The Universe Is Flat — Now What?

Paul Sutter is an astrophysicist at The Ohio State College and the chief scientist at COSI Science Middle. Sutter can also be host of Ask a Spaceman, RealSpace, and COSI Science Now. 

Spoiler alert: The universe is flat. However there's a whole lot of subtlety packed into that innocent-looking assertion. What does it imply for a 3D object to be "flat"? How can we measure the form of the universe anyway? For the reason that universe is flat, is that…it? Is there the rest attention-grabbing to say?

Oh sure, there's.

Stroll the road

First, we have to outline what we imply by flat. The display you are studying this on is clearly flat (I hope), and you realize that the Earth is curved (I hope). However how can we quantify that mathematically? Such an train is perhaps helpful if we need to go round measuring the form of the entire whole universe. [The History & Structure of the Universe (Infographic)]

One reply lies in parallel strains. For those who begin drawing two parallel strains in your paper and allow them to proceed on, they're going to keep completely parallel ceaselessly (or no less than till you run out of paper). That was basically the definition of a parallel line for a pair thousand years, so we ought to be good.

Let's repeat the train on the floor of the Earth. Begin on the equator and draw a pair parallel strains, every pointing immediately north. Because the strains proceed, they by no means flip left or proper however nonetheless find yourself intersecting on the North Pole. The curvature of the Earth itself prompted these initially parallel strains to finish up not-so-parallel. Ergo, the Earth is curved.

The alternative of the Earth's curved form is a saddle: on that floor, strains that begin out parallel find yourself spreading aside from one another (in swanky mathematical circles this is called "ultraparallel"). [I explore the possible shapes of the universe in this video.]

The form of the universe relies on its density. If the density is greater than the crucial density, the universe is closed and curves like a sphere; if much less, it'll curve like a saddle. But when the precise density of the universe is the same as the crucial density, as scientists suppose it's, then it'll prolong ceaselessly like a flat piece of paper.

Credit score: NASA/WMAP Science Group

So there you have got it: You possibly can measure the "flatness" of a construction simply by watching how parallel strains behave. In our 3D universe, we might watch beams of sunshine: If, say, two lasers began out completely parallel, then their long-term habits would inform us necessary issues.

Flat as a (massive) pancake

Keep in mind that measuring the form of the universe is a query for cosmology, the examine of the whole universe. And in cosmology, no person cares about you. Or me. Or photo voltaic methods. Or black holes. Or galaxies. In cosmology we care in regards to the universe solely on the very largest scales; small-scale bumps and wiggles usually are not necessary for this query.

The universe has all kinds of deformations in space-time the place it varies from the superbly flat. Anywhere the place there's mass or vitality, there is a corresponding bending of space-time — that is Basic Relativity 101. So a pair mild beams would naturally collide inside a wandering black gap, or bend alongside bizarre angles after encountering a galaxy or two.

However common all these small-scale results out and take a look at the large image. After we look at very previous mild — say, the cosmic microwave background — that has been touring the universe for greater than 13.eight billion years, we get a real sense of the universe's form. And the reply, so far as we are able to inform, to inside an extremely small margin of uncertainty, is that the universe is flat.

There is no such thing as a spoon

Effectively, that settles that. However this text is not over but, which implies there's extra to the story.

Have you ever ever requested your self if there is a distinction between a cylinder and a sphere? Greater than doubtless not, nevertheless it's by no means too late to attempt new issues.

Take out your piece of paper with two parallel strains on it. Go forward, dig it out of the trash. Wrap one finish round to fulfill the opposite, making a cylinder. Fastidiously observe the parallel strains — they continue to be parallel, do not they? That is as a result of cylinders are flat.

You heard it right here first: Cylinders are flat.

There's an necessary distinction between geometry, the habits of parallel strains, and topology, the way in which an area can get all tousled. Whereas the geometry of the universe may be very effectively measured (once more, it is flat), the topology just isn't. And here is a bonus truth: not solely can we not decide the topology of the universe from observations, however there are additionally no legal guidelines of physics that predict or prohibit the topology.

Along with your 2D piece of paper, you may join the ends just a few alternative ways. Join one of many dimensions usually and you've got a cylinder. Flip one edge over earlier than connecting and you have made a Mobius strip. Join two dimensions, the highest to the underside and one aspect to the opposite, and you've got a torus (aka a donut).

In our 3D universe, there are many choices — 18 identified ones, to be exact. Mobius strips, Klein bottles and Hantzsche-Wendt house manifolds are all non-trivial topologies that share one thing in frequent: for those who journey far sufficient in a single path, you come again to the place you began. Within the case of flipped dimensions, if you come again to your place to begin, you will end up the wrong way up with out having tried to take action in any respect. [Watch: Explaining the Shape of the Universe.]

In fact we have seemed to see if our universe is related like this; we do not see any copies of galaxies, and we do not see the cosmic microwave background intersecting itself. If the universe is pretzeled-up, it is on scales far, far bigger than what we are able to observe.

So do not get too excited by chance of dwelling in a real-life model of the "Asteroids" sport — which, as you now know, is performed on the floor of a donut.

Be taught extra by listening to the episode "What is the form of the universe?" on the Ask A Spaceman podcast, accessible on iTunes and on the Internet at http://www.askaspaceman.com. Because of Greg S. and Michael W. for the questions that led to this piece! Ask your individual query on Twitter utilizing #AskASpaceman or by following Paul @PaulMattSutter and fb.com/PaulMattSutter.

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