My views on Cosmology and Physics
site navigation menu
Book by David Michalets
This is section 10 of 18.
The web page series for Distant Spectral Shifts is based on my book Cosmology Crisis Cleared.
Astronomers have several methods for determining the distance to a galaxy. This begins the topic. Several subsequent sections cover specific methods.
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs) to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.
Almost all astronomical objects used as physical distance indicators belong to a class that has a known brightness. By comparing this known luminosity to an object's observed brightness, the distance to the object can be computed using the inverse-square law. These objects of known brightness are termed standard candles, coined by Henrietta Swan Leavitt.
Two problems exist for any class of standard candle. The principal one is calibration, that is the determination of exactly what the absolute magnitude of the candle is. This includes defining the class well enough that members can be recognized, and finding enough members of that class with well-known distances to allow their true absolute magnitude to be determined with enough accuracy. The second problem lies in recognizing members of the class, and not mistakenly using a standard candle calibration on an object which does not belong to the class. At extreme distances, which is where one most wishes to use a distance indicator, this recognition problem can be quite serious.
A significant issue with standard candles is the recurring question of how standard they are. For example, all observations seem to indicate that Type Ia supernovae that are of known distance have the same brightness (corrected by the shape of the light curve). The basis for this closeness in brightness is discussed below; however, the possibility exists that the distant Type Ia supernovae have different properties than nearby Type Ia supernovae. The use of Type Ia supernovae is crucial in determining the correct cosmological model. If indeed the properties of Type Ia supernovae are different at large distances, i.e. if the extrapolation of their calibration to arbitrary distances is not valid, ignoring this variation can dangerously bias the reconstruction of the cosmological parameters, in particular the reconstruction of the matter density parameter.
That this is not merely a philosophical issue can be seen from the history of distance measurements using Cepheid variables.
In the 1950s, Walter Baade discovered that the nearby Cepheid variables used to calibrate the standard candle were of a different type than the ones used to measure distances to nearby galaxies. The nearby Cepheid variables were population I stars with much higher metal content than the distant population II stars.
As a result, the population II stars were actually much brighter than believed, and when corrected, this had the effect of doubling the distances to the globular clusters, the nearby galaxies, and the diameter of the Milky Way. [Reference:
The problems with assigning stars to 1 of the 3 populations were explained in section Stars.
Walter Baade's discovery is crucial. Hubble's Law was based on relating distances obtained from Cepheids with the measured redshift values of galaxies. The method of measuring each value is important.
The method of measuring the redshift of each galaxy, with or without a Cepheid, is very important.
This book has a section Galaxies with Cepheids.
Several other methods can be immediately discarded.
One is the "standard siren" which is based on a claimed sound extracted from gravitational waves. My book, Predicting Gravitational Wave Detections, described how to predict GW detections by LIGO. This prediction reveals LIGO's deception of declaring a GW detection during specific lunar and solar events. LIGO is an abomination for proper science.
There is no such thing as a gravitational wave, so there can be no standard siren based on that mistake.
A second questionable method uses a supernova.
The often-cited study which claimed to find consistent supernovae, enabling them to be a candidate for a standard candle, was riddled with errors. The most important mistake is the study used variable stars, not supernovae.
The study included a chart of their consistent light curves which never had an extreme brightening which should be a requirement for a claimed supernova. The mistake was confirmed within the paper by including one example of the star's spectrum and its change during the event. The change was consistent with that expected in a Cepheid-type variable star. The dimming of the claimed supernovae was also consistent with a Cepheid.
A supernova is expected to shed mass during the explosive event. That significant change makes it rather unlikely for these extreme events to follow the same luminosity curve. A Cepheid's slow brightening in the course of its light curve spanning a few days is not like a supernova, which abruptly increases in brightness by many magnitudes.
Each supernova is a unique event and is not a candidate for a standard candle.
The Cosmic Distance Ladder continues:
The following four indicators all use stars in the old stellar populations (Population II):
Tip of the red-giant branch (TRGB) distance indicator.
Planetary nebula luminosity function (PNLF)
Globular cluster luminosity function (GCLF)
Surface brightness fluctuation (SBF)
Each of the 4 will be addressed in section NED Distances.
Go to Table of Contents, to read a specific section.
;ast update: 01/08/2022