So today we finish up what I have nicknamed the Dark Trilogy with our look at Dark Flow
& the Great Attractor.
We previously looked at Dark Matter in the first episode and then Dark Energy, and as
I said back when we began these three phenomena do not have too much in common besides
the word Dark.
Also this week we are doing a collaboration episode with fellow Youtuber Joe Scott, who
will be looking at the Great Attractor and you can click over to that video now, this
video will just pause and open his in a new window, or at the end of the episode when
I’ll bring the link up again.
It will also be linked down in the video description for those of you watching from platforms that
don’t allow in-video links.
If you are visiting from Joe’s channel and are new here, you’re encouraged to turn
on the closed captions for this episode as my speech impediment can take a little getting
used to.
Okay, so what is Dark Flow?
As you know just about everything outside of our galaxy and its nearest neighbors is
moving away from us, and the further away they are the faster they are moving away.
Indeed, as we discussed in the Dark Energy video that expansion seems to be accelerating
too, so that it looks like a trillion years down the road almost all the Universe we can
reach will be within a million or so light years of use even though the Observable Universe
will be trillions of light years wide and most of what is in the currently Observable
Universe will have sailed over the horizon leaving us alone.
The speed things move away from us from the Hubble Expansion is proportional to their
distance from us.
Get twice as far away and the velocity is twice as high, so anything already at galactic
distance moving away from us will just keep moving away faster and faster.
But things do not have an exact velocity based solely on their distance from us, much like
everything else they move relative to each other so two galaxies nearby each other might
be moving away from us at a rate of 1000 kilometers a second but one is only moving away at say,
990 km/s and the other at 1010 km/s.
We would expect that.
Same the moons around Mars or Jupiter keep pace with their planets but might be moving
toward us a bit or away from us a bit while performing their own orbits.
Galaxies tend to exist in clumps too, so it is no surprise some might be moving toward
us or away from us, relative to their cluster even if that cluster is receding away from
us.
Local motions, we would expect them all to average out.
Indeed we almost have to since our main way to determine the distance of very distant
objects is to look at how fast they are moving away from us by their redshift.
If we saw those two galaxies form a moment ago, we might not even realize at first they
were near each other, just in the same direction, because we are guessing their distance from
their velocities and they’re not the same.
It can be quite difficult to look at groups of galaxies and figure out what each one’s
velocity relative to that area is, what is called their peculiar velocity.
In this sense we use the word peculiar not to mean strange or odd, we’ll get to the
strange and odd motion in a moment, but in its other meaning, of something belonging
exclusively to.
In fact that is the original meaning, and it meaning strange or odd came later.
Much as I might say that cliff diving is a hobby I consider peculiar to those sorts of
people who like their hobbies suicidally reckless, and how we might say those people are a bit
strange, and so peculiar in that way too.
But I digress, we use the term peculiar velocity to refer to a things velocities when we cut
away the component from Hubble Expansion so we can talk about how group of galaxies move
relative to each other.
Now we would expect that to average out to zero.
So it came as a bit of a surprise in 2008 when we noticed a lot of galaxies seemed to
be moving, in terms of peculiar velocity, in a specific direction.
They were flowing that way for no apparent reason, presumably form a source of gravity
we could not see.
This gave us the term Dark Flow.
Now two things came up right away.
First the motion is headed toward a place we called the Great Attractor, having already
noticed all the way back in the 1970s that galaxies in our supercluster were drifting
that way.
Irritatingly the Great Attractor happens to be in the Zone of Avoidance, whose somewhat
dramatic sounding name probably helped add the mystery value with the public.
It’s a huge chunk of the night sky taking up fully 20% of it.
The Zone of Avoidance sounds like a place you would expect to be where space pirates
hung out or maybe some evil galactic empire.
It used to be called the Zone of Few Nebulae which isn’t much better because now it sounds
like some empty place Cthulu would go for naps.
However Nebulae is what we used to call other galaxies, as I mentioned back in the Dark
Matter episode, we confused them with Nebula in our own galaxy and didn’t realize how
far away and huge they were.
This was a zone of the sky in which few were found.
You could also consider the Zone of Avoidance the second zone of avoidance, since telescopes
can rotate in which way but you usually want to avoid pointing them down at ground.
If you don’t avoid pointing your telescope at ground you won’t see many stars.
The Zone of Audience is just a term that refers to the chunk of the sky our galaxy occupies
and all the stars and dust make it quite difficult to see distant galaxies on the other side
of it.
Hence you avoid looking at that zone if you want to study distant galaxies.
