a


b


c


d


e


f


Figure 9.4: In 2012, NASA astronomers announced that they could predict with certainty the next major cosmic event to affect our galaxy, sun and solar system: the titanic collision of our Milky Way galaxy with the neighbouring Andromeda galaxy. (a) Present day; (b) In two billion years the disc of the approaching Andromeda galaxy is noticeably larger; (c) In 3.75 billion years Andromeda fills the field of view; (d–e) In 3.85–3.9 billion years the sky is ablaze with new star formation; (f) In four billion years Andromeda is tidally stretched and the Milky Way becomes warped; (g) In 5.1 billion years the cores of the Milky Way and Andromeda appear as a pair of bright lobes; (h) In 7 billion years the merged galaxies form a huge elliptical galaxy, its bright core dominating the night sky.


However, in 4.5–5.5 billion years from now, the sun will consume Earth, before collapsing into a white dwarf. Therefore, the last part of this panoramic view most likely will not be observed from Earth.


g h


In Greek mythology, Andromeda was the princess daughter of King Cepheus and Queen Cassiopeia who chained Andromeda to a rock, in order to be sacrificed to a sea monster. She was saved by the hero Perseus. They married, and had nine children. After Andromeda’s death, the goddess Athena placed her in the sky as a constellation, near her beloved husband Perseus and her mother Cassiopeia.


NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas, and A. Mellinger


constitutes 87% of the mass in the universe. Let us discuss dark matter. In galaxy clusters – huge conglomerations of galaxies –


the normal matter, like the atoms that make up the stars and planets, is primarily in the form of hot gas and stars. Tey present the largest structures in the universe held together by gravity. In 1933, astronomer Fritz Zwicky, while examining the motion of the Coma Cluster, concluded there was not enough visible matter to hold these fast-moving galaxies together. Te Coma Cluster is about 330 million light-years from Earth and today is known to contain several thousand galaxies, each housing billions of stars. Zwicky proposed that something invisible was producing additional gravity out there in space. He dubbed this unknown substance dark matter. Unseen dark matter? Astronomers at the time considered his claim to be a crazy idea, just as they had Zwicky’s first proposals of stars made of neutrons and galactic cosmic rays. But in the 1970s astronomer Vera Rubin together with instrument maker Kent Ford confirmed Zwicky’s dark matter findings. She observed that galaxies are rotating so fast that they would fly apart, if


296


the gravity of their observable stars was all that was holding them together. But they are not flying apart, and therefore, a huge amount of unseen mass must be holding them together – at least ten times as much mass as can be accounted for by the visible stars. Although Rubin’s work was initially met with scepticism, her results have been confirmed over subsequent decades. Her observations were the strongest evidence at that time for the existence of dark matter. Currently, we know that dark matter is hypothetical matter


that does not emit or absorb light, but whose presence can be inferred from gravitational effects on visible matter. If it sent out light or electromagnetic radiation, we would see it. If it absorbed light, we would be able to see it as shadows against the background. It is abundant, making up about 26% of the universe’s total mass and energy. Physicists have postulated that dark matter is made up of a


new and as yet unknown elementary particle that only interacts weakly with other known particles, thereby making it difficult to detect. Tis weakly interacting massive particle (WIMP) can only interact through gravity and the weak force. WIMPs


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