Supernovae as Probes of Dark Energy

In the wake of the excitement surrounding the discovery of a Type Ia supernova by a group of University College London (UCL) students and staff (yay!) from the University of London Observatory, I asked my good friend Suhail Dhawan to write a piece about the importance of type Ia supernovae in an attempt to justify this hysteria. Suhail is currently pursuing his PhD from the European Southern Observatory (ESO), Munich and is working on supernovae. Apart from being really smart, he is hilariously funny too (am I the best wingman or what?!).

Check out his fun-filled blog here: http://seventhgradedropout.blogspot.co.uk

Suhail and I have always been very passionate about astrophysics. This post is the kind of stuff that we talk about and believe it or not, plan to do for the rest of our lives, despite the fact that this will most probably lead us to live a life of poverty… Enjoy!

In 1929, Edwin Hubble, at Mt. Wilson observatory, discovered that the universe was expanding and that faraway galaxies were receding from us at a velocity proportionate to their distance. This was truly one of the most remarkable observations. Even Einstein was of the opinion that our universe is static. Hubble’s discovery however, led him to comment that failing to realise this was, in his own words, his “greatest blunder”.

One of the questions that emerged now was to measure how the rate of expansion would change over time. It was initially posited that the expansion should decelerate, since the galaxies attract each other gravitationally. Several cosmologists aimed to pin down the amount of deceleration.

In order to measure the deceleration, one would have to determine the distance to a faraway object. Cosmological distances are measured by a distant object’s apparent luminosity or brightness to its absolute or intrinsic brightness. For this purpose, a 'standard candle' was required. This term refers to a class of astrophysical objects, which has a uniform intrinsic brightness, and therefore, the apparent brightness gives a robust measure of the distance to the object. The term ‘standard candle’ is fairly accurate. Imagine you have a candle and you know its brightness and how far it is from you. Now, if there were to be another candle placed some distance away, you could easily measure its ‘apparent’ brightness. Comparing that with the brightness of the candle you have, you can make a fairly accurate estimate of how far the observed candle is from you.

Supernovae of spectral type Ia were seen to meet the ‘standard candle’ criterion. This spectral type refers to those supernovae, which are created from a binary system in which one of the components is a compact object that accretes matter from a donor star and explodes upon reaching a critical mass limit. As there is a definite mass limit after which the accreting star explodes, Ia supernovae have the same intrinsic luminosities (or brightness).

Accretion of matter by a compact star from a massive donor star.

Image credits: http://www.einstein-online.info/spotlights/galactic-binaries

In the late 90s, two research groups, using these Type Ia supernovae, found that the value of the deceleration was negative. This pointed to a mysterious force that was pushing objects away at an accelerated rate. By fitting the observations to models of the universe from theory, they found that the mysterious fluid, which was termed as 'Dark Energy' makes up nearly three-quarters of the universe!

However, the story with supernovae isn’t complete yet. An improvement in technology and the advent of charge-coupled devices (CCDs) meant that we were able to discover many more of these objects. Upon analysing the data in the early 90s, it was seen that Type Ia supernovae weren't as homogenous a class as originally thought. To overcome the problem, there were several ingenious corrections proposed. It was argued successfully that explosions that were intrinsically brighter also faded away more slowly, an effect attributed to the amount of nickel produced in the explosion. Similar relations between the intrinsic luminosity and colour, and the mass of the host galaxy were found, which were used to calibrate the objects. 

Although these corrections were crucial for the discovery of dark energy, we need more precise constraints to understand its nature. Recent studies have shown that these objects are truly standard when observed in the near infrared wavebands. This is extremely exciting from viewpoint of cosmology. With planned surveys to observe a larger dataset in these bands, out to greater distances, there is optimism about finding new and exotic physics that describes the universe.