The Big Bang theory is the generally accepted version of the origin and further evolution of the Universe. According to the big bang theory, the universe once existed in the form of a dense and hot spot. About 13.7 billion years ago, matter from this point began to “scatter”, and a big explosion occurred.
Such a point is called singularity. This is a state in which all matter is at a point of zero volume, having an infinitely high density and temperature.
Origin of the theory
The theory owes its birth to Albert Einstein and his general theory of relativity. The application of the general theory of relativity in describing the Universe led to problems: the theory did not describe the Universe that the scientific community imagined.
Before general relativity, the universe was viewed as a static object. This idea was so ingrained in the minds of scientists that even Einstein himself initially did not believe in the expanding Universe.
The first to come up with the big bang theory was the Belgian physicist Georges Lemaitre. He solved the equations of general relativity and pointed out that the theory predicted not a static but an expanding Universe.
What happened before the big bang?
Talking about a “before” moment in the context of the Big Bang Theory is meaningless. Before the Big Bang there was no such thing as time; it appeared after. Nevertheless, scientists have guesses about this; they have developed a theory that does not require time. It lies in the fact that the universe is cyclical: each time there is a new big bang, then compression. This assumption means the eternal existence of the Universe, the absence of its beginning and end.
Proven evidence
Next, we will list the proven evidence that originated and, to this day, supports the Big Bang theory. This is existing evidence, extensively researched and validated by astrophysicists around the world. To date, no other unified theory has managed to connect and explain all of the proven phenomena described below.
Evidence for the Big Bang Theory
The big bang theory is generally accepted for a reason: some of its consequences have already been experimentally confirmed.
Redshift
Redshift is an effect observed when observing distant galaxies. The observer will notice a shift of galaxies to the red side of the spectrum.
This happens for the following reasons:
- The wavelength of light passing through space increases. This also results in a redshift;
- galaxies are constantly moving away from each other.
Cosmic microwave background radiation (CMB)
The cosmic microwave background radiation (also abbreviated as CMB), discovered in 1965, is a uniform radiation field detected in all directions across the sky. In the past, the Universe was extremely hot and dense. As it expanded, the radiation dissipated and lost energy. The photons had their wavelengths “stretched” in such a way that we currently observe this residual radiation as microwave radiation — as remaining white noise. These observations fit perfectly with the prediction of a hot and dense early universe.
Some argue that the CMB could be radiation from an older source or the result of processes unrelated to the Big Bang. However, the uniformity and spectrum of the background radiation correspond remarkably well to the predictions of the Big Bang theory. No alternative explanation takes into account the uniformity and detailed fluctuations observed in the CMB.
Expansion of the universe and Hubble’s Law
At the beginning of the 20th century, in 1929, astronomer Edwin Hubble found that all galaxies are moving away from us. The proof was in the Doppler shift detected in the light spectrum of almost all galaxies. Most showed a redshift.
Not only that, Hubble also established a correlation between deviation and radial velocity: the further away a celestial object is, the stronger this redshift, that is, the faster it is moving away from us. This is Hubble’s Law. This law has proven to be so consistent and precise (in the graph, 600 Mpc = 1.95 billion light years) that it is how astronomers measure the distance of galaxies and quasars (which are so far away that parallax measurement does not work). .
The fact that all the galaxies are moving away also implies the fact that, in the past, the galaxies were closer together. According to Hubble’s Law, exactly 13.8 billion years ago, the Universe was a point. What’s more, the time dilation observed in the light curves of distant supernovae also proves the fact that the Universe is continually expanding.
The cosmological principle: the evolution of distant objects
A basic concept of cosmology already discussed is that the further we look, the further “back” in time we also observe. A galaxy 10 million light years away shows what it looked like 10 million years ago. The fact is that very distant (older) objects are also distributed differently and show a different level of evolution than the galaxies closest to us (more recent), so we can draw a kind of timeline of the evolution of galaxies and quasars.
The tendency of matter is to clump together. From atoms forming molecules, from stars forming galaxies, and galaxies forming clusters. The fact is that the further we look, the more large structures disappear. There is no evidence of superclusters of galaxies or structures at a distance greater than 500 Mpc. This means that, in the past, the Universe was much more homogeneous, which supports the Big Bang theory. This fact is called the cosmological principle .
Plenty of light elements
The process of stellar nucleosynthesis consists of the transformation of light elements, such as hydrogen and helium, into heavier elements, such as carbon, oxygen, sulfur and silicon. Supernovae produce even heavier elements, such as gold, mercury, silver and uranium.
The universe is also isotropic on cosmological scales in all directions. The number of galaxies observed per unit area in any direction of space is practically the same, supporting the homogeneity of the Universe in the past.
These recent observations have completely overturned the steady state theory, popular between the 1940s and 1960s, which says that the density of matter in the expanding Universe remains unchanged due to a continuous creation of matter, which contradicts the Big Bang theory. However, mappings from modern telescopes over the last 20 years (like the image above) show that the cosmological principle remains more than confirmed.