In that respect it isn’t too surprising that the Great Attractor lies in that direction,
20% of the Universe lies in that direction after all.
Now when this first got noticed back in the 70’s the assumption was that there was probably
a lot matter we couldn’t see over there.
Some folks took ‘We can’t see it’ to imply it must be some giant super-colossal
black hole, but again the lack of visibility is because our galaxy is in the way.
The main assumption was there was just a ton of galaxies in that direction.
That was still interesting at the time because we had still been assuming the universe was
pretty evenly spread, not all clumped together into lines and walls and huge voids.
Whichever the case, as the years rolled by there were arguments about the method and
how much mass was actually there, which was kind of difficult to rigorously measure with
us not being able to see the stuff closest to it to take measurements either.
It’s estimated to be around 200 million light years away, which is very far away though
also quite close on the scale of the Observable Universe.
The observable Universe is a couple hundred times wider than the volume encompassing our
galaxy and the Great Attractor and several million times bigger in terms of mass and
volume.
In 2008 though we got new data and we saw that there seemed to be a much larger volume
of space moving towards this spot.
For a while some were even thinking the whole Universe might be headed in that direction
and that the Great attractor might not be some spot a couple hundred million light years
away but actually billions of light years away, possibly outside the Observable Universe.
And that’s allowable too.
The Observable Universe is not the whole Universe, just those parts we can see, and we have no
idea how big the whole Universe is, it may indeed be infinite, it may just be a little
bigger than the Observable part, but the main assumption is that it is at least much, much
larger than what we can see.
If the Universe didn’t start off very evenly spread there could have been a super huge
chunk of mass just over the Cosmological horizon in some direction causing a drift.
Now gravity does move at the speed of light, for some reason I still hear folks so it is
instantaneous and I have no idea where that comes from.
Though amusingly if you think of gravity as being made of gravitons, in the same light
is made of photons, that does raise some eyebrows about how gravity itself escapes from black
holes.
So no mass that’s already fled over the cosmological horizon is exerting force on
us anymore.
The Cosmological Horizon is the imaginary line where space is expanding faster than
the speed of light so no light from galaxies beyond that every reaches us again, and also
no gravity.
That doesn’t mean it wasn’t pulling on us in the past or isn’t pulling still on
things nearer to it, causing a net drift, or flow.
Now this stirred a lot of speculation and ideas but before you go off contemplating
those, not only was Dark Flow at the Universal scale never particularly accepted, it more
or less got skewered in 2013 when we got the data from the ESA’s Planck Space Telescope.
It should no evidence of dark flow on the Universal scale.
This does not mean Dark Flow was debunked, because it is just a term for large scale
flow of galaxies in some direction where that direction can’t be seen well enough to get
too specific about the cause.
And by cause I mean the source of the gravity, I’m not implying some weird new force or
phenomena.
It would not be limited to just our area of the Universe and the Great Attractor but by
and large those flows at very distant location won’t be dark to us since we won’t be
sitting in the middle of the flow with its center obscured by our galaxy being in the
way.
At the same time I do not want to imply this is not a mystery.
Or that it is not important.
It is both.
In fact in recent years you may have heard the emergence of the term the Laniakea Supercluster.
This is our Supercluster of galaxies that we live in.
You might not be familiar with it because we used to call our Supercluster the Virgo
Supercluster, but the emerging evidence would seem to point to it just being a decent sized
chunk of a larger construct including a few more super cluster all center around the Great
Attractor.
The Laniakea Supercluster, our new supercluster, centered on the Great Attractor, is half a
billion light years across and contains a total mass of around 10,000 Trillion times
our own sun’s mass.
That’s many thousands of galaxies and many hundreds of galaxy clusters.
The name derives from Hawaiian and means Immense Heaven, presumably because it is impolite
to name everything in Greek, although more likely because we are running out of Greek
words we haven’t already used for something else.
Now when we say things are moving toward it, they actually are not.
They are mostly moving away from it but not as fast as they should be.
As a rule Galactic Supercluster are too far spread out not to be ripped apart by the expansion
of the Universe, regular galactic clusters are not, so our neighboring galaxies ought
to stay with us till the end of time and eventually merge with us.
Which they do all the time anyway, this week’s big science news was astronomers discovering
there were ten times as many galaxies as previously thought and I got messaged dozens of times
about it by folks wondering if it through dark matter and dark energy into the trash
can.
It did not, this is an example of way too much hype in science journalism.
We already knew our galaxy like many of the bigger modern galaxies gobbled up tons of
smaller neighbors.
