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Exploring a New Gravitational Theory
~ Exploring the Origin of the Accelerating Expansion of the Universe ~
Rampei Kimura, Assistant Professor

The Expanding Universe

Although it is a well-known fact that universe is expanding at an accelerating rate, the cause of the expansion is still unknown. I have been working on building a new gravitational theory to explore the origin of this accelerating expansion.

The fact that the universe is expanding was first made known through a study by Edwin Hubble et al. in the year 1929. Through their observation of the galaxy, they discovered that “the farther galaxies move away from the earth faster” and went on to declare that the entire universe continues to expand like a balloon. It was an astounding discovery, which demolished the popular belief of the time that “the universe is still.”
Approximately 70 years later, in 1997, Saul Perlmutter, Brian Schmidt, and Adam Riess revealed from their observations of the type “la supernova” that the universe is rapidly expanding. This is what is known as the accelerating expansion of the universe. Following this discovery, the three men won the Nobel Prize for Physics in 2011.

Why does the expansion of the universe accelerate?

The universe is believed to have began approximately 13.8 billion years ago, owing to a rapid expansion from a high-temperature, high-density state. This is the so-called “big bang,” according to which the universe has been continuing to expand until the present day.

Because of law of universal gravitation, matters (such as stars, galaxies) pulls each other, and thus the universe stop expanding and eventually starts contracting. However, what observation of type Ia supernovae brought us is that our universe is accelerating, which cannot be understood by the standard physics.

The plausible solution for this is that the existence of “dark energy” that has not yet been comprehended. While matters are normally attracted to each other by gravity, dark energy exerts a repulsive force that tries to accelerate our universe. This force is believed to be an enormous energy that is large enough to counteract the effect of gravity of normal matters.

 

Figure 1: Expansion history of the universe containing only matter and of the universe containing matter and dark energy (courtesy: Professor Kimura)
(Balloons in the figure have been modified from https://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379)

Observations show that dark energy may account for approximately 70% of the total energy of the present universe. Even though it fills the entire universe, nothing is known about the real nature of dark energy; researchers from around the world are currently engaged in trying to understand it better.

New Gravitational Theory to Explain Accelerating Expansion

The expansion of the universe can be described by the general theory of relativity propounded by Albert Einstein in 1915. The general theory of relativity is enunciated by the figures in black in the following equation, and as mentioned earlier, it predicts a shrinking universe in the future. However, the expansion of the universe can be explained by adding the term in red, and the result is seen to agree with the observation result. This term in red is known as “cosmological constant.” However, the physical basis of this cosmological constant is not known at the moment.

The general theory of relativity has been verified by various experiments and observations at the scale of the solar system, and there is no room for doubt regarding its validity. However, it is still unknown whether it is applicable on larger scales. Perhaps there exists a theory based on a wider framework that is capable of explaining the accelerating expansion of the universe. I am, therefore, currently working on the construction of a new gravitatinoal theory that describes this phenomenon.

Verification of the Theory by Observation of Gravitational Waves

Proof of the existence of “gravitational waves” has had a great impact on cosmology research in recent years. Gravitational waves are a phenomenon in which space–time distortion propagates similar to a wave. In 2015, Rainer Weiss, Barry Barish, and Kip Thorne successfully detected gravitational waves using the observation device LIGO. This achievement earned them the Nobel Prize for Physics in 2017. Einstein had also predicted the existence of gravitational waves in his general theory of relativity. Now, 100 years after this prediction by Einstein, the existence of gravitational waves has finally been proved.

The fact that gravitational waves can be detected means that we have acquired new and different observation methods. For instance, in the general theory of relativity, the speed of propagation of gravitational waves is known to be the same as the velocity of light. However, in some other gravitational theory, it may be different from the speed of light.

This is of great significance in constructing a new gravitaitonal theory. When developing a theory, various predictions are made using the newly constructed theory, which are then compares with observational results to validate the new theory. If the results do not match, we modify the theory, make predictions once again, and again compare them with observational results. By repeating the process, we get closer to a realistic theory. Using the data of the observation of gravitational waves in addition to the observation of the type la supernovae used thus far, we can further strengthen the verification of the developed theory. With the new gravitational theory introduced in this manner, the form of the hitherto unknown universe can be expected to become clearer.

Realistically speaking, it will take a long time to achieve my goal, which is to try to understand everything about the universe, from its birth to the present day, and its future. Through this research, I would like to understand as much as I can of the universe.

Interview and Composition:Chisato Hata
In cooperation with: Waseda University Graduate School of Political Science J-School

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