Book contents
- Frontmatter
- Contents
- Preface
- Background: what you need to know before you start
- 1 Gravity on Earth:
- 2 And then came Newton
- 3 Satellites
- 4 The Solar System
- 5 Tides and tidal forces
- 6 Interplanetary travel
- 7 Atmospheres
- 8 Gravity in the Sun
- 9 Reaching for the stars
- 10 The colors of stars
- 11 Stars at work
- 12 Birth to death
- 13 Binary stars
- 14 Galaxies
- 15 Physics at speed
- 16 Relating to Einstein
- 17 Spacetime geometry
- 18 Einstein's gravity
- 19 Einstein's recipe
- 20 Neutron stars
- 21 Black holes
- 22 Gravitational waves
- 23 Gravitational lenses
- 24 Cosmology
- 25 The Big Bang
- 26 Einstein's Universe
- 27 Ask the Universe
- Appendix: values of useful constants
- Glossary
- Index
26 - Einstein's Universe
The geometry of cosmology
Published online by Cambridge University Press: 05 June 2012
- Frontmatter
- Contents
- Preface
- Background: what you need to know before you start
- 1 Gravity on Earth:
- 2 And then came Newton
- 3 Satellites
- 4 The Solar System
- 5 Tides and tidal forces
- 6 Interplanetary travel
- 7 Atmospheres
- 8 Gravity in the Sun
- 9 Reaching for the stars
- 10 The colors of stars
- 11 Stars at work
- 12 Birth to death
- 13 Binary stars
- 14 Galaxies
- 15 Physics at speed
- 16 Relating to Einstein
- 17 Spacetime geometry
- 18 Einstein's gravity
- 19 Einstein's recipe
- 20 Neutron stars
- 21 Black holes
- 22 Gravitational waves
- 23 Gravitational lenses
- 24 Cosmology
- 25 The Big Bang
- 26 Einstein's Universe
- 27 Ask the Universe
- Appendix: values of useful constants
- Glossary
- Index
Summary
In the last two chapters we have made a lot of progress in exploring the future and past of the Universe, basically just by using local Newtonian gravity. We argued that the dynamics of an expanding, homogeneous and isotropic cosmology can be calculated from Newtonian gravity, at least if the pressure in the Universe is negligible, because all we need to look at is the local Universe, the part nearest us. The assumption that the Universe is homogeneous guarantees that the rest of the Universe will behave the same as our local region.
In this chapter: we explore the three different geometries that a homogeneous and isotropic cosmology can assume. We see how to construct two-dimensional versions of these, which shows us why there are only three possibilities. We see how astronomical observations can measure this geometry directly.
▷ The drawing under the text on this page illustrates how complicated three-dimensional solid objects could be. Why is the Universe apparently so simple?
But this line of reasoning has its limitations. Even if we calculate the dynamics of the Universe this way, we don't learn what the distant parts of the Universe will look like in our telescopes. The curvature of space, which is not part of a Newtonian discussion, will affect the paths of photons as they move through the Universe. Moreover, if we want to ask deeper questions about the Universe, such as those we pose in the next chapter, then we should know something more about its the larger-scale structure.
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- Chapter
- Information
- Gravity from the Ground UpAn Introductory Guide to Gravity and General Relativity, pp. 383 - 390Publisher: Cambridge University PressPrint publication year: 2003