Abstract
When we look up into the sky at night we see—not a lot if we live in a city, maybe the moon and some bright stars plus a planet or two. However, if we leave the bright city and its street lights behind, the picture changes dramatically. We see thousands of stars, and after watching a while we can discern patterns: we might notice that the stars are not evenly distributed. We might see that there is a band of stars right across the sky, this is our galaxy. If we have binoculars to aid our star gazing, we might notice that some of the dots we thought were stars are indeed tiny blobs, little “nebulae” or galaxies like our own. Using more sophisticated and bigger telescopes reveals even more structure in the distribution of galaxies on the largest visible scales. We are still learning more about the universe, as astronomers, with more sophisticated equipment, continue to make new observations at wavelengths beyond those that are visible to the naked eye, for instance observing in the radio and microwave wavelengths.
The world is everything that is the case.
Wittgenstein
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Notes
- 1.
The “standard model of particle physics” is the theoretical model that explains the properties and interactions of the known subatomic particles, in excellent agreement with the experimental evidence, and therefore agreed upon by the scientific community.
- 2.
Some authors use “time zero” or “Big Bang” to denote the beginning of the universe, but we find both terms not very useful since they might lead to confusion. We therefore shall simply talk about “the beginning” when we mean the beginning of the universe.
- 3.
See Appendix A.1 for an explanation of small and large numbers.
- 4.
Throughout this book we try to avoid not only scientific, technical jargon, but also the use of acronyms where possible.
- 5.
By “radiation” we mean electromagnetic waves, and the particle associated with electromagnetic waves is the photon. When referring to “matter” we usually have in mind both normal matter (the particles that form atoms), and the exotic dark matter (which makes its presence felt only gravitationally). We will discuss electromagnetic radiation in more detail in Sect. 3.2.1, the constituents of the universe in Chap. 5, and in particular dark matter in Sect. 5.4.
- 6.
We use “energy density” synonymously with the probably more familiar term “mass density”, which denotes the amount of material per volume. This is not very rigorous, but common practice in cosmology, as energy and mass are closely related, as we will discuss in Sect. 6.4.2.1.
- 7.
A plasma is one of the fundamental states of matter: the atoms in a gas lose some of their electrons (usually, at standard conditions, all electrons in an atom are bound to the nucleus). We will discuss this in Sect. 5.1.2.
- 8.
The expansion only affects the largest scales, as we will discuss in Sect. 6.4.2.4, the temperature we referred to is that of the Cosmic Microwave radiation.
- 9.
- 10.
A “lightyear” is the distance travelled by light in 1 year, a “parsec” corresponds roughly to a distance of 3 lightyears. The distance units used in astronomy are discussed in Sect. 4.1.
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Malik, K.A., Matravers, D.R. (2019). Introduction. In: How Cosmologists Explain the Universe to Friends and Family. Astronomers' Universe. Springer, Cham. https://doi.org/10.1007/978-3-030-32734-7_1
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DOI: https://doi.org/10.1007/978-3-030-32734-7_1
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