The Hubble-Lemaître law is now widely accepted as a description of how objects move in an expanding universe, with a velocity that’s proportional to their distance. However, that constant of proportionality—the H-naught in the equation, still referred to as the Hubble constant—has continued to be a source of intense study and fierce debate over the past century. Why?
Why Did the Hubble Constant’s Value Change?
The units of the Hubble constant can be best understood as speed per unit distance: how fast something is moving given how far away it is. When Edwin Hubble himself first published the equation, he estimated a numerical value of 500 for this constant, while Georges Lemaître estimated a value of about 625. However, both estimates wound up being wrong: for a simple reason.
Hubble and Lemaître’s results both predated Walter Baade’s discovery that Cepheid variables should, in fact, be split into two different classes, each obeying a different period-luminosity relation. Walter Baade’s revision doubled all of Hubble’s measured distances. So, while the neat linear correlation between velocity and distance remained, the implied value of the Hubble constant dropped dramatically as a result.
Today, the value of the Hubble constant is, well, it depends on who you ask. An immense amount of debate has been poured into this single number. The full story of the Hubble constant and its value spans decades and covers numerous scientific battles, discoveries, and disagreements.
In the late 1990s, astronomer Wendy Freedman used the Hubble Space Telescope to observe standard-candle supernovae out to record-breaking new distances. Freedman and her team arrived at an almost perfect middle-ground value for the Hubble constant, estimating a value of 72 plus or minus eight.
Why Has the Debate Not Been Settled Yet?
Freedman’s seemingly satisfactory value still hasn’t settled the debate. Different teams are still getting different answers even as the possible range of values narrows. This is due in part to an ever-growing array of techniques that can be used to measure the Hubble constant.
Today, standard candles used to measure distances for the Hubble constant include Cepheids, some subtypes of supernovae, and relationships between spiral galaxies’ luminosities and the rotation velocities of their spiral arms. Other methods for calculating distances exploit features like the geometry of gravitational lenses, where large masses distort spacetime so severely that the light from background objects appears bent.
Still others propose using gravitational waves—strange ripples in spacetime—as a new type of standard for estimating distances. Most of these measurements currently estimate that H-naught is around 72 to 75. An even more recent estimate from Wendy Freedman’s team, using observations of extragalactic star clusters, comes in a bit lower, at 69.
Other new techniques have led some astronomers to take a different approach to the Hubble constant. Rather than measuring distances of nearby galaxies, pushing successively further and further away from Earth, these teams use properties of the early universe—for example, the faint leftover electromagnetic signature of the Big Bang—and extrapolate from those first moments of the universe to the modern era. These measurements get a consistently lower answer of about 68.
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^8, 69, 72, 75—the differences may not seem like much. Still, the disagreements amount to fundamentally different pictures of how our universe works, with different values changing our interpretation of how old the universe is, how fast it’s expanding, and even what will happen to it one day.
Moreover, in the past few years, different techniques haven’t come any closer to a number that astronomers can agree upon—the early universe measurements are consistently lower, in the high 60s, while the standard candle measurements mostly wind up in the low 70s. The degrees of error on those numbers have also shrunk, and where there once used to be some overlap, there no longer is. Groups are getting noticeably different answers for what is arguably one of the most important numbers in astronomy.
For now, the debate rages on, with teams attempting ever more careful techniques to try and refine their measurement of Edwin Hubble’s famous constant. Modern teams of researchers have even taken to hiding their final numerical answers even from themselves, performing calculations blind and only revealing the final number once they’re satisfied that the work has been done as perfectly as possible to avoid any risk of bias. Even with these precautions, values derived from different techniques still don’t agree.
Is It Even a Constant?
Some scientists argue that these differences are just artifacts of different teams’ techniques, and that the answer will likely converge somewhere in the middle as our observational precision continues to improve. Others have taken to describing this ongoing debate as the ‘Hubble tension’.
If the different answers are truly all correct, this implies that the Hubble constant might not even be a constant at all. It’s possible that the number governing the expansion of the universe may have changed as a function of time, and that it may have been lower in the early days of the universe.
Today, the Hubble-Lemaître law remains a linchpin of modern astronomy. Research groups are still in hot pursuit of an accurate and unanimous value for the Hubble constant, and even a resolution as to whether or not one single value exists.
Common Questions about the Scientific Debate Over the Hubble Constant
The unit of the Hubble constant is a speed per distance unit. It’s based on how fast galaxies move per the distance between them and the Earth.
The differing values that researchers propose for the Hubble constant vary because of the different techniques researchers use to achieve their results.
First of all, the Hubble constant wouldn’t be a constant since it has different values. And the reason that its value varies might be that the expansion of the universe has changed as a function of time and has been slower than it is now near the beginning.