Images by Date
Images by Category
Solar System
Stars
White Dwarfs
Supernovas
Neutron Stars
Black Holes
Milky Way Galaxy
Normal Galaxies
Quasars
Galaxy Clusters
Cosmology/Deep Field
Miscellaneous
Images by Interest
Chandra for Kids
Multiwavelength
Sky Map
Constellations
3D Wall
Photo Blog
Top Rated Images
Image Handouts
Desktops
High Res Prints
Fits Files
Image Tutorials
Photo Album Tutorial
False Color
Cosmic Distance
Look-Back Time
Scale & Distance
Angular Measurement
Images & Processing
AVM/Metadata
Getting Hard Copies
Image Use Policy
Web Shortcuts
Chandra Blog
RSS Feed
Chandra Mobile
Chronicle
Email Newsletter
News & Noteworthy
Image Use Policy
Questions & Answers
Glossary of Terms
Download Guide
Get Adobe Reader
f(R) Gravity

One possible way to explain the observed acceleration in the expansion of the universe is to change Einstein's theory of General Relativity. The simplest modification is to introduce a cosmological constant, which can be explained by energy that exists in the vacuum. In f(R) gravity and other modified gravity models, scientists go beyond this simple modification. In the f(R) gravity model, spacetime reacts differently to the matter in the universe than it does in General Relativity.

In General Relativity, gravity is a manifestation of the curvature of space and time, where the source of this curvature are all of the forms of mass and energy in the universe. In the absence of any mass or energy spacetime can become completely flat. What f(R) gravity does is allow spacetime to act as a source of its own curvature, so there can still be some curvature even if spacetime is completely empty and the energy is zero. So, as the universe expands and empties out, some curvature remains, resulting in cosmic acceleration.

By making this modification to gravity an additional ("5th") force is introduced. By comparing observations of the masses of galaxy clusters with the predictions of f(R) gravity, the range of this 5th force can be estimated. On distance scales smaller than this range, gravity is stronger than predicted by Einstein's equations. The smaller that this range is, the less effect that this modification to gravity has on the growth of galaxy clusters.

If the cosmological constant is the explanation for cosmic acceleration then the acceleration will continue forever and all galaxies outside the Local Group should eventually disappear from view, resulting in a lonely universe. If f(R) gravity applies then the 5th force will die away in the far future, cosmic expansion will slowly decelerate and a lonely universe will be avoided. It will be many billions of years before either one of these future scenarios can play out.



Return to Abell 3376 (April 14, 2010)