People often talk about colonizing the galaxy,
but today we are going to ask just how far away humanity can stretch its reach. So today we are wrapping up the third year
of Science & Futurism with Isaac Arthur, and I am your aforementioned host, Isaac Arthur. It’s been quite a year, and a long trip
from our first year which only had 15 episodes, not the weekly setup we started part way into
year 2. We closed that first year out by discussing
Interstellar Colonization and I mentioned near the end that you didn’t have to stop
at the Galaxy’s edge. Two years and a hundred episodes later, it
seems appropriate to continue that topic, and to reflect a bit on some of the concepts
we’ve discussed since then and how they impact today’s subject. We have discussed many times how you might
travel to another solar system and colonize it, if you were constrained by the speed of
light. Indeed we tend to assume moving at only a
fraction of that speed. To do that, especially with classic humans
rather than some robotic probe or seed ship, requires massive vessels that are almost miniature
planets themselves, able to contain everything you need to start up an ecosystem at your
destination and keep thousands of people alive either during the flight or in some sort of
stasis to be awoken on arrival. We saw that was possible, maybe even with
modern science and technology, that you could send out ships for century-long journeys. What’s interesting is that in most fiction,
where they often have Faster Than Light or FTL travel methods able to move someone across
a whole galaxy in maybe moments or maybe a few years, almost none of those sprawling
galactic empires ever seems to settle other galaxies. That does makes sense when you have a galaxy
already full of other intelligent life forms, since you can assume other galaxies will have
their own too and not welcome colonists from outside, and it is a long trip just to say
hello. However we see it even in fiction where humanity
has the whole galaxy to itself and no special reason to think neighboring galaxies will
have existing civilizations. When that’s the case, it makes a lot less
sense. If you’ve got a spaceship able to cross
the whole galaxy in a year, crossing to another galaxy should not be a problem. Distances between galaxies don’t scale up
like distances between planets or stars. Stars are typically hundreds of thousands
of times further away from our Sun than other planets are from Earth, and the distance to
the Moon, still the only world a human has set foot on, is about a hundred millionth
the distance to the nearest star. Alternatively galaxies are a lot closer together,
relatively speaking. The Magellanic Cloud Dwarf Galaxies are closer
to some stars in our galaxy than they are to stars on the other side of the galaxy from
them, and even Andromeda, the nearest large galaxy to ours, is only about 20 times further
away than the galaxy is wide. So there’s no reason why, if you thought
the neighboring galaxy was empty of civilizations, you couldn’t make that trip if you’ve
got spaceships that can cross the galaxy in a year, because they can get to Andromeda
in 20. That would barely count as a generational
ark ship, something we can almost do now, and should be child’s play for most galactic
civilizations we know from fiction. The other big thing to keep in mind is that
the space between us and other galaxies is not empty. If we view galaxies as continents, with intergalactic
space as the ocean, there are plenty of little islands to use as waypoints. There are a lot of stars in between, and galaxies
don’t have firmly defined edges either. Also, stars are often ejected from the galaxy,
much like how planets can get ejected from a solar system. We aren’t sure how many of these stars there
are yet, I’ve seen estimates as high as half of stars being intergalactic, but there’s
decent confidence of it being 10% of stellar mass or higher. Equal or lesser populations than galaxies,
it’s still spread over a much larger volume, so these stars are much farther apart, light
centuries not light years, but they make potential waypoints on a trip. Decent ones too, because while most stars
on the outskirts of a galaxy have low metallicity - and so probably not a lot of rocky material
nearby - often these ejected ones were tossed out by passing near the central black hole
of our galaxy and are higher in metallicity. Add to that, while an ejection of a planet
from a system or a star from a galaxy often strips it of its satellites, it also often
does not, and the closer the satellite is to its parent, the less likely it will be
ejected. So the rockier inner planets of a system are
more likely to be retained. That means these waypoints could have plenty
of raw materials to use to refuel and repair, and potentially have planets to settle on. You don’t necessarily have to go sundiving
to capture fuel and raw materials on some Icarus-like plunge into the star to pick up
material, like we saw from the spaceship Destiny in Stargate: Universe, one of the few scifi
franchises to seriously tackle intergalactic travel and timelines. You can do stuff like that too, as we’ve
discussed in the Starlifting episode and will look at more next week in Colonizing the Sun. Today we don’t care about that though for
three reasons. First, as mentioned we have discussed before
how it can be done if you need to, second, odds are many of the stars will host planets
which you can mine more conventionally. But third, you normally don’t stop on interstellar
