Lecture 37 – 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 5-10 years ago.

How we can talk about cosmology….

Large look-back times show the universe when it was different

The scientific basis of modern cosmology

Hubble’s Law

V = H0d

The universe is expanding

A Model Universe

With this “toy” model, you can show:

• Hubble’s Law, v=H0d

• There is no center to the expansion, if you see Hubble’s Law, there is still nothing special about your location

• The physical significance of Hubble’s Constant: the time since the expansion began, thus the age of the universe

The significance of Hubble’s Constant

• H0 has units of km/sec/Mpc

• Units are really 1/sec

• If you put km and Mpc in the same units,

• H0 = 2.26E-18 1/sec

• So 1/H0 = 4.43E+17 sec = 1.4E+10 years

• This is our first estimate of the age of the universe

What time does the Hubble Constant represent?

If the universe is expanding now, what will it do in the future?

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

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)

A sermon on General RelativityIn some contexts, spacetime curvature becomes

pronounced, and space and time become inextricably linked or “coupled”. One example is black holes. Another is the universe as a whole. When we look on cosmological scales c/H0, you cannot view the universe as three dimensional,

with time as a parameter. The universe exists as a 4D entity with strong curvature effects. You

can’t even qualitatively understand what is going on if you stick to a 3D view

An analogy

The surface of the Earth is a 2D surface embedded in a 3D space. Locally, the surface of the Earth looks purely 2D. On scales of the

radius of the Earth, the curvature is unavoidable.