The First Visual Evidence of Dark Energy?

A map of the faint microwave radiation left over after the big bang shows superclusters (red circles) and supervoids (blue circles). Credit: B. Granett, M. Neyrinck, I. Szapudi
A map of the faint microwave radiation left over after the big bang shows superclusters (red circles) and supervoids (blue circles). Credit: B. Granett, M. Neyrinck, I. Szapudi

A new cosmic map has been created by University of Hawaii astronomers showing the fingerprint of dark energy throughout the observable Universe. This is the first time such precise direct evidence of the mysterious force that is believed to be behind the continuing expansion of the Universe. By analysing microwave background radiation (the electromagnetic “echo” left over from the Big Bang), the Hawaii team have looked at the characteristics of the radiation as it passes through supervoids and superclusters. If the theory of dark energy is correct, this cosmic background radiation should cool when passing through superclusters and warm up when passing through supervoids. Analysing a huge amount of data from the Sloan Digital Sky Survey, the researchers have observed what the theory predicts and calculated that there is a 1 in 20,000 chance that their results are random. It therefore seems likely that the effect is caused by the presence of dark energy, giving us the best view yet of this strange energy that appears to permeate through the entire expanding Universe…

This is very exciting news for cosmologists. The theory of dark energy is a tough theory to prove as it is a very difficult difficult phenomenon to observe. Like dark matter, it is called “dark” for a reason; its effects can be felt or observed, but the stuff itself cannot be seen directly (because it is, well, “dark”). However, to explain the observed expansion and acceleration of the Universe, there must be something pervading the entire observable Universe giving the expansion a push. But this push cannot be seen, so astronomers are restricted to searching for dark energy’s effects on observable radiation.

What radiation can be seen everywhere with the same characteristics? Cosmic microwave background radiation, the electromagnetic waves with a temperature of 2.725 Kelvin echoing around the Universe since the Big Bang.

So, using data from the Sloan Digital Sky Survey, the University of Hawaii astronomers collected background radiation data from 100 vast regions of space. Measuring up to 500 million light years across, these regions are known as superclusters and supervoids, filled with galaxies (high density) or virtually empty (low density, mainly a vacuum) respectively. (Recently I wrote about a theory that we might be sitting in the middle of a supervoid region.) Standard theory of an expanding (but not accelerating) Universe suggests that as the background radiation passes into a supercluster, it will gain energy; as it leaves, it will lose the same amount of energy. The converse it true for supervoids. As the radiation passes into a supervoid, it will lose energy, as it leaves it will gain the same amount of energy that it lost in the first place. Therefore, the microwave background radiation should have the same energy before it enters as when it leaves either region of space. Great.

But say if there is dark energy, causing accelerated expansion of the Universe? Now this is where it gets clever. As the radiation enters a supercluster, it will gain energy. During the time the radiation travels through the supercluster, the huge region of space will have expanded at an accelerated rate relative to the radiation passing through it. As the radiation leaves, it will lose some energy, but there will be some energy left over. The net effect will be that the radiation will be warmer than other regions of space. In the supervoid case, background radiation will pass into the supervoid, lose energy and then gain energy as it leaves. However, it won’t gain all of its original energy back if the supervoid is expanding at an accelerated rate. Therefore, background radiation passing through a supervoid will be cooler than when it entered.

Background radiation + Supercluster expanding at accelerated rate = Warmer background radiation.
Background radiation + Supervoid expanding at accelerated rate = Cooler background radiation.

Although this effect has been known to exist, this is the most precise measurements to date, giving it a much better probability that astronomers are seeing a very real effect of dark energy acting on the accelerated expansion of the Universe. Plus, they’ve mapped it (pictured top), regions of cool and warm cosmic microwave background radiation. Engrossing research!

Source: National Geographic


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