A Completely Dark Universe


We live in amazing time. Our Universe has started about 13.8 billion years ago. It was so hot for the first 380 thousand years that even light couldn’t travel through it. It started to travel when the Universe cooled enough for elementary particles known as protons, neutrons and electrons to coalesce and form first atoms allowing light to travel through the youthful Universe. The Big Bang model was confirmed that way: scientists detected exactly that light, which travels from that moment and reaches the Earth only now.

We live being quite sure that this will always be that way. Some civilizations will rise, thrive, develop some techniques and eventually fall due to a catastrophic event and the like. But the idea of the sky full of stars and galaxies seems to be perpetual.

We know that our Sun will die in some 5 billion years leaving us only with its core in a form of white dwarf afterwards. Needless to say, mankind will have to leave the Earth much earlier. But let’s keep such questions for another discussion.

Let’s now suppose the humanity will become a stellar civilization. Nothing would destroy us that way, even a supernova.  Basically, when a civilization reaches the point becoming a stellar one, it becomes immortal for any type of natural events. It may be destroyed only by another one, which would be more powerful. Here you may ask, what it leads us for? Actually, I just want you to suppose that we, mankind, are able to live forever. Now, let’s return to some physics aspects.

We know that the speed of light is a natural velocity limit. Nothing is capable to exceed it as well as no object having mass is capable to reach it. It would require infinite amount of energy for this to happen. However, scientists now know that there IS speed which isn’t restricted by this point. It is a speed of the expansion of the Universe.

In 1920s Edwin Hubble discovered that our Universe isn’t static, it’s expanding. The rate of this expansion is given as Hubble constant. According to this constant we can determine the speed of a moving away object. The further away object is the faster it moves away from us. That tells us that there are objects in the Universe which already move faster than the speed of light relative to us. However, you should consider an object which is too far away to exceed the speed of light. Otherwise, its speed would be not so high.

And here we are going to the most interesting part of our discussion. After the discovery of expansion of the Universe most astrophysicists thought that the expansion rate decelerates with time by the presence of mass. Mass makes its gravitational contribution throughout the whole Universe and the expansion rate should have slowed down by its presence. However, it now turns out that the expansion rate is actually accelerating, the objects are rushing away from each other.

In the late 1990s two teams of astronomers worked on determining the slowing down rate. The process is not an easy to explain and we will keep it for another article. What is relevant for us now is that their results are correct in any way. That was one of the most astounding findings in astrophysics ever and brought the authors (Saul Perlmutter, Brian Schmidt and Adam Reiss) Nobel Prize in physics in 2011.

So, we are sure that the expansion rate is accelerating and it means that the distance which is needed now to exceed the speed of light for a far away object will decrease with time. Eventually the expansion rate will be so high that all the objects which are not gravitationally bound with our local group of galaxies will move away from us faster than the speed of light. You may guess, we will not be able to see these objects anymore regardless of whatever technology we will develop. The whole Universe outside of our local group will be completely dark and empty for us.

If we assume that a civilization will rise over that time, when they reach the point of our modern understanding, they won’t be able to build up the Big Bang model and will know nothing about the Universe lying outside of their local group.

Who knows, maybe we are already in such a position?

Thank you.


Shout to the sky!

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