11 Feb Astronomers Improve Type Ia Supernova Measurement Methods
By Hugh Ross | May 4, 2015
For centuries astronomers were convinced that the universe was infinitely large, structurally static with no beginning. For millennia, however, the Bible has implied that the universe has expanded, and continues to expand, from an actual beginning of space and time. Edwin Hubble upset the astronomers’ infinite universe model in 1929 with observations showing that galaxies are speeding away from one another in direct proportion to the distances separating them.1 The only possible explanation for Hubble’s observations was that the universe had expanded from a beginning dating back several billion years ago.
Hubble’s measurements demonstrated that cosmic expansion had occurred and for a time period between 1 and 30 billion years. Since then, a variety of different methods have produced increasingly accurate measurements of the cosmic expansion rate. The results test competing cosmic creation models.
In the re-release of my book A Matter of Days, I give a progress report on the latest precision measurements.2 Many atheists and, ironically, young-earth creationists assert that the biblically predicted big bang model is in trouble. But cosmic expansion rate measures have proven more and more consistent over the past six decades as measuring accuracy has improved.
Currently, the two most accurate measuring methods (that sample more than just the past billion years of cosmic history) are cosmic microwave background (CMB) and luminosity measurements. The first method measures how rapidly the angular sizes of the hot and cold spots in the CMB grow larger.
The second method uses luminosity measures of type Ia supernovae to determine their distances. To obtain the cosmic expansion rate, distances to the supernovae’s host galaxies must be complemented by measurements of the galaxies’ recession velocities (recession velocity = velocity at which a particular galaxy is moving away from Earth). Determining recession velocities is straightforward. Astronomers simply measure the degree by which a galaxy’s spectral lines are shifted toward the red end of the spectrum. Special relativity states that the faster a galaxy moves away from us relative to light’s velocity the greater the “redshift” of its spectral lines. Today, astronomers can determine a galaxy’s redshift and hence its recession velocity to an accuracy of 1 part in 100,000.
Accurate distance measures to type Ia supernovae out to great distances are possible because (1) type Ia supernovae are very bright (their maximum brightness exceeds that of 5 billion ordinary stars) and (2) all type Ia supernovae manifest the same intrinsic brightness. Stars require a certain minimum mass to go supernova. Type Ia supernovae are white dwarf binary stars where the more massive star gradually steals mass from its less massive partner. As soon as the accreting star attains the minimum supernova mass it experiences a supernova eruption. Since the maximum brightness of a supernova depends upon its mass, all type Ia supernovae at time of maximum light possess the same intrinsic brightness. Thus, astronomers need only accurately measure distances of a few nearby supernovae to gain accurate distance measures for all supernovae. (An object’s brightness is inversely proportional to the square of its distance. Thus, for two objects of equal brightness, one twice as far away will measure four times dimmer.)
Today, astronomers have successfully achieved high-precision distance measurements to several nearby supernovae through a combination of direct and indirect techniques based on plane geometry theorems (namely, if the length of the base of an isosceles triangle and the angles to the vertex of the triangle are known, then the distance to the vertex is also known; see figure).
Figure: Trignometric Parallax Method for Measuring Distances to Nearby Stars.
Because the diameter of Earth’s orbit is known (~186 million miles) and the angles of observation at two opposites points in Earth’s orbit to the nearby star relative to very distant stars, galaxies, or quasars can be measured, the star’s actual distance is also known. This distance determining method works just as well where astronomers use the orbit of a star or a maser source about the center of a galaxy and measure the angles back to Earth.
Diagram credit: Booyabazooka/Wikipedia
As noted in A Matter of Days, the two best methods for determining the cosmic expansion rate agree quite well with biblical descriptions of the cosmic history and, therefore, should greatly increase confidence in its inerrancy. In the time since I wrote the book, refinements made to the type Ia supernova method showed it to be slightly discordant with the CMB method. The best CMB determinations, based on Planck and WMAP satellite maps, showed the cosmic expansion rate to be 67.3 ± 1.23 and 69.32 ± 0.804 kilometers/second/megaparsec, respectively. (A megaparsec = 3.258 million light-years.) The recently refined type Ia supernova method gave a value of 72.5 ± 2.55kilometers/second/megaparsec for the cosmic expansion rate.
In March 2015, a team of 38 astronomers published their analysis of an overlooked systematic effect in the type Ia supernova method. They used data from the Galaxy Evolution Explorer space telescope (GALEX) project to determine star formation rates in the environments of type Ia supernovae. They showed that type Ia supernovae in galaxies where star formation is occurring in the vicinities of the supernovae are slightly dimmer at maximum light than type Ia supernovae in galaxies where no star formation occurs in the supernovae’s neighborhood. When the astronomers corrected for this effect, they found that the type Ia supernova method yielded a slightly smaller value for the cosmic expansion rate. Where they used three techniques (two indirect and one direct) to determine the distances of type Ia supernovae in nearby galaxies, they produced a value of 70.6 ± 2.6 kilometers/second/megaparsec for the cosmic expansion rate; when they used only the direct surveying technique based on the orbits of maser sources about the center of the galaxy NGC 4258, they obtained 68.8 ± 3.3 kilometers/second/megaparsec.
For cosmic model testing purposes the latter value is preferred because the calibration of the supernovae distances are free of any assumptions or possible systematic effects. Astronomers, too, will soon achieve much-improved direct geometric distant measures to NGC 4258 and other nearby galaxies where astronomers have observed type Ia supernova eruptions. As it is, the 38 astronomers have demonstrated that as measuring precision improves and as understanding of possible systematic effects grow, determinations of the cosmic expansion rate become more consistent. This improving consistency provides a progressively stronger case for cosmic creation, for the reliability of the Bible, for old-earth creationism, and for the Bible’s predictive power.
- Edwin Hubble, “A Relation Between Distance and Radial Velocity Among Extragalactic Nebulae,” Proceedings of the National Academy of Sciences, USA 15 (March 1929): 173.
- Hugh Ross, A Matter of Days, 2nd exp. ed. (Covina, CA: RTB Press, 2015), 146–50.
- P. A. R. Ade et al. (Planck Collaboration), “Planck 2013 Results. XVI. Cosmological Parameters,” Astronomy and Astrophysics 571 (November 2014): id. A16.
- G. Hinshaw et al., “Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results,” Astrophysical Journal Supplement Series 208 (October 2013): id. 19.
- George Efstathiou, “H0 Revisited,” Monthly Notices of the Royal Astronomical Society 440 (May 2014): 1138–52.
Published by permission of Reasons to Believe,