Ask Ethan: Where did the Big Bang happen?

Of all the concepts and topics discussed, the Big Bang is the most controversial concept. Of course, this is a rather old scientific theory that has been present since the 1940s, and since the 1960s there has been a myriad of evidence in its favor. The idea is simple: the universe had a beginning. She had a birthday. There was a day that did not have “yesterday”, when matter, radiation and the expanding, cooling Universe, known to us, did not exist until a certain point in time. And yet we are here. Which causes a flurry of questions in any inquiring mind. One of our readers just has such a mind, and he wants to know:

Are there any theories or experiments that can calculate and prove our location in space relative to the Big Bang point? I think that since our observation capabilities are very limited from the position of our planet, it will not be easy to determine the curvature of space. Why do we think that the Big Bang occurred at some point in three-dimensional space? Why do we think the universe is a sphere?

These are very good questions, and they all demonstrate a common view of people about the universe. But are these ideas true?


The evolution of the large-scale structures of the Universe

We often believe that the Big Bang was a real explosion. And the Universe really looked like a huge, energetic and expanding fireball at its earliest stages.
• It was filled with particles and antiparticles of various types, as well as radiation.
• All this expanded and all particles, antiparticles and radiation quanta were separated from each other.
• All this cooled and slowed down as it expanded.

It really sounds like an explosion. In fact, if you could be transported to these first moments, and somehow be protected from all this energy, there would even be a sound that you can hear thanks to the next video.



But it is not without reason that I use the word "expansion" instead of "explosion", describing this phenomenon. An explosion is what happens at one point in space, from which fragments fly away. A supernova is an explosion; a burst of gamma rays is an explosion; detonation of a bomb is an explosion; the operation of a grenade is an explosion.


Artist's view of the 1993J supernova explosion in galaxy M81

But the Big Bang is not an explosion [in English, the Big Bang, Big Bang, literally means “Big Cotton” - approx. transl.]. Speaking of the “hot Big Bang”, we have in mind the very first moment at which the Universe can be described as a state containing particles, antiparticles and radiation. From that moment on, the Universe began to expand and cool according to the laws of the General Theory of Relativity, and we took the path of destroying antimatter, the formation of atomic nuclei and neutral atoms, and as a result of stars, galaxies and large-scale structures visible today. The key to the first question is to understand exactly what the Universe was doing at this moment: at the moment that we can describe for the first time, relying on this platform of the hot Big Bang.



As far as we know, there was no special starting point. There was no “source" from which the universe began. All the evidence speaks of a counterintuitive, but from this no less true conclusion: the Big Bang happened everywhere simultaneously. There is plenty of evidence for this, and the Universe itself gives them to us. The universe, judging by the large-scale structures, clusters of galaxies, the appearance of the afterglow of the Big Bang, the average density of cosmos larger than several hundred million light-years, etc., gives us two important observable facts. Its properties are the same everywhere, and it looks the same in all directions. Physically speaking, the Universe is homogeneous and isotropic.



Such characteristics of the Universe cannot be obtained with the help of an explosion - and that’s the point. In an explosion, the fastest moving fragments turn out to be the most distant, but also the most scattered in space. The greater the distance, the less there should have been galaxies per unit volume - but in the Universe this is not so. In the event of an explosion, one could explicitly indicate its starting point. The universe works so that this point would be only a few million light-years from the Milky Way, on the border of a local group. Statistically, the chances of having such a point, given the presence of more than 170 billion galaxies in the Universe, are 100 times worse than winning the Powerball or Mega Millions lottery.

The fact that the Universe is homogeneous and isotropic suggests that the Big Bang happened at one moment, about 13.8 billion years ago, and in all places the same. But we cannot see him in all places. We see him only where we are. Our review is limited. Therefore, you can often see similar illustrations: how our Universe is seen from our point, with us in the center.


The artist's view of the observable Universe on a logarithmic scale

But this does not mean that the universe is a sphere! We, in fact, can measure the shape of the universe, and impose some restrictions on it. If you go outside and send your two friends in two different directions so that you can see each other, the three of you will form a triangle. Each of you will be able to measure the apparent angle between the other two. After that, you can add these angles and you get 180º - this is the sum of the angles of the triangle.

Any triangle in a flat space.



But space does not have to be flat! It may have negative curvature, like the surface of a saddle, when the sum of the angles is less than 180º. It can be curved positively, like the surface of a sphere, when the sum of the angles is greater than 180º. If you stand at the equator in South America, one of your friends stands at the equator in Africa, and the second at the North Pole, you will find that the difference in angles will be large. The sum of the angles is closer to 270º than to 180º. We have no friends in space, but we have something worse: fluctuations in background radiation. Depending on the curvature of the space, they should look completely different.



We made observations and discovered something surprising: the Universe, as far as we can judge, is flat. Very, very flat. Recent data from the Planck and Sloan Digital Sky Survey experiments indicate that if the Universe is curved either positively or negatively, then it is noticeable on a scale of at least 400 larger than the part of the Universe that we observe. And we can see its part with a diameter of 92 billion light years.



So, the Big Bang happened everywhere simultaneously, 13.8 billion years ago, and our Universe is spatially flat according to our best measurements. The big bang did not occur at any point, and we can judge this by the extreme isotropy and homogeneity of the universe. These properties are so accurate that when we come across heterogeneity with a deviation of 0.01% from the average, we already believe that something is wrong. So, if you claim that the Big Bang happened exactly where you are, and you are standing right in the center of everything that happens, no one will object to you. Just everything and everything in the entire Universe can say the same about itself.