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Cosmology I

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Cosmology I Definition of Cosmology: The scientific study of the universe as a whole; how long ago it came into being, the nature of that beginning, the future ... – PowerPoint PPT presentation

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Title: Cosmology I


1
Cosmology I
  • Definition of Cosmology The scientific study of
    the universe as a whole how long ago it came
    into being, the nature of that beginning, the
    future destiny of the universe, and the physical
    laws that govern it.
  • Timeliness One of the main fields of physics
    and astronomy nowadays. Status now is
    drastically different than 10-15 years ago.

2
How we can talk about cosmology.
  • Large look-back times show the universe when it
    was different

3
The scientific basis of modern cosmology
  • Hubbles Law
  • V H0d
  • The universe is expanding

4
A Model Universe
Demo
5
With this toy model, you can show
  • Hubbles Law, vH0d
  • There is no center to the expansion, if you see
    Hubbles Law, there is still nothing special
    about your location
  • The physical significance of Hubbles Constant
    the time since the expansion began, thus the age
    of the universe

6
If the universe is expanding now, what will it do
in the future?
7
If cosmology is to be a branch of physical
science, there must be an underlying
mathematical structure with quantitatively
testable predictions
  • There are two things going on in the universe
  • Galaxies are flying apart due to the universal
    expansion
  • The force of gravity is acting to pull them back
    again
  • The story of the universe is competition between
    universal expansion and gravity

8
How to describe gravity General Relativity
  • Dynamics takes place in a four dimensional
    spacetime
  • Mass induces warping or curvature of spacetime
  • Spacetime curvature may also exist in the absence
    of mass (cosmological constant)

9
A modern description of the evolution of the
universe
  • The basic mathematical language that we use in
    describing the universe
  • What are the main observational results that have
    been obtained?
  • Will follow Chapter 26

10
Section 26.1, the Hubble Constant and the Age of
the Universesee lecture last Friday
11
Question 1 how can we describe the whole
universe by a simple (to physicists) equation?
Say it with equations!
12
A physical theory of the universe
  • Start with Einstein field equations (too tough)
  • Assume universe approximated by smoothed-out
    paste, characterized by its mean density
  • Assume universe is homogeneous
  • Assume universe is isotropic

Cosmological Principle
Such a mathematical model is called a Friedmann
Universe
13
(No Transcript)
14
The theory of the universe says space is curved
on large scales. What does this mean?
  • I think it is simpler to interpret this statement
    mathematicallygeometry is non-Euclidean.
  • Example of non-Euclidean geometry theorem of
    Pythagoras is not true.
  • Curved surfaces in 3D space have non-Euclidean
    geometry

15
Example1 space with positive curvature the
surface of a sphere
You would have flunked Sophomore geometry
16
Interesting historical aside
  • Karl Friedrich Gauss thought of this, and sent
    out surveyors to test if the geometry in Hanover
    really was Euclidean

17
The case of zero curvature Euclidean space
18
Final case that of negative curvature the
surface of a saddle
19
Within the context of General Relativity, all
three cases of curvature (positive, negative,
zero) are theoretical possibilities.All three
possibilities give universes which expand with
time The question is what kind of universe do
we live in?
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