October 2015 – The Mystery of Dark Energy

Heavens Above!

Heavens Above! is the astronomy section of the Sci@StAnd website, updated each month to highlight a particular phenomenon in this month’s night sky. Last month, we profiled three prominent astronomers of the Renaissance and their discoveries. In this issue, we will discuss the mystery of ‘dark energy’ and how we believe it exists. 

Fig 1. Supernovae explosion in M108 (credit: AAO)

Fig 1. Supernovae explosion in M108 (credit: AAO)

One of the most influential topics in modern astronomy, and undoubtedly the least understood, is the mystery of dark energy. When astronomers look out into the vastness of space, they find that the galaxies are not only moving away from us, but at faster and faster speeds. Who ordered that?

Dark Energy is a term for our ignorance. It is a label astronomers and cosmologists use to describe a phenomenon that we don’t yet understand. You many be familiar with the term ‘Dark Matter’, which is also an unresolved mystery whose label conveys our ignorance. To begin to understand why we are left literally ‘in the dark’ about dark energy, we need to revisit the first years of modern cosmology and the mystery of the nebulae.

In 1922 and 1923, Mount Wilson Observatory Astronomer Edwin Hubble concluded that the Milky Way, our home galaxy, is only one of several other galaxies. This conclusion rested on years of careful research into the distance to what was then known as the nebulae. Popular understanding at the time considered the Milky Way to be the entire universe, and that the faint clouds seen throughout the sky were simply gaseous regions of the Milky Way.

Edwin Hubble, using the work of Harvard scientist Henrietta Leavitt, measured the distances to very particular stars, known as Cepheid Variables, within these ‘nebulae’. By measuring the variability of their light, Hubble was able to work out their true brightness and knowing their apparent brightness as seen from Earth, calculate their distance. What Hubble discovered was that these nebulae are too far distant from foreground stars to exist within our own galaxy. Although running hard against the grain of many notable scientists in the field at the time, Hubble’s groundbreaking discovery is now heralded as the birth of modern cosmology.

Fig 2. Edwin Hubble (credit: AIP.org)

Fig 2. Edwin Hubble (credit: AIP.org)

But Hubble didn’t stop there. In 1929, he formulated the notion of redshift which described the speed at which galaxies are moving away from us. This formulation was the result of measuring the speeds of several galaxies by measuring their spectral light. As a reminder, spectral light is formed by dispersing light through a prisim or grating to separate out its component colours, which reveals the chemistry of the atoms which formed the light. While this is certainly a useful technique for scientists here on Earth, it is the only way that astronomers can understand the universe in great detail.

If an object is moving away from the observer, it’s spectral light will be shifted towards longer wavelengths. This light is said to be redshifted. Hubble noted that the spectral light of almost every galaxy he measured was redshifted, which the sole exception of our nearest neighbour the Andromeda Galaxy (which is gravitationally bound to collide with our Galaxy!). Hubble compared these velocities with the distances he found earlier and discovered a very clear trend. The further away the galaxy, the faster they are moving away from us!

This discovery changed the world overnight. Until this point, many prominent scientists, including Albert Einstein, thought that the universe was static and unchanging. This so-called ‘steady state’ theory was so convincing that Einstein had ‘mutilated’ his equations to fit this conclusion. But what does Hubble’s discovery have to do with the state of the universe? Reverse the clock.

Imagine a point of infinite density. Infinitely hot. If you were to take the motions of the galaxies and reverse them, you’d find they would eventually collapse to a single point. This is the remarkably simple evidence for the Big Bang, a term once used by English astronomer Fred Hoyle as a term of derision.

Now keep in mind that this doesn’t mean that we here are in the centre of the universe. In fact, it means the very opposite. There is no centre of the universe! Imagine you are a bacterium inside a loaf of bread dough. The dough is slowly cooked, and you watch as your bacterium friends all seem to move away from you. But from their point of view, all of the bacterium is moving away from them. This thought-experiment aptly describes the expansion of the universe.

So where are we? The universe is larger than just the Milky Way. The galaxies are very far from us. They are moving away from us. The further distant a galaxy, the faster it moves. And there was a Big Bang. But what about Dark Energy? Where does it fit into the story?

Fig 3. Supernovae Ia across redshift (credit: U Alberta)

Fig 3. Supernovae Ia across redshift (credit: U Alberta)

As recent as 1998, the High-Z Supernova Search Team published observations of several exploding stars. But these weren’t ordinary stars – they were in far off galaxies. The information gathered about their light astounded the astrophysical community. The universe was not only expanding, but it was accelerating.

Their observations have since been corroborated and confirmed by several other groups, each measuring very different phenomena about the universe. All of them reached the same conclusion. In 2011, the Nobel Prize was awarded to three leading members of the search team: Saul Perlmutter, Brian Schmidt, and Adam G. Riess.

But what does this all mean? We must revisit classical mechanics. Issac Newton first formulated a key idea about how the universe behaves. He said that any object in motion will stay in motion unless acted upon by an external force. It will keep moving in that direction at that same speed forever. But the universe isn’t staying at one speed. It’s speed is increasing. Newton says that this increase requires a force, and that each force requires energy. But where is this energy coming from? Why is it there at all? That’s what astronomers and cosmologists are trying to find out.

Theoretical physicists have proposed some very simple (and many not-so simple) solutions to the problem. Many believe that the easiest way to explain dark energy is that it is the cost of ‘having space’. In other words, if space exists then it must have some fundamental energy associated with it. This is often referred to as ‘vacuum energy’ as space is a vacuum as it is largely empty. Some go as far as to postulate that this energy is the result of quantum-mechanical collisions embedded within the fabric of space-time. Basically, two particles are created, annihilate, and there is some non-zero energy left over that pushes space apart.

But the theory behind this postulation is really, really wrong. When theorists try to calculate this ‘vacuum energy’, their predictions are off by a factor of 10 to the 120 power. That’s 10 followed by 120 zeros. The universe would likely have ended by the time you counted that high.

Other theorists are looking towards an additional component to the universe. This quintessence field has been suggested by cosmologists as a way to provide the extra energy through small-scale violations of physical principles and variations in fundamental physics. While this doesn’t sound good to someone looking for a beauty in physics, as Einstein once believed, it does point to a way forward. Maybe. But no tangible evidence of this quintessence has been found.

If this acceleration continues, the universe will likely end it’s fate in what has been dubbed ‘The Big Rip’ where all of time and space will expand so quickly that the universe will tear itself apart. Galaxies, stars, and even atoms would eventually give way to this destructive expansion of the universe. Everything, even time, would cease to exist.

So what is Dark Energy? No one knows.

Next Month: The Largest Stars in our Galaxy

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