The Big Bang Theory

When did the universe as we know it actually come into existence? What event initiated it? How do we know this to be the case?

This website delves deeply into these profound questions, striving to provide a more comprehensive understanding of the most widely-accepted explanation of the origins of our universe:

The Big Bang Theory.

What is The Big Bang Theory?

Contrary to the name, the Big Bang was not necessarily a bang. The term refers to the moment the universe was a small, hot, infinitesimal point, which then began expanding. The theory itself attempts to explain the history and evolution of the universe, where our observations reinforce this explanation. Data that is exclusively concordant with the Big Bang continues to demonstrate that this theory is currently the best and most comprehensive explanation for the timeline of the universe.

One thing to note is that a scientific theory is not a colloquial theory, nor can it become a law; instead, it is the graduation point of a hypothesis. It serves as an explanation for factual observations. Through repeated observation and experimentation, consistent results, meticulous peer review, and a wide consensus among scientists, a theory is perhaps the most prestigious term in science.

Science is one of the most humble activities. As more data is collected, a theory can always be made more precise, it is never considered absolute. That is why it is called a theory — it is subject to enhancement.

The Big Bang is a scientific theory.

Observations

The first step of the scientific method is to make an observation. So, what observations have scientists made that support the Big Bang Theory?

Receding Galaxies

According to our current understanding, there are 118 elements recognized on the periodic table. But right after the Big Bang, the universe would have been encompassed by plasma, which condensed into protons and neutrons. The universe was so hot and energetic that these particles could not come together to form atoms. Over time, however, the universe cooled. This allowed for the strength of interactions between particles to take over. Eventually, protons and neutrons would fuse to form the first hydrogen isotope: deuterium. The formation of the first hydrogen isotopes would then collide with enough energy to form helium, lithium, and heavier elements.

Astronomers are able to study these elements elsewhere in the universe. Particularly, they can measure ratios of hydrogen, helium, and other elements. Based on these ratios, they match what we would expect if the universe was once a very massive star.

This data indicates that the universe was once in a state resembling that of the core of a star. This means a high energy, hot state spanning the contemporary universe would have taken place. Some sort of energetic event would have had to have to given rise to these observations.

This event could be described as the Big Bang.

Cosmic Microwave Background Radiation

Scientists are able to measure wavelengths originating from distant galaxies. By measuring the relative change in wavelengths of galaxies, we can deduce the relative speed and direction they are moving. Astronomer Vesto Slipher is credited with discovering that most of these galaxies are actually moving away from us at incredible velocities, causing them to appear more red due to a stretched wavelength.

At the time of this discovery, Vesto — and everyone else — thought that these galaxies were merely stars within the Milky Way. That was until Edwin Hubble was able to observe a particular type of variable star, called a Cepheid variable star. This star has a direct relationship between its energy output and the time between its pulsations of light. By using this relationship, he could calculate their distances from earth. He discovered that these stars were so far away that it rendered them beyond the Milky Way. This meant that the observations made by Vesto were not just stars within our galaxy. They were entirely new galaxies.

If all of these far galaxies are receding from us, this suggests that we were once located in a single point in the distant past. This would be called the singularity — the infinitesimal point — at which the universe then began to expand.

That moment of initial expansion was the Big Bang.

Big Bang Nucleosynthesis

We have already described how the universe once resembled the core of a star, where its contents were unable to form permanent structures until it cooled. We also explained how almost everything in the universe is moving away from us, which suggests that the visible universe was once a small point that began expanding. Both of these observations are important in understanding one of the most profound discoveries in astronomy: the cosmic microwave background radiation.

If you were to direct an antenna toward any point in the universe, you would detect a signal that is constant and in every direction. This is what happened when two radio astronomers, Arno Penzias and Robert Wilson, detected a signal in their radio telescope that had no apparent source and was constant no matter the time of day or year.

Right after the Big Bang, the universe was very high energy, meaning the wavelength of the emitted radiation was very small. However, similar to how the wavelengths of galaxies stretch as they recede, the radiation left from the Big Bang has also stretched. This means that the once high energy, high frequency radiation has weakened over the universe’s expansion — about 13.8 billion years since the Big Bang first released it. This radiation is now in the microwave portion of the electromagnetic spectrum.

The image to the left maps the radiation that was detected by Penzias and Wilson, and it encompasses the entire universe. This data matches what we would expect to see if a cosmic event took place billions years ago, giving rise to the observable universe today.

This is energy is a relic of the Big Bang.

The Big Bang Model

How are observations accounted for in Big Bang Cosmology?

The timeline of the Big Bang model has many different eras. There was an era of inflation, darkness, the primordial stars, and ultimately the development of planets and galaxies into what we see today. According to the Big Bang model, it was about 370,000-380,000 years after inflation that electrons were able to begin orbiting their nuclei to form atoms.

Before this moment, however, the model depicts the universe as being entirely plasma consisting of quarks and gluons, then giving rise to other subatomic particles. As mentioned, the first isotope of hydrogen formed during the process of Big Bang nucleosynthesis. The measured ratios of hydrogen, helium, and other elements throughout the universe support this.

This observation is incorporated in the Big Bang model as part of the early universe, and it was a driving factor in the formation of structures we see today — whether it be at astronomical or atomic levels. Without the stages of inflation and cooling in the Big Bang model, we would not be able to account for the basic building blocks of matter. This is why nucleosynthesis is such a vital piece of the Big Bang Theory and is reinforced by modern data.

