## How do supernovae show us that the Universe's expansion is accelerating? (Advanced)

I have recently heard that there is research or data that would suggest that the speed of the galaxies at the edge of the Universe are actually increasing with speed as they continue. Do you have any more information on this?

Actually, these results show that the expansion of space is speeding up. Since space expands everywhere, this "speeding up" doesn't just happen to galaxies on the 'edge' of the Universe, but to every part of the Universe.

Because the expansion is a rate (like a velocity), then a changing expansion is a changing rate, or an acceleration. We therefore say that the Universe is undergoing accelerated expansion. These results actually started coming in in the late 1990s and the early 2000s, so they're new but not brand new. The projects that studied them were called The Supernova Cosmology Project, the High-Z Supernova Search Team, and the Supernova Legacy Survey.

You may have heard of dark energy, and this is in fact the same thing: the evidence for dark energy is that the universe is accelerating. Scientists determined this by using something called a 'standard candle.' A standard candle is an astrophysical object that has some characteristic that allows us to determine its total luminosity, even though it's very far away. Since the amount of that luminosity which we receive has to do with the distance to the object, standard candles can be used to figure out how far away an object is. You could conceivably do an experiment to prove this to yourself: if you know that a 60 watt lightbulb gives off a certain amount of energy, and then measure the energy received from a 60 watt lightbulb across the room from you, you could calculate the distance to that lightbulb.

Astronomers can take advantage of standard candles to determine the distance to objects like galaxies, and that's what happened in the projects I just mentioned. Using a type of supernova called a type Ia supernova, astronomers determined both the distance of the galaxy and the redshift of the galaxy. "Redshift" basically told them how much the Universe had expanded since the light left the supernova. The astronomers could then compare distance to expansion, and create a kind of 'expansion history' of the Universe.

It turned out that these results showed that the rate of the Universe's expansion was increasing!

Why does this lead us to the idea of 'dark energy'? Well, the increased rate of expansion means that the Universe is getting bigger and bigger. Since gravity is an attractive force, you would instead expect the Universe to want to get smaller. Gravity should 'pull' the Universe back together again. If the Universe continues to expand, faster and faster, some force or pressure must be 'pushing' it back out. This is the so-called dark energy. Dark energy is consistent with a host of other observations, so the supernova data is actually very, very cool because it gives a different kind of 'check' on the conclusions other teams have drawn.