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The Big Bang theory is the theory that explains the origin of the universe and why it continues to expand.
This theory was created by Georges Lemaître from the discoveries of Edwin Hubble and Albert Einstein.
See also: Big Bang Theory Science Definition
According to the theory, the universe began as an infinitesimally small point that was expanding.
We have several reasons to think that this theory is correct: the universe continues to expand and the temperatures shown by the microwave background (CMB) can only be explained by the Big Bang theory.
All the existing matter in the universe came from the first seconds of the Big Bang.
What is the Big Bang?
This theory explains how the universe in which we all live was formed. According to the Big Bang , all the matter that exists, even that of which we are formed, was created more than 13 billion years ago.
Although it may be useful to imagine the Big Bang as the explosion of a bomb on Earth, according to the European Organization for Nuclear Research (CERN) this analogy has important limitations. While a hydrogen bomb (a type of nuclear bomb) can cause an explosion that expands at 300 meters per second and whose center has a temperature of 100 million degrees Celsius; The Big Bang had an energy of a thousand trillion degrees Celsius at the speed of light (a million times faster than the bomb).
Finally, CERN states that there is a fundamental characteristic that differentiates a bomb from the Big Bang. While a normal explosion expands through air (or space), the Big Bang did not expand through anything: before the Big Bang there was no space, no time. Rather, physicists believe that the Big Bang created and extended space itself. A process that continues to occur to this day.
The creation of the Big Bang theory
As Charlie Wood , a physicist and science popularizer, explains , for much of history the heavens were thought to be eternal and static. However, in the 1920s, the American astronomer Edwin Hubble discovered not only that other galaxies existed besides the Milky Way, but that they were also moving away from Earth , indicating that the universe was expanding.
Following this discovery, Georges Lemaître, a priest and physicist, proposed the Big Bang theory. Lemaître studied physics at the Massachusetts Institute of Technology (MIT) from 1925 to 1927. During his studies , he learned about Hubble’s discoveries about the expansion of the universe. In 1927, after becoming a professor of Astrophysics at the Catholic University of Louvain (Belgium), he postulated his theory using Albert Einstein’s general theory of relativity as a framework. He thought that if the universe was expanding, in hindsight it may have started out as a “primordial atom.”
It can serve you: What is the big bang theory definition?
While this is the theory we currently use to understand the origin of the universe, it was not accepted from the start. In fact, the term Big Bang was not originally coined by Lemaître, but by one of his critics, Fred Hoyle , who, in one of his debates, stated that the idea that “all matter in the universe was created in one big bang in a particular remote time ”was irrational.
However, some cosmologists are of the opinion that this is a bad name for the theory. A better name, according to Paul Steinhardt , a cosmologist at Princeton University, would be “the great stretch,” since the image of an explosion causes a number of misunderstandings. Why? Because an explosion implies a central point and a boundary that expands, and light elements that move faster than heavy elements. However, the expansion of the universe does not follow this pattern: there is no center, it has no borders, and smaller galaxies move apart at the same rate as large ones, and those more distant do so faster.
Still, the name stuck with us. But what evidence do we have for this theory?
What evidence do we have of the Big Bang?
As Matthew O’Dowd , an astrophysicist and associate professor in the Department of Physics and Astronomy at Lehman College, City University of New York, explains , we know that the universe is expanding.
In 1929, Edwin Hubble showed that the rate at which other galaxies were moving away from us was proportional to the distance that separated us. This led astronomers to conclude that the universe was expanding.
Hubble was able to prove his discovery thanks to an important fact: the light of the other galaxies is shifted towards the red ( redshift , concept that we saw in a previous report ). What does this mean? As we told you on another occasion , what we call light is actually electromagnetic radiation that travels in the form of waves. In addition, light can be divided into the different colors.
As light travels through the universe, its wavelength stretches, thus tending towards red . According to O’Dowd , the further a galaxy is from us, the more stretched the length of its light wave will be when it reaches Earth. Furthermore, sometimes the wavelength is stretched so far that the light is no longer perceptible to the human eye.
The most interesting thing is that this stretching of the wavelength of light is proportional to how far away from our planet the galaxy that emits it is.
According to Albert Einstein’s theory of general relativity, this phenomenon occurs because space itself is expanding. In other words, light shifts to red on its way through the universe because it is constantly expanding.
