there's controversial evidence that on the largest scales the universe is flowing it may be that much of the matter in the cosmos is drifting due to the ancient gravitational pull of something outside the observable universe space is not static everything moves planets orbit stars stars orbit within galaxies galaxies whirl within the gravitational fields of giant clusters and of course the universe is expanding distant galaxies are thrust apart from each other as the space between them growers but there's no preferred direction 20 of this motion motion due to the expansion of the universe what we call the Hubble flow is equal in all directions the random motion of galaxies is what we call peculiar motion should also have no preferred direction on the largest scales of the universe there should be no preference for up or down or left or right at least that's what we thought observations of the Cosmic Microwave Background suggest that galaxies clusters across the cosmos may be moving ever so slightly towards the same point beyond the cosmic horizon we call this dark flow Galilean relativity tells us that there is no preferred frame of reference to define absolute stillness the laws of physics work the same no matter your speed and yet there is one frame of reference that in some ways is a little more preferred or at least special than all of the others it's the frame of reference of the cosmic microwave background radiation so CMB the CMB is the leftover heat glow from the hot dense early universe in all directions it appears to be the same temperature around 2.7 Kelvin and hence the same microwave wavelengths actually when we look at the CMB from our moving platform of planet Earth we don't see a perfectly even temperature this motion is due to the sun's orbit around the Milky Way and the Milky Way falling to the Great Attractor one last one soon that motion causes the CMB to be Doppler shifted its wavelengths are little stretched out behind and a little more compacted ahead this leads to the impression of bands of cooler and warmer CMB radiation but if the CMB is asymmetric due to our motion there must be some reference frame some velocity in which there's no observed authorship in any direction if you were in that reference frame you would be still with respect to the CMB in that reference frame if you added together the peculiar velocities of all galaxies in the universe you'd expect them to cancel out after all those galaxies formed from the hot hydrogen plasma that produced the CMB so the peculiar motion of galaxies can be defined as motion relative to the Cosmic Microwave Background the CMB can be used to define this notion of stillness but it can also be used to measure deviations from that stillness doing so has seemed to reveal that the motions of galaxies does not cancel out on the largest scales of the cosmos there's controversial evidence of a slight drift in one preferred direction to explain this I'll need to tell you about the kinematic suniye resolved ovitch or khz effects actually let's just start with the regular old thermal sanyas Eldridge effects so the most massive galaxy clusters in our universe our vast conglomerations of thousands of galaxies and a bathed in a diffuse but searing hot plasma hydrogen and helium with temperatures up to 100 million Kelvin as the photons of the CMB passed through that plasma they still a little bit of its energy as a result when we look at the Cosmic Microwave Background through one of these clusters we see that its energy is boosted just slightly this allows astronomers to find extremely distant galaxy clusters just by studying the CMB kinematic suniye result ovitch effect is much much harder to see than the thermal se effect if a galaxy cluster has some extra peculiar velocity in addition to the Hubble flow then the SC effect adds an extra Doppler shift to the CMB photons that pass through that cluster it sort of resets the apparent velocity the frame of reference of that patch of the CMB to the peculiar velocity of the cluster it gives that part of the CMB a Doppler shift relative to the rest of the CMB and that shift can tell us the peculiar velocity of the cluster this effect is really really tiny and it can only tell us the component of the peculiar velocity that either towards or away from us not side-to-side a KSC measurement from a single cluster isn't very useful but what if you could measure the effect from hundreds of clusters across the observable universe that's exactly what Alexander cash linskey and collaborators did they used the W Maps satellite observations of the Cosmic Microwave Background to build up a peculiar velocity map of around 700 clusters in all directions on the sky and out to billions of light-years the claim is pretty astonishing once you factor out the Hubble flow on average those clusters seem to be drifting in the same direction it's like a lot of the matter in the universe is flowing toward some point beyond the edge of our universe this claim is extremely controversial there's a pretty entrenched idea in cosmology the universe should be homogeneous and isotropic on the larger scales homogeneous means that everything looks roughly the same in all locations there certainly seems to be the case when you look at scales larger than around a billion light years you have basically the same amount of matter everywhere isotropic means that the universe shouldn't have a preferred direction matter should be more stretched out in any particular direction and it shouldn't be moving in