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the big explosion?  Maybe this is not the beginning of the universe

the big explosion? Maybe this is not the beginning of the universe

We thought the universe began with a singularity, but after nearly 100 years, we're no longer so sure. Here because

It doesn't matter where or how we look at the universe. Everywhere there is matter and energy. However, it is natural to assume that all this comes from somewhere. That it had a beginning. Let's try to answer one of the questions that has fascinated scientists for almost 100 years: Did the universe really begin with the Big Bang?

Big bang universe
An illustration of our cosmic history, from the great explosion until today. Credit: NASA/WMAP.

What we know about the universe

Today we know that the universe is expanding and becoming less dense And the weather is getting colder. Despite our immense desire to know what the future holds, it is easier to look back than to look forward. But how far back can we go in the history of the universe? From a mathematical point of view, it reaches infinitesimal dimensions, infinite densities and temperatures. I am So-called singularitiesWe will discuss this further in an upcoming article. The idea of ​​a single beginning for space, time and the universe has been known for a long time The big bang comes. But physically, when we looked closer, we realized that the universe was telling a different story: The Big Bang is not the beginning of the universe.

Einstein's relativity
One of countless scientific tests regarding the theory of relativity Einstein. Credit: Caltech/MIT

Where did the big bang come from?

Like most scientific theories, the Big Bang has its roots in the theoretical, experimental, and observational fields. As for the theory, Einstein presented the theory of general relativity in 1915: it was A new theory of gravity Which sought to overturn Newton's universal gravitation. Although Einstein's theory was more complex, it was not long before it was proven.

  1. In 1916, Karl Schwarzschild He demonstrated his accuracy for the point mass, which he described as a “non-rotating black hole.”
  2. In 1917, Willem de Sitter He applied it to solve an empty universe with a cosmological constant, describing the expanding universe;
  3. Dal 1916 al 1921, Reissner-Nordstrom scaleDiscovered by four researchers, it describes the space-time of a spherically symmetrical, charged mass;
  4. In 1921 Edward Kasner He described an anisotropic (different in every direction) universe devoid of matter and radiation;
  5. In 1922 Alexander Friedman He found the solution to an isotropic (same in all directions) and homogeneous (same in all positions) universe, in which all types of energy exist, including matter and radiation.
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The latter is interesting for two reasons. The first is that he described our universe on ever larger scales, where things look the same, on average, in all directions. Secondly, if we analyze the equations that led him to this solution (Friedman's equations in fact), we will discover that the universe he described cannot be static. But it must expand and contract.

The universe is expanding

Friedman's work was recognized by many, including Einstein, but he was not taken particularly seriously until observations began to support him. In the first decade of the twentieth century, for example, astronomer Silver dress He discovered that some nebulae move faster than any other object within our galaxy. Not only that, because most of them were moving away from us, the smaller, lighter nebulae were moving faster. Then in the twenties, Edwin Hubble He began by measuring the individual stars that were part of these nebulae and eventually determined their distances. Not only were they much farther away than anything else in the galaxy, but those at greater distances were retreating faster than those closer. In short, the universe It was expanding.

the great explosion
The story of an expanding universe that leaves nothing behind the great explosion As a valid explanation for everything we observe. Source: NASA/CXC/M. Weiss

Lemaitre was the first to discover it in 1927. He tried to go back in time to understand its origins and find the so-called “primordial atom.” He realized that the universe was a hot, dense, rapidly expanding whole. This idea was developed later, let's see how.

The evolution of the universe

The universe, as we see it today, is much more developed than it was in the past. On the other hand, the further we look back into space, the further back in time we also look. If so, the objects we see should be younger, less massive, and of less advanced structure. There must also be a point beyond which there are no stars or galaxies.

At some point, the radiation will be so hot that atoms will no longer be able to form, because the radiation will remove electrons from the nuclei they are trying to bond to. The heat was so great that it evaporated even the atomic nuclei. This means that there is a first stage (called the pre-stellar stage) in which nuclear fusion will occur: scientists call it Big bang nucleosynthesis. Not only that, because at this point there should have been many other light elements and their isotopes scattered throughout the universe, long before stars formed.

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Cosmic background radiation

It is one of the two tests (the other being cosmological redshift) that allows us to say “Yes, there was a big explosion.” And the Cosmic background radiation. It was discovered in 1964 by American astronomers Penzias Woodrow WilsonWhich also led them to win the Nobel Prize in Physics in 1978. It is what remained of the Big Bang, or rather the remaining radiation coming from the initial stages of the birth of the universe.

But if we could trace the Big Bang back to approx 13.8 billion years (When the universe was less than 1 second old), why don't we go back one second more to the singularity that was expected to exist when the universe was 0 seconds old? The answer is that it is always risky to make unfounded and incorrect assumptions about reality. But, for example, If the universe started out aloneSo there had to be the same balance between matter and energy to balance their expansion. If there was little matter, the universe would have already “collapsed” in on itself. If there were less of it, things would expand so quickly that the universe would be much larger than it is today.

However, what we can observe tells us that the initial expansion rate of the universe is the total amount of matter and energy in it They balance perfectly. but why?

the great explosion
The blue and red lines represent a traditional scenario the great explosionWhere everything starts with t=0. In the inflationary scenario (yellow line), we will never reach the singularity point, or a single point. One can only think of it as getting smaller and smaller as time keeps going back into eternity. Credit: E. Siegel.

Cosmic inflation

Remember de Sitter's theory from 1917, which postulated that the universe is dominated by a large cosmological constant? It was taken up again in the early 1980s for treatment Cosmic inflation theoryWhich assumes that the universe went through a phase of rapid expansion, shortly after the Big Bang, due to a big bang. Negative pressure. But this theory does not explain what existed before the Big Bang.

Whereas a universe containing matter and radiation (i.e. what we got through the Big Bang theory) can be traced back To uniquenessThe inflationary universe cannot do that. Because of its exponential nature, even if you tried to turn back the clock by an infinite amount of time, space would only approach infinitesimal dimensions with infinite temperature and density, but It will never reach zero.

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This means that the idea that the universe started from a singularity (and hence from the Big Bang) had to be discarded when it was recognized that the inflationary phase preceded the hot, dense, matter-and-radiation-filled phase that we still see today.

Because the Big Bang is not the beginning of the universe

This picture provides us Three pieces of information It is very important and relates to the beginning of the universe, in stark contrast to the traditional history that each of us learned in school. First, the idea that the universe emerged from an infinitely small, dense, hot singularity is incorrect. The picture is largely correct, but there will be a limit beyond which we cannot go back in time.

Second, observations have proven its existence Cosmic inflation Which happened before the Big Bang. The early universe went through a phase of Exponential growth, where all pre-existing components of the universe have literally been inflated. When inflation ended, the universe had heated to a high temperature, giving us the hot, dense, expanding universe we live in today.

the great explosion
Credit: NASA. Dana Perry (Skyworks Digital): Lead Animator Michael McClair (HTSI)

Finally, and perhaps the most important thing to understand, we cannot be certain about how (and if) the universe began. By its nature, inflation erases any remaining information before the final moments. As far as we know inflation It could have gone on foreverIt is preceded by another non-singularity stage, or perhaps it is preceded by a stage that emerged from the state of singularity. Until the day comes when we figure out how to extract as much information as possible from the universe, we have no choice but to do the math With our ignorance. The truth is that the Big Bang happened a long time ago and there is no doubt about it: but it may not have been the beginning we all thought.