometers in wavelength ligo can't see ripples with either smaller or larger wavelengths ligo is like a rowboat on the ocean you're sitting in the rowboat eyes closed you can feel the rise and fall and the rocking of the boat in the choppy ocean you and your boat are sensitive to those waves very tiny ripples on the ocean surface just lap at the hull but you don't notice them and there are also vast rolling waves that are so big that the rowboat feels steady eyes closed you don't notice those either okay now you're on a giant cargo ship it tips forwards and backwards in the ocean due to those giant waves eyes closed on deck you notice that tipping but are oblivious to the little waves that rocked the rowboat and there are also much larger waves a tsunami perhaps or the planet-sized wave of the tide itself there's no human-built ship that is rocked by those waves it's the same with gravitational waves ligo is rocked by the little ripples produced at the instant of merger of stellar remnants like black holes and neutron stars or potentially in supernova explosions but we know the space-time ocean must have swells that are millions to billions of kilometers long produced by the merger of the most massive black holes in the universe or by the slow in spiral of smaller compact objects in our galaxy for that we're building our giant ocean tanker it's the laser interferometer space antenna or lisa but even the largest ships are oblivious to the ocean tide and ridiculously colossal gravitational waves must also exist waves whose single oscillations span the solar system or even the distance to the nearest stars such gravitational waves are expected to be produced in a few different ways black holes with masses of millions to billions of suns live in the cores of most galaxies these super massive black holes can end up orbiting each other when galaxies collide and that slow fatal in spiral produces enormous gravitational waves and there are theoretical sources also cosmic strings topological seams in the fabric of space would produce waves when their kinks become unkinked in the speculative time before the big bang cosmic inflation was driving exponentially accelerating expansion and huge gravitational waves would have been produced in the phase transition that ended inflation all of these space-time tsunamis would be invisible to our tiny human-made machines to see them we'd need a gravitational wave observatory spanning the galaxy unfortunately nature has provided us with just such a network a system of boys scattered across the milky way each fitted with the most perfect measuring devices known to science obviously i'm talking about pulsars to remind you a pulsar is the collapsed core of a massive star masses of at least one and a half suns packed into balls 20 kilometers across and that extreme density converts all of the matter into neutrons they are on the edge of absolute collapse into black holes supported only by weird quantum forces neutron stars tend to channel jets of high energy particles due to their intense magnetic fields they also rotate rapidly with the rotational axis offset from the magnetic poles causing those jets to sweep through space to process when the jets sweep past the earth we see flashes of electromagnetic radiation and we call those flashing sources pulsars some flash every few seconds but the fastest millisecond pulsars can rotate hundreds of times per second just to give you a feel for that this is what a typical pulsar sounds like that's the sound of a trillion trillion trillion tons rotating 1.4 times per second and this is the crab pulsar with its 30 rotations per second and this is a true millisecond pulsar at 174 rotations per second and the fastest known pulsar with its 642 hertz rotation these pulses are not just fast they're exquisitely regular with the most stable millisecond pulsars the exact instant of one of these ticks can be predicted to within one tenth of a millionth of a second several years prior to that individual pulse so nature has graced us with these perfect clocks scattered across the galaxy but how do we use them to detect gravitational waves well the whole thing about gravitational waves is that they change distances which can change the travel time of the pulses from a pulsar leading to a difference in the arrival time of those pulses compared to what was expected here's a simple example imagine a pair of supermassive black holes that are orbiting each other on the other side of the universe as their gravitational waves pass through the milky way they cause space to stretch and contract in succession at right angle to the direction of the wave's motion in what is known as a quadrupole wave if the wave passes through a ring of points those points would move in and out like this okay so imagine these points are pulsars and the earth is at the center of the ring as the wave passes by the signal from these pulsars would have to travel a little further there'd be a lag in the arrival time during that part of the gravitational wave oscillation but for these pulsars the distances traveled are shorter so the signal arrives early after one oscillation cycle the situation reverses if we had pulsars in all directions we could record the so-called timing residuals the lateness or earliness of the pulses and we'd record this sort of shamrock shape where the petals are large timing residuals and the diagonals show very little timing residual of course space is three-dimensional a pulsar signal coming from the direction of the gravitational wave would actually ride that wave keeping pace with it and so may spend a lot of the journey either in the stretched out region of space or in a compacted region those guys have large timing residuals while a pulsar on the opposite side will skip through many peaks and troughs of the gravitational wave so that its timing residual cancels out the end result is a very predictable pattern to the lag experienced by pulsar signals as a function of their position it's the so-called antenna pattern a gravitational wave coming from here towards the earth here causes the largest atomic offsets for pulsars within this structure now a single pulsar isn't enough to map this structure and it's not enough to be even sure that we're seeing a gravitational wave other things can cause shifts in pulsar signals for example neutron stars experience star quakes which can glitch their rotational rates the magnetized plasma between the stars also slows radio waves from pulsars and the relative motion of the earth the sun and the milky way shifts the frequency of pulsars but if you have an array of pulsars a pulsar timing array in fact then this characteristic pattern leads to unmistakable correlations in the arrival times of many pulsar signals so this is probably where you want me to tell you that our galactic array of quantum star clocks has revealed gigantic gravitational waves from some colossal black hole dance or from the big bang or something well maybe but not quite so simple at least not yet instead we may have detected the sum of all such waves the cosmic gravitational wave background let's get to the actual result