COSMOLOGY

COSMOLOGY
The Study of the Universe

INTRODUCTION
 Cosmology is the scientific study of the large scale properties of the universe
as a whole. It endeavors to use the scientific method to understand the origin,
evolution and ultimate fate of the entire Universe.
 Like any field of science, cosmology involves the formation of
theories or hypotheses about the universe which make specific
predictions for phenomena that can be tested with observations.
 Depending on the outcome of the observations, the theories will
need to be abandoned, revised or extended to accommodate the
data.
 The prevailing theory about the origin and evolution of our
Universe is the so-called Big Bang theory.
COSMOLOGICAL CONCEPTS
The main concepts of the Big Bang theory are introduced in the
first section with scant regard to actual observations.
The second section discusses the classic tests of the Big Bang theory
that make it so compelling as the most likely valid and accurate
description of our universe.
The third section discusses observations that highlight limitations of
the Big Bang theory and point to a more detailed model of cosmology
than the Big Bang theory alone provides. As discussed in the first
section, the Big Bang theory predicts a range of possibilities for the
structure and evolution of the universe.
The final section discusses what constraints we can place on the
nature of our universe based on current data, and indicates how
WMAP furthers our understanding of cosmology.
In addition, a few related topics are discussed based on commonly
asked questions.
Big Bang Cosmology
 The Big Bang Model is a broadly

accepted theory for the origin and
evolution of our universe. It postulates
that 12 to 14 billion years ago, the
portion of the universe we can see today
was only a few millimeters across.
 It has since expanded from this hot dense
state into the vast and much cooler cosmos
we currently inhabit.
 We can see remnants of this hot dense
matter as the now very cold cosmic
microwave background radiation which
still pervades the universe and is visible to
microwave detectors as a uniform glow
across the entire sky.
Foundations of Big Bang Cosmology
 The Big Bang model of cosmology rests on

two key ideas that date back to the early 20th
century: General Relativity and the
Cosmological Principle.
 By assuming that the matter in the universe is
distributed uniformly on the largest scales, one
can use General Relativity to compute the
corresponding gravitational effects of that
matter. Since gravity is a property of space-
time in General Relativity, this is equivalent
to computing the dynamics of space-time itself.
 A key limitation of the picture shown here is
that we can only portray the curvature of a 2-
dimensional plane of an actual 3-dimensional
space! Note that in a closed universe you could
start a journey off in one direction and, if
allowed enough time, ultimately return to your
starting point; in an infinite universe, you
would never return.
Foundations of Big Bang Cosmology
 Given the assumption that the matter in

the universe is homogeneous and isotropic
(The Cosmological Principle) it can be
shown that the corresponding distortion
of space-time (due to the gravitational
effects of this matter) can only have one
of three forms, as shown schematically in
the picture at left.
 It can be "positively" curved like the
surface of a ball and finite in extent; it
can be "negatively" curved like a saddle
and infinite in extent; or it can be "flat"
and infinite in extent - our "ordinary"
conception of space.
GEOMETRY OF THE UNIVERSE
Matter plays a central role in cosmology. It turns out that the
average density of matter uniquely determines the geometry of the
universe (up to the limitations noted above). If the density of
matter is less than the so-called critical density, the universe is
open and infinite.
If the density is greater than the critical density the universe is
closed and finite.
If the density just equals the critical density, the universe is
flat, but still presumably infinite. The value of the critical density
is very small: it corresponds to roughly 6 hydrogen atoms per cubic
meter, an astonishingly good vacuum by terrestrial standards! One
of the key scientific questions in cosmology today is: what is the
average density of matter in our universe?
While the answer is not yet known for certain, it appears to be
tantalizingly close to the critical density.

COSMOLOGY

Uploaded by

Copyright:

Available Formats

COSMOLOGY

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

COSMOLOGY

Uploaded by

Copyright:

Available Formats

COSMOLOGY

The Study of the Universe

 The Big Bang Model is a broadly

 The Big Bang model of cosmology rests on

 Given the assumption that the matter in

You might also like