Half of the universe is filled with expansionist alien civilizations, and it’s only a matter of time before they’ll reach us.

OK, that sounded a little sensationalist.

But it’s also the claim of a recent astrophysics paper.

Let’s see how they came to that conclusion and whether we should take it seriously.

We humans have been around for only a blink of an eye compared to the 13 billion years since the big bang.

But even the current age of the universe is itself a blink of an eye compared to the amount of time yet to come.

Stars will continue burning for another hundred trillion years, and the heat death of the universe is a googol years in the future, that’s 1 with a hundred zeros.

From this point of view, we’re living pretty much at the beginning of the universe.

At the same time, when we look at the sky we see… stars.

Galaxies.

But not a galactic empire in view.

We see no evidence of other life out there.

Which may mean that humanity is super early in the history of life in the universe.

Which I guess that’s pretty cool - if you think being first to a party is cool.

But it’s also weird.

See, we have this thing called the Copernican principle, which states that we shouldn’t expect to find ourselves in a particularly special place in the universe.

Named for Nikolaus Copernicus, who showed that the Earth is NOT the center of the universe, as previously thought.

We are in fact on a typical planet, orbiting a typical star in a typical galaxy.

But in principle you can extend this principle to time.

If life can appear for trillions of years, then it’s suspicious that we find ourselves in the first billions.

Of course the Copernican principle has to compete with the Anthropic principle, which states that we can only find ourselves in a location in the universe capable of forming and supporting observers.

That’s why it’s not surprising that we’re on a planet at all, rather than floating in a more typical environment: the freezing void between the galaxies.

So perhaps there’s also an anthropic reason for us to find ourselves at the “beginning of time”.

Maybe the future history of the universe isn’t as hospitable to life after all.

This may sound like a lot of speculation.

I mean, how can we know what the future history of the universe will be?

Well, believe it or not, it IS possible to say something about the earliness of humanity and the future of life just based on our arrival date and the apparent emptiness of the sky.

And today we’re going to talk about a study that tries to do exactly that.

To hear more about the anthropic and copernican principles, and how we can use them to calculate other surprising things about our universe, including our expiry date, check out our video on the Doomsday argument.

But today we’re looking at the paper  by Robin Hanson, Daniel Martin, Calvin McCarter, and Jonathan Paulson.

First these researchers ask the question: how early is humanity?

They try to answer it building a simple model of the emergence and spread of intelligent life in our universe.

And then they use the answer to propose something pretty wild: a proposal that gives their model its name: the grabby aliens model.

But you’ll have to wait to hear what the proposal actually is.

First up, how do you determine our “earliness”.

You start with a function called the appearance rate, which tells us how many  intelligent civilizations are evolving from primordial goop at a given cosmic time.

Using the appearance rate along with our current date, we can work out our civilization’s birth rank: of all civilisations that will ever exist, what proportion came before us?

If most of have already appeared then we are late and have a high birth rank, while if most are still to come then we’re early, with a low birth rank.

To figure out the appearance rate function, the researchers start with what we know about when habitable worlds formed in the past or when they will form in the future.

It turns out that most habitable stars are formed in a pretty short period of cosmic history.

In the first couple billion years, the universe didn’t contain enough heavy elements for rocky planets to form.

And around 20 billion years in the future, there won’t be enough interstellar material left to form new stars.

Habitable star formation will peak at around 15 billion years, not so far away from our current date.

There are a bunch of other factors that cause this peak, which we go into in our episode on galactic habitable zones.

Once a habitable star along with its habitable planets have formed, life needs time to evolve.

And it needs lot of time.

We humans only appeared 4 and a bit billion years after our planet formed.

But how long does this take on average throughout the cosmos?

To answer this, the researchers use the hard-steps model.

To get from primordial goop to intelligent life you need a sequence of fluke events.

On Earth that involved the first formation of self-replicating cells, the evolution of complex Eukariot cells, then multi-cellular organisms, the photosynthesis, sexual reproduction, developing giant brains, and various civilization and technology steps.

These events are all possible “hard steps” - extremely unlikely per-year, but will eventually happen after waiting a very long time.

The number of hard steps is a big deciding factor in how fast the universe can spawn intelligent life.

If there are few hard steps, then civilizations emerge soon after their stars are formed.

If there are many hard steps, it takes much longer.

If we assume that humanity was not particularly lucky, in the sense that we didn’t blast through many more hard steps than chance would normally predict, then it’s possible to crudely estimate the number of hard steps it typically takes to produce intelligent life, like us.

The authors’ best guess is between 4 and 8, admittedly with some major assumptions.

It should also be noted that the hard steps model is simplistic - for example, some steps may be easy, but take a long time anyway, like due to the slow but steady buildup of biological complexity.

Still, the hard steps model lets us write down a mathematical function for the appearance rate of life that depends on a simple parameter - the number of hard steps.

It also lets us say something about humanity’s earliness based on the typical number of hard steps.

The more steps there are in the typical evolutionary pathway, the earlier humanity must be.

Another important part of the story is: what’s the time limit for completing these steps?

How long do these habitable planets remain habitable?

It’s estimated that the Earth will only be habitable for another 1 billion years, after which the sun will be too hot for liquid water to exist on the surface of the planet.

So Earth will have 5 billion habitable years in total.

If all habitable planets only last this long, then new civilisations would mostly stop appearing 5 billion years after the peak of habitable star formation - which, if you remember, is 20 billion years away.

