Lecture 10 Cosmological Concepts

ASTR 340

Fall 2006

Dennis Papadopoulos

The Universe

• The Physical Universe– Material objects, energy, space, time, forces

Are space and time physical ? What is space, what time?

Space is what keeps everything from happening at the same location

Time is what prevents everything from happening at once

Any physical model should include and explain space and time along with every other physical phenomenon. We cannot invoke a preexisting space and time into which we construct the universe. We cannot ask “what happened before the universe existed” or “what is outside the universe”. Space and time are not properties distinct from the universe. Time did not exist before the universe and space does not exist outside it. The universe is not expanding into space or spacetime.

Scientific Cosmology

• What is the universe now? How did it reach this state?• If we trace the evolution of the universe backward in time we can

ask whether or not there was a t=0 point.• If there was we must arrive at the question of initial conditions, how

things were at the earliest moment we can consider.• Cosmology aims at describing these initial conditions and describing

how it evolved from them• Initial conditions must simple (Occam’s razor); e.g. big bang -> a

certain amount of energy and matter, certain physical laws and certain fundamental constants.

• The complexity we observe existed as a potentiality and developed naturally in the subsequent evolution

Fundamental constants of nature

• G, c, h, e, me, mp

• Values of fundamental constants and basic laws of physics determine what is possible in the universe

• If any of these changed even slightly the universe would be different than the one we observe

• e.g. if nuclear reactions were not possible at the densities and temperatures prevailing at the cores of gravitationally bound gaseous nebulae would they be stars? Life?

The Cosmological Principle

• Universe -> sum of all particles and energy, physical laws and spacetime

• Humans do not occupy a privileged position in universe

• Copernican principle ->Earth and solar system do not occupy special place in universe– Does not claim that there is no center– Is there a center somewhere?– Humans as observers

• Cosmological principle ->

Cosmological Principle (cont)

Isotropy and Homogeneity

• Homogeneous -> we see no difference when we change position; there is no preferred position in the universe (translational invariance)

• Isotropic -> no difference when we look at a different direction

• Examples: Surface of uniform cylinder is homogeneous but not isotropic- what about the surface of a sphere – or chessboard ?

• Cosmological Principle (CP)-> universe is homogeneous and isotropic (at a given cosmological time)

CP

• Cosmological principle means that physical laws are assumed to be the same everywhere, too

• The cosmological principle of isotropy and homogeneity, like other scientific hypotheses, is testable by confrontation with data.

• So far, observations support this hypothesis

Tests

Galaxies arranged in superclusters that appear as long sheets surrounded by voids

Cosmological Principle Tested

The Perfect Cosmological Principle

The role of the observer

• Anthropic principle – weak and strong– We don’t know why the constants have these values

or if it is possible to have other values but out of all possible universes ours is special because we exist

• Weak Anthropic Principle (WAP): The conditions we observe in the universe (constants, laws, etc) must be compatible with our existence.

Weak Anthropic Principle (WAP)

Against WAP

Other Anthropic PrinciplesNon-Defendable

Cosmological Principles