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.

The Black Hole Information Paradox

After showing the world that the Universe began as a point of singularity, and the event responsible for the creation of the Universe was the ‘Big Bang’, Stephen Hawking turned his attention towards the mysterious cosmological objects called Black Holes. A black hole is formed when a very massive star runs out of fuel and explodes as a Supernova. The explosion throws away the outer gaseous layers and leaves behind a very massive core packed into a region of very little radius. The black hole has a gravitational pull so strong, even light cannot escape from its surface. The region that demarcates the ‘point of no return’ for light, or as a matter of fact, any other object is known as the event horizon of the black hole.

So the first thought that naturally creeps into our minds is that since the gravitational pull of a black hole is so strong, there should not be any particles or radiation emitted from it. This is precisely what the theory of General Relativity predicts. However, Stephen Hawking showed the world that black holes do in fact emit, if one were to take into account quantum perturbations in and around the event horizon of a black hole. This, quite fittingly, is called ‘Hawking Radiation’.

It is important here to remember one very extravagant statement made by the great physicist John Wheeler, “black holes have no hair!” This most certainly does not imply that black holes are bald. What it truly means is that the properties of a black hole which can be observed from outside its event horizon are dependent on only three classical parameters, its mass, electric charge and angular momentum. All other information (or hair) about particles falling into a black hole is inaccessible to observers outside the event horizon.

A very simple idea about the origin of Hawking radiation can be formulated if we were to consider quantum vacuum fluctuations in the gravitational field of a black hole. A quantum fluctuation is a temporary appearance of energetic particles, as allowed by Heisenberg's uncertainty principle in quantum mechanics. These fluctuations lead to particle-antiparticle production out of pure energy, albeit for a very short period of time. If either of the two particles fell into the black hole and the other escaped the black hole's gravitational pull by a phenomenon known as ‘quantum tunnelling’, the particle which escaped will give rise to Hawking radiation. In order to conserve the total energy in this sequence of events, the particle that fell into the black hole must contain negative energy. This means that the black hole is slowly losing energy and hence, mass. It would eventually evaporate!

Therefore, the information about every particle that ever fell into the black hole, and that of the black hole itself will one day completely disappear. This proposition hits right at the heart of quantum mechanics, which postulates that the behaviour of a particle at any point in the future can be predicted if one were to possess ample information about its current state, which is nicely packed into its 'wave function'. The wave function informs us about a particle's position, velocity, momentum, energy, etc. at any given time. This abrupt loss of information was called the black hole information paradox.

Since millions of black holes have been discovered in the universe, the occurrence of this paradox cannot be quarantined to just one particular situation. If the information paradox exists in the case of a black hole, then it must exist everywhere else in the universe too.

Stephen Hawking remained convinced that information was indeed getting destroyed in a black hole, and that quantum mechanics will have to be suitably modified to take into account these occurrences. This annoyed quite a number of physicists around the world. One of the most offended was John Preskill, who bet Stephen Hawking and was sure that information was not destroyed in a black hole. The winner of this bet would receive an encyclopedia of the winner’s choice, in which information never gets destroyed for sure.

Another notable physicist who was troubled by Hawking’s claim was Leonard Susskind, who wanted to ‘save’ quantum mechanics and publicly declared war against Hawking! He, however, maintained that the two are good friends and their war is only that of ideas. The debate was eventually settled when Susskind showed that information of particles entering a black hole was not lost, but it was smeared or ‘painted’ on its event horizon and got completely scrambled. This is knows as the 'Holographic Principle'. Susskind gave a precise string-theory interpretation, which involved a much higher number of dimensions than the three dimensions of space and one of time. It also took into account the non-zero entropy of slightly longer strings that make up the event horizon of a black hole.

Now that the argument was settled, Stephen Hawking had to gift a copy of ‘Total Baseball: The Ultimate Baseball Encyclopedia’ to John Preskill. Comparing the scrambled and useless information that was emitted by a black hole to ‘burning an encyclopedia’, Hawking remarked that he might as well have gifted Preskill the ‘ashes’.

Kip Thorne, John Preskill and Stephen Hawking. Preskill bet that the information can be recovered, with Hawking and Thorne betting that it is destroyed

Image source: theory.caltech.edu

Hello World!

Hi! My name is Aayush and at the time of writing this blog post, I am studying towards a Masters degree in Astrophysics. This blog has nestled cosily in my mind for at least the last two years. As an Undergrad, I had a lot of raw data in my head, which I had collected from textbooks, popular science magazines and Wikipedia articles, accumulated together in a somewhat incoherent fashion. The essentials for a presumably interesting blog were all there. What was lacking was a backbone. A structure. Something that would provide a seamless link and bring together all these 'essentials' into something that made absolute sense.

Having spent a few months studying Astrophysics (something that I truly love and would not mind doing for the rest of my life), coupled with numerous dinner table conversations with family and friends about what astrophysicists actually do and how beautifully philosophical this study can be, I may have found the elusive backbone and a sense of direction for my imaginary blog, which may finally make it real.

My blog is called 'We are Starstuff'. I have to admit, like many other people in this field, I have been deeply affected and enchanted by the great Carl Sagan. Carl Sagan was an astrophysicist, author, science communicator and in my opinion, a philosopher. He had the uncanny ability to blend philosophy with astrophysics. I use the word 'uncanny' because it upset a large section of people, especially those who doubted science in itself. But what it did to me was something huge. It gave me a broad idea of what I wanted to do with my life. How I wanted to live it. Therefore, I have named my blog after one of the statements that Carl Sagan once made, one of the most profound things a human brain can process, the simple fact that we are indeed, made of starstuff. The quote goes something like this,

"Our Sun is a second- or third-generation star. All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth, the carbon in our genes were produced billions of years ago in the interior of a red giant star. We are starstuff."

Through this blog, I will try and capture the beauty of what astrophysicists, and more generally, scientists do. I will also talk about some rather interesting phenomena, in a fashion that makes it easy for everyone to understand! I don't know how many people will actually read this, but I reached a point in my life where I felt that this blog had to be done.

I'll start by uploading some stuff that I wrote during my undergrad for our annual Physics department magazine called 'Quintessence'.

I sincerely hope that this little endeavour of mine yields maximum output. If I can get even a few people interested in this subject, it would count as a major success! Cheerio.

Artist's rendition of an exploding star, Supernova 1986J