What does this phrase mean? [about "energy density fluctuations"]

[Baby Astronaut]

"The spectrum of energy density fluctuations is scale-invariant (the same amplitude on all scales)."

 

Its context is below and at this link.

 

Quantum effects cause the incoming three-dimensional world to ripple along the extra-dimension prior to collision so that the collision occurs in some places at slightly different times than others. By the time the collision is complete, the rippling leads to small variations in temperature, which seed temperature fluctuations in the microwave background and the formation of galaxies. We have shown that the spectrum of energy density fluctuations is scale-invariant (the same amplitude on all scales). The production of a scale-invariant spectrum from hyper-expansion was one of the great triumphs of inflationary theory, and here we have repeated the feat using completely different physics.

[Martin]

"The spectrum of energy density fluctuations is scale-invariant (the same amplitude on all scales)."

 

Its context is below and at this link.

 

thanks for providing the context. Paul Steinhardt is world class. Here is a recent paper by him

http://arxiv.org/abs/0811.1614 (came out just a month ago, November this year)

 

His ekpyrotic or cyclic model based on colliding branes has not acquired much support. But it was a good try to find an alternative to inflation. Didn't really catch on, or hasn't yet anyway. That article you quote was from 2001 when ekpyrotic looked more hopeful.

 

In his latest, it looks to me like Steinhardt is going along with inflation (instead of continuing to work on an alternative way of explaining early universe features).

 

======================

About your question, what they are talking about is the temperature map of the CMB. There is as much amplitude of variability in the small angle speckles as there is in the large angle blobs. They've studied it in detail with an idea of fluctuation power, whether it depends on scale, or angular size of the blobs. They found that it does not depend on scale!

 

This requires an explanation. The map might have shown a preference for large blobs, with most of the variation being in strong large blotches and only very faint little speckling. Or it might have been biased the other way, and favored small speckles, being fairly unblotchy at large scales but intensely stippled like goosepimples at small scale. But no, no particular scale is favored. So why? How did that happen?

 

Inflation provided a possible scenario for that to occur. Steinhardt came up with an alternative story, which also seemed to offer an explanation---the rippling of the branes during collision. Don't know how really plausible, but its a try.

Edited December 13, 2008 by Martin