The Universe is Flat?

     The answer to this question comes up a lot in conversation, which is yes, the Universe is flat from the data we have collected. But before we talk about the data and why it proves the Universe is flat and how dark energy, matter and the CMB tie into it, lets take a look at what it means for the Universe to be flat or round.

     When people say that the Universe is flat, what they are actually saying is that space itself is flat, meaning the structure of the Universe as a whole. Due to my sufficient lack of a PhD in physics, I do not really understand what it means by the shape of space, but I have learned what would happen if it was flat or round and why those things would happen. If you went out into a space that was flat with two other spaceships and shot lasers from each spaceship to the other two, the beams would form a triangle. When we work with triangles on a flat surface, the total angle between the lines is always 180 degrees. If space was uniformly flat, this would hold true no matter where you shot your lasers from. But, if space was curved out or in, the angle would add up to less or more than 180 degrees, respectively. This can be proven by simply drawing a triangle on a sphere vs. drawing a square on a flat surface. On the sphere, it takes less distance and less angles to end up where you started, if all of the angles and distances are the same and you walk very far along the sphere. But how do we know that the Universe is flat? There are several observations that have been made that support this statement, including, of course, evidence within the Cosmic Microwave Background.

     If space was flat, the calculated density parameter, Omega, would be 1. If it was greater than 1 than it would be positively curved and if it was less than 1 it would be negatively curved. If either of the second options was true, the angles in the lasers would not add up to 180, but if Omega does equal 1, then they would always add to 180. This is where the CMB comes into play. The CMB is the furthest thing from us that we can see. If we used the degrees of fluctuation combined with the age of the CMB, our numbers would show that if the fluctuations were 1 degree apart, the Universe would be flat, but if it was any more or any less than 1 degree, the Universe would be curved. Numerous observational devices have looked at these fluctuations and found that they are about 1 degree, making the Universe close enough to flat with a small margin of error. The issue with this observation is the amount of normal matter and dark matter that we have measured and calculated to be in our Universe. As I said earlier, if the Universe was flat, Omega would be 1. So the Omega calculated from our measurement of the matter in the Universe should be close to 1 to match the data found in the CMB. The measurement of the matter, however, is not 1, but close to 0.3. This means that 70% of the Universe's mass-energy density is unaccounted for. This is where dark energy comes in. Dark energy is what makes up the other 70% of the Universe, allowing all of the data to match up, the Universe to be flat, expanding and accelerating. There are many other independent proofs of dark matter and dark energy, but this is how the CMB ties all of these aspects together and the data produced from this entire system matches the other independent proofs. Below is a video I found after researching all of these topics that summarizes this model, known as the Lambda-CMD model of the Universe.