The expansion of the universe is one of the proofs we have for the Big Bang theory and this theory is consistent with the theory of general relativity. If you want to know more about the redshift and the expansion of the universe, you can see our report on the subject.
Now, there is yet another proof of the Big Bang. O’Dowd states that , if we use the mathematics of the general theory of relativity, we can “go back in time” and calculate what the universe was like in its first moments.
Some cosmologists think that, at time zero, the universe was an infinitesimally small point. Others think not. Why? Because while the theory of relativity is very good, its mathematical machinery would not work to explain what would happen at this infinitesimally small point. However, it is possible to use it to explain what happened in the first moments of life.
In its early days, O’Dowd explains, the universe was as hot, dense, and opaque as the interior of a star – an ocean of protons and electrons. During this time, light roamed this ocean, but in a scattered manner. NASA gives the following example: the light at the beginning of the universe did not move freely as it usually does today on Earth , but was scattered, as it happens when we are in a dense fog. If you want to see a more detailed explanation of this phenomenon,
As the universe expanded, it cooled, and around 400,000 years after its expansion began, the first hydrogen atoms were formed. At that moment, the infrared light that was trapped in this primordial ocean was released. Currently, we can see this light, although no longer as infrared light, but as microwaves because it was stretched as it traveled through space: we call this the Cosmic Microwave Background or CMB.
The CMB is a dim light that fills the entire universe . It even reaches the Earth from all directions with a uniform intensity. This light is the oldest that we can see, although not with our eyes. As Elizabeth Howell , Master of Space Science Studies at the University of North Dakota explains , this light is invisible to us because it manifests as microwaves, a part of the electromagnetic spectrum that we cannot visualize.
However, it is possible to detect it with specialized instruments. In the image below we can see the CMB that NASA has detected in space.
As NASA explains , when we see light from distant objects, we actually see something that has happened many, many years ago, depending on how far away these objects are. For example, if we look at the Andromeda galaxy, the largest galaxy close to our own, we see what it was like 2.5 million years ago. The light from the CMB was emitted approximately 13.7 billion years ago. Therefore, by studying it, we can know the properties of the universe in its early age. But what does this CMB tell us?
According to O’Dowd , the CMB shows a radiation pattern that would be impossible to explain without a much smaller, hotter, and denser universe, so at least the Big Bang theory works up to that point. The CMB shows us, for example, a temperature pattern with very small differences. According to NASA , alternative explanations have been created, but all have failed to explain this phenomenon, so the Big Bang is still our best explanation.
Finally, the patterns seen in the CMB predict that certain galaxies and sets of galaxies would form in certain parts of the universe, which we can observe when we look into space. For these reasons, scientists think that the Big Bang theory is true.
Brief history of the universe
One last interesting question is what happened in the first moments of the Big Bang. How was everything that currently makes up the universe formed? In the following video from the Kurzgesagt channel , you can see this process explained in an animated way. Or you can continue with our detailed explanation.
According to CERN , in the first moments of the Big Bang there was not too much heat for atoms to form, such as hydrogen (the most abundant element in the universe). In these first moments, long before a second is fulfilled (10 at minus 32), there were only quarks and photons. Quarks are the fundamental particles that make up neutrons and protons (particles that in turn make up the nuclei of atoms). This moment is called the epoch of the quarks. If you want to know more about quarks, you can enter our report on the fundamental forces of physics.
As time passed, the universe expanded, causing temperatures to drop. Before the first second of the existence of the universe is fulfilled, in the first microsecond (10 to minus 6), the quarks are grouped into protons and neutrons.
After this first microsecond, the universe was already 100 billion kilometers and the first hydrogen atoms began to form. As it expanded, the universe got colder. After the first 200 seconds began what scientists call the dark ages, since hydrogen gas did not allow light to move and there were stars.
The CMB that we can detect today occurred approximately 375,000 years after the beginning of the universe.
Later, after millions of years, the first stars were formed as can be seen in the graph. Then, a billion years ago, gravity caused atoms to gather together in large clouds of gas, thus forming collections of stars, which we now call galaxies.
The stars, through a process called nuclear fusion, created other atoms like helium, carbon, and iron. Those larger ones exploded spreading these elements in the cosmos. These same elements cooked in the stars are what make up the planets. Finally, on a small blue planet called Earth , plants, animals and us appeared . That is why astrophysicist Carl Sagan said that humans are, at the end of the day, the stuff of ancient stars.