Astronomers are divided over the precise rate at which the universe is expanding, a crucial measurement known as the Hubble constant. This debate, known as the ‘Hubble tension,’ has become a major controversy in the field of cosmology, with implications for our understanding of the universe’s evolution.
The Distance Ladder Dilemma
Measurements of the Hubble constant have traditionally been made using what astronomers refer to as the ‘distance ladder’ approach, which involves a series of steps to calibrate different types of stellar distance indicators in order to work out distances from our galaxy outwards. This technique has been employed since the beginning of the somewhat serendipitous discovery of the expansion of our universe — work that got its beginnings with astronomer V.M. Slipher in the early 20th century.
The ladder of distance depends on a series of distances so that no one rung of the ladder necessarily has an accurate calibration, but rather a stepwise inverting bootstrap running from near stars to distant supernovae. The difficulty is in properly weight the absolute distances of these objects — as the smallest error will be magnified, given its interest to Hubble constant measurements.
Problem with Cosmic Microwave Background
The other primary way to determine the Hubble constant is to use the ancient light traveling through space that astronomers can see today: that which was burned into existence just a few hundred thousand years after the Big Bang, known as cosmic microwave background (CMB) emission. Properties of these ancient photons can be used to indirectly test String Theory, predict how fast the universe was expanding in the very early universe, among other exacting tests.
The measurement of the CMB, however, [carried out by members of the Planck satellite team] gives a significantly lower Hubble constant than we get from using the distance ladder. This difference, referred to as the ‘Hubble tension,’ has led to great controversy and speculation that perhaps the foundation of our cosmological picture may need a radical overhaul.
Conclusion
The now-decade-old controversy over the Hubble constant is a stark illustration of how difficult it can be to tease out the detailed history of the universe being stretched by dark energy. Although new measurements and methods should help to get at the root of Hubble tension, the fate of our grasp on cosmic expansion lies in limbo. The controversy (above) is a testament to the fluid and progressing nature of scientific research, where even the most basic measurements can continue to stymie and intrigue the field.