Photo credit: NASA/WMAP Science Team - Original version: NASA; modified by Cherkash

In science, information may be too complex to relay verbally. A visual representation enables an audience to better grasp the information in the form of a comprehensive model. The Big Bang, like other scientific theories, has a model which is used to explain what we observe in the universe based on an abundance of evidence.

The Big Bang model does not depict an explosion. Instead, it depicts an expansion of space that is still underway at this moment. This is important, as it incorporates the fact that all things in the universe are, on a large-scale view, moving away from each other. This is not just necessarily the motion of the objects themselves, but the space between them actually growing. This part of the Big Bang model accounts for the recession of galaxies from us, which was discovered by the aforementioned Vesto Slipher and confirmed by Edwin Hubble. This observation was crucial for the Big Bang, and it reinforces the modern Big Bang model as an expansion of space.

Photo credit: Astronomy: Roen Kelly

The cosmic microwave background radiation (CMBR) was a milestone discovery for the Big Bang as it fit predictions made decades prior. This radiation is seemingly in concordance with Big Bang nucleosynthesis. Once the universe cooled and atoms formed, space became transparent, allowing light to travel freely without scattering.

The moment light was able to travel through transparent space is mapped by the CMBR mappings. This was just under 400,000 years after the Big Bang. The mapped radiation is the first light that could travel without scattering. Predictions allow us to estimate the wavelength and temperature of this light as it has traveled through space. These predictions rendered the light within the microwave region of the electromagnetic spectrum — and that is where we are able to see it.

The CMBR is accounted for in the Big Bang Model by the afterglow light pattern, just after inflation. It is a predicted consequence of everything that happened within 400,000 years after the Big Bang.

The stages of the universe coincide with each other, showing the timeline of how subatomic particles formed. These eventually cooled and gave rise to atoms. Now that space is cooler, light does not scatter, meaning it can travel. This creates the primordial CMBR. It all unifies to form a coherent model.

ΛCDM Model

ΛCDM is a cosmological model that describes the evolution of the universe. It is based on the principles of general relativity and incorporates dark matter and dark energy. This model serves as a framework for understanding the distribution of matter and energy in the universe.

Lambda (Λ) is the cosmological constant, which was first introduced in Einstein’s general theory of relativity as a part of his field equations. The constant denotes the energy density present throughout spacetime — it causes the accelerated expansion of the universe. It is important for models of the Big Bang, such as ΛCDM, to incorporate this constant because of the observation that the universe is accelerating in its expansion. Without Λ, we are disregarding a fundamental fact of the universe and may consequently create an inaccurate model.

Λ is often related to the elusive dark energy. Dark energy makes up about 70% of the energy density in the universe, so it comprises the bulk of what Λ represents.

CDM stands for cold dark matter — where dark matter is some form of matter that does not appear to interact with electromagnetic radiation. Cold dark matter refers to a particular non-relativistic dark matter, so it moves at speeds far less than the speed of light. Dark matter makes up about 27% of the universe and is an important role in the structure of cosmic bodies, like galaxies and galaxy clusters.

ΛCDM explains the expansion of the universe through dark energy. The dark energy acts as a counteractive force to the gravitational attraction between all matter, including dark matter. The dark energy’s forces exceed that of gravity, causing the universe to expand — and at an accelerating rate, nonetheless.

How do we model the Big Bang?

Photo credit: Universe Today

Assessing the Evidence

Where does the evidence lead us?

Science consistently demonstrates itself to be the most reliable tool for investigating the natural world. And when an idea is proposed, it should face critical analysis. All ideas are subject to peer review, and science embraces this. Science does not account for personal beliefs, feelings, or biases — this is what makes it so reliable. Every idea, no matter how substantiated or sincere, is subject to ridicule.

A hallmark quality in a scientific theory is the standard of evidence. The evidence should be unambiguous, having a clear correlation with the theory it attempts to support. The evidence presented here in support of the Big Bang is what we deem positively indicative of or exclusively concordant with — meaning it has a specific and precise relationship with what it attempts to support. This leaves little room for misinterpretation. It is physical, quantifiable, and demonstrable in nature. It is hard data that can be expressed visually in graphs and models.

The redshifts of galaxies can be quantified and related to their distances from us. This relationship can be modeled via graphs, which is an important quality for the evidence to have. The CMBR can be mapped to form a visual representation of the early universe, which allows us to see our data in action. The composition of elements in our universe can be compared to that of stars, where we can see similarities in the ratios between them. These pieces of evidence are an example of the standard we would expect for a scientific theory.

Of course, most people do not have the necessary training to interpret complex data or conduct their own experiments. But you do not need a lifetime of physics to use simple reason. Your phone, computer, vehicles, microwaves, wireless internet, and lights are all a demonstration that science works. We use rudimentary sciences, like physics, to perform basic tasks. We innately act concordantly with the laws of physics.

We drive slower around sharp turns in order to minimize the force needed to make the turn; otherwise, we may skid. We rely on friction to keep us on our feet — that is why we choose to walk on dry concrete, not ice. Even with every step we take, we are demonstrating Newton’s Laws in action. To an extent, we rely on the principles of physics every day.

With that said, the most reasonable conclusion to draw from the evidence is that science is still reliable — the Big Bang did happen.