However, the current amount of helium present in the Universe is much greater than expected. Stars alone would not be enough to produce the observed amounts of helium. The theory predicts that much of the helium we observe would have been synthesized during the Big Bang, whose extreme environment in the first minutes of the Universe would have provided conditions for the formation of helium independent of the synthesis of stars.
Most modern mathematical models of the Big Bang predict certain abundances of light elements, such as hydrogen, helium, lithium and beryllium. Considering the age of the Universe and the progression of different populations of stars, the predicted estimates of nucleosynthesis in the Big Bang match perfectly with current observations. No other known process can explain the proportions and distribution of light elements as accurately as Big Bang nucleosynthesis.
The night is dark: Olber’s paradox
The simple fact that the night is dark is also proof of the expansion of the Universe, Hubble’s Law and, by extension, the Big Bang. Olber’s paradox says this: if the Universe were static and infinite, we would see infinite amounts of stars, making the night as bright as the day. The fact that the night sky is dark is consistent with the Big Bang theory, which describes a Universe that is finite in age, expanding and constantly evolving.
In its early stages, the Universe was in a state that did not allow light to travel freely (like the hot, dense plasma at the beginning of the Big Bang). Only after the Universe cooled and became transparent (recombination), about 380,000 years after the Big Bang, could light travel vast distances. This means there is a limit to how far we can see.
Furthermore, the Universe is not static, but is expanding. This expansion “stretches” the light from distant galaxies, shifting it to the red end of the spectrum (redshift). Some of this light is so displaced that it moves from the visible range into infrared or even longer wavelengths, making the sky appear dark.
Hypotheses still under research
Like any new area of science, we observe many unknown phenomena, still without a concrete physical and/or mathematical interpretation. These are hypotheses that may or may not be favorable to the Big Bang theory, but it is difficult to answer without further investigation and future discoveries (such as dark matter), which depend on more advanced telescopes, more powerful computers and more sensitive instruments.
Gravitational waves
The detection of gravitational waves could provide indirect evidence of the Big Bang, particularly the inflationary period of the early Universe.
However, gravitational waves can be interpreted differently or have other origins. Although the exact origins of specific gravitational waves may be debatable, their general properties and effects are in line with the predictions of the Big Bang model, especially with regard to cosmic inflation.
The search for gravitational waves is very recent (the first detection only took place in 2015). Advances in technology and greater sensitivity of instruments for detecting gravitational waves can help answer many questions about them.
Dark matter and dark energy
Dark matter and dark energy do not directly invalidate or prove the Big Bang, but they directly influence measurements on a cosmological scale. Dark matter and dark energy are one of the greatest mysteries of current cosmology that, hypothetically, would influence the acceleration of the expansion rate of the Universe and the movement of galaxies and structures, such as superclusters. Today, we observe some discrepancies (such as the Hubble Voltage) that are attributed to the supposed existence of dark energy.
Currently, we do not have enough technology to detect dark matter and dark energy. It is a type of non-baryonic matter that does not interact with anything we know except gravity. A deeper understanding of matter and dark energy will allow us to develop more accurate modeling of the dynamics of the Universe’s expansion.
Primordial black holes
Primordial black holes are hypothetical types of black holes that are believed to have formed in the early Universe, shortly after the Big Bang.
In the early Universe, it is theorized that random fluctuations in density could have been so high in some regions that they could collapse on their own, under their own gravity, forming black holes. This would have happened in the first second after the Big Bang. Unlike black holes formed by collapsing stars, primordial black holes can be incredibly small, potentially as light as asteroids, or even lighter. This makes them extremely difficult to detect.
Researchers are actively looking for signs of these types of black holes using several methods, including gravitational wave observations, gamma-ray astronomy, and gravitational lensing studies. The discovery of primordial black holes would offer a unique way to test several cosmological theories, including inflationary models and the nature of dark matter.
The singularity, the initial state of the universe before the big bang, cannot be described mathematically.
The Big Bang Theory Problem
Stephen Hawking, in his monograph “Large-Scale Structure of Spacetime,” wrote the following:
Myth: The Big Bang Theory describes the beginning of our Universe. In fact, the Big Bang theory itself says nothing about the immediate birth of the Universe. The current concept assumes the existence of energy, time and space, and does not explain their origin.
Myth: The Big Bang was “tiny . ” Many people, trying to visualize the Big Bang (I am no exception), try to compare the Big Bang with everyday objects. This comparison, however, is only true in part of the Observable Universe, not the entire Universe.
Myth: Hubble’s Law violates special relativity . Indeed, distant galaxies are moving away from us faster than the speed of light. However, special relativity only applies to motion through space. In this case, the space itself expands.
Myth: The redshift of receding galaxies is caused by the Doppler effect . Astronomers often refer to the cosmological redshift as if it were a simple Doppler effect. Although they are similar in their action, they have different mechanisms. Doppler redshift is based on the special theory of relativity, which does not take into account the expansion of space. Cosmological redshift is based on general relativity, which takes expansion into account. For galaxies relatively close to us, they may appear identical, however, if you try to describe the redshift of distant galaxies using the Doppler effect, you can come to the wrong result.