What is the Universe Made Of?
One of the key questions that needs to be answered by astrophysicists is what is really out there? And of what is it all made? Without this understanding it is impossible to come to any firm conclusions about how the universe evolved.
PROTONS, NEUTRONS AND ELECTRONS: THE STUFF OF LIFE
You, this computer, the air we breathe, and the distant stars are all made up of protons, neutrons and electrons. Protons and neutrons are bound together into nuclei and atoms are nuclei surrounded by a full complement of electrons. Hydrogen is composed of one proton and one electron. Helium is composed of two protons, two neutrons and two electrons. Carbon is composed of six protons, six neutrons and six electrons. Heavier elements, such as iron, lead and uranium, contain even larger numbers of protons, neutrons and electrons. Astronomers like to call all material made up of protons, neutrons and electrons "baryonic matter".
Until about thirty years ago, astronomers thought that the universe was composed almost entirely of this "baryonic matter", ordinary atoms. However, in the past few decades, there has been ever more evidence accumulating that suggests there is something in the universe that we can not see, perhaps some new form of matter.
WMAP AND DARK MATTER / DARK ENERGY
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4.6% Atoms. More than 95% of the energy density in the universe is in a form that has never been directly detected in the laboratory! The actual density of atoms is equivalent to roughly 1 proton per 4 cubic meters.
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23% Cold Dark Matter. Dark matter is likely to be composed of one or more species of sub- atomic particles that interact very weakly with ordinary matter. Particle physicists have many plausible candidates for the dark matter, and new particle accelerator experiments are likely to bring new insight in the coming years.
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72% Dark Energy. The first observational hints of dark energy in the universe date back to the 1980's when astronomers were trying to understand how clusters of galaxies were formed. Their attempts to explain the observed distribution of galaxies were improved if dark energy was present, but the evidence was highly uncertain. In the 1990's, observations of supernova were used to trace the expansion history of the universe (over relatively recent times) and the big surprise was that the expansion appeared to be speeding up, rather than slowing down! There was some concern that the supernova data were being misinterpreted, but the result has held up to this day. In 2003, the first WMAP results came out indicating that the universe was flat (see above) and that the dark matter made up only ~23% of the density required to produce a flat universe. If 72% of the energy density in the universe is in the form of dark energy, which has a gravitationally repulsive effect, it is just the right amount to explain both the flatness of the universe and the observed accelerated expansion. Thus dark energy explains many cosmological observations at once.
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Fast moving neutrinos do not play a major role in the evolution of structure in the universe. They would have prevented the early clumping of gas in the universe, delaying the emergence of the first stars, in conflict with the WMAP data. However, with 5 years of data, WMAP is able to see evidence that a sea of cosmic neutrinos do exist in numbers that are expected from other lines of reasoning. This is the first time that such evidence has come from the cosmic microwave background.
