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‘Standard candles’ are luminous objects in deep space whose brightness at source is known, and whose distance can therefore be calculated from the intensity of their light as viewed from Earth. They are used to map the universe and study its expansion – the brighter they shine, the longer the stretches of space they can be used to survey.
Supernovae used as cosmological candles have delivered the momentous insight that the expansion of the universe is accelerating. What ground-breaking discoveries could even brighter standard candles enable?
The EU-funded project Cosmological Candles discovered that a specific type of gamma ray burst (GRB) would be suitable as a potential standard candle. The project also delivered insights into the physics and possible causes of these intense flashes of radiation.
Italian astrophysicist Maria Giovanna Dainotti was a fellow in the Cosmological Candles project which was led by coordinator/scientist Dr Lorenzo Amati. The fellowship involved an initial two-year phase at Stanford University and a second, final stage in Bologna, at the Astrophysics and Space Science Observatory of Italy’s National Institute for Astrophysics. It was funded by the European Commission’s Marie Skłodowska-Curie Actions programme.
‘A standard candle is an astronomical object whose luminosity is known or can be derived through well-known correlations,’ Dainotti explains. The main output of the project was the discovery of a correlation involving three parameters that applies reliably to a specific type of GRB,’ she notes.
Across space and time
‘GRBs are among the most luminous objects in the universe,’ Dainotti explains. ‘In just a few seconds, they release the same amount of energy that our sun releases in its entire lifetime.’
At cosmological scale, peering at objects afar means looking into the distant past, as the light we observe has taken a very long time to reach us. Particularly remote GRBs take us back to the era when the very early stars were formed shortly after the Big Bang, Dainotti explains.
‘GRBs are very diverse, and therefore did not appear to be obvious candidates for use as standard candles,’ she adds. Essentially, they are massive explosions, but which can involve vastly different amounts of energy.
Cosmological Candles built on earlier research conducted by Dainotti, and more specifically on a similarity she had discovered in the way that two parameters of these particular blasts from the past relate to each other. Based on her analysis of GRBs, whose afterglow had been observed to plateau, she had identified a correlation between the duration of the plateau phase and the GRB’s luminosity at the end of this stage.
She had also observed a link between the peak luminosity in the prompt emission – the initial phase of the GRB explosion in gamma rays and hard X-rays – and the luminosity at the end of the plateau stage. ‘The aim of Cosmological Candles was to understand if this second correlation is also intrinsic to the physics of GRBs presenting plateaus,’ Dainotti explains. Along with colleagues, she explored this aspect through a series of statistical tests, concluding that it is indeed intrinsic and not due to observational selection effects.
The stars aligned
Combining the two correlations describes long GRBs with a well-defined plateau far more narrowly than before, Dainotti notes. ‘For this subclass, the luminosity at the end of the plateau can be considered as universally derivable from the other two parameters,’ she explains, adding that this luminosity is in fact close to constant for all such GRBs. Therefore, Dainotti concludes that this subclass of GRB could indeed be used as a standard candle.
This was the question Dainotti had set out to address in Cosmological Candles, but the project outcomes exceeded even her own expectations, she notes. Cosmological Candles ended in January 2018, having delivered various leads on the nature and origin of GRBs.
Dainotti attributes this success to the synergies that emerged throughout her fellowship. Her work in the project benefited greatly from the physical, theoretical and observational background and the statistical and machine learning expertise injected by scientists at Stanford, as well as from insight contributed in a wide variety of areas by researchers at her home institute in Italy, she notes.
Further inspiration emerged from contacts she had forged earlier in her career, which had taken her to Stanford via Poland and Japan. ‘The project was an opportunity to build a bridge between people who worked on similar topics, but who had never collaborated before,’ she notes.
Dainotti’s research both in and prior to Cosmological Candles has gained her international recognition. Her work at Stanford continues, currently with the backing of a grant awarded by the American Astronomical Society. It notably involves identifying more GRBs that might one day help to shed new light on the universe, on the star formation rate – and on dark energy, the mysterious force thought to be accelerating its expansion.