voyages to refuel. Oh, in fiction you often do, they tend to
have FTL systems that are non-inertial, a warp drive that requires constant power input
to maintain its speed rather than just coasting along, or wormholes or gates or hyperspace
jumps with maximum ranges that leave you stopped relative to the local area, rather than needing
to burn a ton of fuel to slow down and then more to speed back up when you’re done. Normally in interstellar space you head to
your destination without stopping, because doing so costs you time and gains you nothing. And while I always say it would be nice to
have FTL, it doesn’t really look like it is in the cards, nor do we really know the
logistics involved if it was, since they are different for every hypothetical drive system. So we always try to look at the future assuming
no new physics and see if we can tackle a problem anyway. Normally you wouldn’t want to stop a ship
en route to another galaxy, or so we’d assume, since it will tend to involve a not-quite
straight path between various intergalactic stars and that wouldn’t seem to make sense,
but we’ll be giving that a second look today. We also do have an existing precedent for
stopping an interstellar spaceship. In the Life in a Space Colony series, we examined
a ship called Unity, a large interstellar vessel kilometers long carrying hundreds of
thousands of passengers. After they arrived at their destination they
realized that they did not really need to stop and stay there. They had all the manufacturing ability needed
to take raw materials and build anything in the ship’s structure or colonizing inventory. This was up to and including the colonists
themselves, since they were making journeys of many decades and could easily replenish
their colonist pool simply by keeping a decent portion of them on the ship to breed more
colonists for the next stop. We gave them both life extension and the ability
to freeze people and thaw them out. Although both technologies were handy for
growing the colonists’ numbers, they weren’t truly necessary since people weren’t dying
off and could continue to have children and maintain a crew with the same goals and traditions. So this ship, Unity, decided it could transform
itself from a regular interstellar arkship with one destination in mind into what we
called a Gardener Ship, one that stops at a system, builds a colony, picks up new raw
materials and fuels, and heads off to a new destination. During the flight, they would breed up their
numbers again, and work on turning all those raw materials into colonial gear or replacement
parts and supplies for the ship. We ended up revisiting the crew in the episode
Interstellar Travel Challenges to upgrade how fast they could go and talk about all
the problems one can encounter moving through space that fast. We also visited them again in the Dead Aliens
episode but I consider that non-canon to their tale, which we’ll pick up again today because
it’s handy to have a narrative framing device. So our gardener ship Unity has been slowly
working its way out to the galactic rim, as have various sister ships, and indeed every
so often the ship divides itself up like an amoeba. They can make every part the ship needs so
they can make a new twin ship and do upgrades as new science comes in from home. However, we will still limit them to the 20%
of light speed we gave them in our last visit. We will also ignore that the ship, which first
went to the Tau Ceti than Epsilon Eridani, was headed in the wrong direction for Andromeda,
so they’ve kind of cork-screwed around. Handily Andromeda is in the direction of the
region of the galactic edge closest to us, so we don’t have to cross the whole galaxy
to get there. It’s not quite the fastest route the galactic
edge, which would lie more in the direction of Orion, and we need to head more toward
Perseus to aim for Andromeda, but it is fairly close and a lot better than crossing the whole
galactic disc. That’s also true of both Magellanic Clouds,
we’re closer to them than most of the galaxy is. That’s worth mentioning because ‘intergalactic’
is a bit relative. Andromeda is the nearest big galaxy to us,
but the Magellanic clouds aren’t much further from us than the furthest parts of this galaxy,
and they are no longer the closest known dwarf galaxies. The Sagittarius Dwarf Spheroidal Galaxy is
considerably closer, just 70,000 light years from Earth, and the Canis Major Dwarf Galaxy,
whose status as a galaxy is still debated, is only 25,000 light years away. In all 4 cases, colonizing them is not really
any harder than colonizing the more distant parts of our own galaxy, and there are plenty
of stars in between to use as way points. I will be ignoring them today though beyond
pointing out that they would tend to be settled along with the rest of the galaxy, though
in many cases you will need to cross some areas fairly devoid of stars and pick your
path accordingly. But it also means our ship Unity has arrived
at the galactic rim a long time later, on the path they took it would be at least 30,000
light years and they’ve only been going 20% of light speed, not to mention stopping
for at least a few years once or twice a century to set up a colony. So we last saw them sometime around the 26th
century AD, centuries ahead of us in the twenty-first century, but it is now closer to the twenty-first
hundredth century. They are fifty times further ahead in history
