Figure 9.3: Graphic view of the Milky Way Galaxy, our home galaxy in the universe and home to 400 billion stars as well as our own sun and solar system. The ancient Greeks called it the Milky Circle because it reminded them of milk, and the word galaxy in fact is from the Greek word gala meaning “mother’s milk”. Legend tells that the Milky Way was formed from the milk spurting from the breasts of the Greek goddess Hera, Queen of Heaven. Where drops fell to earth, fields of lilies sprung forth.


The Milky Way is 120,000 light-years across and contains spiral arms of giant stars that illuminate interstellar gas and dust. The sun is in a finger called the Orion Spur. Overlaid is a graphic of galactic longitude in relation to our sun. The galactic year is the time required for the solar system to orbit once around the centre of the Milky Way. The orbit radius is 8 kpc. The solar system is travelling at an average speed of 230 km/s within its trajectory around the galactic centre, at approximately 1/1,300th of the speed of light. Estimates of the galactic year range from 225 to 250 million terrestrial years. The galactic centre, which is located 26,000 light-years from Earth in the direction of the constellation Sagittarius, contains a supermassive black hole called Sagittarius A* discovered in 1974 and estimated to have around 4.5 million solar- masses (see Section 9.1.6.2). Luckily, the black hole is quiet and doesn’t ‘eat’ a lot of material. Measuring the rotation speeds in spiral galaxies provides us with a way to estimate the masses of such galaxies, and to measure how that mass is distributed within them. In 2015 physicists revealed that there were large amounts of dark matter between us and the centre of our Milky Way galaxy. Dark matter seems to be lurking in our own solar system and at the Earth’s location. Maybe tomorrow’s dark matter detectors will be able to find it?


inflation made the universe enlarge itself by a factor of ~1026 ,


meaning that it was a 100 trillion times larger than it had been less than a second before. In another tiny fraction of a second, inflation slowed down to a more leisurely rate of expansion that continues to this day but is accelerating. Te universe cooled to allow the formation of subatomic particles, and later simple atoms. Tese became the building blocks for the universe. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies. Te Big Bang model explains the origin of all known matter, the laws of physics, and accounts for the expansion of the universe as well as a broad range of other phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and Hubble’s Law. CMB radiation is the residual heat of creation from the time


there was a firestorm of radiation and elementary particles – or the afterglow of the Big Bang – streaming through space like the heat from a sun-warmed rock, reradiated at night. When this cosmic background light was released around 380,000 years after the Big Bang in a universe at a temperature of about 3,000°C, it was as hot and bright as the surface of a star, but as the universe expanded CMB cooled to its present- day temperature, measured by radio telescopes at about 2.73 degrees above absolute zero. CMB radiation fills the universe and can be detected in every direction. In 1963 the American radio astronomers Arno Penzias and Robert Wilson were studying faint microwave signals from the Milky Way galaxy when they found a mysterious noise of unknown origin. It soon turned out that the noise was CMB signal, left over from the


early stage in the development of the universe. Tis accidental discovery is today considered a landmark test of the Big Bang model of the universe, and earned the discoverers the 1978 Nobel Prize in Physics.


9.1.2 Dark Energy and Dark Matter


However, with the development of the Big Bang model a number of mysteries and problems have arisen. Two mysteries that are still under intense investigation by cosmologists and astrophysicists are dark energy and dark matter. In 2011, the Nobel Prize in Physics was awarded to the


astrophysicists Saul Perlmutter, Brian P. Schmidt and Adam G. Riess, for their 1998 discovery that the universe is expanding at an accelerating pace. It was a very unexpected discovery, and something was causing it. Physicists came up with the idea that this something is an enigmatic force, called dark energy, which is working against the gravitational forces that are trying to pull the universe inward. Te exact explanation for dark energy is a mystery, but we know how much dark energy there is because we know how it affects the expansion of the universe. Te widespread acceptance is that the universe is 70% dark energy. As Einstein’s famous equation tells us, energy equals mass multiplied by the speed of light squared (E=mc2


), so the


concepts of matter and energy are intrinsically linked. If the universe contains 70% dark energy, this leaves 30% to other mass-energy content. Te normal matter that we can account for and explain with all our experiments and instruments, is only 4%. Tis leaves mass-energy of 26% to a mystery substance called “dark matter”. In terms of total mass, dark matter


295


NASA/Adler/U. Chicago/Wesleyan/JPL-Caltech


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