Abstract
If we want to construct a model of the universe we need to know what is in it and we also need to know the physical properties of whatever it is. In this chapter we will therefore have to answer the questions: what stuff, what kinds of matter, is there and where does it come from. Of course we are interested in the “stuff” from the point of view of cosmology and building our cosmological models. This means that we gloss over or even leave out some stuff that is important in other branches of physics.
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Notes
- 1.
We use the terms “matter” and “stuff” interchangeably.
- 2.
For example, we can think of the “observer at rest relative to the particle” as a scientist measuring the mass of the particle in a laboratory.
- 3.
Changing from one state of mater to another, for example from a solid state to the liquid state, is also known as a “phase transition”.
- 4.
The product of the uncertainty in the position and in the speed would have to be zero, if we could measure both to infinite precision. Experiments have shown this to be wrong.
- 5.
The reactions are, for example, neutron ↔ proton+electron+neutrino, while it is hot in both directions, when it is cooler only from left to right.
- 6.
The neutron decays into a proton, electron and an anti-neutrino. The anti-neutrino is the antiparticle of the neutrino.
- 7.
The universe would be very different if the bound neutrons would also decay, as only protons and electrons would be left to form atoms from, and only hydrogen atoms would exist.
- 8.
The Cosmic Microwave Background has today a temperature of 2.73 K and is present or “shines” everywhere in the universe, keeping normal matter above absolute zero even in the absence of stars.
- 9.
Fritz Zwicky (1898–1974), a Swiss physicist and astronomer, worked mainly at Caltech, introduced “dark matter” to cosmology.
- 10.
The Coma cluster is a large cluster of galaxies, located about 100 Mpc or 300 million lightyears from Earth, containing roughly 1000 galaxies. It is similar, but larger, to the local group of galaxies, see Fig. 4.10.
- 11.
Vera Rubin (1928–2016), US astronomer, her work on spiral galaxy rotation curve established dark matter.
- 12.
The acronyms “WIMP” and “MACHO” are these days firmly established in cosmology, reflecting the rather special sense of humour cosmologists possess.
- 13.
The value for the cosmological constant found taking some quantum field theory calculation at face value is roughly 120 orders of magnitude too large, that’s “12” followed by 118 zeroes, a very large number even by cosmology’s standards!
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Malik, K.A., Matravers, D.R. (2019). What is the Universe Made Of?. In: How Cosmologists Explain the Universe to Friends and Family. Astronomers' Universe. Springer, Cham. https://doi.org/10.1007/978-3-030-32734-7_5
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DOI: https://doi.org/10.1007/978-3-030-32734-7_5
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