Question about accelerating expansion

[exmarine]

Question about accelerating expansion

Why do I see everything backwards? Here is a paragraph from Bernard Schutz’s excellent book on General Relativity, p. 352: (referring to the famous plot from the High-Z Supernova Search Team: Riess, et al, 1998)

The top diagram shows the flux (magnitude) measurement for each of the supernovae
in the sample, along with error bars. The trend seems to curve upwards, meaning that at
high redshifts the supernovae are dimmer than expected. This would happen if the universe
were speeding up, because the supernovae would simply be further away than expected.
Three possible fits are shown, and the best one has a large positive cosmological constant,
which we shall see below is the simplest way, within Einstein’s equations, that we can
accommodate acceleration. The lower diagram shows the same data but plotting only the
residuals from the fit to a flat universe. This shows more clearly how the data favor the
curve for the accelerating universe.

As high-Z objects are older, and more distant “than expected”, wouldn’t that have to mean that the younger objects nearby are not retreating as fast as they once were in the distant past?

I assume that the expectation is from a linear fit extrapolation of the younger nearby supernovae? I attach a Word/pdf of a simple integral that seems to confirm intuition.

 

[kimbyd]

As high-Z objects are older, and more distant “than expected”, wouldn’t that have to mean that the younger objects nearby are not retreating as fast as they once were in the distant past?

No. The accelerated expansion makes it so that all objects, regardless of distance, are receding faster than they would have we didn't have an accelerated expansion. It's just that that discrepancy is most visible at certain specific redshifts.

At redshifts that are too high, the dark energy was such a tiny fraction of the total energy density that it had negligible impact on expansion at that time. At redshifts that are too low, there hasn't been enough time for the very weak dark energy to have had a measurable impact.

I assume that the expectation is from a linear fit extrapolation of the younger nearby supernovae? I attach a Word/pdf of a simple integral that seems to confirm intuition.

It's substantially more complicated than that. The expectation is from an alternate model fit which doesn't include dark energy. That alternate model predicts a power-law relationship between supernova brightness and redshift, so that plotted on a graph of log(z) vs. magnitude (which is a logarithm of brightness), the graph appears linear. But a full physical model is used, rather than a simple linear regression.