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Pineta


Particle Physics and Pony Fiction Experimentalist

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Apr
27th
2019

Dark Energy · 3:00pm Apr 27th, 2019

To one of my past blog posts, Booster Spice gave the random comment:

Dark Energy is Magic?

I can’t remember the context, but I’m taking it as a prompt for this post.

As poets, journalists, and brand designers know, dark is a powerful prefix. You can stick it in front of a suitable noun to create something mysterious and sinister, with a hint of Evil Magic. Dark Money, Dark Web, Dark Force… There are countless opportunities for pop culture references from Star Wars to Game of Thrones.

When academics use the term, this is not what is usually intended. Dark is a shorthand for ‘we can’t see exactly what it is’. The Dark Ages was a name for the period of European history after the decline of the Roman Empire. Dark because, with few written records, historians didn’t know much about it, and it suited the narrative that these centuries were a gloomy period, when Europe was hidden under dark clouds and everyone was on the lookout for witches and other darks things, until the Christians arrived to brighten everything up. In our more enlightened times, serious historians prefer the term Early Middle Ages.

The Dark Age of the Universe is a less controversial term among astrophysicists. It was when the universe was literally dark, after any photons from the cosmic microwave background had been redshifted beyond visible light, but before the first stars lit up, when the universe was around 100 million years old. At that time the universe was dominated by cold Dark Matter.

I did my PhD working on Dark Matter, so it is a story I have told many times, in different ways. It’s a good story. A real mystery – what makes up the majority of the mass of the galaxy? We don’t know. It combines some of the best bits of astronomy and particle physics. Then moving on to how we search for it, which involves sophisticated detector technology in deep underground laboratory, with the glamour of a good spy story.

When telling the story of dark matter, a science writer has to pick their words carefully. Note I wrote “the majority of the mass of the galaxy” not the universe. Dark matter is spread across the cosmos, however a few billion years ago the matter-dominated era of the universe ended. The majority of the mass of the universe is now made up of something even stranger. Something so strange that it's better not to mention it in a dark matter story if we want to keep it within a reasonable word count. Let’s now move on to the sequel, where we will dial up the mysteriousness factor a bit further, and introduce Dark Energy.

While dark matter does not interact much with ordinary matter, it is still matter in the sense that it is made up of particles. Clouds of these dark particles are shaped by gravity and clump around galaxies. Dark energy is a uniform mass-energy spread across space. To summarise the 14 billion-year history of the universe: it began with the Big Bang, where a super-hot dense mix of particles and radiation rapidly expanded. This expansion continues and we can see it today as distant galaxies are moving away from us with speeds that increase the further away they are. The equations governing this describe how the pressure-driven expansion is countered by the gravitational attraction of the total mass. They predicts two possibilities for the Ultimate Fate of the Universe. If the density is big enough, it will reach a maximum size and then collapse, ending in a Big Crunch, however observational data seems to favour the other possibility: that the universe will continue to expand forever.

So far there is no need to bring Dark Energy into this theory – Dark Matter is enough. However in 1998 came the exciting discovery that the expansion of the universe was accelerating – the most distant galaxies are flying away from us even faster than expected. Explaining this required an addition to the model, called Dark Energy, which gives the vacuum throughout the universe an intrinsic density and pressure.

We are now in an era of precision cosmology, where probes such as the Planck space observatory have taken high resolution maps of the cosmic microwave background radiation from shortly after the Big Bang. This has allowed us to calculate cosmological parameters and show that Dark Energy makes up 68% of the density of the universe (dark matter makes up 27% and ordinary matter 5%).

However explaining exactly what dark energy is, is not so easy. It's a mysterious force causing the universe to expand. There are various theories, but we may as well just say Dark Energy is Magic.

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Comments ( 13 )

I'm a laypony when it comes to cosmology and other deeper physics stuff, but how I feel about DM and DE is that the former almost certainly exists, only we haven't figured out what it is yet; while the latter looks like one of those anomalies you get when you are missing something very fundamental about how the universe works. I'm hoping for it to end up as the Ultraviolet Catastrophe of cosmology and astrophysics, and ends up spurring a lot of new and interesting science. :twilightsmile:

Three things Id like to put to you, given the third one I saw earlier today.

Stanford announced the creation of a 4 inch long lithium niobate piezo electric crystal rod, that when driven by the appropiatly coded varying electrical signal, generated a modulate VLF radio signal with a data rate of 100 bits per second. For use with submarine and deep cavern communications. What really upset me though was they said it was useful for occasional text message, when at that speed they should be saying speech to text to morse to text to language translator to speech, with some old dial up webcam video thrown in as well. After all, Voyager is running at 16 bits a second.

The other two however, are one is a proposed model for energy ejection jets by a pulsar utelising an offset positioned rotating multipoint emmitting array, like a stick on the end of a drill, thats bent at an angle, then along the stick theres antenna that emit signals at differnt phases and rates. Suposedly gives a very strange overall beam behaviour.