than the pyramid builders are back in history. They are parked at a last lonely star near
the galactic rim, the Terminus System, and the captain is deciding if they dare jump
farther off and head for Andromeda or abandon the mission, finally stay at a planet unlike
the many hundreds she’s colonized and left behind. Truth be told, she’s been planning this
for millennia, captain of one of humanity’s first interstellar colony ships, even if it’s
been rebuilt and subdivided dozens of times. They could turn around, they could get back
to Earth a good deal faster with the laser highways between stars many worlds have been
creating as they got bigger. They could settle here or turn perpendicular
and help colonize the galactic rim. Indeed they could do all of the above. Spending decades to build new ships, one to
head off on each direction of the rim, one to head off to Earth for those wanting to
see home again, and one to head off to Andromeda. The science officer points this out and that
they probably want a much bigger ship or fleet of ships to do the job. He also points out that individuals don’t
actually have to choose, it’s the year 200,000 AD, none from the original crew are entirely
human anymore, and copying their minds onto some clone bodies or androids isn’t too
hard. They’ve done that before, for a crew member
or colonist who wanted to travel on but also wanted to settle down, folks who had a spouse
or kids who wanted to stay and they couldn’t decide if they wanted to stay or go, so they
did both, making a copy of themselves. Or when the ship subdivided, building a twin
to head off at a different angle to colonize other systems. They have some crew members who have done
that many times, same as they have others who sleep most of the journey. This is the original ship, for a given value
of original, that headed out from Earth 200,000 years ago, and the original captain, for a
given value of original, who piloted it out, and the original science officer, for a given
value of original, who has been nitpicking her plans since Unity was on the drawing board. But the ship can’t make the journey on its
own, so the science officer says. This ship is immense, bigger than when it
left Earth and you could have crammed a major metropolis into that one. But to do this right, they are going to want
a whole fleet and they need to build that here at Terminus. Now we say Terminus is the last star at this
edge of the galaxy but that’s not entirely true. It’s actually an extragalactic system sent
on its way many millions of years ago and just now getting out of the galaxy. In fact with a little bit of nudging, it could
be aimed to reach Andromeda. They could colonize it and just wait. Andromeda is, after all, set to merge with
the Milky Way galaxy in a few billion years and they could get this whole system to arrive
there a good deal sooner than that. There is a highly advanced technology called
a Shkadov Thruster, whose design is actually a very simplistic one. It calls for trillions of cheap mirrors to
be placed around a star so that they bounce all its light in one direction, providing
thrust and allowing you turn a whole solar system into an interstellar spaceship. You can also boost that speed by using the
solar wind of the star as propellant, or modifying starlifting technology to create a giant plasma
drive. Normally, this is no way to cross between
stars. They take half of forever to get up to speed,
but while it takes millions of years for them to get up to even modest interstellar velocities,
it also takes millions of years to travel between galaxies. At their current cruising speed of 20% of
light speed, they will need ten million years to get to Andromeda. It’s been fifty times as long since this
ship was built as between then and when the pyramids were built, and it will be fifty
times as long as their entire past journey to get to Andromeda this way. They could turn the entire Terminus System
into one huge spaceship - technologically speaking it's simplistic - and head off to
Andromeda that way. However it will take longer, many tens of
millions of years at least. Potentially they could use a miniaturised
version of this, turning a gas giant into a giant fusion-driven ship, but fundamentally
this is just a super-sized version of the ship they already have. The captain mulls both option over but rejects
them. She has been dreaming of being the first to
set foot in another galaxy for thousands of years, and that’s the kind of commitment
that borders on the obsessive. Tell me how we do it fastest, that’s what
she want to know, and why a fleet, why not a bigger ship? Why even a bigger ship? The science officer says he’s very dubious
- even with all their technology that can fabricate any part they need to replace - of
being able to cruise the entire intergalactic void for ten million years without breaking
down. That’s not why he wants other ships though,
not as spares. He wants to make stops along the way, and
using astronomical data gathered from some of their colonies a few light centuries back,
which are now civilizations a couple of thousand years old with lots of giant telescopes, they
found a modestly straight path with stars never more than a thousand light years apart. The captain stops him though, and asks why
not a straight path, instead of using Terminus as a giant spaceship with all those mirrors,
why not use a variant of that, turn it into a giant laser to push the ship to near light
