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Space is big.

Really big.

You just won't  believe how vastly hugely mind-bogglingly   big it is.

In fact, the near 100 billion  light year wide observable universe is   only a tiny fraction of the greater bigness  of space beyond.

And for a little while,   we will see more and more of that universe beyond until the cosmological horizons collapse on us   and cut off our view forever.

So how long do we  have, and what will we see before that happens?

There is an absolute limit to our access to the universe beyond our own galaxy.

There’s a limit to what we can ever  hope to explore or send signals to,   and a very different limit to  what we can ever hope to witness.

Today we’re going to explore the latter.

We’re  going to figure out the absolute limit of our   future view of the universe, and of the universe’s  ability to influence us.

Next time we’ll turn it   around and ask: how much of the external universe  can we potentially influence, and even explore?

But let’s start by thinking about the light that we can see from the distant universe.

We know that light doesn’t travel instantaneously.

That means when we look into the distance,   we’re seeing objects as they were in the  past.

We see the moon as it was 1.3 seconds   ago, the Sun as it was 8 minutes ago, and Milky  Way stars as they were 100s to many 1000s of   years ago.

With our greatest telescopes like  the James Webb we’ve seen galaxies whose light   has been travelling to us for many billions of  years.

This is old light from a younger universe.

Although the night sky looks like  a sphere surrounding the Earth,   you can imagine it as a set of shells, which  each shell representing light that has come to   us from a particular distance.The most distant  light we can see is from the cosmic microwave   background radiation - the CMB - which has  been travelling to us for almost the entire   age of the universe—13.7 billion years more  or less.

This is the light emitted by the hot   hydrogen plasma around 300,000 years after  the big bang when the universe became cool   enough to form the first atoms, and at  the same time first became transparent.

Surely this is the limit of our  vision, so how far away is the CMB?

Well, this is where things get confusing.

When  the CMB light started its journey towards us,   the universe was 1100 times smaller than it is  today.

A photon of CMB light observed today was   emitted from particles of primordial plasma that  were around 40 million light years away from the   region that would one day become the Milky  Way.

That’s right next door on cosmological   scales.

But that light was running the wrong  way against the cosmic travelator.

The expanding   universe forced it to travel 13.7 billion  light years in order to reach us just now.

Meanwhile, the galaxies that those CMB blobs  would eventually form into have now been shoved   in the opposite direction to a distance of  46.5 billion light years by that expansion.

We often say that the observable universe is 46.5  billion light years in radius because if you froze   the expansion now, that would be the current  distance of the furthest objects we can see.

The CMB is the practical limit of our view  because at earlier times and greater distances   the universe was opaque.

But it’s possible that one day we’ll get signals from those times.

For example,   neutrinos can pass through the dense primordial  plasma and could take us closer to the big bang.

And there are events in the first  fraction of a second that may   have created gravitational waves whose  influence might one day be detectable.

The earliest time we can ever in principle  observe is the instant of the big bang itself.

The big bang happened everywhere, but if we could  observe the big bang, we would be observing it   as it happened on a spherical shell just  beyond the cosmic microwave background.

We call that shell the particle horizon.

It’s  the absolute limit of the observable universe.

At least for now.

If we wait, signals from more  distant parts will have had time to reach us.

The particle horizon expands, and the shell  from which we observe the CMB also expands,   changing over time.

But there’s a limit to  the amount of universe we’ll be able to see,   even if we wait infinite time.

To figure out that  limit we’re going to need a spacetime diagram.

Long-time watchers have seen these  before, but today we’re going to do   things with the spacetime diagram may  not have seen—at least not from me.

Consider the spherical shells representing light  coming to us from earlier epochs.

Now get rid of 2   of our 3 dimensions of space so we just see a pair  of points on each sphere in opposite directions,   with arrows for the light travelling to us.

Now  sort vertically according to when that light was   emitted.

This is a spacetime diagram—one dimension  of space plotted against time.

