If you look at the starry sky, you start to wonder how far the Universe extends, what is out there, where we have come from, how things come to exist. These philosophical questions are no longer just philosophy; there are in the realm of scientific endevour.
We are quite sure that our Universe started about 15 billion years ago with an explosive fireball called Big Bang. Ever since, Universe has kept expanding. We actually have fossils to prove this: so-called cosmic microwave background radiation is the relic of hot Big Bang remaining to date in the form of about 400 photons per cubic centimeter everywhere in our Universe. These photons not only provide evidence for Big Bang, but also carries information about its history from the time when Universe was thousand times smaller.
Physicists and astronomers worked together in the past few decades to reveal surprising answers to many questions. What we see in the sky are beautiful stars of all colors and sizes. They are, however, only about 0.1% of the Universe. The ordinary matter, namely electrons and baryons (protons and neutrons) we are are made of, make up only up to 5%. The remaining 95% of Universe is still a mystery, and is subject of our research.
Even in the emptiest looking part of the Universe, there are about 300 neutrinos for every cubic centimeter. We have just discovered that they actually have tiny but finite masses. They contribute to somewhere between 0.1% to 10% of our Universe. If on the high end, they have affected the way galaxies had formed.
About 25% of Universe is what is called Dark Matter. They are most likely microsopic particles that are clumped in our galaxy. It is actually these Dark Matter particles that hold our Milky Way galaxy together. If they weren't there, our galaxy will fly apart. The picture (a) shows the measurement of Doppler shift in 21cm line that allows us to determine the rotational speed of other galaxies. The rotational speed is much faster than what the gravitional pull by stars would allow (b). The Earth is revolving around the Sun, and our solar system is revolving in our galaxy, in the sea of dark matter, but there is no sign that we are slowing down. It means that Dark Matter particles are very shy and interact very little with us. This is as much as we know about Dark Matter. However, forthcoming experiments are poised to detect the mysterious Dark Matter particles. We study the existing constraints from the data, develop theories of candidates for Dark Matter particles, and predict their properties. One of the best candidates for Dark Matter is the lightest supersymmetric particle.
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The most mysterious of all is Dark Energy. Study of supernovae billions of lightyears away as seen in the picture below has shown that the expansion of our Universe is accelerating, contrary to the past belief. As Universe expands, we expected that everything thins out, and the expansion slows down. If it is accelerating, however, there must be a form of energy that does not thin out even as Universe expands. In other words, Universe is creating energy all the time. This form of energy then must have negative pressure, so that it gains energy as the expansion of Universe tries to pull it apart. This mysterious energy is called Dark Energy. One such candidate is the cosmological constant, a term once Einstein introduced to his theory of gravity (general theory of relativity) but later retracted. He had regretted that introduction of this term was his biggest mistake. What we may be seeing is that the cosmological constant was necessary after all.
The problem with the cosmological constant is that a naive theoretical estimate of its size is at least 1060 too big compared to the size indicated by observations. This is the worst prediction of theoretical physics ever. It is hoped, however, that deeper understanding in quantum theory of gravity based on the holographic principle may ameliorate the problem greatly. The holographic principle states that all information in a volume can be encoded on its surface. A dramatic reduction in the degrees of freedom! This picture is supported by the only candidate theory for quantum gravity we have, namely string theory.
Another big puzzle is where the anti-matter has gone. At the time of Big Bang, there were equal amount of matter and anti-matter. Most of them have annihilated away into pure form of energy. There was, however, one out of ten billion excess matter over anti-matter, and this tiny excess survived The Great Annihilation. This is us. What caused such a small excess of matter over anti-matter so that we can exist? We develop theories why. One such theory relies on recently-discovered mass of neutrinos. Experiments that study subtle difference between the behavior of matter and anti-matter, in particular in neutral K and B mesons, may also reveal the origin of this tiny excess matter.
How far does the Universe extend? It is actually amazing that microscopic physics, with typical distances smaller than the size of an tom, must have led to the current enormous size of the Universe. It is likely that there was a period of inflation in which the size of Universe has grown exponentially. We study what caused this cosmological inflation.
Contributed by Hitoshi Murayama.