speed so they can make the journey in a fifth of the time. He shakes his head at that, that’s part
of the idea in truth but too simplistic. Even at that speed it’s a 2 million year
journey, and even if they hug the speed of light to get time dilation, so less time passes
for them and the ship, and even with the intergalactic void being far thinner so they get less drag
and collisions to damage or slow the ship, going that long with no resupply is a dubious
proposition. But more to the point, they can’t slow down
when they arrive. The ship’s defense officer objects to that,
if collisions slow the ship, why not let space drag slow them down? When they approach the galaxy they can expand
out a thin sail to get smacked into and slow the ship. Even a magnetic field that will deflect ionized
particles off and exchange momentum with them. The faster you’re going the more each particle
slows you, so once you get down to a modest speed you can use the ship’s regular engine
to finish slowing down. The science officer agrees it is viable, but
not ideal, such a slowdown still takes huge amount of time and distance and would leave
them a huge distance from any possible help and needing to ration out their supplies on
the whole journey with not much margin for error, and on something totally untested. It could work, they could build a massive
laser array and use it like a giant cannon to hurl themselves out of the galaxy at near
light speed then slow down by using the local interstellar medium to break them to more
normal speeds. But he hasn’t lived 200,000 years by throwing
dice and he doesn’t want to leave it so people have to do the same thing in the future. The science officer wants to build a big intergalactic
bridge, using the Laser Highway system, and he wants to colonize each star system along
the way. By doing that they will always be in some
sort of range of civilization and future ships will be able to cross at near light speeds
safely, with each bridge star from Terminus running a laser relay to push ships faster
or slow them down. Done this way people can always go home, or
send follow up missions that will be able to catch up to them, and reinforce them. Done the other way it’s a bit like burning
your ships when you reach a new shore. What they’ll do is build a fleet. Once everyone is up to speed, they’ll accelerate
a bit more, more than they could normally slow down from. Then they’ll transfer some extra fuel from
each ship to one ship, which they’ll all then push on with their prow-mounted lasers,
speeding it up a bit, and it’s got fuel to slow down from a slightly higher velocity. When it arrives at the next target system,
it will get to work building lasers to slow them all down, though they’ve already slowed
a bit pushing that vanguard ship. If the Vanguard fails in its job they can
cannibalize fuel from the various ships, transfer people over, and jettison some mass, to ensure
they can still slow down at the destination system. They’ll have many stopovers to practice
this and get it down right, and after thousands of years of doing century long trips, they’re
confident they can keep the ships running through their normal methods for the few thousand
years most of the intergalactic jumps will take. Better yet, with each system along the way
colonized and with a pushing laser there, they can arrange for critical resupply if
something goes wrong. They can just wait at the next system or slow
down as much as they can and go mostly on ice till rescue arrives, even if that take
thousands of years. Thousands of years sounds like an eternity,
but it’s nothing compared to millions and that your best time for rescue if you aren’t
doing stopovers. You’re also sailing through mostly empty
intergalactic void if something goes wrong here, and probably slower than before, not
plowing through an uncharted galaxy at relativistic speeds. It’s very iffy if a frozen body could be
revived after millions of years too, thousands would probably leave something to work with
in terms of brain structure and is a lot more realistic for a digital copy of a mind to
repair off of. And you only have to go that route if you’re
getting critical failure across the fleet. Most of the time if something goes wrong you
could transfer to another ship and cannibalize some for fuel or material to repair and slow
down. When you’re done, even though it takes around
ten million years, you’ve been busy the whole time founding new colonies that can
still talk to each other and send ships through at very relativistic speeds, enough that the
journey will seem shorter to them from time dilation. Those ships only need enough extra fuel and
supplies to course correct to the next relay point if something goes wrong there, and it
will be a whole colonized system. Subsequent intergalactic trips will occur
at nearly light speed, including follow up colonization missions. What’s more, if it turns out that the galaxy
you’re arriving at is occupied, and ten million years is a long enough time frame
that something might have evolved from chimpanzee to interstellar civilization in that time,
you do have a way to go home. So Unity sets off from the Terminus system
toward the Andromeda Galaxy, and their first waypoint along the way, and assuming nothing
goes wrong, it will take them a bit less than ten million years to arrive, using only fusion
and lasers to do it, technologies probably fully developed by the end of the 21st century. Okay, let’s consider some other scenarios. First, we mentioned that you could mobilize
