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

The absolutely weirdest new kid on the block of new cosmology is dark energy. I'm just sayin'.



I became interested in space back in 1969, when I was fourteen. Looking back, I think that what really attracted me to space was the humoungous, outrageous size of it. When I learnt that all stars are suns, I tried to picture how far away those faint little lights in the night sky had to be in order to look as faint as they do if they really are blazing suns, and the thought of it made my head swim. Then I realized that I liked how immense reality had suddenly become when the stars were so unimaginably far away. I'm a claustrophobiac, and cramped enclosed spaces make me panic.



A cramped enclosed narrow cave, oh no!!!



I don't want to be in the desert, but for all of that, the wide open space of a desert appeals to me.

And what could be more utterly unlike cramped enclosed spaces than a universe stretching for light years upon light years in all conceivable directions?

So I liked the universe because of its sheer size. Soon enough I learnt that astronomers believed that space was expanding, becoming even bigger. Fantastic!

But then I learnt that many astronomers believed that space would stop expanding after a while. After coming to a halt it would start contracting instead. It would contract more and more, become smaller and smaller, crash down upon itself faster and faster until it crash-landed on itself and scrunched itself out of existence in a Big Crunch. Well, let me tell you, that was definitely the worst and scariest thing I had heard of in my life.



Imagine the entire universe going down its own drain!

Many astronomers believed that such a Big Crunch would happen. (And most of them believed that after the Big Crunch, the universe would "bounce back" and rise again from its squeezed-out-of-existence ashes in a Phoenix-like fashion.)



The Big Bounce theory. After collapsing back into a single point, the universe is supposed to "bounce back" and rise again, Phoenix-like.

Anyway, the thought of a collapsing universe certainly wasn't something that I worried about on a daily basis, because honestly, I did realize that such a Big Crunch couldn't happen until many billion of years from now at the earliest! And what does it matter to me or to any of us what happens to the universe many billions of years from now? Well, personally I think that most of us like to picture a future that appeals to us or at least does not repulse us, even if we talk about a future when we ourselves must long be dead. And I cared enough about the distant future of the universe to pay close attention when my astronomical magazines wrote anything about the subject. My magazines told me that the ultimate fate of the universe depended on how much matter it contained. If it contained more than "the crucial density of matter", it would collapse. If it contained less than "the crucial density of matter", it would keep expanding forever.



This graph shows the evolution of a forever-expanding (an open) universe, a critical universe (one that balances on a knife's edge between expanding and collapsing, but which eventually avoids collapse) and a collapsing (a closed) universe. The dotted lines shows a theoretical bouncing universe, for which there is no real mathematically stringent theoretical support.

You can imagine what kind of universe that I, the claustrophobiac, was hoping for. Not a collapsing one, that's for sure!

Over the years, my magazines kept reporting how astronomers kept searching and searching for evidence that the universe would indeed collapse. Much to my resentment, the people who wrote the articles in my astronomy magazines seemed to be hoping that the universe was destined to collapse! Clearly they were hoping for a bouncing and "recycling" universe, and then they didn't mind "living through" the horror of a cosmic collapse! Didn't they feel any compassion at all for us poor claustrophobiacs?

However, to my relief, most inventories of matter in the universe did come to the conclusion that the universe probably wouldn't collapse. But the results were inconclusive.

This search for the ultimate fate of the universe (and the attempts to sort of persuade the universe to push itself over the edge to collapse) went on for years. It went on until 1997, when two independent teams who had been studying distant supernovae dropped the bombshell of the century in the laps of the unsuspecting astronomical community.

They announced that the universe was accelerating.



Ann

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

Now you really need to understand why two teams of astronomers were looking for distant supernovae in order to learn about the evolution and fate of the universe. Well, they did it because supernovae of type Ia are so-called "standard candles", whose brightness is almost always more or less the same, and which can be recognized over vast distances because of their distinctive light curve. When astronomers find a distant supernova of type Ia, they can know how far away it is simply by measuring how exactly how faint it appears to be and compare that with how bright they know the supernova to be. The difference between the supernova's measured faintness and its known intrinsic brightness is a measure of the true distance to that supernova.

Just like stars and galaxies, supernovae have spectra with spectral lines. These spectral lines will be redshifted. By detecting and identifying spectral lines in the supernova's light curve, astronomers can determine the redshift of the supernova. (If you don't remember spectral lines and redshift, read this .)

In more nearby light sources, like moderately nearby galaxies, redshift can be directly translated as the galaxy's current "recession velocity", its current "speed away from us". But that is not necessarily the case with distant supernovae. The reason for this is that redshift doesn't measure a galaxy's current velocity as much as it measures how much space has expanded since the light that reaches us from it was emitted.



Albert Einstein visualized spacetime as an elastic sheet, in which massive bodies make indentations. The expanding universe can be imagined as this sheet growing larger. The light emitted from light sources that are being carried away from us by the "growing sheet of spacetime" will have its light "stretched" by the growing spacetime.

A supernova always goes off at a very specific moment in time, and the light it emits starts fading after a few weeks. The light it emits therefore always originates from a very short period of time. How much the light from the supernova is redshifted is a direct measurement of how much space, or spacetime, has expanded and "stretched" since the light was emitted.

Ah, but the redshift of this supernova doesn't tell us how far away the supernova actually was when it exploded. It doesn't tell us how much or how fast space itself had expanded before the time when this particular supernova went off. It certainly doesn't tell us if the "rate of expansion" of the universe has changed since the supernova exploded.

