Olber's Paradox and the Distant Universe

Robert NemiroffMichigan 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 requiredo Wikipedia, web links, and lectures onlyo 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 Principleo Universe Homogeneous & Isotropic

• Homogeneouso Smooth when averaged outo Example: jello, even fruity jello

• Isotropico Same in every directiono 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

brightnesso 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

• 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 MapCredit: COBE Project, DMR, NASA APOD: 2006 October 7

Our Dusty UniverseCredit: DIRBE Team, COBE, NASA APOD: 2000 November 19

The Cosmic Infrared BackgroundCredit: A. Kashlinsky (SSAI) & S. Odenwald (Raytheon), 2MASS, NSF, NASA APOD: 2002 February 6

ROSAT Explores The X-Ray SkyCredit: S. Digel and S. Snowden (USRA/ LHEA/ GSFC), ROSAT Project, MPE, NASA APOD: 2000 August 19