an entire solar system as one giant space ark, the straight Shkadov Thruster route is
the technologically easiest approach but via starlifting you can accelerate a lot faster
by firing helium out as a propellant, siphoning out heavier elements in the star for construction,
and recycling hydrogen back down for the star to eventually burn perhaps. This same technique can be used galaxy-wide,
and we’ll come back to that in a moment, but it’s worth remembering that galaxies
are not static and that Andromeda is headed toward us. That’s actually quite rare, virtually every
galaxy is moving away from us and the further away they are, the faster they are moving. Some folks ask how far off we could colonize
the Universe, if it was empty, and the answer is that your absolute maximum depends on how
fast your ships can travel versus what the redshift of the target galaxy is. That’s about 20 kilometers per second for
every million light years of distance, or 20,000 kilometers at 1 billion light years,
about 7% of light speed. Our ships were averaging about 20% of light
speed, so it could catch up to a galaxy moving just under 20% of light speed some 3 billion
light years away, though it would take longer than the Universe is old to arrive with such
a slow relative speed, just a little faster toward its destination than that destination
is moving away. Fortunately the closer you get the slower
it will be moving away, but unless you want to take half of eternity to get to your destination
you don’t aim for anything moving away at much more than maybe half your maximum speed. This is part of why when asked I usually say
a billion light years is about as far as humans can colonize without FTL systems. That’s a lot of living space too, many thousands
of galaxies. However we do have other drive options, that
laser slingshot and magnetic sail slowdown method from earlier would probably work, it’s
not ideal for interstellar travel to neighboring stars but it ought to work over much larger
distances, even well short of intergalactic ones. And you could do it at every waypoint. It only takes a year at one-gee of thrust
to get to near light speed and the same to go back down, so you can get away with stopping
between stars as waypoints without losing much time, at least when those distances are
much larger than the normal interstellar scale. Slowing down without fuel by collision or
magnetic braking is all about how fast you are going, how thick the interstellar or intergalactic
dust and gas are, and how big your braking sail is, but think centuries not minutes,
even for an immense sail. That aside, and ignoring our example of Unity
today, it would probably be how I would do it. We also have options for black hole drives
or anti-matter drives or maybe quark fusion or some other new concept, which might allow
higher travel speeds and expand that radius we might colonize a lot. However, I said that was part of the reason
I usually put it at a billion light years. There are three others. First, it does take time to colonize places
and most colonists will want to go the nearest and easiest empty place, so even if you’ve
got ships that could make speed to get to some place four billion light years away,
and do it fast enough to arrive before it ran out of stars to colonize, you’d presumably
want to stop along the way. Second, if you aim to the edge of your speed,
you could arrive when there are no stars left, but more importantly if you are arriving somewhere
2 billion years from now, I’d find it very hard to believe that you’d be arriving at
an unoccupied place. The older the Universe gets, the more likely
life is to develop and get technology. Worlds that already have life have more time
to evolve, most stars live a lot longer than ours does, and the metallicity of new stars
rises, meaning more rocky planets in general. I’m on the extreme skeptic end in terms
of the Fermi Paradox, in that I doubt any civilizations have arisen within a billion
light years of us, see the Dyson Dilemma 2.0 episode or various Fermi Paradox episodes
for explanations of that reasoning, but I’d have a very hard time believing you could
arrive at a galaxy 3 billion years away from us, and maybe 10 billion years ahead in time,
and find it was still absent of intelligent life. Third, the Universe is expanding, and only
a handful of galaxies are near enough to us to stick with us as that happens. However, just as you can move a star you can
move a galaxy, you just build those Shkadov Thrusters around every star and let gravity
tractor it with you. And most of the galaxies within a billion
light years of us are going slow enough you could slow them down this way to stay bound
to us. If you can’t, then any of those colonies
are destined to be forever gone. You won’t ever be able to talk to them again
at some point. Of course you might not care, and anyone sent
on billion year long quests are going to have more time to have diverged from you than humans
have with oak trees, though that problem already exists at the Interstellar level and is probably
beyond manageable even at the galactic scale already. That’s why we discussed alternatives like
the light year wide Birch Planets from the Mega Earths episode. Don’t overlook that option though, as huge
as a galaxy is and as long as billions of years is, you can move one, there’s no tricky
physics involved, it’s just an application of brute force on a galactic level, astronomical