The third one is even weirder. Pirelli, the tyre company, claimed to have at one point, the patent for a neutrino detector, using a large perfect crystal whose structure deformed, generating signals as neutrinos pased through it. Lots of people claim things, but eventually something sticks, then they claim they were right because of the accident?

Then again, unicorn horns have a lot of extremely complex structures including atomic layouts in nudged patterns, with lots of complex neurons triggering them.

There was somethng to do with variable frequency LEDs using Exitons in 2D layered atomic structures that remain useable at room temperature, instead of millikelvin.

I like to think that Dark Energy is the Imaginary part of the Complex solution to GYT, where Matter, Energy is the Real Part. Except given normal Gravity is attractive, that should mean that space and mass is imaginary, to give a negative force, and Dark Energy is real, to give a positive force, repelling? Squeeze everyhting together, and due to reinforing effects being asymetrical, eventhally things go boom?

Newton didnt fall when Quantum Mechanics and Relativity came along, just as Einstein doesnt have to fall if it actually turns out that to combine the two you need a variable speed of light. As long as any variation required follows Relativistic rules and gives rise to Quantum mechanical limitations, then Einstein still holds, and was so close to discovering the effect himself.

If looking back in time shows the stuff moving faster than expected, why cant it be exploding faster than expected at the beginning, since thats when we see it? Because if the univrese was expanding faster Now, wouldnt the Local Hubble constant be higher than in the past? When compared against the plot accross observable time?

5050141
You're right. Dark matter is a clear theory, which we can, in principle, test. There has always been the alternative idea that maybe dark matter doesn't exist and instead there's something we don't understand about gravity, but the observational evidence supports the dark matter hypothesis.

While dark energy isn't really a theory, it's just a statement of what we don't know. It's not so much a matter of trying to test a hypothesis, but to figure out what it is, which we hope will be some new and interesting science.

5050145
Got a bit lost in all that. Just to comment on the Pirelli neutrino detector. That was a story from years ago (around 2003). They had an ambitious idea to use a neutrino beam to send information through the Earth. Potentially this could be a fraction of a second faster than an optical fibre signal on the surface, which would have real commercial value (for sending buy/sell messages to stock markets).

But as neutrinos are so weakly interacting you would need a large accelerator to produce a beam and a huge underground detector to detect them. The necessary scale meant the idea didn't seem viable. However they proposed to build a new type of detector based on coherent scattering on sapphire crystals. It was not clear how this would work and all the physicists I have spoken to about it thought it was a pretty crackpot idea. But we were quite happy for a private company to fund such a research project.

We haven't heard anything since.

PresentPerfect
Author Interviewer

I always found dark matter to be more strange than dark energy. Dark energy does have simple explanation that, at least to my knowledge, fits the data fairly well, having a non-zero cosmological constant is a trivial addition to general relativity, and is a concept, that space time isn't almost as old general relativity itself. On the other hand, dark matter, while having vast amount of evidence for it, seems to indicate we have a large gap in our knowledge of the how the universe works. First, it evidence points towards that should be cold dark matter, dark matter that slowly compared to light, that is non-baryonic as the data points toward there isn't enough baryonic dark matter in the universe. The best remaining theories require new fundamental particles. That is a huge change to me in comparison to having a non-zero cosmological constant. I like to note I am not an expert, so you can take all of this with a grain of salt.

*Intrigued, thoughtful look* Fascinating...

Random mustache!! :moustache:

5050236
Good to know if I ever get bored of writing physics pony stories, there is another fandom where I could apply my skills.

5050252
Interesting points. Here we move away from experimental physics and take steps towards the dark mysterious world of aesthetics and philosophy. How can we say if one idea is stranger than another? Is the cosmological constant an elegant way to fit the data to the theory with fewest parameters? Or is it just a fudge factor? Is it extravagant to postulate the existence of an entire new universe of dark particles just to explain observations, or is it unreasonable to think we should already know of nearly everything? Why shouldn't there be much more to universe than we can currently see?

5050268
Thanks

Space is slightly repulsive. Interestingly, if we assume that repulsiveness varies with the density of the universe, one comes up with PhantomEnergy, where everything gets destroyed in a Big Rip.

I never got why physicists believed that the universe would be anything other than flat:

I expect the initial expansion and gravity must balance so the at τ = ∞, expansion equals 0. That has always been mine intuition.

5050426
A flat universe looks like an example of fine tuning - why should the parameters be set so the expansion is precisely balanced by the gravitational attraction. This was, in due course, explained by inflation theory.
https://www.symmetrymagazine.org/article/april-2015/our-flat-universe

5050535

Everything has to equal 0; otherwise, we would have an excess, or deficit, to explain.

The strangest hypothesis for dark matter I've ever seen a qualified physicist propose is that some stars are sentient and can move themselves, thus explaining Parenago’s Discontinuity:


https://www.centauri-dreams.org/2019/04/26/probing-parenago-a-dialogue-on-stellar-discontinuity/

5063306
Yeah, I think there are simpler explanations for those observations, but certainly an interesting idea.

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