Neutron Stars and the Hubble Constant

One of most contentious questions in cosmology right now surrounds the rate of expansion of the Universe. Different observations have been yielding contradictory results, but astronomers think that they will soon be capable of measuring it very precisely using merging neutron stars.

Universal expansion, an idea that was controversial for a good part of the early 20th century, is characterised by a number referred to as the Hubble Constant. This number might be the closest we have to the number 42, the fictional answer to the ultimate question of life, the Universe, and everything, as it allows cosmologists to calculate both the size and age of our Universe.

For decades scientists have measured this number to increasing levels of accuracy, but with lower uncertainty in individual observations has come an increasing disparity between the different methods used to do the measuring in the first place. Look at distant exploding stars and you get a value of 74 km/s/Mpc, but measurements of the cosmic microwave background suggest it’s only 67 km/s/Mpc (an Mpc is a term used by astronomers that is known as a megaparsec, a distance equal to 3.262 x 10⁶ light-years).

The problem is so significant that cosmologists are beginning to question whether they might need some new physics to explain what they are seeing. A challenge to our fundamental understanding of the Universe sounds like it might be a blow for astronomy, but, to the contrary, this is exactly the sort of thing that theorists get excited about.

Now, new research led by the Galician Institute of High Energy Physics (IGFAE) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), suggests that we might soon have a new method for measuring the Hubble Constant using a precise, yet elegant and simple, technique that could even reveal whether the number varies throughout time and space. And Australia is uniquely positioned to lead the experiments.

The idea that the Universe is expanding comes from Edwin Hubble… no wait, from Vesto Slipher, an astronomer at the Lowell Observatory in Arizona. Slipher observed the redshift of distant galaxies and related these redshifts to velocity. This is something that is often mistakenly credited to Hubble.

But Hubble was the first to observe that the redshift was directly proportional to the distance of the galaxies from Earth, publishing his seminal paper in 1929. And by first, I mean second.

Belgian priest and scientist Georges Lemaître had already done the same two years earlier, but his contribution was less well known with certain key parts of his work omitted in the English translation of his paper. For those that love a bit of a conspiracy, there is every chance that Hubble himself had something to do with that.

Either way, Hubble, working with Slipher’s data, observed that the redshift of galaxies was directly proportional to their distances from Earth, and that allowed him to formulate a simple law (now known as the Hubble-Lemaître law) with a simple constant of proportionality – the Hubble Constant.

How does this work?

The light emitted by galaxies is like a fingerprint that can be used to identify the atoms, elements, or molecules that are present. Astronomers analyse this light by separating it into its component colours through a prism. The result is a spectrum of colours, crossed with discrete lines.

As an example, the spectrum of the element sodium is dominated by two lines at very specific wavelengths of 589.0 nm and 589.6 nm known as the Sodium D-lines. On Earth, the spectrum for sodium looks the same every time it is measured.

But when the Sodium D-lines are found in the spectra of distant galaxies, their wavelengths are shifted towards the red, or longer, wavelengths. The lines are still there, but they have moved such that their wavelengths have increased.

Hubble’s interpretation of all this was that intergalactic space was expanding, stretching the space-time between galaxies, and any light from galaxies as it travelled towards us, thus increasing its wavelength. His initial value of the expansion rate was approximately 500 km/s/Mpc, suggesting that the Universe was only about 2-billion years old.

Geologists at the time had already shown that the age of the Earth was around 3-billion years, so Hubble’s value was a little problematic. But with increasingly precise observations and with an independent method of calculation using the cosmic microwave background, astronomers were fairly confident at the beginning of this century that the correct value was 72 km/s/Mpc, give or take a bit.

That number meant that the Universe was about 13.6-billion years old, which fit much better with the ages of heavenly bodies that had been calculated separately.

Fast forward 20 years and the two main independent methods of determining the Hubble Constant are giving two incompatible results. But astronomers now have a new tool in the shed that they can use to work out the Hubble Constant, and it is by observing the gravitational waves generated by collisions of black holes.

However, this technique is still in its infancy and results have a high degree of uncertainty. But OzGrav alumni Prof Juan Calderón Bustillo, OzGrav Chief Investigator Dr Paul Lasky of Monash University, and their collaborators have now proposed a simple method to increase the accuracy of measurements done in this way by using neutron stars.

The problem is that we haven’t yet built a gravitational-wave observatory that can detect the comparatively high-frequency signals generated by neutron stars. However, here’s the good news. Scientists have already presented the Australian government with a proposal to build an observatory called NEMO (the Neutron Star Extreme Matter Observatory) that would be capable of doing exactly that.

NEMO is a priority goal identified in the Australian Academy of Science’s astronomy Decadal Plan and could be operational in less than a decade if funding for the $100-million observatory goes ahead.

With new technology that would be sensitive to higher-frequency gravitational waves, data from NEMO could be used to more accurately calculate the distance to merging neutron stars by helping astronomers work out their orientation to Earth. Instead of uncertainty in the measurement of the Hubble Constant of 16% when using black holes, it would then be as low as just 2%.

And it would allow astronomers to check whether the Hubble Constant is really ‘constant’, as currently thought, or whether it varies throughout space and time.