por The big Bang Theory It is a cosmological theory to explain the origin of the universe and the one that currently has the most acceptance in the scientific community. He claims that the universe began with a big bang, about 13.8 billion years ago, and has been continuously expanding ever since.
From this great explosion matter, time and space arose, which later became galaxies and star systems, including our own Milky Way, the solar system and finally ourselves.
The theory originated in 1915, with Albert Einstein’s equations of relativity, which among other things predict the expansion of the universe, a fact with which the German scientist never felt comfortable.
However, the Belgian astronomer George Lemaitre, when studying relativity, stated that if the expansion were true, obviously the universe must have a starting point. In 1927 Lemaitre published an article where he exposed his ideas about the origin of the universe, which he called «the primordial atom».
The American astronomer Edwin Hubble promoted the new theory, confirming in 1929 that galaxies are moving away from each other and also from us.
Going back in time, the galaxies must certainly have been much closer than they are today. And therefore there must have been an instant when all matter was incredibly compressed, occupying an infinitely small space: a singularity.
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Characteristics of the Big Bang Theory
The term «big bang» was coined by physicist Fred Hoyle in 1940, who was not sympathetic to the idea, so he referred to it derisively, calling it «that big bang.» Hoyle was convinced that the universe was stationary.
Although its name leads us to think of some catastrophic event, physicists and cosmologists now believe that it was neither a big one nor a cataclysm that sent galaxies flying in all directions.
But it was so powerful that the four fundamental interactions of Physics were unified during those first moments.
Main postulate of the theory
The entire universe was originally in an incredibly hot and dense state, and then suddenly expanded while slowly cooling. That expansion continues today.
The big bang doesn’t explain how the original singularity came to be, let alone what came before it. What it explains is what happened to the universe during the early times when the singularity ceased to be.
When did it happen
Scientists estimate that the big bang happened 13.8 billion years ago and it is not possible to know what happened before, since time, together with space and matter, were created at that precise moment.
where it happened
It was not a localized event. It turns out that the more distant the objects we see with the most powerful telescopes, the further back we go back in time to the big bang, no matter which direction you look.
what happened next
After the big bang, the temperature dropped and the subatomic particles we know were formed: protons, neutrons and electrons, to give rise to atoms.
During the big bang, gravity, the unifying force of attraction of matter, arose, as well as the other fundamental interactions.
The first chemical elements formed were hydrogen, the simplest of all, and then helium and lithium, in a process called nucleosynthesis. Over time, immense clouds of these elements gave rise to the first galaxies.
Theoretical foundations of the big bang
The big bang is based on:
-The equations of the theory of relativity proposed by Einstein.
-He standard model of particleswhich describes the structure of matter in terms of the fundamental particles and the interactions between them.
-He cosmological principle, which affirms that the universe is homogeneous and isotropic, when we see it on a larger scale. This means that its properties are identical in all directions and the laws of Physics are the same everywhere.
Of course we know that there are accumulations of matter, separated by spaces of much less density. From that point of view, the properties of the universe certainly differ. But the scale covered by the cosmological principle is much larger than that.
According to the cosmological principle, the universe does not have a center, nor does it have borders or limits, because preferential places simply do not exist.
It is therefore concluded that the universe has an origin in time and therefore a finite age, although it is not yet clear whether its extension is finite or infinite.
Stages of the universe according to the Big Bang theory
Scientists distinguish three great stages, the first of a universe very primalthe second of the primeval universe proper and the third the stage of the formation of structures.
During the first two, the universe was dominated first by radiation and then by matter.
radiation stage
During this era, energy was in the form of photons, massless elementary particles that make up light. Thanks to them, electron-positron pairs of matter and antimatter were created, which annihilate when they meet, once again emitting energy in the form of photons.
However, at some point matter slightly predominated over antimatter, which later led to the appearance of the first subatomic particles.
