According to the standard cosmological model, the age of the Universe is estimated at around 13.8 billion years. This estimate was obtained thanks to the combination of several measurement methods based in particular on the study of the cosmic microwave background and the acceleration of the expansion. One of the currently open questions in cosmology concerns the observation of stars whose age seems to be older than that of the Universe, a problem called the "paradox of age". This is particularly the case of the star HD 140283, whose age has been estimated at 14.27 billion years, forcing astrophysicists to reconsider measurement methods and observational error margins.
For more than 100 years, astronomers have observed a strange star located in the constellation of Libra, some 190 light years from Earth. It speeds across the sky at 1.3 million km/h. But more interestingly than that, HD 140283 — or Methuselah — is also one of the oldest known stars in the Universe.
In 2000, astrophysicists sought to date the star using observations made via the European Space Agency's (ESA) Hipparcos satellite, which estimated an age of 16 billion years. Such a result was rather disconcerting. As astronomer Howard Bond of Pennsylvania State University pointed out , the age of the universe — determined from observations of the cosmic microwave background and accelerating expansion — is 13.8 billion years. “It was a very serious difference .
Taken as is, the predicted age for the star poses a major problem. How could a star be older than the Universe? It was clear that Methuselah — named for a biblical patriarch who is said to have died aged 969, making him the oldest character in the Bible — was old, as the metal-poor subgiant is mostly made of hydrogen and helium and contains very little iron. Its composition meant that the star must have appeared before iron became a common element.
Bond and his colleagues took it upon themselves to determine whether the initial number of 16 billion years was accurate or not. They studied 11 sets of observations recorded between 2003 and 2011 by the Hubble Space Telescope's Fine Guidance Sensors, which note the positions, distances and energy yield of stars. By acquiring parallax, spectroscopic, and photometric measurements, astrophysicists could make more accurate measurements.
“One of the uncertainties associated with the age of HD 140283 was the precise distance from the star. It was important to get it right, because we can better determine its luminosity, and from there its age — the higher the intrinsic luminosity, the younger the star. We were looking for the parallax effect, which meant we looked at the star every six months to look for the change in position due to Earth's orbital motion, which tells us the distance explains Bond.
There are also uncertainties in the theoretical modeling of stars, such as the exact rates of nuclear reactions in the core and the importance of downward-scattering elements in the outer layers. The researchers worked on the idea that remnant helium diffuses deeper into the core, leaving less hydrogen to burn through nuclear fusion. With the fuel used faster, the age is lowered.
“Another important factor was, among other things, the amount of oxygen in the star says Bond. HD 140283 showed a higher oxygen to iron ratio than expected, and since oxygen was not abundant in the Universe for a few million years, it again indicated a lower age for the star.
Bond and his collaborators estimated the age of HD 140283 at 14.46 billion years, a significant reduction from the previously claimed 16 billion. This was, however, still older than the age of the Universe itself, but astrophysicists posed a residual uncertainty of 800 million years which Bond believed made the age of the star consistent with the age of the Universe, although not entirely accurate.
“Like all estimates, age is subject to both random and systematic error says physicist Robert Matthews of Aston University. “Overlapping error bars give an indication of the likelihood of conflict with cosmological age determinations. In other words, the best-estimated age of the star conflicts with that of the derived age of the Universe [determined by the cosmic background], and the conflict can only be resolved by pushing the error bars at the extreme limit .
Refinement of the age of HD 140283 continued with improvements. A 2014 follow-up study updated the star's age to 14.27 billion years. “The conclusion we have come to is that the age is about 14 billion years and, again, if you include all sources of uncertainty — both in the measurements of observation and in theoretical modeling — the error is about 700 or 800 million years, so there is no conflict because 13.8 billion years is within the star's error bar says Bond.
But what could potentially make the Universe younger than this particular star? “There are two options, and the history of science suggests that in such cases the reality is a mixture of both. In this case, it would be sources of observational errors that have not been fully understood, as well as some gaps in the theory of the dynamics of the Universe, such as dark energy parameters. , which has been the main driver of cosmic expansion. for billions of years now explains Matthews.
The solution to the age paradox requires a better understanding of the dynamics of the Universe, in particular its rate of expansion. This video summarizes the current problem regarding divergent measurements of the Hubble constant:
Matthews suggests the possibility that the current "age paradox" reflects the temporal variation of dark energy, and therefore a change in the rate of acceleration - a possibility that theorists believe is potentially compatible with ideas about the fundamental nature of dark energy. gravity, such as causal set theory. New research on gravitational waves could help resolve the paradox.
To do this, astrophysicists would examine the ripples in the fabric of spacetime, caused by neutron star or black hole mergers, instead of relying on the cosmic microwave background or the monitoring of nearby objects such as Cepheid variables and supernovae to measure the Hubble constant — the former giving a rate of 67 km/s/Mpc and the latter a rate of 73 km/s/Mpc. A better understanding of the dynamics of the expansion would indeed make it possible to estimate a more precise age of the Universe.
