## How Do We Know the Universe is Flat? Discovering the Topology of the Universe

Whenever we talk about the expanding Universe, everyone wants to know how this is going to end. Sure, they say, the fact that most of the galaxies we can see are speeding away from us in all directions is really interesting. Sure, they say, the Big Bang makes sense, in that everything was closer together billions of years ago.

But how does it end? Does this go on forever? Do galaxies eventually slow down, come to a stop, and then hurtle back together in a Big Crunch? Will we get a non-stop cycle of Big Bangs, forever and ever?

We’ve done a bunch of articles on many different aspects of this question, and the current conclusion astronomers have reached is that because the Universe is flat, it’s never going to collapse in on itself and start another Big Bang.

But wait, what does it mean to say that the Universe is “flat”? Why is that important, and how do we even know?

Before we can get started talking about the flatness of the Universe, we need to talk about flatness in general. What does it mean to say that something is flat?

If you’re in a square room and walk around the corners, you’ll return to your starting point having made 4 90-degree turns. You can say that your room is flat. This is Euclidian geometry.

But if you make the same journey on the surface of the Earth. Start at the equator, make a 90-degree turn, walk up to the North Pole, make another 90-degree turn, return to the equator, another 90-degree turn and return to your starting point.

In one situation, you made 4 turns to return to your starting point, in another situation it only took 3. That’s because the topology of the surface you were walking on decided what happens when you take a 90-degree turn.

You can imagine an even more extreme example, where you’re walking around inside a crater, and it takes more than 4 turns to return to your starting point.

Another analogy, of course, is the idea of parallel lines. If you fire off two parallel lines at the North pole, they move away from each other, following the topology of the Earth and then come back together.

Got that? Great.

Now, what about the Universe itself? You can imagine that same analogy. Imaging flying out into space on a rocket for billions of light-years, performing 90-degree maneuvers and returning to your starting point.

You can’t do it in 3, or 5, you need 4, which means that the topology of the Universe is flat. Which is totally intuitive, right? I mean, that would be your assumption.

But astronomers were skeptical and needed to know for certain, and so, they set out to test this assumption.

In order to prove the flatness of the Universe, you would need to travel a long way. And astronomers use the largest possible observation they can make. The Cosmic Microwave Background Radiation, the afterglow of the Big Bang, visible in all directions as a red-shifted, fading moment when the Universe became transparent about 380,000 years after the Big Bang.

When this radiation was released, the entire Universe was approximately 2,700 C. This was the moment when it was cool enough for photons were finally free to roam across the Universe. The expansion of the Universe stretched these photons out over their 13.8 billion year journey, shifting them down into the microwave spectrum, just 2.7 degrees above absolute zero.

With the most sensitive space-based telescopes they have available, astronomers are able to detect tiny variations in the temperature of this background radiation.

And here’s the part that blows my mind every time I think about it. These tiny temperature variations correspond to the largest scale structures of the observable Universe. A region that was a fraction of a degree warmer become a vast galaxy cluster, hundreds of millions of light-years across.

The Cosmic Microwave Background Radiation just gives and gives, and when it comes to figuring out the topology of the Universe, it has the answer we need. If the Universe was curved in any way, these temperature variations would appear distorted compared to the actual size that we see these structures today.

But they’re not. To best of its ability, ESA’s Planck space telescope, can’t detect any distortion at all. The Universe is flat.

Well, that’s not exactly true. According to the best measurements astronomers have ever been able to make, the curvature of the Universe falls within a range of error bars that indicates it’s flat. Future observations by some super Planck telescope could show a slight curvature, but for now, the best measurements out there say… flat.

We say that the Universe is flat, and this means that parallel lines will always remain parallel. 90-degree turns behave as true 90-degree turns, and everything makes sense.

But what are the implications for the entire Universe? What does this tell us?

Unfortunately, the biggest thing is what it doesn’t tell us. We still don’t know if the Universe is finite or infinite. If we could measure its curvature, we could know that we’re in a finite Universe, and get a sense of what its actual true size is, out beyond the observable Universe we can measure.

We know that the volume of the Universe is at least 100 times more than we can observe. At least. If the flatness error bars get brought down, the minimum size of the Universe goes up.

And remember, an infinite Universe is still on the table.

Another thing this does, is that it actually causes a problem for the original Big Bang theory, requiring the development of a theory like inflation.

Since the Universe is flat now, it must have been flat in the past, when the Universe was an incredibly dense singularity. And for it to maintain this level of flatness over 13.8 billion years of expansion, in kind of amazing.

