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Dark energy

One of the two greatest mysteries of the universe


Kavli Institute for the Physics and Mathematics of the Universe

In 1999, two research groups reported their observations that the expansion of the universe has been accelerating for the last seven billion years. This expansion, which began with the Big Bang, had been predicted to ultimately decelerate because of the gravitational force that draws matter together. Professor Masahiro Takada, of the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at the University of Tokyo, describes the reaction to this surprising observation. “It’s as if you threw a ball upwards and it kept accelerating higher into the sky, rather than returning to earth.” The accelerating expansion of the universe has been attributed to dark energy.

Einstein’s equations of general relativity

Figure 1: Einstein’s equations of general relativity
Gμν on the left side of the equation is the quantity known as Einstein’s tensor, a geometrical description of space-time which, when applied to the universe, describes the extent of its expansion. The right side of the equation is the distribution of all matter and energy contained in the universe. This relationship holds true at every time (instant) in the history of the universe’s expansion. Einstein attempted to produce a steady-state universe solution (static universe) by adding the cosmological constant to the left side of the equation, but when this constant is moved to the right side, it is considered to be a component of energy. In general, including the scenario where the cosmological constant changes over time, this new component is known as dark energy.
© 2014 The University of Tokyo.

Dark energy first made an appearance in theoretical research in the early twentieth century, before anyone was even aware that the universe was expanding. In his theory of gravity, known as the general theory of relativity, Albert Einstein formulated an equation that showed that the development of the size of the universe is equivalent to the amount of matter and energy it contains (Figure 1). Einstein, who believed that the size of the universe would be constant over time, altered his equation to produce a model of a static universe by adding a term known as the cosmological constant, a repulsive force to counteract the pull of gravity. Known as Einstein’s cosmological constant, this term compromises the mathematical beauty of the theory, and when Edwin Hubble discovered in 1929 that the universe is expanding, Einstein promptly abandoned it.

Ironically, the cosmological constant that Einstein later called the greatest mistake of his life was revived in the 21st century as the source of the repulsion that produces the accelerating rate of expansion. The generalized form of this cosmological constant is dark energy. As described by the equation, the total amount of this dark energy steadily increases with the universe’s expansion, a characteristic which cannot be readily explained by current models of physics.

Researchers across the globe are now advancing plans to reveal the physical properties of dark energy through observations. The spotlight of attention is focusing particularly on dark matter, which accounts for most of the matter in the universe. With its constituent particles mutually attracted by gravity, dark matter is the polar opposite of dark energy, which pulls matter apart. As accumulated mass (energy) is distributed throughout space, the density of dark matter was high when the universe was small; in other words, the effect of dark matter was then dominant. As the universe grew, the density of dark matter decreased and the strength of dark energy increased. Professor Takada notes that “The shift to an increasing rate of expansion that occurred seven billion years ago must have been due to the reversal of their relative strengths at about that time.” If we can shed light on the competition between dark matter and dark energy through observations, we will be able to better understand the nature of dark energy (Figure 2).

Expansion of the universe from the Big Bang, 13.8 billion years ago, and the competition between dark matter and dark energy: Two scenarios

Figure 2: Expansion of the universe from the Big Bang, 13.8 billion years ago, and the competition between dark matter and dark energy: Two scenarios
1) Dark energy steadily increases as the universe expands, it eventually dominates dark matter and the universe continues to expand at an increasing rate.
2) Previously-dominant dark energy is extinguished in the future for some reason, dark matter dominates over dark energy, the universe reverses to a pattern of decelerating expansion, and eventually shrinks and collapses to a point.
© 2014 The University of Tokyo.

To do so, a new map of the universe will be needed; one made by taking snapshots of the universe from remote areas to our nearby neighborhood and ranging in time from the past to the present, to determine how the distribution of matter has changed. Dark matter is invisible to the human eye, but its distribution can be reconstructed using the effect known as gravitational lensing, in which the path of light is bent by gravity. A grand cosmological census is needed; one in which the universe will be explored seven billion years back in time, requiring sufficient precision to detect the minuscule bending due to gravitational lensing, and adequate coverage of a wide region of the night sky to explain the effects of dark energy on expansion.

In the vanguard of these global endeavors, the Hyper Suprime-Cam (HSC), an ultra-wide-field, prime focus camera mounted on the Subaru Telescope, began full operations in March 2014 (Figure 3). Showcasing Japanese technology, this is the world’s first instrument that can observe in high resolution a broad and deep field of space. About 200 researchers from Japan, Taiwan, and the United States of America have gathered to carry out this cosmological census, and will together create a new map of the universe over the next five years. With an even larger observational project is underway in the West for completion in ten years, we are today at the dawn of an era of large-scale and high-precision exploration of the universe.

The Hyper-Suprime Camera, 3.3 tons in weight and approximately 3 m in height

Figure 3: The Hyper-Suprime Camera, 3.3 tons in weight and approximately 3 m in height.
National Astronomical Observatory of Japan/HSC collaboration.

Leading the HSC research team, Professor Takada explains that “We may learn that dark energy has unexpected characteristics. It’s possible that general relativity theory, the underlying assumption of our cosmological theory, is mistaken; perhaps we don’t truly understand the nature of gravity at all. If that is the case, the map is likely to differ widely from our expectations. It’s incredibly exciting research.” The map of the universe obtained in five years’ time may well depict something that exceeds the imagination of the research community. This research into dark energy, the controlling force of the universe, is also a validation of our cosmological understanding.

Interview/text: Azusa Minamizaki. Translation: Tony Atkinson.

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