The Astrophysics Spectator

Issue 1.7, November 17, 2004

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November 17, 2004

We turn our eye to cosmology with this issue of The Astrophysics Spectator. Cosmology is the one topic in astrophysics that has a philosophical pull. Cosmology is the study of the universe; the touchstone of cosmology for most people is that our universe appears to have a definite beginning.

The theory behind cosmology is actually quite simple in the epochs that are observable. The rate of expansion of the universe can be derived from Newtonian mechanics. The basic forces acting on the universe are pressure and gravity, with gravity the dominant force in the current epoch. The one hypothetical piece of physics that is sometimes invoked is the gravity associated with the vacuum, an effect that is represented by a variable called the cosmological constant.

The astronomy of cosmology, unlike the theory, is quite difficult, because our tools for determining the distance of a galaxy are difficult to use. With great effort, the type 1a supernova has been developed as a distance measure and has been used to test the theory of cosmic expansion. These results have produced surprises.

In our commentary for this week, we discuss the invocation of new physics to explain poorly-understood observations.

Jim Brainerd


Swift. The Swift satellite is set to launch on November 17, 2004 from Cape Canaveral. This satellite is designed specifically to detect and localize gamma-ray bursts. (continue)

More news.


Invoking New Physics. Sometimes we see in astronomy processes that only happen outside of the laboratory. But the complexity of astronomical phenomena also creates effects from known physics that cannot be modeled with current computer programs, and there is always the chance that a systematic error or a poorly understood bias will mislead us. So when does an astronomical observation justify the invocation of a new physical theory? (continue)

More commentaries.

New Background

Cosmic Expansion. The more distant the galaxy, the more its light is shifted to the red. The simple interpretation of this observation is that the redshift is the Doppler shift of the radiation from a galaxy moving away from us. This is the origin of the theory that the galaxies are moving apart in an expanding universe. (continue)

Basic Theory of Cosmic Expansion. The theory for the expansion of the universe at the current epoch is quite simple: the galaxies are moving away from us with a velocity that is affected only by gravitational forces. In the most conservative theory, only matter experts a gravitational force, which is a deceleration, but recent observations have forced theorists to consider a repulsive gravitational force from the vacuum that is just now in our current epoch affecting the expansion of the universe. (continue)

Current Research

Cosmology with Type 1a Supernovae. The best analysis of the expansion of the universe at high redshift is from the study of type 1a supernovae. This analysis of supernovae at redshifts between 0.01 and 1.2 finds that the supernovae at high redshift are dimmer than expected in the matter-dominated theory of cosmological expansion. Does this imply that the vacuum gravitates, or is this a consequence of a systematic error or of poorly-understood physics? (continue)


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