Starting in 1912 the American astronomer Vesto Slipher discovered that the spectra of “spiral nebulae” (now called galaxies) displayed large redshifts. This was different from the spectra of stars, which showed an even mix of red and blue shifts. By 1917 he had measured the Doppler shifts of 25 spiral nebulae, with only three displaying blue shifts. He concluded, rightly, that the spiral nebulae were outside of the Milky Way and moving away at great speed. Slipher had laid the groundwork for soon-coming important discoveries in astronomy.

A Puzzling Contradiction

In the late 1920s, Georges Lemaître and Edwin Hubble discovered that the universe is expanding. By comparing the distances to galaxies with their redshifts (from Slipher), they determined the rate at which the universe is expanding, now known as the Hubble-Lemaître constant, H₀. However, the initial estimate led to a puzzling contradiction.

Assuming that the expansion of the universe has been slowing down since the Big Bang due to the attractive force of gravity, the first estimate of H₀ implied that the universe was only 1.5 billion years old. This was a problem because, even in 1920s, evidence from Earth’s geology suggested that our planet was much older than that.

Over time, more precise measurements of H₀ reduced this discrepancy, but a tension remained between the “Hubble age” of the universe and the age of individual objects within our galaxy, particularly globular clusters.

Globular clusters are dense, spherical collections of hundreds of thousands to millions of stars that are believed to be among the oldest objects in our galaxy. They likely formed early on from the same gas cloud that later collapsed to form the Milky Way. The low abundance of heavy elements like iron in globular cluster stars, sometimes less than 1 percent of what’s found in the Sun, suggests they formed before significant star formation had occurred.

A Cosmological Constant

In the 1980s, age estimates for the oldest globular clusters ranged from 16 to 20 billion years, again conflicting with the Hubble age which was then estimated at 10 to 15 billion years. This discrepancy motivated scientists to reconsider the idea of a cosmological constant.

The cosmological constant, originally proposed by Albert Einstein, is an extra term in his equations that counteracts the attractive force of gravity, causing the expansion of the universe to accelerate. If the universe is accelerating, it would have been expanding more slowly in the past, meaning it would have taken galaxies longer to reach their current separations. This would make the universe older than the Hubble age calculated assuming a constant expansion rate.

However, as estimates of globular cluster ages were revised downward due to improved understanding of stellar evolution, the discrepancy with the cosmic expansion age diminished. At the same time, other cosmological evidence, including observations of the cosmic microwave background (the afterglow of the Big Bang), the distances to Type Ia supernovae, and the large-scale distribution of galaxies began to support the idea of a universe with a cosmological constant.

Over the last couple of decades measurements of H₀ have become ever more precise. This has uncovered a new tension. The “local” value, determined from the cosmological distance ladder using cepheid variables and supernovae, is about 5-sigma (5 standard deviations) from the value determined from the background radiation for the early universe. In other words, there is only one chance in 3.5 million that the two measurements really do agree. 

Four categories of explanation have been offered to solve this tension: systematic errors in the measurements, local anomalies, new physics beyond the standard Big Bang model with a cosmological constant, or early universe modifications. Measurement systematic errors seem less probable with every passing day. Recently, observations with the James Webb Space Telescope have confirmed the Hubble Space Telescope calibration of Type Ia supernovae with cepheids in nearby galaxies (see Live Science, “James Webb telescope confirms there is something seriously wrong with our understanding of the universe”).

Overthrowing the Big Bang?

I don’t pretend to know the solution to the H₀ constant tension. One thing I am fairly certain about: overthrowing the Big Bang theory is not in the offing. Note that there is an H₀ constant, and there is cosmic background radiation, both of which point to an earlier era when the universe was much smaller, denser, and hotter. Furthermore, the estimates of H₀ don’t really differ from each other that much in absolute terms: 73 km/s/Mpc for the local value and 67 km/s/Mpc for the early value (note: the units are read as “kilometers per second per megaparsec”).

You can also turn the problem around and ask how the ages of the oldest objects constrain the H₀ constant. Cimatti and Moresco (2023) calculate that the ages of the oldest stars constrain H₀ to be <73 km/s/Mpc. This is compatible with both estimated values of H₀.