How the geometric shape of our universe is flat?

                   

Our universe began with the Big Bang. So we see most galaxies moving away from us in all directions. Really interesting! Big Bang means everything was together billions of years ago.  Now everyone is interested in how the universe will end? Will it remain on forever? Or, will galaxies eventually slow down, stop, and reassemble through the Great Contraction? Will we always and forever have a continuous expansion-contraction-expansion-contraction cycle of the Big Bang? It will not happen. Because, current astronomers have concluded that since the universe is flat, it will never collapse on itself and start the Big Bang again. 

Even though the universe is known to be flat, the biggest thing is still unknown to us. We still don't know if the universe is finite or infinite. 

 

Figure: Big Bang Theory; The universe is expanding from a single point. (Credit: grandunificationtheory.com) 

So, what does it mean for the universe to be "flat"? Why is this important, and how do we know the universe is flat? Before talking about the flat shape of the universe, it is necessary to talk about flatness in general. If you are in a corner of a square room and look around the other corners of the room. Then, you will return to your starting position by making four 90-degree turns. Then you can say that the room is flat. This is Euclidean geometry. If you were to travel the same way on the Earth's surface, you would start at the equator and turn 90 degrees to the North Pole. A further 90-degree turn will bring you back to the equator. Take another 90-degree turn to return to your starting position.  In the first scenario it takes four turns to return to the starting point, in the other scenario it takes only three. Because the geometry of the surface you walk on decides what happens after a 90-degree turn?

  

Figure: Starting from one corner of a flat square room and making four 90-degree turns to return to the starting point. Similarly, three turns are required for a spherical earth. (Credit: Fraser Cain)

Let's imagine a more extreme example. Suppose you are walking around inside a hole. Now it takes more than four turns to return to the starting point. 

Again imagine two parallel lines originating from the north pole of the earth. Parallel lines follow the geometrical shape of the two worlds and move away from each other and converge again. 

Now the same analogy can be applied by thinking about the universe. Now imagine a rocket flying billions of light years into space. If the rocket makes 90-degree turns and returns to the start. But it cannot return in three or five, exactly four turns will be required. Which means the topology of the universe is flat. Which is totally intuitive, right? Because, without any direct logical evidence, we feel the flatness to be true. 

This cosmic radio background radiation plays a role in creating the geometric shape and topology of the universe. If the universe were somehow curved, these temperature variations would appear distorted compared to the actual size of the structures we see today.

So astronomers were skeptical about it. They needed direct evidence to be sure, and so they set out long ago to test this hypothesis.   

                          Image: Cosmic microwave background radiation ( CMBR) Credit: NASA

It took astronomers a long time to prove that the universe is flat. Astronomers use the largest observations they can make. They began observing and researching the cosmic radio background radiation in the universe.  The cosmic radio background radiation of the universe is electromagnetic radio waves created immediately after the Big Bang. This radiation is attenuated and still exists in the universe. The light after the Big Bang is seen as a redshift in all directions. On the eve of the release of the radiation, the temperature of the entire universe was about 2,700 degrees Celsius. This moment was finally cool enough for light particles to roam the universe. The expansion of the universe spanned the 13.8 billion years of travel of these light particles and transformed the microwave spectrum into a temperature just 2.7 degrees above absolute zero.  

If the universe is curved, it can be returned to its original position in fewer than four turns. This means, the universe will be closed in nature and contractible. And if more than four turns are to be taken, then the universe will be free in nature and destined to expand forever. 

With the most sensitive space-based telescopes available, astronomers can detect even small continuous changes in the temperature of the background radiation. These miniature temperature variations correspond to the largest scale structure in the observable universe. The region, with a  temperature fraction of a degree, forms a massive galaxy cluster several million light-years across. This cosmic radio background radiation plays a role in creating the geometric shape and topology of the universe. If the universe were somehow curved, these temperature variations would appear distorted compared to the actual size of the structures we see today.

Figure: If this universe were inhomogeneous, our observed cosmic radio wave radiation data would be distorted compared to the current universe. (Credit: NASA/WMAP Science Team) 

The universe is flat meaning that two parallel lines will always be parallel. The two lines will never meet. Unfortunately, even though the universe is flat, the biggest thing is still unknown to us. We still don't know if the universe is finite or infinite. If the curvature of the universe can be measured, then we can say that we are in a finite universe. Later we will be able to measure beyond the observable universe and realize what its true size is. Earlier, cosmologists used to say that the universe is flat and the effect will be in the future. If the universe is curved, it can be returned to its original position in fewer than four turns. This means, the universe will be closed in nature and contractible. And if more than four turns are to be taken, then the universe will be free in nature and destined to expand forever.

 

 

         Figure: The invisible force that governs gravity (green grid) (Credit: NASA/JPL-Caltech) 

In 1998, scientists discovered dark energy. Which is amazingly accelerating the expansion of the universe. Whether this universe is open or closed, flat or curved, it is expanding. This expansion is going to accelerate forever.

I hope that an understanding will come from reading what the universe actually means to be flat. In conclusion, the idea of ​​a flat universe is proven by the background radiation of radio waves.  

                                                         Video Credit: Fraser Cain 

          

   

Source: phys.org, and Universe Today