Home SCIENCE Ask Ethan: Is our Universe truly matter-dominated? | by Ethan Siegel | Starts With A Bang! | Oct, 2023

Ask Ethan: Is our Universe truly matter-dominated? | by Ethan Siegel | Starts With A Bang! | Oct, 2023


Many examples of stars, nebulae, gas, dust, and other forms of matter can be seen interacting both within the Milky Way and beyond. In every instance, we see lots of evidence for absorption and emission, but no evidence that any astrophysical object is primarily made up of antimatter as opposed to matter. (Credit: Hubble Heritage Team (AURA/STScI), C.R. O’Dell (Vanderbilt), NASA)

The laws of physics don’t prefer matter over antimatter. So how can we be certain that distant stars & galaxies aren’t made of antimatter?

Here in our own backyard, matter is common, while antimatter is rare. In fact, except for high-energy reactions that produce equal amounts of matter and antimatter — things like electron-positron pairs, for example — there’s absolutely no antimatter found anywhere that we look. All of the planets, stars, gas, dust, and more within our Milky Way is made of matter and not antimatter. All of the galaxies we look out at beyond our own are made of matter and not antimatter. Galaxy clusters and the large-scale cosmic web point to everything being made out of matter and not antimatter. Somehow, all of the normal stuff, the stuff of the Standard Model, is all “matter” in our Universe, with practically no antimatter at all.

Most of the time, we ask the big question of baryogenesis: how did the Universe come to be made of matter and not antimatter? But before we even get there, are we truly, absolutely certain that the Universe is made of matter, and that there isn’t some large collection of antimatter out there? That’s what Tim Thompson wants to know, asking:

“how do we know that it’s predominantly one over the other? Can we tell, from a distance, if a system is matter or antimatter? For example, for a galaxy millions of light years away, which we only observe via photons emitted, what is it that tells us its matter vs antimatter?”

It’s a great question. And thankfully, astronomy and astrophysics holds the answer.

The production of matter/antimatter pairs (left) from pure energy is a completely reversible reaction (right), with matter/antimatter annihilating back to pure energy. And yet, the fact that we see that everything in our Universe is consistent with being made of matter, and not antimatter, teaches us that perhaps something must have occurred, early on, to create a matter/antimatter asymmetry. (Credit: Dmitri Pogosyan/University of Alberta)

Whenever matter and antimatter meet in the Universe, they annihilate, and matter-antimatter annihilation produces a very specific signal. When a particle of matter collides with its antimatter counterpart, it typically results in the production of two photons (in the center-of-momentum frame of reference of the collision) with equal energies and opposite momenta. An electron annihilating with a positron, for instance, produces two photons of…

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