Breakthroughs in astronomy in the early part of the twentieth century were the first of many scientific confirmations of the biblical cosmic creation model. It was the discovery of cosmic expansion that first persuaded astronomers that the universe had a beginning. Detailed measurements of the expansion of the universe throughout its history led to the recognition that space and time were created and that a Causal Agent beyond space, time, matter, and energy created our universe of space, time, matter, and energy.1

Cosmic Expansion Rate Tension
Lately, however, tension has arisen between present-day and very distant (13.8 billion years ago) measurements of the cosmic expansion rate. Last year, a team led by Nobel laureate Adam Riess used Cepheid variable stars to calibrate the distances to local galaxies hosting type Ia supernovae. Based on this calibration, Riess’s team determined that the universe is expanding at a rate of 74.03 ± 1.42 kilometers per second per megaparsec.2 (1 megaparsec = 3.26 million light-years.)

This value differs, however, from the one based on a detailed map of cosmic microwave background radiation (CMBR), a map that reveals the state of the universe when it was only 380,000 years old. The CMBR-based cosmic expansion rate = 67.4 ± 0.5 kilometers per second per megaparsec.3

This difference between local (present-day) and very distant (13.8-billion-years-ago) determinations of the cosmic expansion rate = 6.6 kilometers per second per megaparsec. This difference amounts to a 4.4 standard deviation discrepancy. Such a large discrepancy implies that the difference is unlikely to be a statistical fluke. It is much more likely due to systematic uncertainties (instrumental effects and/or overlooked physical effects that push the measurements either above or below the true value) and/or unforeseen physics (overlooked physical phenomena).

Since the age of the universe is inversely correlated with the cosmic expansion rate, the local cosmic expansion rate measurements imply that the universe is 9 percent younger (1.2 billion years younger) than the 13.8 billion years implied from the CMBR-based cosmic expansion rate. Some young-earth creationist leaders have jumped on this discrepancy to claim “that there is something seriously wrong with the big bang model,”4 that “the big bang model is false,”5 and “Big Bang age estimates often contradict one anther.”6

Resolution of the Tension: Tip of the Red-Giant Branch Stars
About ten months ago, I wrote a blog demonstrating several ways that the tension between the CMBR-based and the Cepheid-supernovae-based measurements for the cosmic expansion rate could be resolved.7 Thanks to new sets of measurements, the means for resolution is now evident.

Astronomers have known for more than a decade that there was a second, independent method for calibrating the distances to type Ia supernovae, the basis for determining the cosmic expansion rate during the universe’s past half history. That method was to use “tip of the red-giant branch” stars.

The tip of the red-giant branch (TRGB) is an astronomical distance indicator. It uses the luminosity of the brightest red-giant branch star in a galaxy as a standard candle to determine the distance to that galaxy. In every galaxy, the brightest red-giant branch star manifests the same intrinsic brightness. Therefore, a measure of such a star’s apparent brightness yields the galaxy’s distance through the inverse square law (the luminosity of a light-emitting object decreases with the square of its distance), given that astronomers possess an accurate geometric distance to a nearby tip of the red-giant branch star.

A few months ago, a team of thirteen astronomers led by Wendy Freedman and Barry Madore (a classmate of mine at the University of Toronto), published the most comprehensive determination of the cosmic expansion rate based on TRGB.8 They used 20 eclipsing binary stars in the Large Magellanic Cloud (see figure 1) to determine, by far, the most accurate geometric distance to that galaxy and, hence, to its brightest red-giant branch star. That distance = 162.04 ± 0.30 million light-years.9

Figure 1: Large Magellanic Cloud, the Closest Galaxy to the Milky Way
Image credit: European Southern Observatory

Based on the new geometric distance to the Large Magellanic Cloud and their observations of TRGB in galaxies hosting type Ia supernovae, the Freedman-Madore team determined that the cosmic expansion rate over the past seven billion years = 69.8 ± 0.8 kilometers per second per megaparsec.10 This value is discordant with the CMBR-based cosmic expansion rate by only 2.4 kilometers per second per megaparsec.

In their paper, Freedman and Madore’s team presents a diagram showing the history of cosmic expansion rate measurements using the three methods described here: CMBR, Cepheid variable stars, and TRGB. Figure 2 below is their diagram.

Figure 2: Cosmic Expansion Rate Measurements over the Past Two Decades. The shaded portions show the probable measuring errors.
Image credit: Wendy Freedman et al., Astrophysical Journal

Whereas increasing measuring precision has resulted in greater tension between the CMBR and Cepheids determinations, the opposite trend has occurred for the CMBR and TRGB determinations. In fact, the difference between the CMBR and TRGB is now small enough to attribute to the probable measuring errors.

Resolution of the Tension: Dark Energy and the Local Hole
There are other likely reasons for the remaining difference between the CMBR and TRGB determinations. I mentioned one in my June 24, 2019, blog.11 Dark energy, the dominant component of the universe, will more strongly accelerate the cosmic expansion rate when the universe is old and its space surface is larger than when it is young and its space surface is smaller. The latest and best measurements of dark energy imply that the universe should be expanding about one percent faster (about 0.7 kilometers per second per megaparsec faster) now than when it did when the universe was only 380,000 years old.

Another likely reason has been the subject of several recent research papers.12 A local underdensity of galaxies and galaxy clusters would cause an increase in the local value of the cosmic expansion rate. A study based on the CLASSIX galaxy cluster survey revealed a 30 percent underdensity of matter in a local region with a diameter of 330–450 million light-years.13 Inside this underdense region, observations of the cosmic expansion rate will be about 4 kilometers per second per megaparsec greater than for the rest of the observable universe. Distance indicators outside this local underdense region will reduce this enhanced cosmic expansion rate but not to the point of making the enhancement trivial. The local underdense region probably enhances the TRGB determination by 1.0–1.6 kilometers per second per megaparsec.14

The combination of the new TRGB cosmic expansion rate determination, the impact of dark energy, the adjustment arising from the local underdensity, and the range of probable errors in all the measurements now fully resolves the tension in the cosmic expansion rate determinations. The biblically predicted big bang creation model is not in trouble; there is no cosmic age discrepancy. The latest observations demonstrate that the more we learn about the universe the more evidence we accumulate for what the Bible thousands of years ago taught about the origin and history of the universe.

Check out more from Reasons to Believe @Reasons.org