Inflationary Theory: The Discovery of Dark Matter

FROM THE LECTURE SERIES: Redefining Reality: The Intellectual Implications of Modern Science

By Steven Gimbel, Ph.D., Gettysburg College

The Big Bang theory was a breakthrough that had many repercussions for the world of cosmology. While it created a plausible explanation for the existence of the entire universe, it also came with a plethora of inconsistencies that scientists failed to explain. 

Representation of the evolution of the universe over 13.77 billion years.
The inflationary theory was paramount in answering a lot of questions raised by the Big Bang theory and eventually led to the discovery of the existence of dark matter. (Image: NASA/WMAP Science Team/Public domain)

In the 1970s, there were a lot of questions raised about the problems that arose as a result of the Big Bang theory. For instance, the correctness of the Big Bang theory would imply the presence of a lot of magnetic monopoles in the universe. Yet, not one has ever been discovered. It also raised questions about the cosmological principle and the uniform, isotropic nature of the universe. Furthermore, the Big Bang theory postulated a flatness in the universe. Why was this so, and how did it happen? 

Inflationary Theory—Bridging the Gaps of the Big Bang

All these questions, and many more, were answered when the American Alan Guth and the Russian Andrei Linde came up with the notion of inflation, which said that the early universe had expanded very rapidly, blowing itself up by a factor of 1050, in a very short time, right after the Big Bang. This notion was able to account for the fundamental questions against the Big Bang theory.

But, as is common in science, the answers given by inflationary theory were met with even more questions. For instance, what could have caused an inflationary event of this magnitude? 

Learn more about Big Bang cosmology.

Gaps in Inflationary Theory 

Despite the fact that the Big Bang itself was such a unique event with inconceivable forces such that this violent explosion of space-time was indeed plausible in its aftermath, it became pressing to know what its drivers were.

One possibility that was studied intricately was the Higgs field, which gives rise to the Higgs Boson. Even today, this just remains a possibility, one that has only been experimentally verified very recently. 

Then, another question posed against the inflationary theory is that of smaller-scale inequalities. The cosmological principle that is so necessary for the Big Bang theory to make sense means that the universe, both before and after the inflationary event, should be completely smooth; however, when we see the universe today there are galaxies, and clusters of galaxies, interspersed in space.

The inflationary theory soon came up with an answer to this question, namely that the early, pre-inflation universe would have been sufficiently closed to create the equilibrium necessary for the homogeneity and isotropy we see now. But this equilibrium would not have been completely perfect, and there would have been inequalities distributed throughout space in a random way, similar to the way it is seen today. So, if the universe inflated as posited by Guth and Linde, it would have magnified these irregularities, thereby creating the seeds necessary for the formation of galaxies and the birth of the universe as we see it today. 

To this theory, however, skeptics have even more questions. The problem that could have arisen is that the irregularities could have been magnified at a stage when the universe was still too hot, and while protons and electrons may still have been able to be formed, the energy density would have made them move around so much that they would not have been sufficiently attracted by gravity to the irregularities, which is what was needed to form galaxies. 

The response to this problem is what gave rise to the study of one of the most perplexing fields in cosmology and physics: dark matter.

From the Inflationary Theory to Dark Matter

While the concept of inflation in the early stages of the universe could have planted the seeds for galaxies, conditions were still far too intense for them to grow. However, while it was in fact too hot for normal matter to have been overheated and not exposed to the required gravitational force, matter itself was of a different type at that time. This was a different kind of matter that was there at the beginning of the universe to start the process of creation—a kind of matter that was invisible: dark matter.

Upon hearing this notion for the first time, it admittedly does sound sketchy. But in reality, there are a lot of reasons to support the existence of the mysterious dark matter. 

This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on The Great Courses Plus.

The Case For Dark Matter

For a long time, astronomers had been struggling with the question of the mass of astronomical objects, such as stars, and even galaxies. There are two ways to figure out the weight of a large, compound object. One can measure the weight of the entire object, or one can weigh the constituents separately and add up their masses to get the mass of the whole.

Scientists followed both approaches to find the masses of galaxies. They counted the stars in a galaxy, figured the mass of different stars, and summed them up. On the other hand, they also used the effects of gravitation to figure out the weight of the galaxy as a whole entity, in accordance with Einstein’s general theory of relativity, according to which mass warps space, leading light through the shortest path. So, light warp around a galaxy could signal to its mass. 

Representation of general theory of relativity displaying space-time warped by the mass of the sun and the earth.
According to Einstein’s theory of relativity, mass warps the speed of light; this phenomenon was used to determine the weight of galaxies, leading to the discovery of dark matter. (Image: vchal/Shutterstock)

Now, while these methods should ideally have led to the same answer, they did not. The galaxy as a whole was monumentally heavier than the total masses of all the stars in it. This difference could only be explained by hidden mass in the galaxy. Since we couldn’t observe this mass, it was dark. 

In 1975, American astronomer Vera Rubin came up with the same conclusion when examining the rotation of galaxies. In order for galaxies to behave the way they were observed to be behaving, their mass had to be six times the mass of all the visible stars. Thus, the majority of the mass in the universe is unobservable.

Learn more about dark matter and dark energy.

The Search For WIMPs

This majority stuff of the universe is one of the biggest mysteries in cosmology. Its dark nature prevents it from having an electromagnetic footprint, leaving only its gravitational footprint to reveal its presence. 

In a quest for an interesting acronym, physicists ending up naming these as WIMPs: Weakly Interacting Massive Particles. They had to be massive since they had gravitational effects, and the ‘weakly’ comes from their lack of other interactions, though it is not necessarily a reference to the weak nuclear force. 

These particles are theoretically needed to reconcile our theories of matter, and physicists have for long played with quarks and leptons to try to create a recipe for dark matter, but to no avail. Later, it was realized that supersymmetry allowed the standard model of matter to explain these particles.

Still, we haven’t ever found them in our world, despite the fact that many of the brightest minds have searched for them. The quest for WIMPs goes on to this date, and millions are spent every year in an attempt to learn more about dark matter.  

Common Questions about the Inflationary Theory

Q: What is the inflationary theory in science?

The inflationary theory in science is the theory that explained the rapid expansion of the early universe by a large factor, within a very short time, and it manages to solve a lot of inconsistencies in the Big Bang theory.

Q: How does the inflationary theory add to the Big Bang theory?

The inflationary theory, by positing that the universe expanded by a factor of 1050 right after the Big Bang, within an extremely small period of time, solves the fundamental questions about the Big Bang theory. Furthermore, it also ultimately led to the discovery of the existence of dark matter.

Q: What observations led to the discovery of the existence of dark matter?

The existence of dark matter was uncovered when scientists discovered that the masses of all the visible stars in galaxies were much, much less than the comparative gravitational effects exerted by the galaxies. This led to the belief that an invisible form of matter added mass to these galaxies.

Keep Reading
The Big Bang Theory Explained
The Origin of Mass
How Einstein Solved the General Theory of Relativity