Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the Universe, part 10: The weirdest new kid on the block

In the 1990s, two teams of astronomers were put together to study the evolution of the universe, specifically to study how much the universe had decelerated, slowed down, since it was born in the Big Bang.

Because of course the universe had slowed down since then. How could it not?

When the Big Bang theory started to gain acceptance, its proponents thought that the Big Bang itself was slightly comparable to to a powerful rocket engine that is fired once and sends its rocket going:



When the rocket engine has been fired, the rocket may have gotten enough energy to leave the Earth altogether.



A successful rocket launch.



But if the initial boost is not enough, the rocket is going to fall back to the Earth again. (Sorry about including a weird graph here, but this was the best "crashing rocket image" I could find.)

But in any case, if there is just a single boost, the rocket is going to slow down from its top speed, even if that speed is enough to break it free of the Earth's gravity.

That's how the Big Bang was viewed, as a tremendous rocket engine boost which sent the universe flying. The question was whether or not the initial speed that the universe was given would be enough to break it free of its own gravity, or if it would be forced to fall back onto itself again.

(When the theory of inflation became accepted, it was thought that the universe had been given two initial boosts, one at the precise moment of the Big Bang, and one a fraction of a nanosecond later, at the onset of inflation. Big deal. We are still talking about a situation where the universe has been given all of its "flying energy" within a fraction of a nanosecond after its birth.)

And since it had been given all of its "outward velocity" at its own beginning, it must have been slowing down ever since.

In the 1990s, two teams of astronomers were put together to study how much the universe had slowed down since its creation. By measuring how much the universe has been slowing down, the teams hoped to predict if the slow-down was enough to eventually make the universe stop dead in its track and then collapse in a Big Crunch.

The teams were the High-z Supernova Search Team, led by Adam Riess, and the Supernova Cosmology Project, led by Saul Perlmutter. Both teams were going to look for very distant supernovae of type Ia. Supernovae of this kind are considered to be "standard candles", whose brightness is always more or less the same. These supernovae can be identified by the shape of their light curves, and they are also enormously bright. Typically, a supernovae of type Ia will reach a peak brightness of -19.3, which is about five billion times brighter than the Sun. Because these supernovae are so incredibly bright, the can be seen across billions of light years.



Several supernovae of type Ia. Their light curves are extremely similar.

The problem, when we are talking about supernovae, is that they are rare, there is no way of predicting when one will go off. After all, the last time a bright supernova could be seen from the Earth in the Milky Way was in 1604! So in order to detect any distant supernovae at all, the two supernovae team who were trying to find out about the evolution of the universe had to phtograph large swaths of the sky at regular intervals, and then carefully scrutininize every new image of an old patch to see if anything new might have popped off since last time! And when they found something suspcious, they had to use a large telescope to confirm that they had indeed discovered a new supernova of type Ia.



This is what it looked like, pretty much. Two sets of images are scrutinized. A tiny difference is detected. The Hubble Space Telescope is used to confirm that the tiny blur is a distant type Ia supernova.

But what could the two supernova teams learn from distant supernovae about the fate of the universe? You will have to wait for my next post to find out. But, well, I will give you a clue. Take a look at this graph:



This graph plots how bright the supernovae are, compared with how bright they would be if the expansion of the universe was unchanging and constant. This m-M brightness, as it is called, is plotted against how "redshifted" the supernovae are. The redshift is called z.

Bottom line: The astronomers fully expected to see the vertical line slowly "sink down", which would indicate that the universe was indeed slowing down. The real question was to find out by how much the vertical line was deviating downwards. Nobody, but nobody, expected the vertical line to become non-vertical by going up.

But that is what the two teams saw.

Ann