Inferring Dark Matter from Cosmic Shear

Inferring Dark Matter from Cosmic Shear

(Originally posted December 09, 2016 on Blogger)

Science is awesome, if for no other reason than it brings people of all nationalities together with the goal of advancing humanity’s collective knowledge of... well everything we can imagine and more. From the ISS, to international collaborations planning missions to Mars; science is to humanity, what math is to the universe; the one language not confounded due to an over-built ziggurat.

The European Southern Observatory (ESO) is a perfect example of an international collaboration in science. From the ESO comes a recent paper published in the Monthly Notices of the Royal Astronomical Society which may have implications for our fundamental understanding of the early universe.

As it stands, what we know of the early universe is that there was a point in spacetime when matter was created; nucleosynthesis.

We all know what matter is; it’s the stuff that makes you and me, the stars, pickles, and peacocks... all that stuff is made up of baryonic matter. Yet this form of matter makes up less than 5% of all matter in our universe. To further our intrigue, baryonic matter (and I assume dark matter as well) aren’t really things, but properties that all energy exhibits. And we all know what energy is... or do we? It seems to me we know energy more by what it does, rather than by what it is; nuclear, chemical, electromagnetic...

So if you watch popular videos like this guy on YouTube: you may walk away feeling satisfied “that mass and energy are the same thing”. Wrong. If they were the same then that patent clerk’s famous equation would look more like this: E = m In fact, in Einstein's original paper, the formula equated mass to energy divided by the speed of light (causality) squared, because he wanted to know what mass was, realizing that it isn't anything, but a property of energy. Here is some simple arithmetic anyone can do to demonstrate just how wrong the notion that energy and matter are the same thing. I’ve written this before a few posts ago, but if you were to individually weigh the protons, neutrons, and electrons in any of the elements on the periodic table, you’ll find that the respective sums of each element’s weight do not equal the atomic weight shown on the table. So the next time you hear the phrase, "...converting mass into energy", you'll know enough to know that's just plain terrible.

All that aside, in the early universe when “normal” (baryonic) matter was synthesized, it seems apparent that another form of matter must have formed as well; dark matter. This is evidenced by the structure of our universe and the structures within it. No one is sure ‘what’ dark matter is yet... axions? Teeny-tiny, itty-bitty primordial black holes (not likely), MACHOs (Massive Compact Halo Objects), or the WIMPs they pick on (Weakly Interacting Massive Particles)? It isn’t yet known. Like energy (not coincidentally), we know of dark matter through inference.

Back in the 1980s, when I was more interested in building treehouses, it was a popular idea in the academic world of cosmology that there was this hot dark matter (HDM) that existed throughout the universe. It began with large-scale cosmic structures like large quasar groups, galaxy filaments, and superclusters (like the recently-discovered Hercules-Corona Borealis Great Wall whose maximum dimension is 10 BILLION light years across), then fragmented down to smaller and smaller structures like our own Milky Way.

However, this HDM theory didn’t meld well with observations, and a new cold dark matter (CDM) theory of structure was fit into place. In CDM theory of structure, the universe starts out with smaller structures that collapse under their own gravitational effects, leading to larger and larger structure formation (like that Herculean superstructure). A bottom-up hierarchy... the opposite of HDM theory’s top-down model.

So what’s the big deal with this ESO paper? Before I TRY to explain the paper (which I’ll probably do terribly), it’s important to know that CDM theory of structure is an integral part of what is considered the standard model of Big Bang cosmology; the Lamda-CDM model. By extension, it also plays a key role in inflationary theory (what happened before the “big bang”). And though inflationary theory doesn’t have a standard model... yet... any apparent discrepancies in our (our as in their) understanding of CDM theory could have very important implications for our understanding of the early universe. So it’s this point where I’ve made full circle (partial circle back to my second paragraph), and can make a feeble attempt to summarize the paper.

In what is dubbed the Kilo-Degree Survey (KiDS), scientists at the ESO in Chile pointed a very powerful telescope called the VLT survey telescope towards the sky. They imaged sections of sky (tomography) eventually covering 450 square degrees of it; the largest area yet imaged which included 15 million galaxies! With powerful fancy-named software, they resolved these images to find that this inferred dark matter, which makes up the cosmic web, appears less dense, and less clumpy than previously determined by imaging done by the WMAP & Planck satellites.

They determined this by looking at how light bends near superstructures. Light bends around large structures like galaxies due to gravitational effects in what is known as gravitational lensing. But KiDS looked for the gravitational lensing that occurs on a much, much larger scale. It looked at how light bends around superstructures (those superclusters, filaments, etc..); gravitational lensing on this scale is called cosmic shear; a very subtle bending of light that when calculated gives inference to CDM structure, and it seems to be inconsistent with the Planck data. Of course, this doesn’t mean everything we have come to understand to this point is wrong, in fact this makes future missions to look wider and deeper into our universe all the more important; I mean you Euclid satellite! And you too Large Synoptic Survey Telescope! This potential new discovery is far from being an obstacle to progress, but more of a fuel to churn the unstoppable engines of scientific inquiry to ever greater reaches of the universe!

Here is a link to the freely-available paper:

...and a link to KiDS:

As for inflationary theory, and the interesting fact that most models of inflation point towards an eternal universe, check out this documentary created by my friend SkyDivePhil & his wife on YouTube:

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