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Expanding Universe and Dark Energy
-96% of the Universe Is Yet to Be Unknown-

Kei-ichi Maeda
Professor, Faculty of Science and Engineering, Waseda University

The 2011 Nobel Prize in Physics was awarded to professors Saul Perlmutter (University of California, Berkeley), Brian Schmidt (Australian National University), and Adam Riess (Johns Hopkins University), for the reason that "the discovery of the accelerating expansion of the Universe through observations of distant supernovae." This groundbreaking study pointed out the existence of dark energy, the biggest mystery in the universe and now becoming a popular topic, for the first time in history. Let me explain how great this discovery is.

1.Einstein's blunder and expanding universe

The universe is expanding. This was revealed by Albert Einstein's general relativity. He put forward the idea of general relativity in 1915. This is an extension of the special relativity he proposed in 1905, and transformed the idea of spacetime at the time. According to the theory, a heavy object or large amount of energy distorts spacetime, which explains gravitation (universal gravitation). The fact that spacetime is changeable existence opened the way to make the universe an object of scientific study.

Einstein thought the universe is a finite and eternal absolute object, and tried to confirm it in his own theory. However, his thought was denied. At a loss, he slightly revised the theory by newly adding the cosmological constant. Gravity is a force with which all objects attract each other, and the universe would crush without any other force at work. So he introduced the cosmological constant as a repulsive force (force to resist each other), thereby presented an everlasting, static universe. In 1922, however, Alexander Friedmann pointed out the possibility of an expanding universe, which was corroborated in 1929 by Edwin Hubble's observation.

How can we know that the universe is expanding? To grasp the image more easily, think about it in a two-dimensional space. Suppose the surface of the earth is the entire universe, and we live in the surface of the earth. An expanding universe means that the earth, that is, the surface of the earth, is expanding. How would the expansion be observed those living on the surface of the earth? Suppose the observers are in Tokyo. When the earth is expanding as shown in figure 2, the palaces on the earth go away from Tokyo, with more distant ones faster than nearer ones.

Hubble observed the fact that the more distant a galaxy is, the faster it goes away, and thereby concluded the universe is expanding. Subsequently, this theory of expanding universe incorporated more solid arguments about the Origin of the Elements by Gamow and others, and became a standard theory of the universe called Big Bang cosmology. Later, when it became clear the theory of an expanding universe is correct, Einstein reportedly referred to the introduction of the cosmological constant as the "biggest blunder of my life."

Figure 2:Japanese islands at a certain time and those after a small time passed. When the earth is expanding, looking from Tokyo, places recede with a speed proportional to their distance.

Figure 1:Mr. and Mrs. Albert Einstein visiting Waseda University, November 1922. The figure on the right is Masasada Shiozawa, the fourth Dean of Waseda University. (Courtesy of Waseda University Archives)

2.New mystery of the universe: The discovery of dark energy

Figure 3:An object thrown up in the air decreases upward speed and eventually falls on the ground. Similarly, the universe's expansion speed decreases if only gravitation is in force.

Although Big Bang cosmology is a highly successful theory which can be described as a monument of modern science, it does not mean, of course, that we acquired full knowledge of the universe. Physics in very high-energy region (i.e., Quantum gravity theory, Superstring theory) has not been solved yet, and we know little about the beginning of the universe. On the other hand, in the current understanding of the universe, which is assumed to have been explicated enough, there emerged a big mystery: the expansion of the universe is accelerating.

The universe began by an expansion called Big Bang, and the expansion velocity gradually decreases. This is similar to the phenomenon that an object thrown up in the air loses upward speed because of gravity, and eventually returns on the ground (Figure 3)*1. The expansion of the universe also slows down gradually, as objects pull each other back due to the gravitation working between objects. This is a decelerated expansion.

In order to know the actual state of cosmological expansion, you should find what the expansion law discovered by Hubble used to be in the past. For that purpose, it is necessary to observe in what speed distant celestial objects are receding, because seeing the far distant space means seeing the past of the universe.