Light is travelling   through space and time towards us, forming a light cone—in particular, our past light cone.

The boundaries of the cone are at 45  degrees.

This is the angle light makes   on our spacetime diagram.

Slower speeds  have steeper angles, and that means only   things inside or on this cone can potentially  send a signal to or travel to our location in   spacetime.

Events outside the past lightcone  can’t get signals to us here in our present.

Our particle horizon is at the bottom of the light  cone, and the CMB is just above it.

But there’s a   problem here.

Remember we constructed this light  cone by layering a series of shells that represent   the current size of those regions.

But the  universe has been expanding, so at earlier times   those regions were smaller.

These lines represent  the past and future expansion of the universe.

The big bang singularity has all space  compacted into a point, after which it   expands.

That expansion at first slows due to the  gravitational pull of all matter in the universe,   but after a while dark energy takes over and the   expansion begins to accelerate.

And that started just a few billion years ago.

While the lightcone encompasses more and more  stuff into the past, at some point it starts   to encompass less and less physical space.

It  comes to a point at the particle horizon.

Even   at the big bang, the particle horizon doesn’t  encompass nothing, nor does it encompass   everything.

It represents a finite portion of the  compacted everything at the beginning of time.

It’s easier to see that if we make  a modification to our diagram.

Imagine we redefine the tick marks on the x-axis  so that instead of representing absolute distance,   they represent points fixed to the expanding  space.

These could be galaxies for example.

Currently those reference galaxies follow these  lines.

But we can stretch the graph differently   at each layer to straighten the lines, so that now  each tick represents more and more space going up.

We’ll label that axis with the distances  those galaxies will be from us at the present moment.

But at the bottom of the diagram, each tick  really represents a tiny physical distance.

That distance approaches zero towards the very bottom of the plot, but at any non-zero time after the   big bang, we can see that our particle horizon  encompasses a finite amount of the universe.

We just created a new coordinate system.

These  are comoving coordinates—they co-move with the   expansion of the universe.

Our past lightcone in  this system is curved—light travels many ticks   per unit time at the bottom but fewer going up.

Let’s make one more change—we’re going to adjust   the time axis so that each tick represents less  and less time as we get closer to the big bang,   to the same degree that space is compressed.

Do this right and our lightcone and all light   speed paths make straight 45 degree  lines like they’re supposed to.

With our new tool in hand, we can really see how  our particle horizon really changes over time.

As we move forward in time  our past lightcone follows us.

In the regular coordinate system, it still  converges to a point, but in comoving coordinates   we see that it encompasses more and more of the  universe.

Naively, it might seem that there’s no   limit—if we want to see any distant point we  just have to wait long enough.

And that would   be the case if it weren’t for dark energy.

The fact that the expansion of our universe is   accelerating means there’s an absolute upper limit  on how much of the universe we can ultimately see.

But to understand the future limits of  that view, we need to understand some   new horizons—horizons that aren’t getting  bigger.

In fact they’re racing towards us.

In an expanding universe like ours, more  distant points seem to be moving away   from us more quickly.

A point that’s far  enough away will be moving away from us at   the speed of light.

While objects can’t  move through space faster than light,   space itself can move at any speed relative to other space, and it can carries objects with it.

So, there’s a sphere that surrounds us where the  recession velocity equals the speed of light.

It’s   called the Hubble horizon, after Edwin Hubble, the  guy who discovered the expansion of the universe.

The hubble horizon is currently 14 and a half billion light years away.

Now you might think it would be impossible to see things further than this.

After all, light   emitted by galaxies in that distant space will be pulled away from us like fish swimming against a current.

But we can see the cosmic microwave background, and that’s 46.5 billion light years.

So what gives?

In fact we routinely observe galaxies that were  moving away from us faster than light at the time   they emitted the light that we see.

To understand  how this can work, imagine you’re a photon.

You’re spat out by a bright star in a galaxy  millions of light years from of the Milky   Way.

