What does the acceleration of the universe mean?

The Universe is expanding around us. When we measure the rate the Universe is expanding now and compare that to the rate in the past, it seems the Universe has been speeding up over the past 6 Billion years or so.

What evidence is there that the universe is expanding?

If we look at objects across the Cosmos, we notice that the further away the object is, the more its light is stretched to redder colours. This is expected if the Universe is in motion: as light travels through expanding space, the light stretches with the space it is travelling in, making it appear redder. The phenomenon is called redshift. In partciular, measuring the light from supernova has been used in this way to trace back the expansion of the Universe more than 10 billion years and uncover the acceleration.

How does this relate to Dark Energy?

Dark Energy is meant to be energy that is part of space itself. When the Universe expands, the amount of dark Energy per volume stays constant (but of course the Universe is bigger). This material has, according to General relativity, the almost magical ability to accelerate the expansion of the Cosmos. Observations of supernova suggest that more than 70% of the Universe is made up of Dark Energy.

What could this mean for the ultimate fate of the universe?

If the Universe’s Dark Energy doesn’t disappear (or change over time), it will cause the Universe to expand faster and faster over time, until our part of the Universe (us and the nearest few galaxies) loses contact with the rest of the Universe. We will look out onto a vast emptiness and slowly fade away into oblivion.

What are supernovae?

Supernovae are the violent deaths of stars; there are two types, but for measuring distances I will mainly focus on the explosions of stars called white dwarf stars. When our sun runs out of nuclear fuel, its interior will collapse down to a ball of Carbon and Oxygen about the size of the Earth (but a million times denser). This is a white dwarf. If our sun were born as a binary (a star system consisting of two stars orbiting around their common center of mass), then it is possible, after the white dwarf is created, that the second star in the binary can shed material onto the white dwarf and increase its mass. At a certain point, gravity can cause this huge nuclear powder keg to ignite into an explosion 5 billion times brighter than our sun – this is a Type 1a supernova.

Where might future research take us?

I can honestly not say. It would be nice to figure out what Dark Energy is (or even if it really exists), and thereby better understand why the Universe is expanding and its ultimate fate.

Prof. Brian P. Schmidt is an astrophysicist at the Australian National University’s Mount Stromlo Observatory.