Big Crunch: It may sound like a science fiction script, but the theory that in the past all matter that exists was concentrated in a singularity, hot and dense, is highly accepted and supported by observational evidence. The Big Bang, or Great Expansion, marks the moment when time itself came into being, matter began to expand, and the Universe began to cool. Current and accurate measurements show that the Universe is 13.8 billion years old.
“What existed before the Big Bang?”. This question, intuitively natural as it is, has no answer. Because, in fact, there is no “before” the Big Bang. The arrow of time came into being with the Big Bang, and it always moves forward.
13.8 billion years later, we can see some evidence of the expansion of the Universe, which is still ongoing. One example is galaxies moving away from each other (except those on a collision course like the Milky Way and Andromeda).
Since the Universe has a starting point, what would the end be? The Big Crunch theory is highly disseminated and intuitive! Basically, at some point the gravitational pull of matter would slow down the expansion of the Universe and then it would collapse again. It’s not as complicated as it sounds, it’s as simple as throwing a rock in the air. When we throw something up, we can gradually see that object slowing down, at some point stopping its upward movement, and then coming back down. This is due to the gravitational pull between the stone and planet Earth. And this idea is easily carried over to the expansion of the Universe: Big Bang, galaxies move away, eventually they would slow down and go back to a final big collapse.
But today, we already know that this is not the future of the Universe. And the big culprit is Dark Energy.
If the Big Crunch were real, what would be expected is that the speed at which objects move away would decrease over time. And here there is a great advantage of astronomy: we are able to observe the past of the Universe. As some objects are extremely distant, the light that arrives here represents the reality of that object some many millions of years ago. Therefore, we were able to know the history of the distance between objects. And what we observe is not a brake. Not a constant speed. But an acceleration.
Instead of moving away more slowly over time, galaxies move away faster and faster. Returning the stone we threw to the top, this means that instead of the stone braking and descending, it starts to rise faster!
We call Dark Energy the force responsible for this acceleration. The acceleration of the Universe is an observed and well stipulated fact. Dark Energy is the explanation we have today for this fact. What exactly is this energy and its nature? We need to find out! But basically the characteristic of dark energy is that it has a strong negative pressure. This reflects as a force that opposes the force of gravity. That is, while gravity attracts, dark energy repels! And we know that about 70% of the Universe’s scale is actually Dark Energy.
Does it still sound like science fiction? Reality explained by science may seem more interesting than a movie script!