Initially you find yourself in a region  space that’s moving away from the Milky Way at   3c — three times your own speed of light.

You’re dragged with that space and so the   Milky Way seems to be racing away from you.

But  because you’re moving towards the Milky Way,   you escape that patch of space that’s traveling  at 3c.

That patch moves in the opposite direction   to you.

Now you find yourself in a region  of space that’s traveling a little less   quickly away from your destination.

You’re still  dragged away from the Milky Way, but not as fast.

After billions of years of losing ground, you  eventually find yourself at the Milky Way’s   Hubble horizon where space is receding at exactly  1c - your own speed.

The Milky Way finally stops   moving away from you, and as you struggle across  that horizon and can finally make progress.

You have to retrace a lot of ground—billions of  light years in fact—but eventually you reach the   Milky Way, then the Earth, then fall into a  telescope where you reveal secrets about your   ancient origin to a group of bipedal apes that  evolved long, long after you started your journey.

The reason that light from beyond the Hubble  horizon can reach us is that for the first several   billion years after the big bang, the Hubble  horizon was expanding.

Galaxies were moving away   at a more or less constant speed, in fact actually  slowing down a bit.

That means the point where   the recession speed equaled the speed of light  was getting further away from us.

This enabled   light from more distant objects to creep inside  the Hubble horizon and make their way to us.

By the way this is great news by the way—otherwise we would never have seen the CMB and probably couldn’t have   proved that the Big Bang happened.

But this won’t always be the case.

Like I said, the expansion   of the universe is accelerating.

At some point  the Hubble horizon will stop moving away and   actually collapse back on us.

The boundary of  light-speed expansion will eventually surround   the local group of galaxies and stop there,  walling us off from the rest of the universe.

Only the light that can cross the Hubble horizon before it collapses can ever reach us.

So now we introduce our final horizon—the horizon  that’s going to determine our ultimate view of   the universe.

This is the cosmological event horizon.

It's the boundary beyond which no signal can ever be   received, no matter how long we wait.

Light coming  from beyond the cosmological event horizon won’t   make it across the Hubble horizon before it  collapses.

The event horizon started out much,   much larger than the Hubble horizon—63 billion light years in comoving distance,   but it shrinks over time, because light released  at later times has less time to get to us.

It’s now around 16 billion light years in  radius.

At some point—in 10 billion years   give or take—the cosmological event horizon  will merge with the collapsing Hubble horizon.

At that point we’ll never be able to  see new events from beyond this horizon.

But that doesn’t mean the universe will suddenly  go dark.

This is also when we get our broadest   view of the universe.

Light from the most  distant points that we will ever see will   then cross the shrinking Hubble horizon just in the nick of time.

Those photons will struggle against space   that’s moving just less than light speed, and  will eventually make their way into our local   bubble of non-expanding space.

They’ll carry  with them light from the cosmological event   horizon as it was in the beginning of time,  when it was 63 billion light years in radius.

And that will become the final  size of our particle horizon.

We’ll be able to see things that are currently 63  billion light years away.

That’s about half again   the size of our current view.

But we won’t  have that view forever.

While the photons   continue to trickle in from the collapsed horizon for eternity, they will be increasingly   redshifted—sapped of energy—from their long  fight against the outward flow of space.

When we first get close to our our full  panorama of the universe in about 10 billion years,   the galaxies will shine mostly in infrared light.

After a few hundred billion years,   even the most energetic photons will be stretched  to the point that we’d need a radio antenna   larger than our contracted Hubble horizon  to see them.

The sky would finally be dark.

So that is the extent of humanity’s  potential to see the universe.

There is one way we can see further—and that’s  by traveling as fast as possible to escape   our collapsing Hubble horizon.

We’ll then be in a different Hubble horizon with different view.

But we’ll save that for next episode, when we  explore the limits of our ability to influence   and explore the universe.

How far can humanity eventually expand and when   we need to leave to avoid getting trapped  forever in our local bubble of space time.

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