To know this Hubble expansion law of the past, we must measure the distance to and the receding speed of the distant celestial objects. However, they are dark and therefore it is difficult to measure how far away they are. The above-cited three Nobel laureates, then, turned their attention to supernovae. A supernova is an explosion phenomenon accompanying an enormous energy, shines almost as luminous as an entire galaxy, and thus useful in observing the far distant space. Of all the supernovae, they focused on Type Ia supernova*2. The absolute luminosity of Type Ia supernova can be estimated through measuring the change of its brightness, and by comparing the absolute luminosity with the apparent luminosity observed, we can obtain the distance to it. The three researchers observed many supernovae tracing back to when the universe was half its present size, and as a result concluded that the universe is expanding with accelerating speed.

Figure 4:The proportion of the components of the universe. 96% is unknown matter. As to dark energy, comprising three quarters of the universe, we know nothing of what it is.

The universe model based on general relativity cannot explain the accelerating expansion of the universe with speed. There must be something like a repulsive force to resist gravity pulling back the cosmological expansion. The cosmological constant, which Einstein introduced to think of a static universe, was to play this exact role. In fact, the accelerating expansion of the universe now discovered is explainable when we bring in an appropriate value of the cosmological constant. The cosmological constant, which Einstein once referred to as the "blunder," is brought back to life. There is no theory, however, to explicate its value to regard it as the cosmological constant, and the cause of the accelerating expansion became known as dark energy.

Later, more detailed information about the components of the universe has been revealed through studies including a precise observation of the cosmic background radiation*4 by the WMAP satellite*3. According to the results, matter consists of atoms, which are familiar to us, account for only 4%, 22% is still unknown dark matter, and remaining 74% is dark energy (Figure 4).

While dark matter is matter that localizes around a galaxy and makes a structure, dark energy is a very mysterious existence, suggested only by the accelerating expansion of the universe. Nothing has been revealed as to what it is, and it may not be matter at all. In fact, there exists an effort to explain the accelerated expansion of the universe by conceiving a gravitation theory in a cosmological scale which is different to general relativity. If general relativity, which has been considered correct for 100 years, is not correct on a cosmological scale, it poses a significant problem affecting the basis of modern Physics. Clarifying this dark matter issue, the biggest issue of the modern cosmology, will surely be honored with a Nobel Prize. I have been excited these days to the fact that in the near future, as in early 20th century when relativity and quantum theory emerged, a new big leap might be seen in the development of Physics.

*1:If the initial velocity is very fast, the object gets out of the sphere of gravitation to reach the outer space. Even in that case, the upward speed decreases constantly.
*2:Of supernovae, those that the absorption line of hydrogen is not seen in its optical spectrum is called Type I, and the Type I supernovae which absorption line of silicon is seen in its optical spectrum are specifically called as Type Ia.
*3:Wilkinson Microwave Anisotropy Probe, an artificial satellite launched by NASA to observe the cosmic background radiation of the universe.
*4:Microwave radiation emanating from every direction of the universe, which provides the ground for the Big Bang cosmology.

Kei-ichi Maeda
Professor, Faculty of Science and Engineering, Waseda University

Professor Maeda was born in Osaka in October, 1950
After earning his doctoral degree at Kyoto University in March 1980, he took various positions such as researcher at the International Centre for Theoretical Physics in Trieste, Italy and the Paris Observatory, and assistant at the Department of Physics, Faculty of Science, The University of Tokyo. He assumed the position of professor at the School of Science and Engineering (present: School of Advanced Science and Engineering), Waseda University in April 1989. During March to September 2006 he concurrently assumed the position of visiting professor at Trinity College, University of Cambridge.
Area of specialization: general relativity and cosmology.
His publications include Topology of the Universe [Uchu No Toporoji], Einstein's Time [Ainshutain No Jikan], Lectures on Gravitational Theories [Juryoku Riron Kougi] and others.