any one direction more than any other so the idea of a universe wide dark flow surprised a lot of people and made others very skeptical other research teams have been quick to try to refute this claim and the strongest challenge came from the team behind the Planck satellite Planck is the latest cosmic microwave background probe and it's produced a much more detailed map compared to W map analyzing around a thousand clusters using the new data the Planck satellite team say that no dark flow is apparent however push Lincoln team as well as some other researchers also redid their work with the plant data and claim the dark flow is still very real but let's talk about the implications of this finding assuming it's real the direction of the claimed flow is towards the constellations Centaurus and Hydra in fact it's towards something beyond the cosmic horizon in the same direction as these constellations this is an odd coincidence we already know that there's something very massive in that direction it's the so-called Great Attractor since 1973 we've noticed that galaxies in the local part of the universe seem to be drawn in that direction due to an unseen gravitational influence we now think that this might be the center of the Laniakea super cluster which is a vast cluster of clusters that encompasses hundreds of millions of light years and several hundred entire galaxy clusters but the Great Attractor is not causing the dark flow because that flow seems to affect galaxies across the observable universe or two and a half billion light years at least far beyond the gravitational reach of Laniakea so what causes dark flow if it's real then the leading explanation is that it's a relic of a gravitational attraction towards something beyond the edge of the observable universe now that part of the greater universe can no longer influence us is moving away from us faster than the speed of light and is now forever beyond the reach of light or tea or anything however that wasn't always the case in the earliest instance of the Big Bang the observable universe was compressed into a subatomic scale things that are now beyond the cosmic horizon were close enough to affect us gravitationally then the event known as cosmic inflation caused an exponential expansion that threw these once neighbors far far apart yet the relics of their influence may remain what if somewhere beyond our cosmic horizon there exists a region of much more stuff a different bubble of observable universe with more galaxies more clusters more dark matter such a region may have given our entire universe such a gravitational tug that even 13.7 billion years later we still see a faint drift in that direction whether or not that flow is real is still unresolved we need to better understand why the different teams get different results however we may also need a better map of the Cosmic Microwave Background so that we can measure more clusters to greater distances in higher precision however if dark flow does turn out to be real we may have detected for the first time the influence of a neighboring region of the greater universe beyond the horizon of observable space-time last week we talked about the actual rules by which Feynman diagrams can be used to describe real interactions in quantum electrodynamics now let's look at your comments a couple of you pointed out that the fireman diagram vertex representing interactions between an electron positron and photon is not by itself a valid diagram and that is right and probably I should have mentioned that in the episode in fact vertices are really just building blocks for firemen diagrams by themselves they don't obey momentum conservation this is something we didn't get into incoming and outgoing particles must obey energy and momentum conservation for example in order to conserve momentum and annihilating electron and positron one must produce two photons not 1 Samuel bender asks whether electrons and positrons in barbar scattering have to remain on their respective sides of the diagram or whether they can cross paths or actually the spatial positions and even directions of motions of particles in the diagrams don't mean much at all each incoming and outgoing particle is identified with a numerical position and momentum and we don't need to worry about drawing the diagrams accurately to represent those so in short the answer is yes the final directions of motions can be anything within the realms of possibility of physics jojo model asks what if one incoming electron is entangled to another electron well it's a really good question the answer is that the states of the outgoing particles are also entangled with that other electron upon measurement of the properties of the outgoing particles weird entanglement correlations can still occur in the case of single entanglement say of two electrons measurement of the properties of one of the electrons appears to influence the state of the other as though that influence propagates back in time along the path of the measured electron but if that measurement instead happens after the electron undergoes an interaction represented by a fireman diagram then we need to think of that retro causal influence as also propagating through the infinite possible interactions within the virtual space of the Fineman diagram squid master started studying physics and got a tattoo of a Fineman diagram then switched majors to economics squid I'm sorry that you and physics broke up but that's a relationship that leaves its mark on you for life