Smaller Errors for the Hubble Constant Constrain Dark Energy

Smaller Errors for the Hubble Constant Constrain Dark Energy

What is the ultimate fate of the universe? Will it expand forever or will it eventually collapse back into a singularity? The latter option was pretty much ruled out by the discovery that cosmic expansion began accelerating roughly 5 billion years ago. Scientists refer to the cause of the accelerating expansion as dark energy, largely because they currently have little idea of the cause’s identity. Now the question becomes whether this dark energy will simply move most of the objects in the universe beyond our ability to detect them or actually rip the fabric of space-time. Not only does dark energy paint a bleak future for life in the universe, it exhibits an extraordinary degree of fine-tuning.

Edwin Hubble first discovered cosmic expansion eighty years ago. Since that time, astronomers sought to determine the expansion history to greater and greater precision. Initially, Hubble described the expansion rate using a single number, now referred to as the Hubble constant. Although the expansion rate of the universe varies through time, astronomers still use the Hubble constant to set the scale of the universe. Thus, more precise measurements of the Hubble constant’s value translate into a better understanding of the universe’s age, how the expansion rate has changed, and what might have caused that expansion to accelerate.

Currently, measurements of the Hubble constant do not significantly constrain what form the dark energy might take. The simplest form would be a constant value. This cosmological constant, similar to the one originally introduced by Einstein when he developed his general theory of relativity, would not rip the fabric of space-time. But other more complicated forms of dark energy might cause the universe to rip apart. Or they might eventually decay and return the universe to a more mundane expansion—one that could even permit a collapse. In order to differentiate between these possibilities researchers seek to make more precise measurements of the Hubble constant.

Astronomers use Cepheid variables to measure the Hubble constant. They also use Type Ia supernovae. However, the distances derived from the supernovae must be calibrated using the Cepheids. By means of the Hubble telescope, a collaboration of physicists and astronomers remeasured Cepheids from a number of galaxies that recently hosted Type Ia supernovae as well as NGC 4258. These new observations reduced a number of error sources, resulting in a factor of 2.2 reduction in the Hubble constant error bars.

The new results tighten the constraints on possible dark energy candidates. The cosmological constant still stands as the simplest explanation consistent with the new results, although the best value indicates a future ripping of space-time. The team expects future measurements to narrow down the possibilities even further.

This research adds to the considerable body of evidence for the existence of dark energy and, as a consequence, the bleak future for life in the universe. Christian theology accommodates such implications because the Bible teaches that this universe has a limited lifetime. Once its purpose is complete, God will destroy this universe and create a new one where all those who have accepted His invitation will live for eternity.