If you’ve followed developments in astronomy over the past few years, you may have heard of what is known as the “crisis in cosmology,” which makes astronomers wonder if there is something wrong with our current understanding of the universe. This crisis revolves around the rate of expansion of the universe: measurements of the rate of expansion in the present universe do not match measurements of the rate of expansion during the early universe. Without a clue as to why these measurements might not agree, astronomers are unable to explain the inequality.
The first step in solving this puzzle is to try new methods of measuring the rate of expansion. In an article published last week, researchers at University College London (UCL) suggested that we may be able to create a new, independent measure of the rate of expansion of the universe by observing collisions between black hole and neutron star.
Let’s stand back for a minute and discuss where things are now. When we look into the universe, galaxies farther away seem to move away from us faster than closer ones because space itself is expanding. This is expressed by a number known as Hubble’s constant, usually written as the speed (in kilometers per second) of a galaxy 1 megaparsec (Mpc) away.
One of the best ways to measure Hubble’s constant is to observe objects known as Cepheid variables. Cepheids are stars that regularly lighten and darken, and their brightness happens to coincide with their period (the time it takes to darken and lighten again). The regularity of these objects makes it possible to estimate their distance, and an examination of many Cepheids reveals a Hubble constant of about 73 km / s / Mpc. Type 1A supernovae are another common object of known magnitude and yield a Hubble constant hovering around 73 km / s / Mpc.
On the other hand, you can measure the extent of the universe at its earliest stage by observing the afterglow of the Big Bang known as Cosmic Microwave Background Radiation (CMB). Our best measurement of the CMB was taken by the European Space Agency’s Planck spacecraft, which released its final data in 2018. Planck observed a Hubble constant of 67.66 km / s / Mpc.
Estimated values of Hubble’s constant. Black represents measurements from Cepheids / Type 1A supernovae (73 km / s / Mpc). Red stands for CMB measurements in the early universe (67 km / s / Mpc). Blue shows other techniques, the uncertainties of which are not small enough to allow you to decide between the two. Photo credit: Renerpho (Wikimedia Commons).
The difference between 67 and 73 is not enormous, and at first the most likely explanation for the difference seemed to be an instrument error. Subsequent observations, however, have narrowed the error bars in these measurements to such an extent that the difference is statistically significant. A crisis!
This is where the UCL researchers hope to intervene. They propose a new way of measuring Hubble’s constant that does not rely in any way on the other two methods. It starts with a measurement of gravitational waves: the waves in spacetime caused by the collision of massive objects like black holes. The first gravitational waves were only recently discovered in 2015 and have not yet been associated with any visible collisions.
Lead researcher Stephen Feeney explains, “We have not yet seen any light from these collisions. However, advances in the sensitivity of devices for detecting gravitational waves, as well as new detectors in India and Japan, will lead to a huge leap forward in the number of these types of events we can detect. “
Gravitational waves allow us to determine the location of these collisions, but we must also measure the light of the collisions if we are to measure their speed. A collision between a black hole and a neutron star could be exactly the type of event that would produce both.
If we see enough of these collisions, we could use them to make a new measurement for Hubble’s constant.
The LIGO gravitational wave detector in Louisiana. Photo credit: Caltech / MIT / LIGO Laboratory.
The UCL team used simulations to estimate how many collisions between black holes and neutron stars could occur over the next decade. They found that Earth’s gravitational wave detectors could pick up 3,000 of them before 2030, and of those around 100 will likely produce visible light as well.
That would be enough. Hence, we could have a brand new measurement of the Hubble constant by 2030. We don’t yet know whether the new measurement matches the CMB measurement or the Cepheid / Type 1A measurement, or does not match both. But the result, whatever turns out, will be an important step in solving the puzzle. It could either stall the crisis in cosmology or make it more serious, forcing us to take a closer look at our model of the universe and admit that we don’t know more about the universe than we thought.
More information: “Black hole neutron star collisions can settle disputes over the expansion of the universe.” UCL.
Stephen M. Feeney, Hiranya V. Peiris, Samaya M. Nissanke, and Daniel J. Mortlock, “Perspectives for Measuring Hubble Constant with Neutron Star Black Hole Fusions.” Physical Examination Letters.
Featured image: a black hole devouring a neutron star. Photo credit: Dana Berry / NASA.