Existence of dark energy.

Science has taught us a lot about how the world and the universe work, but there are still many mysteries that elude us. One of those mysteries is dark energy, the force that supposedly counters gravity and aids the increasingly rapid expansion of the universe. Because we cannot observe dark energy, there are still many questions regarding its existence. Researchers from the University of Portsmouth and LMU University Munich have conducted a study and found that dark energy explains certain phenomena in our universe with a statistical chance of 99,996 percent; an almost certainty. Work on dark energy has already resulted in a Nobel prize.

Cosmic background
When you watch tv, but tune in on a frequency that is not used for tv broadcasts, you observe something called white noise. A small part of this grainy pattern is caused by the so-called cosmic microwave background radiation (CMBR). This is left-over radiation from the Big Bang, and is thought to protrude into all parts of the universe. While we see a dark sky at night, a sufficiently sensitive telescope is able to sense the background radiation as a faint glow.

Shift
Because of the Doppler effect, planets, stars or even galaxies can cause a so-called redshift or blueshift when they are moving away from us, or nearing us, respectively. When a heavenly body moves away from us, it 'stretches' the radio waves it emits, which means there is a shift that leans towards the 'red end' of the spectrum. By measuring blue- or redshifts, we can say something about whether a planet, star or galaxy is moving towards us, or is moving away from us.

At the centre you see the observed radiation pattern (black vertical lines) if a heavenly body would not be changing its distance with respect to our own earth. When it moves towards us, the wavelengths of the radiation are 'compressed' and we observe a 'blueshift'. It also works the other way around, which is called a redshift.

What actually happens to the emitted radio waves.

Expanding universe
The CMBR can also undergo blue- or redshifts, and this can tell us something about the expansion of the universe. Astronomers have created models of how 'hot' certain parts of the universe are, as temperature correlates with the level of redshift caused by the expansion of the universe. This makes sense because compression causes an increase in temperature, while expansion would drop it instead. Researchers have already found that the speed at which something moves away from us is proportional to its distance. Basically, it means that the expansion of the universe goes faster as it is moving away from us. The ever-increasing speed at which the universe expands can be compared with blowing up a balloon: it is not like point A and B move away from each other, but it is more like everything in between 'stretches'.

Expansion at different time points (t). The increase in distance between galaxies when comparing t=2 with t=1 is larger than when comparing t=1 to t=0.

Models
Everything stated above is quite well established. What is not quite established is what causes the accelerating expansion of the universe. It is especially peculiar because we have gravity pulling galaxies together, yet the universe does not seem to cease its expansion. This mysterious force, called dark energy, ought to explain it, but it has not yet been observed. The researchers from the University of Portsmouth and LMU University Munich used models incorporating large parts of the universe with the corresponding CMBR and calculated whether a phenomenon such as dark energy could explain the hotter and colder parts. They showed that it is 99,996 percent likely (Sigma 4,4) that dark energy is the cause of what we observe.

Outlook
While this study gives us more certainty about whether we need a force like dark energy to explain our observations, we still have not learned anything about what it actually is. Scientists already calculated that dark energy makes up almost three quarters of all the universe's content, with matter filling up the remaining part. Actually, visible matter comprises only a few percent of the universe, the rest being filled up by the equally mysterious dark matter. It is quite clear that we still have much to learn about the universe, but at least we know pretty sure now that these 'dark forces' exist.

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