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More about Dark Energy

Let us know more about dark energy, a mysterious form of energy that is yet to be identified.
Previously, we learned 71 percent of the energy budget of the universe, it’s so-called dark energy. Then, how do we know 71 percent is dark energy? This is the subject today. Let’s get started. So this is the Hubble diagram, Hubble used this to discover the expansion of the universe. This is plotting velocity against a distance for each galaxies outside our Milky Way galaxy. And this plot is showing more distant galaxies are moving away from us faster, and this was the expan-, the evidence of the expansion of the universe. This was the evidence of the expansion of the universe.
If you imagine the universe is something like this pancake, with raisins inside, then say, Milky Way galaxy, we’re here, in this raisin, and in this raisin, nearby raisin is moving away from us from five centimeter to 10 centimeter. But more distant raisin, for example this one, is moving away from us, 10 centimeter to 20 centimeter, so this raisin is more distant, and then it’s moving away from us faster. So if we imagine this kind of pancake type of universe, then this is exactly, um, showing the expansion of the universe. So that’s why, um, in this kind of pancake situation, this Hubble’s plot, Hubble’s diagram, is showing the expansion of the universe, just like in a pancake universe.
Okay, at the time of the Hubble, Hubble used C5 variable stars to measure distances to other galaxies. But nowadays, researchers use Type 1A supernova to measure distances to galaxies. So this is one example of Type 1 supernova, it’s very, very bright. One of the advantage of using Type 1 supernova, it’s much, much brighter than C5 variable stars, in fact it’s one million times brighter than C5 variable, so, we can measure the distances to more distant, much more distant galaxies. For example, this is just explosion of one Type 1 supernova, but, it’s almost as bright as its host galaxy, and host galaxy has 100 billion stars, so explosion of one star is as bright as 100 billion stars.
What is Type 1 supernova? Type 1 supernova, we believe, researchers believe, is a white dwarf binary system. So here’s a white dwarf star, and it has a companion star, and as a companion star gets old, it accretes materials from companion star, these are dust, and gas, to the white dwarf star, and as white dwarf star get more mass, more and more massive— when it reaches 1.4 times solar mass, then its electron degeneracy pressure cannot sustain its gravity anymore, and it collapse(s) and explodes as Type 1 supernova. So this is Type 1, the model of the Type 1 supernova researchers believe.
And then in 1998, two teams of researchers made use this Type 1 supernova to measure the distances of galaxies, just like Hubble. One advantage of using Type 1 supernova is that because it always explode(s) at Chandrasekhar mass, its brightness is always the same, so we know the intrinsic brightness. If you know the intrinsic brightness, we can use them as standard candle. For example, if you know the brightness of the candle, by, if the candle looks faint, that means candle is far away. If the candle looks bright, candle is nearby. In a similar way, because we know the intrinsic brightness of Type 1 supernova, if supernova looks faint, that means farther away, if supernova looks bright, it’s nearby.
So we can use Type 1 supernova to measure the distances to galaxies. So this is the modern version of the Hubble diagram. Those two teams of researchers used Type 1 supernova to measure distances. And then this is redshift which is distances, and against magnitude, magnitude is brightness; because this is magnitude, the larger number means fainter. And then, very, and then these data points are individual supernova. And very interestingly, and very surprisingly, those brightness of Type 1 supernova were a little bit fainter than expected, so as you can see those data points are a little bit above than the expected this model. What does that mean?
So that means if these galaxies are fainter, because the expansion of the universe is not just expansion, but this expansion is accelerating, pushed by dark energy, so this is a discovery of the dark energy, and a discovery of the accelerating expansion of the universe. Because of the discovery, these gentlemen received a Nobel Physics Prize in 2011. So this is how we know 71 percent of the total energy budget of the universe is dark energy. But then, what is dark energy?
In quantum mechanics you might think this so-called zero-point energy, this is so, in quantum mechanics, you cool down the particles to absolute zero temperature, but still this Heisenberg’s Uncertainty Principle, so you cannot determine the position of the particles very, very accurately, so there’s some remaining fluctuations even at zero temperature, and if there’s a fluctuation that be associated energy, this is the so-called zero-point energy. And this zero-point energy might be the dark energy because it exists even at zero temperature and it’s associated with space. Let’s, but let’s compare the amplitude of this zero-point fluctuation and the dark energy. Dark energy’s energy density is something like 10 to the minus 24 gram per cubic meter.
But the energy density of zero-point energy is 10 to the 99 gram per cubic meter. So, unfortunately this zero-point energy density is much, much larger than that of dark energy. In fact, it’s order of 122 too large. This is the biggest discrepancy be, in physics, between theory and experiments. So, this problem is unfortunately not solved. So, dark energy still remains mystery and it is one of the greatest mysteries in modern physics.

Just like dark matter, dark energy is there, but we can’t see it. What is it exactly?

Prof. Goto will discuss more about dark energy. This video will supplement the previous article you’ve read about dark energy.

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Mysteries Of The Universe

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