「暗能量」淺釋(3之3)

Dark energy rules the universe

Karl S. Kruszelnicki, 08/20/13

Last time, I talked about dark energy and how it makes up the missing 69 per cent of the universe. I also talked about how it's been increasing the rate of expansion of the universe.

Why did the effect of dark energy show itself only five billion years ago? Why not earlier, or later?

Well, it was because there has always been a battle between gravity, which is an 'attractive' force and dark energy, which is a 'repulsive' force. About five billion years ago was when dark energy finally got bigger than gravity.

So first, gravity – gravity is caused by the regular matter and the dark matter that's present in the universe. Gravity has always been trying to slow down the expansion of the universe, but the effect of gravity has been decreasing as the matter in the universe has thinned out.

Okay, second – dark energy. Dark energy seems to be an inherent property of space-time. But not everybody agrees – it's in no way fully proven yet. There seems to be a certain amount of dark energy in each cubic metre of the universe and so, if dark energy is an inherent property of space-time, well then, as the universe expands, it 'creates' more space-time and therefore, it also creates more dark energy, according to some theoretical physicists.

It turns out that about five billion years ago was when the repulsive force of dark energy began to outweigh the attractive force of gravity.

However, the amount of this dark energy, in any given cubic metre of the universe, seems to be always the same and seems to be very, very small. As a comparison, it's roughly a millionth of a millionth of a millionth of a millionth of a millionth of the matter in our planet's crust. But space is very, very big and there are relatively few planets and stars, so, over the whole universe, dark energy adds up to be about 69 per cent of everything.

One big question is: "What is the Universe expanding into?" and, of course, one slightly facetious answer is: "The future".

But another big question is: "What happens in the future?"

Well, in the short term of the next billion years or so, the Sun will heat up so much that life on Earth will be hard-pressed to survive. In five billion years, our Sun will expand to swallow Mercury, probably Venus and possibly the Earth.

But in the long term, dark energy will dominate.

Now, it all depends on what dark energy turns out to be. Now, if the amount of dark energy in each cubic metre is constant, then the universe will thin out and space will be cold and empty. The rest of the universe will disappear from view. This is called the 'Big Chill'.

In 100 billion years, the clusters of galaxies that fill the sky would stretch to the breaking point, leaving us with isolated clumps of galaxies, separated from each other by huge emptiness. The relative speeds of separating galaxies will be faster than the speed of light, so we won't be able to see them. Our home galaxy could be the only galaxy visible in an otherwise empty universe.

Inside our galaxy, the frugal, low-mass stars will burn their fuel slowly, but only for 10 to 20 trillion years. New stars will occasionally form, but most of these will burn out within 100 trillion years. In a cosmic sense, it will truly be the Big Chill.

But let's make a different assumption. Let's assume that dark energy is something different and that the amount of dark energy in each cubic metre increases with time. In that case, we are heading for a different scenario – the 'Big Rip'.

Under the so-called Big Rip scenario, everything gets torn apart by the increasing dark energy, about 50 to 100 billion years in the future. This will be the end of everything.

About 200 million years before the end, the stars in our galaxy get ripped apart from each other, and fly off into empty space. One year before the end, the planets in our solar system get flung into empty space. One hour before the end, the atoms that make up our planet explode away from each other. One tenth of a billionth of a billionth of a second before the end, the electrons get ripped away from the central nucleus in their atom. And at one ten thousandth of a billionth of a billionth of a second before the end, the actual protons and neutrons in the nucleus of atoms get torn apart from each other.

Darth Vader in the Star Wars saga got it wrong. It's not 'The Force' that controls the universe – it's dark energy.

© 2013 Karl S. Kruszelnicki Pty Ltd, Published 20 August 2013

http://www.abc.net.au/science/articles/2013/08/20/3829393.htm

「暗能量」淺釋(3之2)

How do we know how fast the universe is expanding?

