An Odd Case (in Space) for Gigantism

In a media release by Alexia Lopez, a PhD student at the University of Central Lancashire (UCLan), there has been a new observation in deep space, on staggering magnitudes of distance, that have the potential to shake the current theories of cosmology and our understanding of the universe as we know it.

The Giant Arc can be seen near the middle of this image. Shared by Alexia Lopez/Jeremiah Horrock
The Giant Arc is the line of galaxies roughly in the centre of this image (Alexia M. Lopez/Jeremiah Horrock)

The recent observation — nay, discovery — in question is a humungous, nearly symmetrical “arc” of galaxies, found in a portion of the sky within the constellation Boötes. That would make this structure a Gemini Sun, which totally makes sense, seeing that disrupting a cosmological paradigm is a very Gemini thing to do. The aptly named Giant Arc spans a whopping 3.3 billion light years, and is located about 9.2 billion light years from Earth. To put this into perspective, given that the observable universe has a radius of about 46 billion light years, the width of this megastructure is a good 7% of the longest distance we can currently fathom. This figure dwarfs the diameter of our own galaxy (the Milky Way), which comes in at a paltry 0.0000011%. I will leave it to the Americans to express that in football fields. Moreover, the light from the galaxies in the Giant Arc has taken 9.2 billion years to reach us, and hence we are actually observing their embarrassing tubby time snapshots — seeing them as they were 9.2 billion years ago, when the universe was roughly half the size that it is now.

According to the article, Lopez and her adviser Roger Clowes, from the Jeremiah Horrocks Institute at UCLan, together with collaborator Gerard Williger (University of Louisville, USA), made the discovery using singly-ionised magnesium (Mg II) absorption systems towards quasars from the Sloan Digital Sky Survey (SDSS).

Simply put, the team was able to draw their findings by detecting the emission of electromagnetic waves from the above mentioned quasars. Quasi-Stellar Radio Sources, or Quasars for short, are relatively young galaxies that appear to be stars at first glance. This is due to an extremely active, and hence insanely bright, supermassive black hole at the center of the galaxy. Matter near the black hole is pulled into the accretion disk surrounding it, and superheated to millions of degrees (Kelvin) due to friction between the particles, gas and dust. This heat is what makes the quasar an epicenter of emission for electromagnetic waves.

Mg II absorption systems were used due to their propensity to exist in gas around galaxies and in galaxy groups. They were also used because they have a very distinct absorption spectrum. What this means, is that Mg II ions (charged particles) would absorb certain discrete wavelengths of the radiated electromagnetic waves. This absorption can be detected by analyzing the overall spectrum of light from these quasars that hit our measuring devices on Earth; thin gaps in this light spectrum would represent the wavelengths absorbed by the Mg II ions.

The mechanism for determining an absorption spectrum.
The typical (well, watered down) arrangement for determining absorption spectra.

As seen in the image above, is the mechanism to determine the absorption spectrum of a certain element or ion. In the case of the Giant Arc, the Hydrogen Sample would be replaced with the abundant Mg II ions in the quasars, and this would also involve non-visible light spanning the rest of the electromagnetic spectrum — For example, X-rays, gamma rays and ultraviolet rays. The Mg II ions have a signature ‘doublet’ feature, which is a prominent pair of lines on their absorption spectrum that come as quite the convenience to your professional spectral analyst.

Now, one could hold a stance that abuses the anthropic principle and just say that this has happened, simply because it, much like anything else, can. And of course, it’s easy to dismiss it as a chance event and wash our hands clean of the whole problem. The thing is, the likelihood of all these Mg-II absorption systems aligning to form this large of a structure is less than 0.0003% — Certainly a small enough chance to raise a few eyebrows.

And indeed, many eyebrows are cocked; high enough to hide the average Physicist’s receding hairline. But why?

Remember at the top when I said this discovery had the potential to start an upheaval against current cosmological theories? Truly, our current theory of the universe’s formation is not able to plausibly predict or even allow the existence of this large a structure.

The Cosmological Principle, the “central dogma” of cosmology as it were, describes our universe’s inception, growth and distribution of matter. As it is widely agreed upon that the universe expanded from a singularity (a single point in space that, oddly enough, encapsulated all of space at the point of the Big Bang), it follows that the cosmos must be isotropic and homogeneous. Ergo, the large-scale structure and distribution of matter in the universe more or less looks the same when the observer looks in any direction, from any given point (albeit allowing a tiny level of fluctuation, which are inflated from the tiny quantum fluctuations in the very early universe; these fluctuations are what makes the existence of galaxies possible).

So, it is obvious that this Giant Arc would cause a huge stir. Due to its staggering size, it appears to disrupt the homogeneity and isotropic nature of the universe. Now, it is up to the greatest minds of our generation to ponder whether there is something more to this stick in the mud, or if this is merely all a coincidence.

Lopez explains, “When viewed on such a large scale, we expect to see a statistically smooth distribution of matter across the Universe based on the Cosmological Principle introduced by Einstein (to make the maths easier) that the Universe is isotropic and homogeneous. It means that the night sky, when viewed on a sufficiently large scale, should look the same, regardless of the observers’ locations or the directions in which they are looking. The key question is: what is ‘sufficiently large’?”

It is rather troublesome to know that we are now faced with a question that manages to sound so simple, and yet be so laced with elaborate consequences. Sure, we know our observable universe is impressively large, and that it is constantly expanding (at an accelerating rate, no less!), but…

… That has no bearing on the real size of the “rest” of the universe. In short, the universe could be much larger than what we can see, and the Giant Arc could be just a slightly less dainty, yet completely probable fluctuation in the overall distribution of matter.

On the other hand, however, there is a unique semblance of symmetry in this structure that is still being looked into. The fact that it formed as a nearly smooth curved “line” of galaxies rather than a spheroid group is also an anomaly worth showering with academic affection.

This is a fairly new development, having only been presented by Lopez at the 238th virtual meeting of the American Astronomical Society, which was held from 7–9 June this year. The Physics community will have a field day if it is found that this lump in space has a legitimate reason behind it. That being said, it would be equally — if not more — exciting to find out that it’s really just the severed trunk of the fabled Space Mammoth. That’d be way cooler, anyway.

Future economist of thought!

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