In this case, it seems like we’re a little bit early.

However, there’s one big consideration that totally changes the picture.

There might be other habitable planets that last a lot longer than the Earth.

M-type stars, also known as red dwarfs, can live for thousands of times longer than our sun, meaning they could have planets that remain habitable for trillions of years.

If it’s possible for life to evolve on these planets, then new civilisations could continue to emerge for trillions of years into the future.

In this case, we are very early.

The experts are split on whether red dwarfs are habitable.

Since they’re very dim, orbiting planets need to be very close to the star in order to have liquid water, which may be necessary for life.

But planets closer to their star are more likely to be tidally locked.

This can cause erratic climates and make any potential inhabitants vulnerable to solar flares.

The authors of this paper argue that life could well evolve on red dwarf planets.

After all, why should all life have the same requirements as Earth life?

But this isn’t the scientific consensus, and notably these researchers are not planetary scientists nor astrobiologists.

So there are two main deciding factors of our birth rank: the number of hard steps and the maximum lifetime of habitable planets.

By throwing many combinations of these two parameters into their model, the researchers generated this diagram.

Let’s talk through it.

Each point represents a choice of hard-step-number and maximum planet lifetime, and the color of that point tells you our birth rank.

Cold colours mean we’re typical, and hot colours mean we’re early.

If only short-lived planets like Earth are habitable and there are few hard steps, then we were born at a pretty typical period of the cosmos, about a 25% of civilisations were born before us.

If red dwarfs are habitable but it’s easy for life to evolve, we’re in something like the first 0.1% or even first thousandth of civilisations.

Finally, if there are both long-lived habitable planets and many hard steps, then virtually all civilization will result from very drawn-out evolutionary processes trillions of years in the future.

There will be a billion billion times more civilisations born after us than before us.

OK, now we get to the wild proposal of the paper.

The researchers ask the question: if red dwarfs are habitable and there are many hard steps, is there any other way to explain our apparent earliness besides extreme chance?

The idea is to come up with some plausible scenario that limits the number of possible future civilizations, even in the case that we should expect them based on the emergence of habitable worlds and the hard steps model.

The so-called grabby aliens model hypothesises the existence of a cosmic deadline for the birth of new civilisations: some catastrophic event that will happen one day in the future that prevents any new intelligent life from emerging.

But what could cause this deadline?

Well, we could.

And by we I mean all civilizations that emerge up to that deadline.

The model claims that at some point in the future, all habitable planets will be occupied by rapidly expanding alien civilizations.

No new life has the chance to evolve, because every planet is already occupied by space-fairing advanced life.

To reach this conclusion: the model assumes that every intelligent civilization has some chance of becoming a grabby civilisation: a civilisation that expands in every direction through space at close to the speed of light, making visible changes wherever they go, and that they stick around for a long time.

When we look at life on Earth, it seems that life has a common tendency to aggressively expand into new territories and niches.

When thinking about our own society today, some might be skeptical that we’re going to be able to expand through the cosmos close to the speed of light any time soon.

But even if it takes us a million years to work out how to expand into space and another million to cross the galaxy at 10% light speed, this delay will have basically no effect on cosmological timescales.

The authors needed to tune the parameters of the model to cut off just the right amount of future births of new civilisations to give us a typical, run-of-the-mill birth rank.

If grabby civilizations aren’t common enough, they won’t prevent enough future births to make us typical rather than early.

But if they’re too common, then we couldn’t have evolved in the first place as Earth would have already been colonised.

That actually allows us to estimate the number of civilizations needed to make sure that humanity isn't too special by being too early.

But for that we also need to estimate the speed of grabby expansion.

We do that with surprisingly useful bit of information: the fact that we haven’t yet seen any evidence of aliens through our telescopes.

Counterintuitively, this fact implies that grabby expansion is incredibly fast, around a third of the speed of light.

How does this follow?

Basically, if we could see grabby aliens in our skies, the aliens must be moving slowly, since they are in our neighborhood but haven’t reached us yet.

Instead, if we assume they move very fast, then we wouldn’t see them until they basically arrive on Earth.

The emptiness of our skies and the constraint of making us typical observers is enough to determine all of the parameters of this model.

The researchers then use the model to run a simulation of the emergence of intelligent civilizations over time, in which some civilizations become grabby and spread through the universe, ending with a universe totally full of life.

The simulation tells us all sorts of weird and surprising things about how space is populated.

At least, given the assumptions of this model.

It tells us that the colonization of the universe has already begun: in fact, around half of the volume of the universe is currently colonised.

It predicts that only around one in a thousand intelligent civilisations eventually become space-faring and spread through the cosmos.

This could be because most civilisations destroy themselves before they get the chance, or choose not to take to the skies.

Finally: it tells us roughly when we should expect to meet those grabby aliens.

It gives us a best-guess waiting time of around half a billion years.

Don’t expect to meet any aliens any time soon, even if they do fill half the universe.

I guess they’re not in the Milky Way just yet.

I’m not going to say whether the conclusions of this paper are right.

Remember, this is a model, and it’s conclusions are only as good as the assumptions behind the model.

But this paper is a nice example of how we can figure out a lot given very little - in this case, given literally nothing, at least in terms of our observation of the existence of other civilizations.

But if these authors are right - and that’s a big big if - then it may be that our empty sky is one piece of evidence for a universe filled with aliens rushing in to grab this rare remaining patch of empty space time.