If you’ve been following the development of astronomy over the last few years, you may have heard of the so-called “cosmology crisis,” which makes astronomers wonder if there might be something wrong with our current understanding of the universe. This crisis revolves around the speed at which the universe is expanding: measurements of the rate of expansion in the current universe do not align with measurements of the rate of expansion during the early universe. Without an indication of why these measurements may disagree, astronomers cannot explain the differences.
The first step in solving this mystery is to try new methods for measuring the rate of expansion. In an article published last week, researchers at University College London (UCL) suggest that we may be able to create a new, independent measure of the rate of expansion of the universe by observing collisions of black holes and neutron stars.
Let’s go back for a minute and discuss where things stand at the moment. When we look at the universe, galaxies that are further away seem to move away from us faster than those closer to us because space itself is expanding. This is expressed in a number known as the Hubble constant, which is usually written as the speed (in kilometers per second) of a galaxy per megaparsec (Mpc).
One of the best ways to measure the Hubble constant is to observe objects known as Cepheid variables. Cepheids are stars that regularly lighten and darken, and their brightness simply conforms to their period (the time it takes for them to darken and lighten again). The regularity of these objects makes it possible to estimate their distance, and the study of many Cepheids gives us a Hubble constant of about 73km / s / Mpc. Type 1A supernovae are another common object with some brightness, and they also give a Hubble constant of about 73km / s / Mpc.
On the other hand, you can measure the expansion of the universe during its earliest phase by observing the subsequent glow of the Big Bang, known as cosmic microwave background radiation (CMB). Our best CMB measurement was made by the European Space Agency’s Planck spacecraft, which published its final data in 2018. Planck observed a Hubble constant of 67.66km / s / Mpc.
The difference between 67 and 73 is not huge and at first the most likely explanation for the difference seems to be a tool error. However, through subsequent observations, the errors in these measurements were narrowed enough that the difference was statistically significant. A real crisis!
Here, UCL researchers hope to intervene. They propose a new method for measuring the Hubble constant, which in no way relies on the other two methods. It starts with measuring gravitational waves: waves in space-time caused by the collision of massive objects such as black holes. The first gravitational waves were discovered very recently, in 2015, and they are not yet associated with visible collisions.
As lead researcher Stephen Feeney explains, “we have not yet found light from these collisions. But advances in the sensitivity of gravitational wave detection equipment, along with new detectors in India and Japan, will lead to a huge leap forward in how many 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 from the collisions if we want to measure their speed. A black hole-neutron star collision could be the exact type of event that would trigger both.
If we see enough of these collisions, we could use them to create a new measurement for the Hubble constant.
The UCL team used simulations to calculate how many collisions with a black hole and a neutron star could occur in the next decade. They found that Earth’s gravitational wave detectors could capture 3,000 of them before 2030, and about 100 of them are likely to also produce visible light.
That would be enough. As such, by 2030 we may simply have a brand new measurement of the Hubble constant. We do not yet know whether the new measurement will agree with the CMB measurement, or with the Cepheid / Type 1A measurement, or disagree with both. But the result, whatever it turns out to be, will be an important step in solving the puzzle. It can either stop 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 there are many more things we do not know about the universe than we thought.
Learn more: “Collisions with black holes and neutron stars may resolve the dispute over the expansion of the universe.” UCL.
Stephen M. Feeney, Hiranya W. Peiris, Samaya M. Nissanke and Daniel J. Mortlock, “Prospects for Measuring the Hubble Constant in a Neutron Star-Black Hole Fusion.” Physical examination letters.
Recommended image: A black hole absorbing a neutron star. Credit: Dana Berry / NASA.