But a precise measurement of the actual distance to the supernova, which can be done by measuring how faint it appears to be compared with how bright it is known to be, will tell us exactly how far away it is.

Now imagine a supernova which goes off in a universe where the "rate of expansion" has remained perfectly constant. In such a universe, the redshift and "velocity" of the supernova corresponds exactly to the actual distance of it, because the "velocity of space itself" has never changed. But imagine, instead, that the universe has been slowing down, which is what the two teams of astronomers who were looking for supernovae had been expecting to find. How would that have affected the appearance of the supernova? Well, it would have been unexpectely bright!



(This is a picture of a comet, not a supernova, but I just thought I needed a picture here to lighten things up.)

Why would a supernova look unexpectedly bright for its redshift if the universe was slowing down?

Frankly I'm not going to make a too detailed argument to you about this, because there is a nagging aspect of it that I really don't understand myself. Suffice it to say, therefore, that astronomers insist that a far-away supernova in a decelerating universe will appear brighter than it would in a universe where the expansion rate is constant, let alone a sujpernova in a universe that is accelerating.

My problem is that when I try to figure the whole thing out, my mind insists that a distant supernova would look fainter than it "ought to" if the universe was slowing down. But the entire astronomical community have been over the results of the two supernova teams with a fine-toothed comb, and if the faintness of the distant supernovae and the corresponding unexpectedly great distance to them was due to a decelerating universe and not an accelerating one, then you can bet your boots that one astronomer or another would have caught that glaring mistake by now.

So instead of confusing you by calling attention to the thing that I don't understand myself, I'll just repeat the astronomers' argument:

If the expansion rate of the universe has changed, then redshift can't be used as a reliable distance indicator to far-away objects. Redshifts tell only half the story, and you have to measure the exact distance to far-away objects with a known intrinsic luminosity and compare that with their apparent luminosity as well as with their redshifts. That will tell you not only how far away the distant supernova really is, but also how the universe has expanded before and after the light of the supernova was emitted.

Astronomers say that if the universe is slowing down, then the distant supernova will appear to be "too bright" compared with the distance that is indicated from their redshift. But if the universe was to be accelerating, which everyone would have thought was a total impossibility before the "supernovae redshift teams" presented their results, then the distant supernovae will appear to be "too faint for their redshifts".

That was the result that the two teams were getting. The universe wasn't slowing down. It wasn't even expanding at a constant rate.

It was accelerating.

Ann

The suggestion that the universe is accelerating came as a complete shock to almost all astronomers. To see how weird the idea is, imagine that you are at a baseball match and watch a player hit the ball perfectly with his bat. The ball flies out across the playing field in a perfect, high arc.



People are ooh-ing and aahh-ing as the ball flies higher and higher. But wait a minute... isn't it going to come down? Oh, look! Goodness! What's happening? Look, the ball is picking up speed! It starts flying over the city outside! And look! It's disappearing at the horizon! It's a bird, it's a plane, it's a speeding baseball!



It isn't coming down!!!

To most astronomers, an accelerating universe is just as counter-intuitive as a baseball that refuses to come down. So if the universe is indeed accelerating, why is it doing that? No one knows, but astronomers have at least come up with a name for the culprit responsible: Dark energy.



I thought that this "illustration" of dark energy was as good as any.

Dark energy is even weirder than dark matter. Dark matter is thought to be some kind of strange particles in the universe which add gravity and structure to the universe, even though they can't be seen in any other way. But dark energy, well - that is thought of by many as being an unexplained part of the very space of the universe, the very fabric of it.

I think people often tend to take the fabric of space, the emptiness of it, for granted. What we tend to care about are the things that are found in space. Astronomers have spent so much more time describing stars, planets, gas clouds, galaxies and other "objects in space" than they have taken an interest in space, the "spaciness of space", the fact that there is a lot of space out there where stars, planets and people can exist.

Imagine if there was no space. Imagine if everything was packed close, without any breathing space. (Me the claustrophobiac have a minor panic attack at the thought of it.) Compare the idea of a universe with no space with the idea of having no house to live in, but still being in possession of all the things you own, and being forced to store all those things in no space at all.

Some people and some astronomers have indeed thought about the space of the universe. Einstein thought a lot about it.



Four-dimensional spacetime, according to Einstein.

Interestingly enough, the Big Bang theory says some very interesting things about space. Many people who ask about the Big Bang wonder where in the universe it took place. Can we point in the direction of the sky toward the place where the Big Bang went off?



I googled "Big Bang" and came up with this image, among others. Surely this picture suggests to us that the Big Bang was an explosion which took place in a pre-existing black space of some kind?

But according to the Big Bang theory, the "space" of our universe didn't exist before the Big Bang. Space itself, the emptiness of space, the "volume of space" was created in the Big Bang. Don't ask where in the universe the Big Bang took place, because it took place everywhere in the universe. The universe is encompassed by the Big Bang.

Dark energy can be seen as as an innate tendency of the emptiness of space to keep growing, to keep expanding outwards in all direction at an ever-increasing pace.



This "illustration" of "expanding space" will have to do.

Astronomers are still at a loss to explain dark energy. Many of them would like the concept to go away. After all, how can anyone play baseball if the baseballs refuse to come down after you have sent them flying?

But dark energy is gaining more and more acceptance, nevertheless. This is what Wikipedia says on the matter:

It would seem that the universe is taking us along for a ride.

[img]http://www.tzofit.co.il/var/img/1504[/img]

Ann