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 grows, but there's no preferred direction to any 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, 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 galaxy 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.
The 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."
More on that one last one soon...
That motion causes the CMB to be doppler-shifted.
Its wavelengths a 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 doppler-shift 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 Sunyaev-Zeldovic, or "KSZ", effects.
Actually, let's just start with the regular old, Thermal Sunyaev-Zeldovic effects.
So the most massive Galaxy clusters in our Universe are vast
conglomerations of thousands of Galaxies, and are 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 steal 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 Sunyaev-Zeldovic effect is much, much harder to see than the Thermal SZ effect. If a galaxy cluster has some
extra peculiar velocity in addition to the Hubble Flow,
then the SZ 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's either Towards or Away from us
not side-to-side .
KSZ 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 Kashlinskey 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, Kashlinsky and team, as well as some other researchers, also
re-did their work with the Planck 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 Supercluster,
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. Its moving away from us faster than the speed of light and is now forever beyond the reach of light,
or gravity, 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 Dark 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 Feynman 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 Feynmen 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 an
annihilating Electron and Positron, one must produce two photons not one.
Samuel Bender asks whether electrons and positrons in Bhabha-scattering have to remain on their respective sides of the diagram,
or whether they can cross paths?
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 Modding asks: what if one incoming electron is entangled to another electron?
Well, that'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 Feynman 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 Feynman diagram.
Squid Master started studying physics and got a tattoo of a Feynman 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.