Our galaxy is a big cannibal gobbling up its neighbors, in fact it’s probably not our
galaxy as it looks like we originated with the late Sagittarius Dwarf Galaxy.
Everybody knows about the Large and Small Magellanic clouds, our two dwarf satellite
galaxies, but as we learn more we can see the gutted carcasses of an awful lot of them
floating around in us or near us, over 50 that have been identified though some are
disputed and there could be more we ate so long ago we can’t see the traces.
The big new discovery as a big discovery, but it was of the ‘finally some proof’
variety, not ‘Oh My God our theories are in ruins’ variety.
They found a lot of dwarf galaxies when they looked far away in space and thus also far
back in time.
It was confirmation of what was expected, that there were a lot of smaller dimmer galaxies
in the ancient universe most of which have merged together in modern times.
They are going to keep merging too as the eons meander along, till you don’t have
galactic cluster anymore, just single big galaxies, bigger than now, floating alone
in the void with huge and ever-increasing distances to their neighboring galaxies.
This is the confusion we see with the Great Attractor, again it isn’t that all these
galaxies are moving toward it, as I said, they are not.
It is probably not moving toward itself either since it is probably just a bunch of galaxies
maybe slightly more densely packed than normal, or maybe just in the rough center of a fairly
large supercluster.
Things are moving away from it, but slower than they should be because there is a lot
of gravity working against the Hubble Expansion presumably caused by Dark Energy.
It does not necessarily mean this will all be torn apart though, just that if left to
itself it will be.
The Great Attractor has become a popular place in science fiction to have some mega-civilization
that dwarfs even the galaxy-spanning Kardashev 3 civilizations we discussed in the recent
episode on the Kardashev scale.
As I mentioned way back in the Black Hole Farming episode, Civilizations at the End
of Time, the way we predict the Universe to look in a few trillion years will almost certainly
not be what it looks like if we ever get out and start colonizing other solar systems or
anyone else has.
And as I mentioned in the Kardashev Scale episode, you can move galaxies, same as you
can move stars, something we showed in the Shkadov Thrusters episode was very effort
intensive but not actually very high tech at all.
If it turns out we have not only this whole galaxy to ourselves but the whole supercluster,
it might make sense for us to race out ahead of the expansion and start shoving galaxies
back toward us.
We are not at the center of either our traditional Virgo Supercluster or the larger Laniakea
Supercluster we think is centered around the Great Attractor, but with Dark Flow helping
bind these things together it makes it a lot easier to push them to keep them together.
So rather than a sphere of galaxies around our own being rescued from Hubble Expansion
you might see such a sphere centered more around the Great Attractor, especially if
there’s a handful of other civilizations living in the area who might want to team
up and would probably be pretty evenly distributed throughout it.
That is a case where teaming up is good since you’d end up with a larger overall number
of galaxies than the combined number each would get working on its own.
But if we have the place all to ourselves we might as well grab as much as we can, and
in the same way we might expect a galactic empire to re-center itself from Earth to some
place a bit more central to use as a capitol you might see the same thing happen for the
supercluster and get some multi-galactic empire’s capitol located in the Great Attractor controlling
tens of thousands of galaxies.
I don’t think that is terribly likely, especially without at least some form of faster than
light communication if not travel, but I always seem to be in a minority in thinking FTL is
not in the cards.
So good fuel for science fiction authors perhaps, beyond the Zone of Avoidance, deep inside
the Great Attractor, lies the seat of an ancient empire ruling over 10,000 galaxies.
So that’s Dark Flow, the mysterious motion of galaxies toward the Great Attractor, and
it is still a mystery but hopefully less of one right now.
That will also close out our look at Dark Matter, Dark Energy, and Dark Flow.
Next week we will be coming back much closer to home for a look at colonizing the outer
solar system and the week after that we will be taking a look at some more astronomy to
demystify a lot of the terms and concepts like we did today, and look at some of the
more bizarre stars, in the Stellar Compendium.
I’d like to thank Joe Scott from Answers with Joe for teaming up with me on this topic
and if you’d like to hear some more fun stuff about the Great Attractor you can head
on over to his channel to see that now, using the in-video link or the one in the episode
description.
And you can also come by the Facebook Group or sub-reddit, both titles Science and Futurism
with Isaac Arthur, to talk with others but some of these concepts.
You can also try out any of the episodes and playlists on this channel or over at the website,
IsaacArthur.net.
Again, next week is Colonizing the Outer Solar System, and until then, thanks for watching,
and have a great day!