In fact, astronomers estimate that the Universe must have been flat to 1 part within 1×10^57 parts.

Which seems like an insane coincidence. The development of inflation, however, solves this, by expanding the Universe an incomprehensible amount moments after the Big Bang. Pre and post inflation Universes can have vastly different levels of curvature.

In the olden days, cosmologists used to say that the flatness of the Universe had implications for its future. If the Universe was curved where you could complete a full journey with less than 4 turns, that meant it was closed and destined to collapse in on itself.

And it was more than 4 turns, it was open and destined to expand forever.

Well, that doesn’t really matter any more. In 1998, the astronomers discovered dark energy, which is this mysterious force accelerating the expansion of the Universe. Whether the Universe is open, closed or flat, it’s going to keep on expanding. In fact, that expansion is going to accelerate, forever.

I hope this gives you a little more understanding of what cosmologists mean when they say that the Universe is flat. And how do we know it’s flat? Very precise measurements in the Cosmic Microwave Background Radiation.

Is there anything that all pervasive relic of the early Universe can’t do?

## What Are The Biggest Mysteries in Astronomy?

Black Holes? Dark Energy? Dark Matter? Alien Life? What are the biggest mysteries that still exist out there for us to figure out?

“The more I learn, the more I realize how much I don’t know.” These are the words of Albert Einstein. I assume he was talking about Minecraft, but I guess it applies to the Universe too.

There are many examples: astronomers try to discover the rate of the expansion of the Universe, and learn a dark energy is accelerating its expansion. NASA’s Cassini spacecraft finally images Saturn’s moon Iapetus, and finds a strange equatorial ridge – how the heck did that get there? Did the Celestials forget to trim it when it came out of the packaging?

There have always been, and, let’s go as far as to say that there always will be, mysteries in astronomy. Although the nature of the mysteries may change, the total number is always going up.

Hundreds of years ago, people wanted to know how the planets moved through sky (conservation of angular momentum), how old the Earth was (4.54 billion years), or what kept the Moon from flying off into space (gravity). Just a century ago, astronomers weren’t sure what galaxies were (islands of stars), or how the Sun generated energy (nuclear fusion). And just a few decades ago, we didn’t know what caused quasars (feeding supermassive black holes), or how old the Universe was (13.8 billion years). Each of these mysteries has been solved, or at least, we’ve a got a pretty good understanding of what’s going on.

Science continues to explore and seek answers to the mysteries we have, and as it does it opens up new brand doors. Fortunately for anyone who’s thinking of going into astronomy as a career, there are a handful of really compelling mysteries to explore right now:

Is the Universe finite or infinite? We can see light that left shortly after the Big Bang, 13.8 billion years in all directions. And the expansion of the Universe has carried these regions more than 45 billion light-years away from us. But the Universe is probably much larger than that, and may be even infinite.

What is dark matter? Thanks to gravitational lensing, astronomers can perceive vast halos of invisible material around all galaxies. But what is this stuff, and why doesn’t it interact with any other matter?

What is dark energy? When trying to discover the expansion rate of the Universe, astronomers discovered that the expansion is actually accelerating? Why is this happening? Is something causing this force, or do we just not understand gravity at the largest scales?

There are supermassive black holes at the heart of pretty much every galaxy. Did these supermassive black holes form first, and then the galaxies around them? Or was it the other way around?

The Big Bang occurred 13.8 billion years ago, and the expansion of the Universe has continued ever since. But what came before the Big Bang? In fact, what even caused the Big Bang? Has it been Big Bangs over and over again?

Are we alone in the Universe? Is there life on any other world or star system? And is anyone out there we could talk to?

Shortly after the Big Bang, incomprehensible amounts of matter and antimatter annihilated each other. But for some reason, there was a slightly higher ratio of matter – and so we have a matter dominated Universe. Why?

Is this the only Universe? Is there a multiverse of universes out there? How do I get to the Whedonverse?

In the distant future, after all the stars are dead and gone, maybe protons themselves will decay and there will be nothing left but energy. Physicists haven’t been able to catch a proton decaying yet. Will the ever?

And these are just some of the big ones. There are hundreds, thousands, millions of unanswered questions. The more we learn, the more we discover how little we actually understand.

Whenever we do a video about concepts in astronomy where we have a basic understanding, like gravity, evolution, or the Big Bang, trolls show up and say that scientists are so arrogant. That they think they know everything. But scientists don’t know everything, and they’re willing to admit when something is a mystery. When the answer to the question is: I don’t know.