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The Question

(Submitted July 11, 1997)

I recently read a book which gave reference to the Loh and Spillars test which can be used to find a value of the Hubble Constant. Unfortunately, the book did not give many details of this method nor give any findings for this test. Do you happen to know anything about the Loh & Spillar test? Also do you know of any results this test has provided?

The Answer

The method of Loh and Spillar uses the photometric redshifts of nearly 1000 galaxies to determine cosmological parameters. A photometric redshift is measured by observing the flux of a galaxy through different filters, and combining this information to determine the Spectral Energy Distribution (SED) of the galaxy. This is different than the typical method of determining redshifts, which uses the shift of narrow spectral features. The photometric method concentrates on the broad features and overall shape of the spectrum. It is easier to obtain the photometric data than the spectroscopic data for dim galaxies. Using imaging detectors helps even more when multiple galaxies can be measured simultaneously.

The results of their test indicate that the Universe is "flat", one of three possible overall geometries, and the one that results in the Universe decelerating to zero expansion rate as time approaches infinity.

You will find a lot of details about the method at the web page:

http://astrowww.phys.uvic.ca/grads/gwyn/pz/index.html

Regards,

Padi Boyd
for the Ask an Astrophysicist Team

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