The past decade has seen a significant growth in cosmological observations that have placed increasingly tighter constraints on the cosmological model and the basic parameters that describe it. While we have an excellent phenomenological model a more fundamental picture is largely missing, considering both the very earliest times where high-energy processes are relevant and in the late-time universe, where we are in the curious position of living in a universe that is 95% dark. Continua a leggere Cosmology on Safari
A team of researchers led by Robert Quimby at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) has announced the discovery of a galaxy that magnified a background, Type Ia supernova thirtyfold through gravitational lensing. This is the first example of strong gravitational lensing of a supernova confirms the team’s previous explanation for the unusual properties of this supernova.
La più grande ‘caccia’ alle supernovae sta per iniziare il prossimo mese di Agosto. Per cinque anni, il programma scientifico denominato Dark Energy Survey avrà lo scopo di esplorare le esplosioni stellari cosmiche che saranno utilizzate come ‘candele standard’ per misurare con una precisione sempre più elevata l’espansione dell’Universo. Lo scopo della survey è quello di ottenere nuovi dati per comprendere gli effetti dell’energia scura, quella enigmatica componente che sta determinando una espansione accelerata del cosmo.
DES is operated by an international collaboration of researchers from 25 institutions and consortia, including six universities in the UK. It will use a massive new 570 Megapixel camera (DECam) installed on the four-meter diameter Blanco telescope, high in the mountains of Chile. The instrument was commissioned in September and October 2012, and this was followed by a period of science verification from November through February 2013. “Thanks to the extreme sensitivity of the camera and to the large area of sky that can be imaged through the telescope at once (about 15 times the size of the full moon), we expect DES to find more supernovae than any previous experiment. During the verification phase, we have already identified at least 200 good candidates“, said Chris D’Andrea, a researcher at the University of Portsmouth’s Institute of Cosmology and Gravitation. More than just numerous, these supernovae are very old, with the light from the most distant having travelled towards Earth for over 8 billion years. Of particular interest are Type Ia supernovae, which all have nearly the same luminosity when they reach their brightest phase.
By comparing the brightness of Type Ia supernovae, scientists in DES will be able to determine accurately the distance to the supernovae and measure how the Universe has expanded over time.
This method was used in the Nobel Prize-winning research that led to the discovery of the accelerated expansion of the universe 15 years ago. While those researchers used a few dozen supernovae in their study, DES will find over 3500 of these objects. This glut of data poses a challenge for the team to analyse. “Traditionally, astronomers have identified supernovae by analysing the spectrum of light from candidates. Because DES will give us so many candidates – we already have hundreds just from the commissioning phase – we don’t have the resources to do this for each individual candidate supernova. We need to use other techniques to confirm which of the objects we observe really are exploding stars“, said D’Andrea. An alternative method for identifying supernovae is to monitor changes in the brightness and colour of their light over time. However, the scientists also need to know how much the Universe has expanded since the star exploded. This information can be gathered by analysing the spectra of light from galaxies in which supernovae have occurred, unlike a supernova, a galaxy does not quickly fade away. “DES is a long-term survey – we may not know whether some of our candidates are ‘real’ supernovae until the end of the project. However, in collaboration with Australian researchers, our team has recently been awarded 100 nights of time on a telescope in Australia over the next five years. The Anglo-Australian Telescope has the ability to take spectra of nearly 400 galaxies at the same time. With the first of these nights scheduled for September, it won’t be that long before we can start to accurately classify the supernovae candidates discovered in DES“, concluded D’Andrea.
Lo scorso anno, una serie di osservazioni relative all’esplosione di una nana bianca nei dintorni nella galassia NGC 1365 ha permesso ad un gruppo di ricercatori della Australian National University a raccogliere una grande quantità di dati su quella che essi ritengono sia una delle migliori ‘candele standard’ che viene utilizzata dagli astronomi come strumento di misura delle distanze cosmiche.
“We know how a candle of a particular brightness grows fainter as it is moved further away from us. So, if we know the true brightness of the candle (in this instance, supernova SN 2012fr) and we measure its observed brightness, we can then calculate the interceding distance”, said Michael Childress. Supernova SN 2012fr left a chemical fingerprint which has been analysed by a team of researchers led by Childress from the ANU Research School of Astronomy and Astrophysics and which also includes Nobel Laureate Professor Brian Schmidt. Their data shows unprecedented, and quite unusual, layering in the material that was burnt and ejected in the explosion, especially silicon and iron. Two distinct layers of silicon were found: one thick, outer layer that had faded by the time the supernova reached its peak brightness on 12 November 2012 (16 days after the initial explosion), and one deeper layer that hardly changed for several weeks after the explosion.
As it turns out, SN 2012fr is not just another supernova but a really interesting case.
“Since it was discovered within a day of explosion, we were able to study it in greater detail than almost any supernova ever discovered” Childress said. “Because we know the distance to its host galaxy (NGC 1365), this supernova actually lets us better calibrate all Type Ia Supernova observations to measure distances in the Universe, using what we call the ‘standard candle’ technique”. Despite its unusual layers, SN 2012fr appears to still be classified as a so-called ‘normal’ Type Ia Supernova, which Professor Schmidt used in his Nobel Prize winning work to discover Dark Energy, and it also presents a key link in our cosmic distance ladder. “Our analyses of SN 2012fr will increase the precision of which we can measure distances outside of our own galaxy, as well as improve our understanding of these explosive events and our ability to use them in the hunt for Dark Energy, the source of the accelerated expansion of the Universe”, said Childress.
arXiv: Spectroscopic Observations of SN 2012fr: A Luminous Normal Type Ia Supernova with Early High Velocity Features and Late Velocity Plateau
Stella Novae: Past and Future Decades – This conference will bring together experts and young scientists in the research field of novae. Since the last conference on novae in 2002 new developments in nova research have emerged. Continua a leggere Stella Novae: Past and Future Decades