From time to time when I hear of an exciting astrophysics discovery I wait a day and see it reported on all over the main stream media.
So when I saw that the current theory for Dark Energy had been proven, I was surprised that it wasn't reported everywhere.
My only thought was that it was just too difficult to explain.
So as a personal challenge, here is my explanation:
Back Story One (The Cosmological Constant)
Shortly after publishing his General Relativity Theory, Einstein started checking his work and ran into a sticky problem. Since gravity is an attraction force only, with no counter acting repulsive force, the Universe should already have collapsed.
Since all of the scientists of the time believed that the Universe was generally static Einstein assumed there was a serious flaw in his theory and went to work.
The result was a new term in his gravitational equation called the cosmological constant. This new part of the General Relativity theory states that space is constantly expanding by a force inherent within itself. Thus the constantly expanding space within the Universe was preventing the Universe's gravitational collapse.
Ten years later Hubble discovered the expanding Universe and Einstein labeled the cosmological constant “his greatest blunder”.
Back Story Two (The Cosmic Microwave Background)
In its first second the Universe “inflated” from the smallest possible piece of space to a size about 1/1000 of its current size. In that same second the Universe became filled with matter and anti-matter which proceeded to attract and annihilate each other leaving behind a Universe expanding with its own momentum and full of very high energy, short wavelength photons.
Over time, as these photons traveled through an expanding Universe, their wavelengths became longer and longer as the Universe stretched “beneath” them.
If you point a radio telescope in any direction you can record these photons that have been traveling towards us for over 13 billion years and they have very long wavelengths. (This is the proof of the “Big Bang” theory.)
Back Story Three (A Big Crunch?)
With the “repulsive gravity” (aka “Cosmological Constant”) theory in the trash basket scientist then determined there were only two possibilities for the future of the Universe. Either the Universe was expanding with enough momentum that it would expand forever (like how the Viking spacecraft has escaped the solar system) or as Einstein feared it would ultimately pull itself back into a “big crunch”. In either case the expansion of the Universe was slowing down due to the pull of gravity, it was simply a question of how quickly it was slowing down.
A lot of resources and brain power was put in to determining which of these futures was in store.
One of the side discoveries of this scientific quest was the discovery that 74% of the energy of the Universe was missing.
In 1998, cosmology was shaken at its foundations as two research teams presented their findings. They had discovered that while the expansion of the Universe had initially been slowing down, now the expansion is accelerating! (They won the 2011 Nobel Prize for this discovery).
The good news for scientists was that the missing 74% of the Energy of the Universe had been found. But what was this “dark energy”?
Einstein’s biggest blunder, the Cosmological Constant came out of the trash bin.
If space is inherently repulsive (aka growing), then as the Universe grew it would become even more repulsive because there was just more stuff to grow and the expansion of the Universe would accelerate over time just as is observed.
However, a good and reasonable idea is not a proof.
The Proof of Dark Energy
If you made it this far then maybe you are daring enough for the rest.
As we just learned, these photons get longer as they travel through empty space because space is expanding “beneath” them.
What would happen to these photons if they traveled through an area of space with a lot of mass (like a galaxy cluster) which would be curving space and kind of bunching it up. (This is called a “gravity well” although I would think a “gravity bowl” would be a better name.)
Imagine yourself on a swing. At the highest point you are not moving at all, then you accelerate down to the lowest point which is the fastest speed and then slow down until you are at the top on the other side and come to a stop.
The same type of thing happens to these photons. As they enter the “gravity bowl” they start to “accelerate” (which because of special relativity makes their wavelengths shorter because they can’t go faster). As they leave the “gravity bowl” they “slow down” (which makes their wavelengths longer). When it is all done the wavelength should be the same as when they entered the gravity bowl. Actually a little bit longer because they traveled through some expanding space.
If the “Dark Energy” / “Cosmological Constant” theory is correct then there is something called the “Sachs-Wolfe Effect” (actually the Late-time integrated Sachs–Wolfe effect, aka Rees–Sciama effect).
The Sachs-Wolfe Effect says that if space is always expanding, then the “bowl” is expanding over time also.
Image a large clay bowl on a potter’s wheel.
If there is no friction and you release a marble at the lip of the bowl, the marble will travel up to the lip of the bowl on the other side stop, return back to the center and up to the exact place where the marble was released.
However, if after the marble is released you start widening the bowl, the bowl will become shallower. The marble instead of traveling to the top of the bowl on the far side and stopping, it travels over the edge of the now shallower side. This would look like something was accelerating the marble as it traveled through the bowl.
This is exactly what happens with a photon traveling through a “gravity bowl”. It appears to be accelerated compared to its twin traveling through empty space.
This effect can only happen (we think) because the space itself is expanding and the “Dark Energy” / “Cosmological Constant” theory is correct. And it has been proven to happen with an error possibility less than .004%