timelines and energy needs, but you have both. We have no idea where the closest civilization
to us is, might be within a few hundred light years, might be none in the whole Observable
Universe, and with all the time it takes light to reach us and for our ships to arrive there,
especially in intergalactic terms, what we can see now doesn’t necessarily mean much
compared to when you arrive. A galaxy a billion light years away might
be absent technological civilizations a billion years ago, when that light left, but would
it still be a few billion years from now, when you arrived? We can’t know, but for my part an unused
and dead solar system is one we should always claim if there’s nobody around there or
nearby asserting their own claim. And I would expect an alien civilization to
do the same, not because they or we are aggressive and hostile, but because a random space rock
around a lifeless star just has less inherent value than a tree or cat or a dog or a person
or even an inanimate statue someone has carved. And while some might claim otherwise, every
breath they take puts the lie to that claim that they think their life is no more valuable. I don’t know many people who say otherwise
and I don’t believe the few who do genuinely believe that, so when someone asks why colonize
other places, be it other planets or other stars or even other galaxies, I always feel
it’s the wrong question. Not why would you, if you could, but why wouldn’t
you? I don’t know if humanity is destined to
colonize other galaxies, I’d never support doing so if someone else already lived there,
but should it turn out that intelligent life is that rare I think we should, and as we’ve
seen today it is on the table, even if we never figure out how to make warp drives or
wormholes. As we head into 2018, two generations after
we last set foot on the moon, I think it does help to remember that the sky is not the limit
and that we have potential new frontiers for billions of years to come. And we are going to keep on exploring them
next year. Until next time, thanks for watching, and
have a great year!
I think there has to be two (at least) distinct groups of colonization efforts.
(1) If 1 to 3 is the normal stuff, then 5 is to use all gathered resources in one time and place to escape the death of the universe somehow, and number 4 is to spread to the largest possible area which can send resources to one central location for that given agreed upon time window (given universe expansion and lightspeed limitations).
It needs to be separated like this because the two will be very distinct, yet intertwined, yet only the latter will remain at the end. And there will be every variation people can think of throughout trillions of years of drug-use, biological modification and technological modification and so forth. One group of people perhaps no larger than 12 might decide to test The Myth of Sisyphus, and decide to do repetitive pointless labor for billions of years to see if it is impossible to will themselves into being content or even happy about the situation which goes against every gene we have. Then imagine some civilization spawned from our journeying vessel, which is a trillion times stranger, to us, than this. Its their right to do whatever they deem is worthwhile, and the term "worthwhile" will change drastically. After all, that is a term born in an era where humans had limited calories to acquire maximum successful procreation. If it didn't require minimal calories and/or succeed towards procreation, it is a pointless endeavor as far as modern humans think. Yet entire civilizations will completely reject this train of thought, even before they remove the genes which makes them innately want to follow that behavior.
PS: Both and every variation in-between and outside either side of these, will spawn from our vessel, whichever culture we have on our vessel. The most atheist vessel in the universe will eventually have a group of religious people form a religious colony for either purpose (or a gradient mix). Which begs the question, is it our right to stop them using our gathered resources to start such a colony? Even if they are all clones of you, or me? I think its the possibility that some vessels will have one person on them, for just this reason, so that every colony follows his/her path, with automated robots.
How would you possibly fix the problem with evolution?
Even on an interstellar scale this would prove difficult. And on an glactic, not to mention intergalactic, scale even more!
Even on a birch planet this would be an issue. Keep in mind how different humans of this earth are and this is only a single planet!
Surely there shouldn't be a single "common 21st Century human" a few thousands of years, millions of years on (inter)galactic scale. Every system could be vastly different from humans although they all did start out as normal human colonies.
When Andromeda and wilky way collide; You certainly would not want to be greeted by the Warhammer 40k Tyranids or Nekrons or the Borg.
So what could be the answer?
Uploading genetic code and sending it frequently to al colonies so that they would reproduce humans via enhanced cloning and the downloaded genepool.
This would allow evolution and all colonies to always be up to date genetically.
Still, this would mean that intergalactic humans are millions (!) of years behind the galactic humans. Which, "evolutionally" speaking, is a looooooooong ass time!