Karl S. Kruszelnicki, 08/13/13

Last time, I talked about dark energy, how it was recently discovered, and how it makes up the missing 69 per cent of the 'energy budget' of the universe.

Now, you might have come across the concept that the universe is expanding. What dark energy does is make the expansion accelerate -- so today the universe is expanding at a rate about one-quarter times faster than it was five billion years ago.

Back in the 1920s, the astronomer Edwin Hubble announced his discovery that the universe was expanding. The galaxies were moving further apart.

Hubble used the new 100-inch Mount Wilson telescope to look at galaxies. He measured their Doppler shift. That's the change in frequency, due to the speed of the galaxy. He found that practically all galaxies were moving away from us. This shifted the colour of their light towards the red end of the colour spectrum -- hence the phrase, 'red shift'. Hubble also found that the further away a galaxy was; the bigger was its red shift.

Since the 1920s, the astronomers assumed, thanks to gravity doing its 'sucking' thing that this expansion would mellow and slow down.

But in May 1998, cosmologists held a conference called The Missing Energy in the Universe. Some of them presented their data showing that the universe had strangely increased its rate of expansion, about five billion years ago and, as a result, the universe was bigger and emptier than we previously thought.

Of the 60 scientists at the conference, 40 accepted the revolutionary new findings. Now, this was very unusual. Generally, a poll is not taken at scientific meetings asking if the scientists do or do not accept a result. The science of 'dark energy' was just beginning -- like a baby.

But, how did astronomers measure that the universe was expanding at a decreasing rate until about five billion years ago and then it gently began to expand at a faster rate?

Answer -- they used 'exploding stars' as a 'standard candle'.

Exploding stars? What are they?

Well, there are a few different mechanisms by which stars can explode. Our astronomers looked at type 1a supernovae, which start off as a 'white dwarf'.

Now, a white dwarf is a 'degenerate' star that has finished its 'normal' burning, and is now in the later stages of its life. This stage is the common end point of evolution for stars that started off with a mass between eight and 10.5 times greater than the mass of our Sun. The star has now cooled down and collapsed. It's about the size of our Earth, but has roughly the mass of our Sun, so it's very dense (about one tone per mil). There are about eight white dwarfs in the nearest 100 star systems.

To make a type 1a supernova, imagine that a white dwarf and a big star are in a tight orbit around each other. The white dwarf 'sucks' some hot gas from the bigger star onto its surface. Once this layer gets a metre-or-two thick and the white dwarf reaches a mass of about 1.38 times the mass of our Sun, the whole star explodes, like a giant hydrogen bomb.

The white dwarf erupts outwards at about five to 20,000 kilometres per second. After three weeks, the explosion reaches about 10 billion times the brightness of our Sun and then it fades over the next few months. These supernovae are all close to the same brightness, because they all explode at around the same mass.

Now, I mentioned 'standard candle'? What is that? Well, a standard candle is something that has a fixed and known brightness.

You see, a major part of astronomy is measuring the distance to stars and galaxies and the like. But, how do you tell the difference between a faint star that is close and a bright star that is distant? After all, to your naked eye, they each have the same brightness.

Luckily, type 1a supernovae all pretty well reach the same maximum brightness. So, if it's bright, it's close, and if it's faint, it's far away. This is how a type 1a supernova can make a reasonable standard candle. We can measure their actual distance by their brightness and the red shift will give us their supposed distance.

Two separate teams of astronomers measured many type 1a supernovae. The younger and closer ones were where they expected them to be, but the older and more distant ones were not. They were fainter and further away. It was as though the expansion rate of the universe had sped up about five billion years ago.

The astronomers were seeing the fingerprint of dark energy -- the missing 69 per cent of the universe. So, next time, I'll talk about why dark energy made the expansion of the universe increase -- and what it will do in the future …

© 2013 Karl S. Kruszelnicki Pty Ltd, Published 13 August 2013

http://www.abc.net.au/science/articles/2013/08/13/3824492.htm