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Olber's Paradox and
the Distant Universe
Robert Nemiroff
Michigan Tech U.
Physics X: About This Course
• Officially "Extraordinary Concepts in Physics"
• Being taught for credit at Michigan Tech
o Light on math, heavy on concepts
o Anyone anywhere is welcome
• No textbook required
o Wikipedia, web links, and lectures only
o Find all the lectures with Google at:
 "Starship Asterisk" then "Physics X"
o
http://bb.nightskylive.net/asterisk/viewforum.php?f=39
Considering Our Universe as a Whole
• The Cosmological Principle
o
Universe Homogeneous & Isotropic
• Homogeneous
o
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Smooth when averaged out
Example: jello, even fruity jello
• Isotropic
o
o
Same in every direction
Example: room with the lights out
Olber’s Paradox
• Why is the sky dark at night?
o
Why is it bright during the day?
• Assume the universe is infinite
• Assume stars all have the same surface
brightness
o
Surface brightness does not depend on distance
Olber’s Paradox
• Point in any direction
• That direction goes through empty space
but ends on a star
• Every direction should be as bright as the
surface of a star
• The sky should be bright at night
• What’s wrong with this picture?
http://hyperphysics.phy-astr.gsu.edu/Hbase/astro/imgast/olbers.gif
Dense Forest Analogy
•
•
•
•
Picture you are in a dense forest
Trees are found in every direction
Every direction you point ends on a tree
Therefore, every direction is tree-bark
brown
Olber’s Paradox: Possible Solutions
•
•
•
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Universe is finite in size
Universe is finite in age
Dust blocks out most light
Light gets too redshifted to see
• Which do you think is correct?
o
Take a minute to think about it!
Olber’s Paradox: Solution
• All of those have some affect BUT
• The finite age of the universe is the most
important factor
• Light just can’t get to us from distant stars
The Sky IS bright at night
• Background radiations in all energy bands
• The sky is never completely dark at any
wavelength
• Olber was correct after all!
COBE All-Sky Map
Credit: COBE Project, DMR, NASA
APOD: 2006 October 7
Our Dusty Universe
Credit: DIRBE Team, COBE, NASA
APOD: 2000 November 19
The Cosmic Infrared Background
Credit: A. Kashlinsky (SSAI) & S. Odenwald (Raytheon),
2MASS, NSF, NASA
APOD: 2002 February 6
ROSAT Explores The X-Ray Sky
Credit: S. Digel and S. Snowden (USRA/ LHEA/ GSFC),
ROSAT Project, MPE, NASA
APOD: 2000 August 19