Type 1a Supernovae and Dark Energy

[chris1969]

Hello everyone

I hope you don't mind me asking a real silly question about the original evidence for dark energy. I've been reading about how Type 1a supernovae provided the initial evidence that the expansion of the universe was accelerating. They are able to do this because they are standard candles i.e they all have the same intrinsic brightness - therefore the fainter they seem the further away they must be.

The part I don't get is what this information adds to the original observations made by Hubble. These showed that the velocity of objects (as measured by the redshift) increased with distance (as measured by some form of standard candle). If velocity is increasing with distance and time then this implies that the rate expansion of the Universe is increasing i.e accelerating. If this is the case, what is it that the Type 1a supernovae add to the story.

I'm conscious that I'm misunderstanding something at a very basic level, so an idiot's level
explanation would be appreciated!

And many thanks in advance.

Chris

 

[inflector]

The part I don't get is what this information adds to the original observations made by Hubble. These showed that the velocity of objects (as measured by the redshift) increased with distance (as measured by some form of standard candle).

Hubble's observations point to a constant expansion rate, not an accelerating one.

If velocity is increasing with distance and time then this implies that the rate expansion of the Universe is increasing i.e accelerating.

That's not right. There was no acceleration according to Hubble's observations.

Consider a very long stretchy nylon rope that you cannot see the end of that's being stretched by trucks over the horizon at the rate of 1% per minute. If you measure two points separated by one meter, you'll get a velocity of separation of 1cm per minute, if you measure two meters, you'll get a velocity of separation of 2cm per minute. If you measure two points 100 meters apart, you'll get 1m per minute velocity of separation. So the velocity increases linearly with the distance measured.

This is what happens with a uniformly expanding universe and this is what Hubble observed. The further away you look, the faster the object will be moving away from you when the universe is expanding at a constant rate. This was thought to be the case until the Type 1a supernova observations of 1998.

If this is the case, what is it that the Type 1a supernovae add to the story.

These observations showed that the expansion was not at a constant rate but was accelerating slightly. This was a very surprising finding because to this day no one knows why this acceleration occurs.

For there to be an acceleration there needs to be a source of energy causing that acceleration. This is why dark energy was proposed. It is a placeholder name for whatever is causing the acceleration in the expansion of space in the universe.

 

[chris1969]

Many thanks for this, the rope analogy is very helpful. I've got another couple of questions.

Firstly, am I right in thinking that the increase in velocity with distance is because the rope (ie space) is being stretched?

My second question is why we were able to identify the acceleration from the type 1s but not from the other standard candles?

Many thanks once again.

Chris

 

[inflector]

Many thanks for this, the rope analogy is very helpful. I've got another couple of questions.

Firstly, am I right in thinking that the increase in velocity with distance is because the rope (ie space) is being stretched?

Not stretched exactly more like it is expanding from within. This only happens between galaxies and galaxy clusters. But it acts like space itself is expanding.

My second question is why we were able to identify the acceleration from the type 1s but not from the other standard candles?

I'm not qualified to discuss the particulars of the experiment but you can find the papers for the 1998 discovery by Riess et al. and subsequent confirmation by Perlmutter et al. here:

http://www.arxiv.org/abs/astro-ph/9805201
http://arxiv.org/abs/astro-ph/9812133

But the main thing is that these measurements were more accurate because the Type 1a distance measuring technique was more accurate than prior measurements. The new observations showed that rather than there being a straight-line relationship between distance and speed there was a curve. The curve indicates acceleration. In other words, they found that the speed difference between very old (which is the same as very far away) galaxies was less than the equivalent distance between newer galaxies.