Dark Flow & The Great Attractor

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 Ill bring the link up again. It will also be linked down in the video description
for those of you watching from platforms that dont allow in-video links.

If you are visiting from Joes channel and
are new here, youre 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 theyre not the same.

It can be quite difficult to look at groups
of galaxies and figure out what each ones 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, well 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.

Its 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 isnt 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 didnt 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 dont avoid pointing your telescope
at ground you wont 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 isnt 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
70s the assumption was that there was probably a lot matter we couldnt see over there. Some folks took We cant 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. Its 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 thats 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 didnt 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 thats 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 doesnt mean it wasnt pulling on
us in the past or isnt 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 ESAs 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 cant be seen well enough to get too specific about the cause. And by cause I mean the source of the gravity,
Im 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
wont be dark to us since we wont 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 suns mass. Thats 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 havent 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 weeks
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 its 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 cant 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 dont 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 isnt 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 theres 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
youd 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 empires
capitol located in the Great Attractor controlling tens of thousands of galaxies. I dont 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 thats 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. Id like to thank Joe Scott from Answers
with Joe for teaming up with me on this topic and if youd 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!.

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