Do you find our galaxy too confining? Do you long for the vast expanses of extragalactic space? Now, you can make it to the edges of the observable universe in only a few decades! In this week's "Ask a Physicist," we'll offer you a practical guide for the intergalactic traveler.

Illustration by esk6a

A few weeks ago, I put out a call for questions about the universe for the opportunity to win an advance copy of my exciting new book. This week's winning question comes from the awesomely named AbedNadir (Cameron, for realsies) who asked:

What impact will the expanding universe have on humans exploring space?

Can I say, first, that I love this question, not least because it's really quite audacious. The universe is expanding, sure, but individual galaxies aren't. In other words, to even matter, you have to start thinking about travel to fairly distant galaxies – millions of light-years. Given that the furthest manned mission so far has been to the moon (1.3 light-seconds), this takes some serious gall.

Also, it turns out that if you want to figure out all of the implications of a realistic trip to another galaxy, especially ones billions of light-years away, the physics becomes a bit hairy. This means that I got to do a really fun calculation that (as near as I can tell) nobody's done before. I normally don't do this, but for those of you who'd like to see a fairly technical write-up and a little code to do all of the detailed calculations, go nuts!


If you'd rather just get the gist of what would be involved (for a sci-fi story that you're working on, maybe?), I offer the following primer. Of course, I fully expect the comments section to be awash in theories about other oddities of space.

Illustration by esk6a

You'll need to move fast

First and foremost, a trip across the cosmos isn't going to be easy, and it's certainly not going to be quick. The so-called "Event Horizon" of the universe – the most distant places that we could travel to – are about 16 billion light-years away now and they're only getting more distant as time goes on.


To make any sort of reasonable headway, we're going to have to send ships traveling insanely close to the speed of light – and by this, I mean that the peak speeds are going to be roughly 99.9999999999999999998% of c. Stuff will hit you and at those speeds, you will die. I strongly recommend figuring out a solution to this problem before you set out on your trip, and before you simply blurt out "magnets," let me just remind you that not all rocks and particles are electrically charged. And at those speeds, even the ordinary photons that surround us in the Cosmic Microwave Background would appear to glow at tens of billions of degrees, bombarding us with high-energy gamma rays. As an upside, we'll get to Hulk-out.

The universe will still age tens of billions of years during your trip. The sun and the earth and everything you've ever cared about will have long since been destroyed. Not that it matters much. You won't be able to loop back to earth, anyway. It is most decidedly a one-way trip.

Finally, it takes a little while to get up to speed. You may want to send robots who won't get bored or who don't feel any adverse effects from tremendous g-forces. I won't begrudge you that, and for most of what follows, it doesn't matter anyway.

Just to make things as "realistic" as possible, let's figure out how things would work in a spaceship that accelerates at one-g (so it feels like earth-normal gravity in the ship, with the back as "down"). Around midway through, you'll start to decelerate at the same rate, at which point, you and all of your stuff will be tossed to the front of the ship as though you were in free-fall.

Illustration by esk6a

Time runs slow

So far, everything has sounded pretty dire, but traveling at near light-speeds has some advantages as well. One of the great predictions of special relativity is that the clocks of moving observers seem to run slow, and the closer you are to the speed of light, the slower that your clock seems to run.

A one-g acceleration is surprisingly swift. After only a year or so, you'd be traveling at close to the speed of light, so for the vast majority of the billions of years that it'd take to reach another galaxy, you'd be cruising along at nearly the speed of light, which means that your personal clock will start to run very, very slow.


By the time you reach a cruising speed (the one with all of the 9's above), you'll age about 10 billion times slower than the stationary chumps in the rest of the universe.

Illustration by esk6a

Distant galaxies are a moving target

Just because a galaxy is 5 billion light years away now doesn’t mean that it would take 5 billion years to get there, even at the speed of light. Every day distant galaxies get further and further away. And what with the universe accelerating, any given galaxy gets more and more distant the longer you wait. I guess what I'm trying to say is that we should probably leave now, before things get worse.

Surprisingly, there's another problem with an expanding universe: the expansion acts like a drag and slows your ship down. This is the same effect that causes light from the big bang to to get redder and redder (lower and lower energy) as time goes on. Just to keep at a constant speed, you need to keep your foot on the gas.


The drag on the universe only really seems to matter once you get really close to the speed of light – which is why there is a cruising speed. No expanding universe, and you'd just get closer and closer to the speed of light the further you go.

Illustration by esk6a

In an accelerating universe, you can only go so far

We live in a universe of dark energy, which means that the universe is not only expanding, but accelerating. This is a bigger deal than you might think. In an old-fashioned universe made only of Dark Matter and dumb old atoms, the universe gets bigger with time, sure, but it does so at a fairly slow rate. The most distant galaxies get further and further away, but they do so slowly enough that, if you really cared to, you could eventually catch up to them.

Not so in an accelerating universe.

In an accelerating universe, there is a event horizon beyond which you can’t ever reach, no matter how quickly you travel. For our universe, that means that anything further away than about 16 billion light-years (right now) is totally off limits. Let me make this clear: there is no "edge of the universe" in the real sense of hitting a brick wall. There's just a furthest distance that you can reach, with more distant galaxies pulling away from you like a Charlie Brown-esque football.


This is to say nothing of other potential weird effects like the fact that you can’t be sure of what you’ll reach when you get to your destination or even if the galaxy hasn’t already been destroyed in some sort of collision.

A Trip to the End of the Universe

Suppose you actually wanted to make a trip to the Restaurant at the End of the Universe – or at least, the end of our visible universe. How long would it take?


While you should feel free to take a look at the technical calculations, the money shot is here:

Credit: Me, as a result of one of the most fun calculations I've done in a while.


You can see the horizon right there. No matter how long you keep at it, you'll never get more than the galaxies that are currently about 16 billion light-years.

But there's a truly amazing aspect to all of this. Despite traveling for about 20 billion years or so (as measured by the universe), the entire trip will only seem to take about 45 years to you, the astronaut. 45 years! Seriously, you could do it in a single human lifetime, and even faster if you didn't mind being crushed by higher g-forces or you were a robot.

Stranger still, most of that time is simply spent getting up to speed. It would take 28 years to get to Andromeda – and that's the nearest big galaxy to us. If somehow, all evidence to the contrary, we don't live in a Dark Energy universe – and hence, no ultimate horizon – 45 or 50 years (rocket-time) would get you pretty much anywhere.


And what's the downside (besides, as I keep needing to remind you, the fact that every stray atom is flying at your ship at something like a million times the energy in the LHC)?

Well, there's the cost. For a trip of this magnitude, you're going to need something with a little more punch than ordinary rocket fuel. A supply of equal amounts of matter and antimatter have 100% efficiency, but even so, you're going to need a lot of it. Previously, I did a calculation show that it would take about 530 times as much matter-antimatter as cargo to reach a relatively nearby star about 20 light-years away.

That's nothing.

To reach the most distant galaxies, you’d need a matter-antimatter supply the size of the moon to carry about 500kg of cargo. You should feel free to speculate below on ways to try to collect the requisite matter and antimatter en route. But in any case, it's going to be a rough trip.

Don't forget your towel.*

* Though, fair warning, bringing it along would require a matter-antimatter drive roughly the size of Mimas.**

** That's the one that looks like the Death Star.
