Seeing the universe through dark matter lenses

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Here is a riddle for the riddle-loving Sphinx: What is the ring which is made of galaxies, and what is the lens which is made from the invisible? If the Sphinx hadn’t surrendered her life to Oedipus and lived until Einstein published his paper regarding gravitational lensing in 1936, she would know the answer to the first part of the riddle: an Einstein ring.

Nonetheless, despite her shrewdness, she couldn’t answer the second part of the riddle with full certainty. Her best guess would be dark matter, but really, nobody knows.

The famous Einstein ring is a product of strong gravitational lensing. It occurs when a high concentration of mass with a gravitational attraction is so strong that it bends light passing by. The concentration of mass acts as a lens, focusing galaxies hidden behind it, which are tens and hundreds of megaparsecs away, onto our telescopes.

Their distorted and multiplied images form an arc or — if one’s in luck — a full gleaming circular ring.

“The first sighting of an Einstein ring took place in 1979,” sniggers the knowledgeable Sphinx, “validating Einstein’s Theory of General Relativity (GR). Since then, other gravitational lensing events have been spotted, with notable effort using my personal favourite Hubble Space Telescope.”

She relishes in explaining the other two major classes of lensing. Weak lensing: smaller distortions of background galaxies which often remain undetectable without applying statistical corrections. Weak shear signals produced in images are analysed to constrain the lensing mass. Microlensing: a relatively small lensing mass periodically channels more light to our telescopes, without producing distorted images of the light sources.

It is theorised that the Universe’s mass is comprised of 85% dark matter

Gravitational lensing has offered astronomers insight into the mass distribution of the Universe. Most observed gravitational lenses simply don’t contain enough stellar or visible mass to bend passing light rays. Therefore, astronomers propose the existence of invisible dark matter in the lensing masses.

“So, I am correct,” cackles the Sphinx, “dark matter indeed makes up the invisible lens.” Before we discuss whether the Sphinx’s statement is truly indisputable, we must have a general idea of what dark matter is, or could be. It is theorised that the Universe’s mass is comprised of 85% dark matter.

Dark matter is a non-interactive, non-baryonic and hardly detectable form of matter which contributes massively to the Universes’ gravitational field.

Dark matter could exist as massive compact halo objects (MACHOs). Similar to dark matter, MACHOs are non-luminous astronomical bodies.

However, MACHOs can also be constituted of dim stellar objects such as brown dwarfs and neutron stars, which are made of baryonic matter. Astronomical observations of gravitational lensing and the cosmic microwave background (CMB) have also indicated that existent MACHOs cannot account for the overwhelming abundance of dark matter in the Universe.

Dark matter could also exist as a form of weakly interacting massive particles (WIMPs). The WIMP theory is more widely accepted and theoretically plausible. It states that dark matter is a new elementary particle, yet it is excluded by the Standard Model of Particle Physics.

And last, dark matter could just… not exist at all. Numerous modified gravity (MOG) theories suggest that, outside the four fundamental forces of nature, there are additional forces which are misconstrued as dark matter. Most plausible MOGs are alternatives to GR or consider it as a special case.

The most recent theory postulates an undiscovered new form of gravity as a potential ‘fifth force’

A prominent contestant of GR and the hypothesis of dark matter is the theories of modified Newtonian dynamics (MOND). They modify Newton’s law of gravitation by replacing the inverse square dependence on distance with an inverse linear relation.

The most recent theory postulates an undiscovered new form of gravity as a potential ‘fifth force’. Valiant effort by two researchers from Czech Academy of Science, Skordis, and Zlosnik, proved that this particular theory of MOND matches both gravitational lensing and cosmic microwave background data.

Nevertheless, the physics community still holds by GR and the dark matter hypothesis. The former has simply been proven correct through uncountably many experiments, and the latter pertains to a systematic and satisfying model of cosmology. In addition, MOGs, especially MONDs, fail to construct any feasible or meaningful cosmological models.

So, is the Sphinx correct? We won’t know until the day dark matter and its (non-)existence are fully understood.

For now, we shall find harmony in our differences, and together, gaze up at the coldly glimmering stellar rings, scattered across the ever-expanding dark night sky.

Image: Xiaoyao Yin

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