nanograv the north american nanohurts observatory for gravitational waves made an announcement at the american astronomical society meeting in early january nanograve is a collaboration of over 100 us and canadian researchers and itself is one of several groups that together make up the international pulsar timing array which combines many of the largest radio telescopes on the planet to monitor around 100 millisecond pulsars nanograv's own pulsar timing array consists of 47 pulsars which it monitors with the green bank observatory in west virginia and with the arecibo observatory in puerto rico but i should say it previously monitored with arecibo you might have heard the iconic arecibo is no longer with us after its hurricane damage cable snapped causing a catastrophic collapse on december 1st we had the great honor to visit arecibo in 2019 and we shot some 180 video there i invite you to climb the catwalk with me one last time it's a tragedy to have lost such an important workhorse for our exploration of the universe but nanograf's findings are now part of arecibo's broad and enduring legacy while nanograv didn't detect a single resolvable gravitational wave its 12.5 years of observations have tentatively detected the gravitational wave background this is the sum of all gravitational waves too weak to be detected individually but which together form this universe wide through of activity the gwb doesn't produce the telltale quadrupole antenna pattern rather it results in more subtle correlations in the timing residuals between pulsars individual fluctuations in this background will result in a correlation across all pulsars in the array and the entire background results in this overlapping set of correlations that can be extracted with sophisticated statistical techniques ultimately the result will be a distribution of wave frequency a spectrum for the background and the shape or the slope of that spectrum will help tell us whether the gravitational wave background is produced by supermassive black holes cosmic strings or events during the inflationary epoch or a combination of all of the above but the current nanograph result isn't quite sensitive enough to measure the shape of the spectrum yet it's just revealed a signal correlation in pulsar timing residuals that can't be explained by other known processes if this is real then there's no question that the gravitational wave background will become better and better resolved as we add pulsars to the timing array and we stare at it for longer and we add more and larger radio telescopes the promise is a deeper understanding of the weirdest things in the universe and if we manage to prove the existence of say cosmic strings or inflation or other weird physics this way then we have opened a window into realms of physics beyond our current imagination all it takes is our galactic fleet of star clocks bobbing in the ripples across the ocean of spacetime before we get to comments i want to let you know about overview a new science show from it's okay to be smart's johansson here is a quick trailer some stories are best told from above and we are taking you up there overview is a new series on pbs terra that explores the planet from the lens of a drone [Music] come wander the skies with us as we show you the marvels of our world from a different point of view okay today we're covering comments on the last two episodes that one on the dark universe the physics of dark matter particles and then the one where we explained how time must run slow in gravitational fields the deck to the 16th power asks if i'm asking if dark matter is made of particles okay i'm not sure of the answer to your exact question so i'll just answer the question that you're asking if i'm asking namely is dark matter made of particles the answer is we have no idea it might be particles and lots of people think it is but it also might be larger chunks of stuff that don't emit or absorb light like black holes or failed stars but dark matter may also not be matter at all it could be that general relativity breaks down at large scales these days most researchers are favoring some kind of matter and probably some kind of particle but we just don't know for sure philip rightly points out that in fact you can tell the difference between a lab in freefall versus a lab floating in the absence of gravity because in free fall there's a difference between the strength of the gravitational field between the floor and the ceiling right that's a subtlety that i skipped the equivalence principle is technically only true in a uniform gravitational field so if the phil's strength or direction doesn't change over the region of your experiment then there's no way that experiment can tell the difference between gravity or acceleration or between free fall or the absence of gravity earth's gravitational field changes with distance from the earth and there's also the tidal effect the direction of down shifts slightly from one point to the next but that's why gdenkan experiments are so powerful you get to imagine situations as unrealistically ideal as you like gustain abell typically hears that there are four fundamental forces in physics weak strong electromagnetic and gravity but if gravity is really due to space-time curvature surely it's fundamentally different to the other three which appear to be governed by their own quantum fields and communicated by force carrying particles so this is the million dollar question literally that's roughly the nobel prize money it is reasonable to think of gravity as a force it has a field and that field is the fabric of space-time actually more accurately the force field of gravity is the curvature of space-time more more accurately it's the metric tensor which describes that curvature remember a fuel is just something that takes on a numerical and maybe a vector value everywhere in space so gravity does have its own field but does it have its own particles well many maybe most physicists working in the area think a theory of quantum gravity will reveal that the gravity field space-time curvature itself will quantize to give a particle of gravity the graviton by the way you can also go the other way it's possible to describe the other three forces in geometric terms just like we do gravity it's called yang milsgage theory but actually the whole what is a force thing is something i wanted to delve into in more detail so let's just do an episode shall we many of you noticed that apparently i got it backwards when i said that your feet age more than your head or that your butt is ticking faster than your head well technically you're right time slows down the deeper you go into a gravitational field so most people's feet experience less time and so age less than their heads do shout out to those who pointed out that this also assumes a significant fraction of one's life spent not lying down sorry to presume occasional verticality there folks in my case my defense is that i'm australian and so i grew up upside down we actually film space-time with me suspended from the ceiling and just flip it in post and yes we have an enormous hair gel budget you