Cosmologists believe that this stage lasted about 700,000 years, and in it the following periods are distinguished:
initial stage
It starts 10-43 seconds after the big bang and includes:
-The Planck era, when the four fundamental interactions – electromagnetic, strong nuclear, weak nuclear and gravity – constituted a single fundamental force.
-The era of unification, which occurred 10-36 seconds later, when gravity separated from the other forces, but the others remained fused in what is called GUT (grand unified theory) as the universe expanded and cooled.
the great inflation
From 10-36 to 10-33 seconds, in which the universe experienced a growth spurt, it cooled and rapidly decreased its density, as a consequence of the expansion.
This is how the universe grew from something less than the point of a pin, to a sphere the size of several suns like ours, all at great speed.
particle formation
The growth of the universe slowed down without stopping and the first elementary particles arose: protons, electrons and neutrons.
Creation of light atoms
After three minutes, protons and neutrons collided to form the first nuclei. Then these nuclei met and light atoms were formed.
appearance of light
Paradoxically, the high temperatures of the early universe did not allow light to appear until some 380,000 years after the big bang.
But then the universe had already cooled enough to allow the formation of neutral hydrogen, with which photons -carriers of light- could move great distances without obstacles.
matter domination
The universe, previously opaque due to high density, became transparent to radiation and matter took over.
In this way the first conglomerates were formed, thanks to the action of gravity and the universe began to acquire its present form. It is the stage of the formation of structures.
Star and galaxy formation
Gravity caused the gas clouds to collapse to form the first stars, which later became associated with galaxies. Experts believe that this happened about 400 million years after the big bang.
epoch of dark matter
The expansion of the universe has not stopped, on the contrary it seems to have accelerated.
Now scientists believe that there is a matter other than the matter we can see, called dark matterwhich is responsible for this accelerated expansion.
evidence
radiation cosmic background
The big bang is still observable today, despite the time elapsed, through the radiation that comes from the most distant places in the universe.
The cosmic background of microwave radiation (cosmic microwave background) was discovered in the mid-1960s by two researchers from Bell Laboratories: Arno Penzias and Robert Wilson.
It is the glow left behind by the big bang, something that theory had already pointed out in advance, but that could not be detected until the experiments of Penzias and Wilson.
The Hubble–Lemaitre law
In 1929 Edwin Hubble affirmed that the universe is expanding, and for eight years he was in charge of collecting the necessary data to prove it at the Mount Wilson observatory, California.
In this way he enunciated the following law, in which the speed v with which galaxies recede from us is proportional to the distance R.being h Hubble’s constant:
v = HR
Where H = 22 x 10-3 m/(s⋅light year). This simple form of the law is valid when dealing with galaxies not too distant.
Uniform distribution of distant galaxies
The Hubble Space Telescope confirms that distant galaxies are evenly distributed, in accordance with the cosmological principle.
Apparent magnitude of distant galaxies
The greater the redshift, the greater the apparent magnitude of a distant galaxy, which means that the wavelength of its light stretches as it travels through an expanding universe.
Problems and criticism
In theory there are many points that remain obscure, for example scientists still do not know what triggered the great inflation.
On the other hand, many experts are not happy with the fact that time, matter and space did not exist before the big bang, since some think that time has always existed.
Of course, cosmological theories point to large-scale phenomena and are refined or discarded by new discoveries. The scientists hope to settle discrepancies such as the following:
entropy problem
Entropy was abnormally low during the early moments of the universe, and cosmologists are unable to explain the increase in entropy to today’s levels.
horizon problem
This problem refers to the fact that the speed of light is finite and nothing travels faster than it, however, regions that during the big bang could not be in contact because of their separation, it turns out that they were in thermal equilibrium. .
flatness problem
It is believed that we live in a flat universe, however the big bang theory does not offer a physical mechanism that satisfactorily explains why.
Magnetic monopoles problem
The big bang theory predicts the existence of magnetic monopoles, but so far they have not been found. Every time we try, sectioning a magnet always results in smaller magnets with north and south poles, never separate magnetic poles (monopoles).
Other questions about the theory are: where did the singularity originate from? And how did the…