Channel: PBS Space Time

Views: 533,691

Rating: 4.95192 out of 5

Keywords: Space, Outer Space, Physics, Astrophysics, Quantum Mechanics, Space Physics, PBS, Space Time, Time, PBS Space Time, Matt O’Dowd, Astrobiology, Einstein, Einsteinian Physics, General Relativity, Special Relativity, Dark Energy, Dark Matter, Black Holes, The Universe, Math, Science Fiction, Calculus, Maths, Graviational Waves, LIGO, VIRGO

Id: L9L_gNogNes

Length: 18min 25sec (1105 seconds)

Published: Wed Feb 17 2021

Reddit Comments

[u/Garthak_92]

Guess what I'm watching tonight before bed.

[u/irrationalrules]

Are there estimates/theories about the energy density of the GWB outside of the observed spectra?

Do current models give us an idea of what is going on in the spectra that cannot currently be observed by LIGO/VIRGO and the Pulsar timing array?

In particular, does anyone know if the energy distribution / total energy of the gravitational waves can be estimated (assuming full knowledge of the contents and processes within a given region of space) and do we currently assume, that the gravitational field has some sort of UV cutoff, like the other fields?

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I mean, High-frequency noise on the Gravitational field could for instance be generated by all kinds of momentum exchanges, even from individual particles that undergo acceleration (including interactions of atoms or subatomic particles, due to the other 3 forces). This could imply that, in regions with regular matter, a lot of activity in the GWB occurs as noise in the ultra-high frequency regime (due to heat, EM interactions, blackbody-absorption/emission of photons, radioactive decay, etc.).

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However - at the most basic level, my first question should have been:

How do GWs contribute to a galaxy's gravitational well?-

- They could either intrinsically not be considered "local" somehow (i.e. part of the stress-energy-momentum tensor) and therefore not be adding to the overall (static) curvature of spacetime. Makes no sense to me - at least from the perspective of an observer outside the wave, the galaxy didn't lose any energy (yet) - But perhaps my intuition fails me here, as I'm only considering it in terms of basic (special) relativity, so the merger hasn't happened yet for this observer...

- They could be a form of energy like any other - seems more reasonable in my head...

But: If so, what happens after a wave has passed - is the well still deeper than from the merged BH alone? Or does it abruptly rise when the GW passes (since it disperses the energy content radially around the merger at the speed of light)? Think about when you are deep below a planetary surface - the planetary shell material above your altitude cancels its gravitational pull on you (no net force downwards), but the additional time dilation from its material is still present (at least when the shell is dense enough).

So, do star systems near the merger still "feel" some of the original time dilation (be in the well of the wave around it) for some time or is is more or less insantaneously lifted because it doesn't project the "gravitational influence" of its energy content backwards due to its speed? To be clear: I can see that the gravitational "pull" from the merged (remnant) BH is reduced (accordingly to the BH mass loss) at the instant the energy of the gravitational wave has passed completely - but is the time dilation also lifted at the same rate or does it linger until the wave is further out?

Hope that someone here knows enough General Relativity to enlighten me on this one...

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My thoughts are also going in the direction of dark matter contributors:

What if there is so much energy in the high frequency GWB noise near galaxies, that it significantly increases the total energy of the region? After all, Black Hole mergers lose a lot of mass sending out those relatively low frequencies - wouldn't higher frequency GWs carry even more energy (per wavelength, as for sound)?

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As a disclaimer - I don't want to suggest that GWB energy would be sufficient as a dark matter explanation, but it could change the energy density we would expect (in halos around galaxies) to some extent and may even be a factor to consider when calculating the required dark matter / regular matter ratio - even if it turns out its effects are miniscule compared to the matter part of the equation.

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Of course, all of this is purely speculative and I'm totally not an Astrophysicist, but it seems like there is a lot more to find out here... For instance - could the activity of virtual particles (in the vacuum) absorb the UV part of the GWB spectrum and how would this be manifest in the resulting range of higher frequency GWs? Just thinking out loud here - don't shoot me, if this sounds too much like MOND to you or is just complete nonsense for any other reason.